AP Chemistry Flashcards: Complete 9-Unit Course Review

Cartes de ce paquet

1. Carte 1

   Question

   What does one mole count?

   Réponse

   Exactly 6.02214076 × 10^23 representative particles.

2. Carte 2

   Question

   What does a peak in an element's mass spectrum represent?

   Réponse

   An isotope with a particular mass-to-charge ratio; for singly charged monatomic ions, the position tracks isotopic mass.

3. Carte 3

   Question

   What does an empirical formula show?

   Réponse

   The lowest whole-number ratio of the elements' atoms in a compound.

4. Carte 4

   Question

   How does a mixture differ from a pure substance at the particle level?

   Réponse

   A mixture contains chemically distinct representative units in variable proportions; a pure substance contains one element or compound with fixed composition. Different isotopes do not make an elemental sample a mixture.

5. Carte 5

   Question

   Which particles make up an atom's nucleus?

   Réponse

   Protons and neutrons. Electrons occupy the space outside the nucleus.

6. Carte 6

   Question

   What does a larger binding energy on a PES spectrum mean?

   Réponse

   More energy is required to remove that electron, so it is held more strongly by the nucleus.

7. Carte 7

   Question

   How does atomic radius generally change across a period and down a group?

   Réponse

   It decreases from left to right as effective nuclear charge rises, and it increases down a group as additional electron shells increase distance and shielding.

8. Carte 8

   Question

   What typical ion charge do Group 1 metals form?

   Réponse

   +1, by losing their one valence electron.

9. Carte 9

   Question

   How do you convert moles to particles?

   Réponse

   Multiply by Avogadro's number: particles = moles × 6.022 × 10^23 mol^-1.

10. Carte 10

    Question

    How is average atomic mass estimated from isotope data?

    Réponse

    Add each isotopic mass multiplied by its fractional abundance.

11. Carte 11

    Question

    How is an element's mass percent in a compound calculated?

    Réponse

    Divide the mass contributed by that element by the compound's molar mass, then multiply by 100%.

12. Carte 12

    Question

    How can measured elemental composition reveal a sample's purity?

    Réponse

    Compare the measured mass fraction with the fraction expected for the pure compound; a mismatch indicates another component.

13. Carte 13

    Question

    How do you build a ground-state electron configuration with the Aufbau principle?

    Réponse

    For ordinary ground states, move through the periodic table in atomic-number order, filling each s, p, d, or f block as it appears. The subshell capacities are s², p⁶, d¹⁰, and f¹⁴. For example, Br is [Ar] 4s² 3d¹⁰ 4p⁵.

14. Carte 14

    Question

    What does the relative area or height of an ideal PES peak indicate?

    Réponse

    The relative number of electrons in the corresponding subshell.

15. Carte 15

    Question

    How does first ionization energy generally change across a period and down a group?

    Réponse

    It increases from left to right as effective nuclear charge rises, and it decreases down a group as distance and shielding make a valence electron easier to remove.

16. Carte 16

    Question

    Why do elements in the same group form similar compounds?

    Réponse

    Their ground-state valence patterns repeat, including which outer subshells are full or partly full. That leads to similar bonding and typical ion charges.

17. Carte 17

    Question

    How do you convert a sample's mass to moles?

    Réponse

    Divide its mass by its molar mass: n = m/M.

18. Carte 18

    Question

    Which mass-spectrum interpretation lies outside the usual single-element model used in this deck?

    Réponse

    Assigning peaks in mixtures or peaks from multiply charged or polyatomic species; the standard model uses singly charged monatomic ions of one element.

19. Carte 19

    Question

    What does the law of definite proportions state?

    Réponse

    Every pure sample of a given compound has the same element mass ratios.

20. Carte 20

    Question

    Why can two samples of the same mixture have different compositions?

    Réponse

    Mixture components are physically combined, so their relative amounts are not fixed by a chemical formula.

21. Carte 21

    Question

    How does Coulomb's law connect charge and separation to attraction?

    Réponse

    Attraction grows with the magnitude of the charge product and decreases with the square of the separation distance.

22. Carte 22

    Question

    Which PES electrons usually appear at the highest binding energy?

    Réponse

    Core electrons closest to the nucleus, because they feel the strongest nuclear attraction.

23. Carte 23

    Question

    How does electron affinity generally change across a period and down a group?

    Réponse

    Electron gain generally becomes more favorable from left to right across a period and less favorable down a group as distance and shielding increase. Stable subshell patterns create substantial exceptions.

24. Carte 24

    Question

    Why are alkali metals generally more reactive down the group?

    Réponse

    Their valence electron is farther from the nucleus and easier to remove.

25. Carte 25

    Question

    How many moles are in 18.0 g of H₂O?

    Réponse

    About 0.999 mol. Use 18.0 g ÷ 18.02 g mol^-1.

26. Carte 26

    Question

    An element is 75% isotope 10 and 25% isotope 11; what is its average atomic mass?

    Réponse

    10.25 u. Calculate (0.75 × 10) + (0.25 × 11).

27. Carte 27

    Question

    A compound is 40.0% C, 6.7% H, and 53.3% O by mass; what is its empirical formula?

    Réponse

    CH₂O. For a 100 g sample, convert each mass to moles and divide by the smallest amount.

28. Carte 28

    Question

    A 10.0 g impure sample contains 8.5 g of the target compound; what is its mass-percent purity?

    Réponse

    85%. Calculate (8.5 g ÷ 10.0 g) × 100%.

29. Carte 29

    Question

    Which electrons are removed first when a transition metal forms a cation?

    Réponse

    Electrons in the occupied orbital with the highest principal quantum number: 4s before 3d. For example, Fe²⁺ is [Ar] 3d⁶.

30. Carte 30

    Question

    A PES spectrum has peaks proportional to 2, 2, and 6 electrons; which configuration fits?

    Réponse

    1s² 2s² 2p⁶, the configuration of Ne.

31. Carte 31

    Question

    How does electronegativity generally change across a period and down a group?

    Réponse

    It increases from left to right across a period and decreases down a group as atomic size and shielding increase.

32. Carte 32

    Question

    What empirical formula results from Al³⁺ and O²⁻?

    Réponse

    Al₂O₃, because two Al³⁺ ions balance three O²⁻ ions.

33. Carte 33

    Question

    How does a particle's mass in atomic mass units relate to its molar mass?

    Réponse

    The numerical value is the same: a molecular or formula-unit mass of x u corresponds to a molar mass of x g mol^-1.

34. Carte 34

    Question

    What does the tallest isotope peak usually indicate in a simple mass spectrum?

    Réponse

    The most abundant isotope, assuming comparable detection response and singly charged ions.

35. Carte 35

    Question

    How much oxygen is present in 25.0 g of a compound that is 32.0% oxygen by mass?

    Réponse

    8.00 g O. Multiply 25.0 g by 0.320.

36. Carte 36

    Question

    What does a particle diagram with two unbonded species in changing ratios represent?

    Réponse

    A mixture, because more than one particle type is present and the ratio is not fixed in a formula unit.

37. Carte 37

    Question

    What distinguishes valence electrons from core electrons?

    Réponse

    Valence electrons are available for bonding or ion formation; main-group valence electrons occupy the outermost shell, while transition metals may also use (n−1)d electrons. Core electrons mainly shield nuclear charge.

38. Carte 38

    Question

    Why can PES peak groups reveal an atom's occupied subshells?

    Réponse

    Electrons in different subshells require distinct removal energies, producing separate binding-energy groups.

39. Carte 39

    Question

    How do ion radii compare with neutral atoms and within an isoelectronic series?

    Réponse

    Cations are smaller than their neutral atoms, while anions are larger. Among species with the same electron count, more protons pull the electrons closer and produce the smaller radius.

40. Carte 40

    Question

    What formula is expected for a compound between a Group 2 metal M and a Group 17 nonmetal X?

    Réponse

    MX₂, because M forms M²⁺ and X forms X⁻.

41. Carte 41

    Question

    When is a covalent bond considered nonpolar?

    Réponse

    When the bonded atoms have identical or very similar electronegativities, so the shared electron density is distributed approximately evenly.

42. Carte 42

    Question

    Why does a bonded pair of atoms have an equilibrium bond length?

    Réponse

    At that separation, attractive and repulsive interactions balance at minimum potential energy.

43. Carte 43

    Question

    How are particles arranged in an ionic solid?

    Réponse

    Cations and anions occupy a repeating three-dimensional lattice held by electrostatic attraction.

44. Carte 44

    Question

    What model explains bonding in a metal?

    Réponse

    Positive metal cores are held together by attraction to mobile, delocalized valence electrons.

45. Carte 45

    Question

    How do you construct a Lewis diagram?

    Réponse

    Count total valence electrons, adding electrons for a negative charge and subtracting them for a positive charge. Choose a skeleton, connect atoms with single bonds, complete terminal duets or octets, and place remaining electrons on the central atom. Add multiple bonds if needed, then check the electron total and formal charges.

46. Carte 46

    Question

    What does resonance mean in a molecule or ion?

    Réponse

    Resonance uses two or more valid Lewis diagrams with the same atom arrangement but different electron placement. The actual electron distribution is a hybrid; equivalent contributors have equal weight.

47. Carte 47

    Question

    What determines molecular shape in VSEPR theory?

    Réponse

    Electron domains around the central atom arrange to minimize repulsions.

48. Carte 48

    Question

    How does an ionic bond differ from a covalent bond?

    Réponse

    Ionic bonding is attraction among oppositely charged ions in an extended structure; covalent bonding uses shared electron density between atoms.

49. Carte 49

    Question

    What happens to potential energy when bonded atoms are pushed much closer than equilibrium?

    Réponse

    Potential energy rises sharply because nucleus–nucleus and electron–electron repulsions dominate.

50. Carte 50

    Question

    Why are many ionic solids brittle?

    Réponse

    A shifted lattice can align like charges, creating strong repulsion that splits the crystal.

51. Carte 51

    Question

    What molecular shapes arise from two electron domains with no lone pairs and from three domains with zero or one lone pair?

    Réponse

    Two bonding domains give linear with a 180° angle. Three domains with no lone pairs give trigonal planar with 120° angles; replacing one bond with a lone pair gives bent with an angle slightly below 120°.

52. Carte 52

    Question

    Why are metals electrically conductive as solids?

    Réponse

    Their delocalized electrons can move through the solid when an electric field is applied.

53. Carte 53

    Question

    How is formal charge calculated for an atom in a Lewis diagram?

    Réponse

    Formal charge = valence electrons − nonbonding electrons − half the bonding electrons.

54. Carte 54

    Question

    Why can't electronegativity difference alone classify a bond as ionic or covalent?

    Réponse

    Bonding lies on a continuum. A larger difference means more ionic character, but the element types and especially the compound's properties give the best classification.

55. Carte 55

    Question

    Which shapes and bond-angle trends arise as lone pairs replace bonds in four electron domains?

    Réponse

    Four bonds give tetrahedral with ideal 109.5° angles. One lone pair gives trigonal pyramidal with smaller angles; two lone pairs give bent with typically smaller angles again because lone pairs repel more strongly than bonding pairs.

56. Carte 56

    Question

    What feature of a potential-energy curve represents bond dissociation energy?

    Réponse

    The energy difference from the curve's minimum to the separated-atoms limit.

57. Carte 57

    Question

    When does an ionic compound conduct electricity?

    Réponse

    When molten or dissolved so its ions can move; not as a rigid solid lattice.

58. Carte 58

    Question

    What is a substitutional alloy?

    Réponse

    An alloy in which atoms of a similar size replace some host-metal atoms in the lattice.

59. Carte 59

    Question

    How do two, three, and four electron domains map to hybridization?

    Réponse

    Two domains map to sp, three to sp², and four to sp³, with ideal angles of 180°, 120°, and 109.5°. Hybridization involving d orbitals is outside this deck’s scope.

60. Carte 60

    Question

    What usually makes one resonance contributor more favorable than another?

    Réponse

    Smaller formal-charge magnitudes, appropriate negative charge on more electronegative atoms, and complete valence shells where applicable.

61. Carte 61

    Question

    How many sigma and pi bonds are in single, double, and triple bonds?

    Réponse

    A single bond has one sigma bond; a double has one sigma and one pi bond; a triple has one sigma and two pi bonds. Head-on sigma overlap is stronger than side-by-side pi overlap.

62. Carte 62

    Question

    Why is a polar covalent bond polar?

    Réponse

    Unequal electronegativity creates an uneven sharing of electron density and partial charges.

63. Carte 63

    Question

    Which molecular shapes arise as lone pairs replace bonds in five electron domains?

    Réponse

    Five bonds give trigonal bipyramidal; four bonds and one lone pair give seesaw; three bonds and two lone pairs give T-shaped; two bonds and three lone pairs give linear.

64. Carte 64

    Question

    How do ionic charge and ionic radius affect attraction between ions?

    Réponse

    Larger charge magnitudes and smaller ionic radii produce stronger attraction because the charge product increases and the ion centers are closer.

65. Carte 65

    Question

    Why do ionic solids often have high melting points?

    Réponse

    Many strong Coulombic attractions throughout the lattice must be overcome to free the ions.

66. Carte 66

    Question

    What is an interstitial alloy?

    Réponse

    A smaller atom occupies holes between host-metal atoms, often making lattice layers harder to slide.

67. Carte 67

    Question

    What shape has six bonding domains and no lone pairs on the central atom?

    Réponse

    Octahedral.

68. Carte 68

    Question

    Which elements commonly form incomplete octets in stable Lewis diagrams?

    Réponse

    Hydrogen forms a duet, and electron-deficient central atoms such as boron or beryllium can have fewer than eight electrons.

69. Carte 69

    Question

    How do bond order and atomic size affect covalent bond length and strength?

    Réponse

    Within a comparable bond family, higher bond order gives shorter, stronger bonds. Larger bonded atoms generally give longer bonds, which are often weaker because their orbitals overlap less effectively.

70. Carte 70

    Question

    What bonding model best fits a sample that is malleable and conducts as a solid?

    Réponse

    Metallic bonding with mobile, delocalized electrons and nondirectional attractions.

71. Carte 71

    Question

    What shape has six electron domains, five bonds, and one lone pair?

    Réponse

    Square pyramidal.

72. Carte 72

    Question

    Which lattice should have stronger attractions: MgO or NaCl, assuming similar separations?

    Réponse

    MgO, because the charge product for Mg²⁺ and O²⁻ is larger than for Na⁺ and Cl⁻.

73. Carte 73

    Question

    Why are pure metals often malleable?

    Réponse

    Metal cores can shift while the mobile electron sea maintains nondirectional attraction instead of exposing fixed like-charge planes.

74. Carte 74

    Question

    What is the best Lewis structure for CO₂?

    Réponse

    O=C=O, with two lone pairs on each oxygen and no formal charges.

75. Carte 75

    Question

    What shape has six electron domains, four bonds, and two opposite lone pairs?

    Réponse

    Square planar.

76. Carte 76

    Question

    What limitation does an odd total number of valence electrons create for a Lewis diagram?

    Réponse

    At least one electron must remain unpaired, so not every atom can have a complete paired-electron octet.

77. Carte 77

    Question

    What does a higher bond order do to a bond's potential-energy curve?

    Réponse

    It generally places the minimum at a shorter internuclear distance and makes the well deeper, corresponding to a shorter bond and a larger bond-dissociation energy.

78. Carte 78

    Question

    When can a carbon–carbon double bond produce geometric isomers?

    Réponse

    When each carbon has two different substituents. The pi bond restricts rotation, so distinct spatial arrangements can persist.

79. Carte 79

    Question

    When may a third-period central atom exceed an octet in a Lewis diagram?

    Réponse

    When the valid electron count and lower formal charges favor an expanded valence shell, as in species such as SF₆.

80. Carte 80

    Question

    How do you decide whether a molecule with polar bonds is polar overall?

    Réponse

    Add the bond-dipole vectors using the molecular shape; symmetry may cancel them, while an asymmetric arrangement leaves a net dipole.

81. Carte 81

    Question

    Which interparticle forces act between all atoms and molecules?

    Réponse

    London dispersion forces, caused by temporary and induced dipoles.

82. Carte 82

    Question

    What four broad solid types does this deck compare?

    Réponse

    Ionic, metallic, molecular, and covalent-network solids.

83. Carte 83

    Question

    How do gas particles differ from liquid particles?

    Réponse

    Gas particles are much farther apart and move independently; liquid particles stay close but can move past one another.

84. Carte 84

    Question

    What relationship connects pressure, volume, amount, and temperature for an ideal gas?

    Réponse

    PV = nRT, with absolute temperature in kelvins and units consistent with R.

85. Carte 85

    Question

    What does temperature measure in kinetic molecular theory?

    Réponse

    The particles' average translational kinetic energy.

86. Carte 86

    Question

    What two ideal-gas assumptions fail most clearly for real gases?

    Réponse

    Particles have nonzero volume and experience intermolecular attractions.

87. Carte 87

    Question

    How is molarity defined?

    Réponse

    Moles of solute per liter of solution: M = n/V.

88. Carte 88

    Question

    What must a correct particulate diagram of NaCl(aq) show?

    Réponse

    Separated Na⁺ and Cl⁻ ions in a 1:1 ratio, each surrounded by oriented water molecules.

89. Carte 89

    Question

    Which separation method removes an insoluble solid from a liquid?

    Réponse

    Filtration: the solid stays as residue while the liquid passes as filtrate.

90. Carte 90

    Question

    What does “like dissolves like” mean at the particle level?

    Réponse

    A solute tends to dissolve when new solute–solvent attractions can compete with the attractions disrupted in the pure substances.

91. Carte 91

    Question

    What happens when matter absorbs electromagnetic radiation?

    Réponse

    Its particles move to an allowed higher-energy state when the photon energy matches the energy gap.

92. Carte 92

    Question

    Which equations connect photon energy, frequency, and wavelength?

    Réponse

    E = hν and c = λν.

93. Carte 93

    Question

    What is the Beer–Lambert law?

    Réponse

    A = εbc: absorbance equals molar absorptivity at the chosen wavelength times path length times concentration.

94. Carte 94

    Question

    What molecular features generally strengthen London dispersion forces?

    Réponse

    More electrons and a more polarizable cloud strengthen temporary dipoles; greater contact area and accessible π-electron density can also strengthen the attraction.

95. Carte 95

    Question

    Why do molecular solids usually have low melting points and fail to conduct electricity?

    Réponse

    Distinct molecules are held together by relatively weak intermolecular forces, while their valence electrons stay localized in bonds and lone pairs.

96. Carte 96

    Question

    How do particles move in a solid?

    Réponse

    They vibrate about fixed positions and do not translate past one another.

97. Carte 97

    Question

    What graph shapes connect V or P with T(K) or n for an ideal gas?

    Réponse

    All four are straight lines through the origin: V versus T(K) at fixed n and P; P versus T(K) at fixed n and V; V versus n at fixed P and T; and P versus n at fixed V and T.

98. Carte 98

    Question

    At the same temperature, which gas has the greater average molecular speed: He or Xe?

    Réponse

    He. Both have the same average kinetic energy, but KE = ½mv² means the lower-mass particles move faster.

99. Carte 99

    Question

    Why do real gases deviate more at high pressure?

    Réponse

    Particles are crowded, so their own volume is no longer negligible compared with the container volume.

100. Carte 100

     Question

     Which relationship describes dilution when solute amount is conserved?

     Réponse

     M₁V₁ = M₂V₂.

101. Carte 101

     Question

     Why does an aqueous ionic solution conduct electricity?

     Réponse

     Dissolved ions are mobile and carry charge through the solution.

102. Carte 102

     Question

     Which property lets simple distillation separate two liquids?

     Réponse

     A sufficient difference in volatility or boiling point, so the vapor is enriched in the more volatile component.

103. Carte 103

     Question

     Why are many ionic compounds soluble in water but poorly soluble in a nonpolar solvent?

     Réponse

     Water can form strong ion–dipole attractions that stabilize separated ions; a nonpolar solvent cannot provide comparable attractions.

104. Carte 104

     Question

     Which molecular transition is commonly associated with microwave absorption?

     Réponse

     A transition between quantized rotational energy levels.

105. Carte 105

     Question

     What frequency corresponds to a 600. nm photon?

     Réponse

     5.00 × 10^14 s^-1. Use ν = c/λ with 600. nm = 6.00 × 10^-7 m.

106. Carte 106

     Question

     What is the absorbance to two significant figures when ε = 2.0 × 10² L mol^-1 cm^-1, b = 1.00 cm, and c = 0.0020 M?

     Réponse

     0.40. Use A = εbc.

107. Carte 107

     Question

     What conditions allow hydrogen bonding between two molecules?

     Réponse

     One molecule must donate an H covalently bonded to N, O, or F, and the other must provide a lone pair on N, O, or F. A molecule can be a donor, an acceptor, or both.

108. Carte 108

     Question

     Why are covalent-network solids often very hard with high melting points?

     Réponse

     A continuous network of strong covalent bonds must be disrupted to deform or melt the solid.

109. Carte 109

     Question

     Why do a substance's solid and liquid phases usually have similar molar volumes?

     Réponse

     Their particles remain in close contact in both phases, even though liquid particles can move past one another.

110. Carte 110

     Question

     How is a gas mixture's total pressure related to its component pressures?

     Réponse

     Ptotal = ΣPi; each partial pressure is the pressure that component would exert alone in the same volume and temperature.

111. Carte 111

     Question

     What microscopic events create gas pressure?

     Réponse

     Gas particles collide with container walls and transfer momentum.

112. Carte 112

     Question

     Why do intermolecular attractions matter more for gases at low temperature?

     Réponse

     Particles move more slowly, so attractions can alter their paths and promote condensation.

113. Carte 113

     Question

     What is the final concentration after 50.0 mL of 2.00 M solution is diluted to 200.0 mL?

     Réponse

     0.500 M. Use M₂ = M₁V₁/V₂.

114. Carte 114

     Question

     What must a particulate representation of a solution communicate?

     Réponse

     The relative concentrations of its components and the particle-level interactions among those components.

115. Carte 115

     Question

     What causes components to separate in chromatography?

     Réponse

     They differ in attraction to the stationary phase and the mobile phase, so they travel at different rates.

116. Carte 116

     Question

     Why are many polar molecular solutes soluble in water?

     Réponse

     Dipole attractions or hydrogen bonds with water can replace the solute–solute and water–water attractions disrupted during mixing.

117. Carte 117

     Question

     Why does an atom produce discrete spectral lines?

     Réponse

     Its electrons can occupy only quantized energy levels, so only photons matching allowed energy differences are absorbed or emitted.

118. Carte 118

     Question

     How does photon energy change when frequency doubles?

     Réponse

     It doubles because E = hν.

119. Carte 119

     Question

     Why is a calibration curve useful in spectrophotometry?

     Réponse

     It relates measured absorbance to known concentrations, letting an unknown concentration be read by interpolation within the linear range.

120. Carte 120

     Question

     How does an ion–dipole attraction form, and how does it compare with dipole–dipole attraction?

     Réponse

     An ion attracts the oppositely charged end of a polar molecule. Ion–dipole attractions tend to be stronger than dipole–dipole attractions.

121. Carte 121

     Question

     Which solid type is usually both conductive and malleable?

     Réponse

     A metallic solid, because its delocalized electrons move and its nondirectional bonding tolerates layer shifts.

122. Carte 122

     Question

     How does a crystalline solid differ from an amorphous solid?

     Réponse

     A crystalline solid has long-range repeating order; an amorphous solid lacks that long-range periodic arrangement.

123. Carte 123

     Question

     How is a gas component's partial pressure found from mole fraction?

     Réponse

     Pi = XiPtotal.

124. Carte 124

     Question

     How does heating a fixed-volume gas affect its pressure in the ideal model?

     Réponse

     Pressure rises because faster particles collide with the walls more forcefully and frequently.

125. Carte 125

     Question

     Why can attractions make a real gas's measured pressure lower than the ideal prediction?

     Réponse

     Attractions pull approaching particles away from the walls, reducing momentum transfer during wall collisions.

126. Carte 126

     Question

     How many moles of ions result from complete dissolution of 0.20 mol CaCl₂?

     Réponse

     0.60 mol ions: 0.20 mol Ca²⁺ plus 0.40 mol Cl⁻.

127. Carte 127

     Question

     How should water orient around Cl⁻ in a particle model?

     Réponse

     Its partially positive hydrogen ends point toward Cl⁻.

128. Carte 128

     Question

     Can filtration separate dissolved components of a liquid solution?

     Réponse

     No. Dissolved particles pass through the filter with the solvent; filtration only retains an insoluble solid.

129. Carte 129

     Question

     Why do nonpolar molecular solutes often dissolve in nonpolar solvents?

     Réponse

     Both rely mainly on compatible London dispersion forces, so mixing can replace the attractions disrupted in the separate substances.

130. Carte 130

     Question

     What does a shorter absorbed wavelength imply about an energy transition?

     Réponse

     A larger energy gap because E = hc/λ.

131. Carte 131

     Question

     What is the energy of a photon with frequency 5.0 × 10^14 s^-1?

     Réponse

     3.3 × 10^-19 J. Multiply by Planck's constant: E = (6.626 × 10^-34 J·s)(5.0 × 10^14 s^-1).

132. Carte 132

     Question

     How does doubling cuvette path length affect absorbance in the linear Beer–Lambert range?

     Réponse

     Absorbance doubles if concentration and molar absorptivity stay constant.

133. Carte 133

     Question

     How can noncovalent interactions affect a large biomolecule?

     Réponse

     Attractions between molecules or between different regions of the same molecule help set its shape, which strongly affects its properties and function.

134. Carte 134

     Question

     Why does an ionic solid usually fail to conduct as a solid?

     Réponse

     Its ions are fixed in lattice positions. The same substance conducts when molten or dissolved because the ions can then move.

135. Carte 135

     Question

     Why does a gas have no definite shape or volume?

     Réponse

     Its widely spaced particles move constantly and experience minimal interparticle attraction, so they spread through the available container.

136. Carte 136

     Question

     What graph shapes show the inverse pressure–volume relationship for a fixed amount of ideal gas at constant temperature?

     Réponse

     A plot of P against V is a decreasing curve, while P against 1/V is a straight line through the origin.

137. Carte 137

     Question

     At the same temperature, do different ideal gases have different average kinetic energies?

     Réponse

     No. Average translational kinetic energy depends only on absolute temperature.

138. Carte 138

     Question

     Under which conditions is ideal-gas behavior most accurate?

     Réponse

     Low pressure and high temperature, where particles are far apart and attractions matter least.

139. Carte 139

     Question

     What particle-level feature distinguishes a solution from a heterogeneous mixture?

     Réponse

     A solution—whether solid, liquid, or gas—is uniform throughout; a heterogeneous mixture has regions or phases with different compositions.

140. Carte 140

     Question

     How should water orient around Na⁺ in a particulate model?

     Réponse

     Its partially negative oxygen end points toward Na⁺.

141. Carte 141

     Question

     In paper chromatography, why does one solute spot travel farther than another?

     Réponse

     It interacts more strongly with the mobile phase or more weakly with the stationary phase. With known phase polarities, that travel difference can reveal relative solute polarity.

142. Carte 142

     Question

     What energy competition helps explain whether an ionic solid dissolves?

     Réponse

     The energy needed to separate lattice ions competes with the energy released when ion–solvent attractions form.

143. Carte 143

     Question

     Which molecular motions commonly absorb infrared radiation?

     Réponse

     Bond vibrations whose changing dipole can interact with the radiation.

144. Carte 144

     Question

     Why must wavelength be converted to meters in c = λν when c is in m s^-1?

     Réponse

     Consistent units are required so meters cancel correctly and frequency comes out in s^-1.

145. Carte 145

     Question

     How can fingerprints on a cuvette affect a visible-light absorbance reading?

     Réponse

     They can absorb or scatter extra light, making measured absorbance too high and the inferred concentration too high.

146. Carte 146

     Question

     What causes and controls the strength of dipole–dipole attractions?

     Réponse

     Opposite partial charges on neighboring polar molecules attract. Strength increases with larger molecular dipoles and depends on how favorably the dipoles are oriented.

147. Carte 147

     Question

     Why is graphite conductive and soft while diamond is insulating and hard?

     Réponse

     Graphite has delocalized electrons within its sheets, so it conducts, and its layers can slide, so it is soft. Diamond has a rigid three-dimensional network of localized covalent bonds, making it hard and insulating.

148. Carte 148

     Question

     Why are gases much more compressible than liquids?

     Réponse

     Gas particles have large empty spaces between them; liquid particles are already close together.

149. Carte 149

     Question

     What volume does 0.500 mol CO₂ occupy at 1.00 atm and 300. K if it behaves ideally?

     Réponse

     12.3 L. Use V = nRT/P = (0.500 mol)(0.08206 L atm mol^-1 K^-1)(300. K)/(1.00 atm).

150. Carte 150

     Question

     Why does a lighter gas effuse faster than a heavier gas at the same temperature?

     Réponse

     Its particles have a higher average speed because equal average kinetic energy is shared by less mass.

151. Carte 151

     Question

     How does finite particle volume affect a real gas at very high pressure?

     Réponse

     The free volume available for particle motion is smaller than the container volume assumed by the ideal model.

152. Carte 152

     Question

     How should 250.0 mL of 0.100 M NaCl be prepared from solid NaCl?

     Réponse

     Dissolve 0.0250 mol NaCl, or 1.46 g, then dilute to exactly 250.0 mL in a volumetric flask.

153. Carte 153

     Question

     What changes in a particle diagram when a solution is diluted without losing solute?

     Réponse

     The solute-particle count stays constant while solvent volume and particle spacing increase.

154. Carte 154

     Question

     Why is fractional distillation better than simple distillation for liquids with close boiling points?

     Réponse

     Repeated vaporization–condensation steps enrich the vapor in the more volatile component more effectively.

155. Carte 155

     Question

     Why are oil and water usually immiscible?

     Réponse

     Water's strong hydrogen-bond network isn't replaced by equally strong water–oil attractions, so the substances separate into phases.

156. Carte 156

     Question

     Which molecular transition is commonly associated with ultraviolet or visible absorption?

     Réponse

     A transition between electronic energy levels.

157. Carte 157

     Question

     Which photon carries more energy, blue light or red light?

     Réponse

     Blue light, because it has shorter wavelength and higher frequency.

158. Carte 158

     Question

     Why is absorbance often measured at the wavelength of maximum absorbance in Beer–Lambert analysis?

     Réponse

     It gives the largest concentration-sensitive signal, and the flat top near the maximum makes small wavelength-setting errors less influential.

159. Carte 159

     Question

     What creates a dipole–induced-dipole attraction, and what controls its strength?

     Réponse

     A permanent dipole distorts a nearby nonpolar particle's electron cloud and creates an attractive temporary dipole. A larger permanent dipole and a more polarizable nonpolar partner make the attraction stronger.

160. Carte 160

     Question

     How do stronger intermolecular forces affect vapor pressure, boiling point, and melting point?

     Réponse

     They lower vapor pressure and raise boiling point. Melting point often rises too, but the trend is less direct because melting rearranges rather than fully separates particles.

161. Carte 161

     Question

     How do particles behave in a liquid?

     Réponse

     They stay in close contact while moving and colliding continuously. Temperature and interparticle attractions affect their arrangement and motion.

162. Carte 162

     Question

     Why must Celsius temperature be converted to kelvins in gas-law calculations?

     Réponse

     Gas-law proportionalities require an absolute temperature scale whose zero corresponds to zero extrapolated thermal motion.

163. Carte 163

     Question

     How does raising temperature change a Maxwell–Boltzmann speed distribution?

     Réponse

     The distribution broadens, its peak lowers and shifts right, and a larger fraction of particles have high speed.

164. Carte 164

     Question

     Why does the ideal-gas model treat collisions as elastic?

     Réponse

     It assumes total kinetic energy is conserved in particle–particle and particle–wall collisions.

165. Carte 165

     Question

     How many moles of solute are in 75.0 mL of a 0.400 M solution?

     Réponse

     0.0300 mol. Multiply 0.400 mol L^-1 by 0.0750 L.

166. Carte 166

     Question

     For equal solution volumes drawn at the same scale, what shows which solution is more concentrated?

     Réponse

     The more concentrated diagram contains more solute particles in that equal volume.

167. Carte 167

     Question

     How do differences in intermolecular attractions let distillation separate a liquid solution?

     Réponse

     They give the components different vapor pressures, so the vapor is enriched in the more volatile component.

168. Carte 168

     Question

     What comparison helps predict whether two liquids will be miscible?

     Réponse

     Liquids with similar types and strengths of intermolecular attractions are more likely to mix uniformly.

169. Carte 169

     Question

     How can an absorption spectrum help identify a substance?

     Réponse

     Its allowed energy gaps produce a characteristic pattern of absorbed wavelengths that can be compared with known spectra.

170. Carte 170

     Question

     How does absorbing or emitting a photon change an atom's or molecule's energy?

     Réponse

     Absorption raises the species' energy by exactly the photon energy; emission lowers it by the same amount.

171. Carte 171

     Question

     What macroscopic evidence can support that a chemical reaction occurred?

     Réponse

     Evidence can include gas formation, precipitate formation, a persistent color change, or an energy change, interpreted with particle-level changes.

172. Carte 172

     Question

     What does a net ionic equation include?

     Réponse

     Only the dissolved or reacting species that undergo chemical change; spectator ions are omitted.

173. Carte 173

     Question

     What must a correct particulate reaction diagram conserve?

     Réponse

     The number of atoms of every element and the total charge.

174. Carte 174

     Question

     What distinguishes a chemical change from a physical change?

     Réponse

     A chemical change rearranges bonds into new substances; a physical change alters state or arrangement without changing chemical identity.

175. Carte 175

     Question

     What does a balanced equation's coefficient ratio provide?

     Réponse

     The mole ratio among reacting and produced species.

176. Carte 176

     Question

     What is the equivalence point of a titration?

     Réponse

     The point where titrant and analyte have reacted in the stoichiometric ratio given by the balanced equation.

177. Carte 177

     Question

     What defines a precipitation reaction?

     Réponse

     Aqueous ions combine to form a sparingly soluble solid.

178. Carte 178

     Question

     What happens in a Brønsted–Lowry acid–base reaction?

     Réponse

     A proton transfers from the acid (donor) to the base (acceptor). In aqueous solution, H₂O can play either role.

179. Carte 179

     Question

     What does oxidation mean in a redox reaction?

     Réponse

     Loss of electrons and an increase in oxidation number.

180. Carte 180

     Question

     What particle-level change confirms that a process is chemical?

     Réponse

     Atoms rearrange into new combinations, producing substances with different compositions.

181. Carte 181

     Question

     Which ions are spectators when AgNO₃(aq) reacts with NaCl(aq)?

     Réponse

     Na⁺ and NO₃⁻. The net ionic reaction is Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

182. Carte 182

     Question

     How does a particulate diagram reveal the limiting reactant?

     Réponse

     After forming the maximum product allowed by the ratio, none of the limiting reactant remains while excess reactant particles do.

183. Carte 183

     Question

     Is melting ice a chemical or physical change?

     Réponse

     A physical change. H₂O molecules remain H₂O while their arrangement and motion change.

184. Carte 184

     Question

     How is the limiting reactant identified from given amounts?

     Réponse

     Convert each reactant to the same product amount using the balanced equation; the smaller product amount identifies the limiting reactant.

185. Carte 185

     Question

     How does an endpoint differ from an equivalence point?

     Réponse

     The endpoint is an observed signal such as indicator color change; the equivalence point is the exact stoichiometric condition.

186. Carte 186

     Question

     How is complete combustion of a hydrocarbon in excess oxygen classified, and what products form?

     Réponse

     It is a redox combustion reaction that forms CO₂ and H₂O.

187. Carte 187

     Question

     What are the conjugate acid and conjugate base in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻?

     Réponse

     NH₄⁺ is the conjugate acid of NH₃, and OH⁻ is the conjugate base of H₂O.

188. Carte 188

     Question

     What does reduction mean in a redox reaction?

     Réponse

     Gain of electrons and a decrease in oxidation number.

189. Carte 189

     Question

     Which common changes are physical rather than chemical?

     Réponse

     Phase changes and the formation or separation of mixtures are physical when each substance keeps its composition.

190. Carte 190

     Question

     How are strong soluble electrolytes written in a complete ionic equation?

     Réponse

     As separated aqueous ions; solids, liquids, gases, and weak electrolytes stay intact.

191. Carte 191

     Question

     A diagram starts with six A particles and four B₂ particles for 2A + B₂ → 2AB; what remains after completion?

     Réponse

     One B₂ remains. Six A consume three B₂ and form six AB.

192. Carte 192

     Question

     Why is dissolving NaCl in water normally classified as a physical change?

     Réponse

     Na⁺ and Cl⁻ separate and become hydrated, but retain their chemical identities. Removing the water recovers NaCl; the shift from ion–ion to ion–dipole attractions does not by itself form a new substance.

193. Carte 193

     Question

     What mass of AgCl can form from 25.0 mL of 0.200 M AgNO₃ mixed with excess Cl⁻?

     Réponse

     0.717 g AgCl. The 1:1 reaction gives 0.00500 mol AgCl; multiply by 143.32 g mol^-1.

194. Carte 194

     Question

     What calculation finds unknown analyte moles at equivalence?

     Réponse

     Use titrant moles, n = MV, then apply the balanced-reaction mole ratio.

195. Carte 195

     Question

     Which feature identifies an acid–base, redox, or precipitation reaction?

     Réponse

     Acid–base reactions transfer protons, redox reactions change oxidation numbers through electron transfer, and precipitation reactions form a sparingly soluble solid.

196. Carte 196

     Question

     What is the net ionic equation for strong acid–strong base neutralization?

     Réponse

     H⁺(aq) + OH⁻(aq) → H₂O(l).

197. Carte 197

     Question

     What is the oxidation number of sulfur in SO₄²⁻?

     Réponse

     +6. Four oxygens contribute -8 total, so sulfur must be +6 to give -2 overall.

198. Carte 198

     Question

     Why can gas bubbles alone be ambiguous evidence of reaction?

     Réponse

     Bubbles may also come from boiling or dissolved gas escaping, so the context and particle identities must support a chemical change.

199. Carte 199

     Question

     How is melting ice represented as a balanced physical-change equation?

     Réponse

     H₂O(s) → H₂O(l). The formula and atom count stay the same because only the physical state changes.

200. Carte 200

     Question

     What does a particle diagram show when no reaction occurs after two aqueous ionic solutions mix?

     Réponse

     All ions remain separated and solvated, with no new bonded particles, precipitate, or gas.

201. Carte 201

     Question

     Why is rusting iron a chemical change?

     Réponse

     Iron atoms form new iron-oxide substances through electron transfer and new bonding.

202. Carte 202

     Question

     For 2H₂O₂(aq) → 2H₂O(l) + O₂(g), what volume of O₂ forms from 0.100 mol H₂O₂ at 298 K and 1.00 atm?

     Réponse

     1.22 L O₂. The mole ratio gives 0.0500 mol O₂, then V = nRT/P.

203. Carte 203

     Question

     A 25.0 mL monoprotic acid sample requires 20.0 mL of 0.150 M NaOH; what is the acid concentration?

     Réponse

     0.120 M. At 1:1 equivalence, moles acid = 0.0200 L × 0.150 M, then divide by 0.0250 L.

204. Carte 204

     Question

     Which salts does the minimum solubility rule in this deck treat as soluble?

     Réponse

     All salts containing Na⁺, K⁺, NH₄⁺, or NO₃⁻ are treated as soluble in water.

205. Carte 205

     Question

     How are the strengths of a conjugate acid and its conjugate base related?

     Réponse

     A stronger acid has a weaker conjugate base, and a stronger base has a weaker conjugate acid.

206. Carte 206

     Question

     How are oxidation and reduction half-reactions combined into one balanced equation?

     Réponse

     Multiply them so electrons lost equal electrons gained, add the half-reactions, then cancel electrons and any identical species on both sides.

207. Carte 207

     Question

     How do molecular, complete ionic, and net ionic equations differ?

     Réponse

     Molecular equations keep compounds intact, complete ionic equations split strong soluble electrolytes, and net ionic equations remove spectators. All three conserve atoms and charge.

208. Carte 208

     Question

     How should coefficients change particle counts in a reaction diagram?

     Réponse

     They set whole-particle ratios while preserving each particle's chemical formula.

209. Carte 209

     Question

     Is separating a mixture by distillation a chemical or physical change?

     Réponse

     A physical change. Components change phase and location but keep their chemical identities.

210. Carte 210

     Question

     What equation results from Cu → Cu²⁺ + 2e⁻ and Ag⁺ + e⁻ → Ag?

     Réponse

     Cu + 2Ag⁺ → Cu²⁺ + 2Ag. Multiply the silver half-reaction by 2 and cancel 2e⁻; both atom counts and net charge then match.

211. Carte 211

     Question

     How is average reaction rate found from a reactant concentration?

     Réponse

     Use the negative concentration change divided by elapsed time, adjusted by its stoichiometric coefficient when comparing species rates.

212. Carte 212

     Question

     What does a rate law express?

     Réponse

     It shows how the measured rate depends on reactant concentrations. In rate = k[A]^m[B]^n, m and n are the orders in A and B, and m + n is the overall order.

213. Carte 213

     Question

     A plot of ln[A] versus time is linear; what is the order in A and its integrated rate law?

     Réponse

     First order: ln[A]t = ln[A]0 − kt, so the plot's slope is −k.

214. Carte 214

     Question

     What is an elementary reaction?

     Réponse

     A single step in a mechanism whose rate law follows directly from its reactant molecularity.

215. Carte 215

     Question

     What two collision conditions are needed for reaction?

     Réponse

     Sufficient collision energy and a productive molecular orientation.

216. Carte 216

     Question

     What does activation energy represent on a reaction-energy profile?

     Réponse

     The energy difference from the reactants to the transition state. The reaction coordinate tracks the step's structural progress, not elapsed time.

217. Carte 217

     Question

     What must the elementary steps of a valid mechanism do when added?

     Réponse

     Cancel intermediates and reproduce the overall balanced reaction.

218. Carte 218

     Question

     How is a proposed mechanism tested against kinetics?

     Réponse

     Its derived rate law must agree with the experimentally measured rate law.

219. Carte 219

     Question

     What does a pre-equilibrium approximation assume?

     Réponse

     A fast reversible step reaches equilibrium before a later slow step consumes its intermediate.

220. Carte 220

     Question

     What does each peak on a multistep energy profile represent?

     Réponse

     A transition state for one elementary step.

221. Carte 221

     Question

     How does a catalyst increase reaction rate?

     Réponse

     It provides an alternate mechanism with a lower activation-energy pathway.

222. Carte 222

     Question

     Why does crushing a solid reactant usually increase its reaction rate?

     Réponse

     Crushing increases exposed surface area, so more reactant particles can collide with the other reactant each second.

223. Carte 223

     Question

     How is reaction order found from initial-rate data?

     Réponse

     Compare trials where one reactant concentration changes while the others stay constant, then match the rate factor to the concentration factor.

224. Carte 224

     Question

     A plot of [A] versus time is linear; what is the order in A and its integrated rate law?

     Réponse

     Zero order: [A]t = [A]0 − kt, so the plot's slope is −k.

225. Carte 225

     Question

     What is the rate law for the elementary step 2A + B → products?

     Réponse

     rate = k[A]²[B]. This inference is valid because the step is elementary.

     

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226. Carte 226

     Question

     How does raising temperature change a Maxwell–Boltzmann energy distribution and reaction rate?

     Réponse

     The distribution shifts and broadens toward higher energies, so a larger fraction of collisions exceeds the activation-energy threshold and can react.

227. Carte 227

     Question

     How is ΔH read from a reaction-energy profile?

     Réponse

     ΔH = energy of products − energy of reactants.

228. Carte 228

     Question

     What is a reaction intermediate?

     Réponse

     A species formed in one mechanism step and consumed in a later step, so it cancels from the overall equation.

229. Carte 229

     Question

     Why can't overall reaction coefficients usually supply rate-law exponents?

     Réponse

     The overall equation hides the mechanism; exponents come from experiment unless the reaction is a stated elementary step.

230. Carte 230

     Question

     How does pre-equilibrium remove an intermediate from a rate law?

     Réponse

     Use the fast-step equilibrium relation to express the intermediate concentration in terms of stable reactants.

231. Carte 231

     Question

     What does each valley between peaks represent on a multistep profile?

     Réponse

     A reaction intermediate.

232. Carte 232

     Question

     Does a catalyst change ΔH or the equilibrium constant?

     Réponse

     No. It changes the pathway and rates, not reactant/product energies or the equilibrium composition.

233. Carte 233

     Question

     For 2A → B, how are disappearance of A and appearance of B related?

     Réponse

     Reaction rate = -(1/2)Δ[A]/Δt = Δ[B]/Δt.

234. Carte 234

     Question

     How do the units of k depend on a rate law's overall order?

     Réponse

     They must make the rate unit M s^-1: zero order uses M s^-1, first order s^-1, and second order M^-1 s^-1.

235. Carte 235

     Question

     A plot of 1/[A] versus time is linear; what is the order in A and its integrated rate law?

     Réponse

     Second order: 1/[A]t = 1/[A]0 + kt, so the plot's slope is +k.

236. Carte 236

     Question

     What is molecularity?

     Réponse

     The number of reacting particles in an elementary step, such as unimolecular or bimolecular.

237. Carte 237

     Question

     How does raising temperature affect k in the qualitative Arrhenius model?

     Réponse

     k increases, often sharply, because a larger fraction of collisions can reach the transition state. Arrhenius-equation calculations are outside this deck’s scope.

238. Carte 238

     Question

     A reactant falls from 0.80 M to 0.50 M in 30. s; what is its average disappearance rate to two significant figures?

     Réponse

     0.010 M s^-1. Use -(0.50 − 0.80) M ÷ 30. s.

239. Carte 239

     Question

     How does a catalyst differ from an intermediate in a mechanism?

     Réponse

     A catalyst is consumed early and regenerated later; an intermediate is formed early and consumed later.

240. Carte 240

     Question

     For 2NO₂ → NO₃ + NO (slow), followed by NO₃ + CO → NO₂ + CO₂ (fast), what rate law is predicted?

     Réponse

     rate = k[NO₂]². The first step is elementary and rate-limiting, so its molecularity sets the observed rate law.

241. Carte 241

     Question

     On a multistep reaction-energy profile, which feature often identifies the rate-determining step?

     Réponse

     The step with the largest activation barrier measured from its preceding valley to its peak.

242. Carte 242

     Question

     If changing [B] leaves rate unchanged, what is the order in B?

     Réponse

     Zero order, so [B]^0 = 1 in the measured rate law.

243. Carte 243

     Question

     What mechanism changes can binding, acid–base, or surface catalysis introduce?

     Réponse

     They can orient reactants, lower activation barriers, or create new bound, protonated, or deprotonated intermediates and elementary steps; the catalyst is regenerated.

244. Carte 244

     Question

     What is special about a first-order reaction's half-life?

     Réponse

     It is constant and independent of starting concentration: t1/2 = ln 2/k. Radioactive decay is a common first-order example.

245. Carte 245

     Question

     Why is a termolecular elementary collision uncommon?

     Réponse

     Three particles must collide simultaneously with suitable energy and orientation, which is much less probable than one- or two-particle events.

246. Carte 246

     Question

     On a reaction-energy profile, how are reverse activation energy, forward activation energy, and ΔH related?

     Réponse

     Ea,reverse = Ea,forward − ΔH. The reverse barrier is measured from products to the same transition state.

247. Carte 247

     Question

     Why can correct orientation matter even above the activation energy?

     Réponse

     The colliding reactive sites must align so old bonds can break and new bonds can form along the reaction pathway.

248. Carte 248

     Question

     How does detecting a proposed reaction intermediate affect a mechanism claim?

     Réponse

     It supports a mechanism that contains that intermediate, but it doesn't prove that mechanism is unique.

249. Carte 249

     Question

     For 2NO ⇌ N₂O₂ (fast equilibrium), followed by N₂O₂ + O₂ → 2NO₂ (slow), what observed rate law results?

     Réponse

     rate = kobs[NO]²[O₂]. Start with rate = k₂[N₂O₂][O₂], use [N₂O₂] = K[NO]² from the fast equilibrium, then substitute.

250. Carte 250

     Question

     What does the highest point of a one-step energy profile represent?

     Réponse

     The transition state, an unstable arrangement at the top of the activation barrier.

251. Carte 251

     Question

     What sign does q have for an endothermic system?

     Réponse

     Positive, because the system absorbs heat from the surroundings.

252. Carte 252

     Question

     How does an exothermic reaction appear on an enthalpy diagram?

     Réponse

     Products lie below reactants, so ΔH is negative.

253. Carte 253

     Question

     What condition defines thermal equilibrium?

     Réponse

     Objects in contact have the same temperature, so there is no net heat transfer.

254. Carte 254

     Question

     What equations relate heat capacity and temperature change to heat transfer?

     Réponse

     Use q = mcΔT with specific heat capacity, or q = nCₘΔT with molar heat capacity.

255. Carte 255

     Question

     Why is temperature constant during a phase-change plateau?

     Réponse

     Added or removed energy changes interparticle potential energy instead of average kinetic energy.

256. Carte 256

     Question

     What does ΔHrxn describe?

     Réponse

     The heat absorbed or released at constant pressure for the reaction exactly as written under the stated conditions.

257. Carte 257

     Question

     How is reaction enthalpy estimated from average bond enthalpies?

     Réponse

     ΔHrxn ≈ Σ(bonds broken) − Σ(bonds formed).

258. Carte 258

     Question

     What is the standard enthalpy of formation of an element in its standard state?

     Réponse

     Zero by definition.

259. Carte 259

     Question

     In a Hess’s law calculation, how should a step change when the target needs twice its reverse?

     Réponse

     Reverse the equation, double every coefficient, and multiply its ΔH by -2.

260. Carte 260

     Question

     How can energy cross a system boundary during a process?

     Réponse

     As heat or work. Heat transferred to or work done on the system increases its energy; heat transferred from or work done by the system decreases it.

261. Carte 261

     Question

     How does an endothermic reaction appear on an enthalpy diagram?

     Réponse

     Products lie above reactants, so ΔH is positive.

262. Carte 262

     Question

     How are heat gained by a system and heat lost by its surroundings related in an isolated setup?

     Réponse

     qsystem = -qsurroundings.

263. Carte 263

     Question

     In coffee-cup calorimetry, how is reaction heat related to solution heat?

     Réponse

     qrxn = -qsolution when calorimeter heat is negligible and pressure is constant.

264. Carte 264

     Question

     What heat is required to melt n moles at the melting point?

     Réponse

     q = nΔHfus.

265. Carte 265

     Question

     How does reversing a reaction change ΔH?

     Réponse

     It reverses the sign of ΔH.

266. Carte 266

     Question

     Why is breaking a bond endothermic?

     Réponse

     Energy must be supplied to separate atoms against their bonding attraction.

267. Carte 267

     Question

     How is ΔH°rxn calculated from standard enthalpies of formation?

     Réponse

     ΣνΔHf°(products) − ΣνΔHf°(reactants).

268. Carte 268

     Question

     How does multiplying an equation by 3 affect its ΔH?

     Réponse

     Multiply ΔH by 3 because enthalpy change scales with reaction amount.

269. Carte 269

     Question

     Why can an exothermic dissolution warm the solution?

     Réponse

     The solution warms because forming solute–solvent attractions releases more energy than is absorbed in separating the original particles. The net potential-energy decrease raises particle kinetic energy and temperature.

270. Carte 270

     Question

     Does an energy diagram's activation barrier determine ΔH?

     Réponse

     No. ΔH depends on reactant and product energy levels, while the barrier controls kinetics.

271. Carte 271

     Question

     Why does heat flow from a warmer object to a cooler object?

     Réponse

     Energy transfers through collisions until their average kinetic energies, and therefore temperatures, equalize.

272. Carte 272

     Question

     How much heat warms 100.0 g of water by 5.0°C?

     Réponse

     2.1 kJ. Use q = (100.0 g)(4.184 J g^-1 °C^-1)(5.0°C).

273. Carte 273

     Question

     How are the molar enthalpies of a phase change and its reverse related?

     Réponse

     They have equal magnitudes and opposite signs, such as ΔHcond = -ΔHvap and ΔHfreeze = -ΔHfus.

274. Carte 274

     Question

     How does doubling every coefficient in a thermochemical equation affect ΔH?

     Réponse

     It doubles ΔH.

275. Carte 275

     Question

     Why is forming a bond exothermic?

     Réponse

     Atoms move to a lower-potential-energy bonded arrangement and release energy.

276. Carte 276

     Question

     What formation equation defines ΔHf° for CO₂(g)?

     Réponse

     C(s, graphite) + O₂(g) → CO₂(g), forming exactly one mole from elements in standard states.

277. Carte 277

     Question

     What should happen to intermediate species when equations in a Hess’s law calculation are added?

     Réponse

     They cancel, leaving the target overall reaction.

278. Carte 278

     Question

     If the surroundings warm during a process, what is the likely sign of qsystem?

     Réponse

     Negative; the system likely released heat to the surroundings.

279. Carte 279

     Question

     For a profile with reactants at 40 kJ and products at 10 kJ, what is ΔH?

     Réponse

     -30 kJ for the reaction as drawn.

280. Carte 280

     Question

     Assuming no phase change, what determines the final temperature when two substances exchange heat in an insulated container?

     Réponse

     Energy conservation: q_warm + q_cool = 0. Use each substance's mass, heat capacity, and initial temperature to solve for the common final temperature.

281. Carte 281

     Question

     How would heat loss to the room affect an exothermic calorimetry result?

     Réponse

     The observed temperature rise is too small, so the calculated magnitude of released heat is too low.

282. Carte 282

     Question

     What heat expression covers warming a liquid without a phase change?

     Réponse

     q = mcΔT, not nΔHphase.

283. Carte 283

     Question

     If forming 1 mol of product has ΔH = -50 kJ mol^-1, what is q when 2 mol forms?

     Réponse

     -100 kJ. Use q = nΔH = (2 mol)(-50 kJ mol^-1).

284. Carte 284

     Question

     Breaking reactant bonds requires 500 kJ, and forming product bonds releases 650 kJ; what is the estimated ΔH?

     Réponse

     -150 kJ, from 500 − 650.

285. Carte 285

     Question

     For CO(g) + ½O₂(g) → CO₂(g), what is ΔH°rxn if ΔHf°[CO] = -110.5 and ΔHf°[CO₂] = -393.5 kJ mol^-1?

     Réponse

     -283.0 kJ. Use -393.5 - [-110.5 + ½(0)], since ΔHf°[O₂(g)] = 0.

286. Carte 286

     Question

     In a Hess’s law calculation, two valid steps have ΔH values +25 kJ and -60 kJ; what is the combined ΔH?

     Réponse

     -35 kJ, provided the equations add to the target reaction.

287. Carte 287

     Question

     Why is “bonds breaking releases energy” incorrect?

     Réponse

     Bond breaking absorbs energy; the overall reaction releases energy only when forming new bonds releases more than breaking old bonds requires.

288. Carte 288

     Question

     How would melting appear on an energy diagram?

     Réponse

     The liquid lies above the solid, so ΔHfus is positive; the diagram represents a physical, endothermic change.

289. Carte 289

     Question

     Can two objects at the same temperature exchange energy microscopically?

     Réponse

     Yes, but their energy transfers balance, so there is no net heat flow.

290. Carte 290

     Question

     Why must the calorimeter's heat capacity be included when it isn't negligible?

     Réponse

     The apparatus can absorb or release heat, so include q_cal = C_calΔT in the energy balance: q_process + q_solution + q_cal = 0.

291. Carte 291

     Question

     What makes chemical equilibrium dynamic?

     Réponse

     Forward and reverse reactions continue at equal rates even though macroscopic concentrations stay constant.

292. Carte 292

     Question

     For aA + bB ⇌ cC, what is the concentration-form expression for Q?

     Réponse

     Q = [C]^c / ([A]^a[B]^b), using current rather than necessarily equilibrium concentrations.

293. Carte 293

     Question

     What does K much greater than 1 indicate?

     Réponse

     Products predominate at equilibrium, though K says nothing about reaction speed.

294. Carte 294

     Question

     How does reversing a reaction change its equilibrium constant?

     Réponse

     K becomes 1/K.

295. Carte 295

     Question

     Can a reversible system reach equilibrium when it starts with only products?

     Réponse

     Yes, if the reverse reaction is possible. The equilibrium composition depends on temperature, initial amounts, and volume or pressure.

296. Carte 296

     Question

     How do Q and K predict reaction direction?

     Réponse

     Q < K shifts forward, Q > K shifts reverse, and Q = K means equilibrium.

297. Carte 297

     Question

     Which species are omitted from a heterogeneous equilibrium expression?

     Réponse

     Pure solids and pure liquids because their activities are effectively constant.

298. Carte 298

     Question

     How does increasing a dissolved reactant's concentration or a gaseous reactant's partial pressure affect equilibrium at constant temperature when other Q terms are initially unchanged?

     Réponse

     It lowers Q relative to K, so the system shifts toward products until Q = K again. Changing the amount of a pure solid or liquid omitted from Q does not cause this shift while that pure phase remains present.

299. Carte 299

     Question

     What does a flat concentration-time graph mean at equilibrium?

     Réponse

     Each concentration is constant, not necessarily equal to the others.

300. Carte 300

     Question

     For A ⇌ B in one fixed volume, a particulate model shows 16 A and 0 B initially, then 4 A and 12 B at equilibrium. What changed, what predominates, and what is Kc?

     Réponse

     The net change was forward: 12 A particles became 12 B particles. B predominates at equilibrium, and Kc = [B]/[A] = 12/4 = 3.0 because both counts come from the same fixed volume.

301. Carte 301

     Question

     What can Ksp tell you about a salt's solubility, and when can two Ksp values be compared directly?

     Réponse

     Ksp > 1 indicates a soluble salt. For salts with the same dissolution stoichiometry, a larger Ksp generally means greater molar solubility; across different stoichiometries, calculate molar solubility before comparing.

302. Carte 302

     Question

     What is the common-ion effect on solubility?

     Réponse

     Adding an ion already in the dissolution equilibrium usually decreases the solid's molar solubility.

303. Carte 303

     Question

     How does uniform dilution shift an aqueous equilibrium based on the stoichiometric powers in Q?

     Réponse

     It shifts toward the side with the larger sum of stoichiometric coefficients for dissolved species included in Q. If the sums are equal, dilution causes no shift by this effect; pure solids and liquids remain omitted.

304. Carte 304

     Question

     What happens if a reversible reaction starts with reactants only?

     Réponse

     The forward rate is initially largest; products form, the reverse rate grows, and the rates eventually become equal.

305. Carte 305

     Question

     What is the purpose of an ICE table?

     Réponse

     To organize initial, change, and equilibrium concentrations using reaction stoichiometry.

306. Carte 306

     Question

     Can a reaction with a very large K be slow?

     Réponse

     Yes. K describes thermodynamic equilibrium position, while rate depends on kinetics and activation energy.

307. Carte 307

     Question

     What happens to Q immediately after product concentration increases?

     Réponse

     Q increases; if it rises above K, the reaction shifts toward reactants.

308. Carte 308

     Question

     How does multiplying every reaction coefficient by 2 affect K?

     Réponse

     The new equilibrium constant is K².

309. Carte 309

     Question

     For A ⇌ B, Kc = 4.0 and initially [A] = 1.0 M and [B] = 0, what are the equilibrium concentrations?

     Réponse

     [A] = 0.20 M and [B] = 0.80 M. Let x form: Kc = x/(1.0 − x) = 4.0, so x = 0.80 M.

310. Carte 310

     Question

     What macroscopic properties stay constant at equilibrium?

     Réponse

     Properties such as concentration, color, and pressure remain constant when external conditions are fixed.

311. Carte 311

     Question

     How does decreasing volume shift a gaseous equilibrium?

     Réponse

     Toward the side with fewer moles of gas, if the two sides have different gaseous mole counts.

312. Carte 312

     Question

     For N₂ + 3H₂ ⇌ 2NH₃, what is Kc?

     Réponse

     Kc = [NH₃]² / ([N₂][H₂]³).

313. Carte 313

     Question

     For CaF₂(s) ⇌ Ca²⁺ + 2F⁻, how is Ksp written in terms of molar solubility s in pure water?

     Réponse

     Ksp = s(2s)² = 4s³ because [Ca²⁺] = s and [F⁻] = 2s.

314. Carte 314

     Question

     What does K much less than 1 indicate?

     Réponse

     Reactants predominate at equilibrium.

315. Carte 315

     Question

     How does decreasing a dissolved product's concentration or a gaseous product's partial pressure affect equilibrium when other Q terms are initially unchanged?

     Réponse

     It lowers Q and drives a net forward reaction until equilibrium returns. Changing the amount of a pure solid or liquid omitted from Q does not cause this shift while that phase remains.

316. Carte 316

     Question

     Does equilibrium mean the reaction has stopped?

     Réponse

     No. Both directions continue, but equal rates produce no net macroscopic change.

317. Carte 317

     Question

     Why does adding NaF reduce CaF₂ solubility?

     Réponse

     The added F⁻ raises Qsp, shifting the dissolution equilibrium toward solid CaF₂.

318. Carte 318

     Question

     For N₂ + 3H₂ ⇌ 2NH₃, what is Kp when P_N₂ = 0.50 atm, P_H₂ = 1.50 atm, and P_NH₃ = 0.25 atm?

     Réponse

     0.037. Use Kp = (P_NH₃)²/[(P_N₂)(P_H₂)³] = (0.25)²/[(0.50)(1.50)³]. Use equilibrium partial pressures directly; Kc↔Kp conversion isn't assessed.

319. Carte 319

     Question

     What happens to Q when a gaseous equilibrium mixture is compressed at constant temperature if products have fewer gas moles?

     Réponse

     Q falls relative to K, so the reaction shifts toward products.

320. Carte 320

     Question

     How do K and Q transform when a reaction is reversed, its coefficients are multiplied, or reactions are added?

     Réponse

     They follow the same algebra: reversing takes the reciprocal, multiplying every coefficient by c raises the value to the power c, and adding reactions multiplies their K or Q values.

321. Carte 321

     Question

     When is the small-x approximation acceptable?

     Réponse

     When x is small relative to the initial concentration and the final result confirms the neglected change is suitably small.

322. Carte 322

     Question

     What graph feature shows a disturbance followed by re-equilibration?

     Réponse

     A sudden or gradual concentration change followed by new constant plateaus while rates return to equality.

323. Carte 323

     Question

     If Q = 0.20 and K = 5.0, which direction is favored next?

     Réponse

     Forward, because Q < K.

324. Carte 324

     Question

     At equilibrium, are reactant and product concentrations equal?

     Réponse

     Not necessarily. They are constant, while forward and reverse rates are equal.

325. Carte 325

     Question

     CaF₂ has Ksp = 3.2 × 10^-11 in pure water; what is its molar solubility?

     Réponse

     2.0 × 10^-4 M. If the molar solubility is s, then [Ca²⁺] = s, [F⁻] = 2s, and Ksp = 4s³.

326. Carte 326

     Question

     For N₂ + 3H₂ ⇌ 2NH₃, how is Qp written?

     Réponse

     Qp = (P_NH₃)²/[(P_N₂)(P_H₂)³], using the current partial pressures rather than necessarily equilibrium values.

327. Carte 327

     Question

     How does heating shift an endothermic forward reaction?

     Réponse

     Toward products, and K increases because temperature changes the equilibrium constant.

328. Carte 328

     Question

     Why do both forward and reverse rates change as equilibrium is approached?

     Réponse

     As reactant and product concentrations change, the collision frequencies for the two directions change until their rates match.

329. Carte 329

     Question

     CaF₂ has Ksp = 3.2 × 10^-11. What is its molar solubility in 0.10 M NaF?

     Réponse

     About 3.2 × 10^-9 M. With [F⁻] ≈ 0.10 M, Ksp = [Ca²⁺][F⁻]² gives s = (3.2 × 10^-11)/(0.10)². The common ion lowers solubility but does not change Ksp at constant temperature.

330. Carte 330

     Question

     What concentration data must be used to calculate Kc?

     Réponse

     Equilibrium concentrations, each raised to its stoichiometric coefficient and excluding pure solids and liquids.

331. Carte 331

     Question

     What is a Brønsted–Lowry acid?

     Réponse

     A proton donor.

332. Carte 332

     Question

     How is pH defined?

     Réponse

     pH = -log[H₃O⁺].

333. Carte 333

     Question

     What is Ka for HA + H₂O ⇌ H₃O⁺ + A⁻?

     Réponse

     Ka = [H₃O⁺][A⁻]/[HA].

334. Carte 334

     Question

     How does stabilizing a base affect its basicity and the strength of its conjugate acid?

     Réponse

     It makes the base weaker and its conjugate acid stronger. A more stable base is less willing to accept H⁺.

335. Carte 335

     Question

     What is a Brønsted–Lowry base?

     Réponse

     A proton acceptor.

336. Carte 336

     Question

     At 25°C, what are Kw and the relationship between pH and pOH?

     Réponse

     Kw = [H₃O⁺][OH⁻] = 1.0 × 10^-14. Taking negative logarithms gives pH + pOH = 14.00.

337. Carte 337

     Question

     What is Kb for B + H₂O ⇌ BH⁺ + OH⁻?

     Réponse

     Kb = [BH⁺][OH⁻]/[B].

338. Carte 338

     Question

     Why can lowering pH increase the solubility of a salt containing a basic anion?

     Réponse

     H₃O⁺ consumes the anion, pulling the dissolution equilibrium toward more dissolved ions.

339. Carte 339

     Question

     What are conjugate acid–base pairs?

     Réponse

     Species that differ by exactly one proton.

340. Carte 340

     Question

     What is the pH of 1.0 × 10^-3 M HCl?

     Réponse

     3.00, assuming complete dissociation and negligible water contribution.

341. Carte 341

     Question

     How are pKa and pKb defined?

     Réponse

     pKa = -log Ka, and pKb = -log Kb.

342. Carte 342

     Question

     Why does acid strength increase across a row of comparable hydrides?

     Réponse

     Increasing electronegativity stabilizes the conjugate base and polarizes the H–A bond.

343. Carte 343

     Question

     What is an amphiprotic species?

     Réponse

     A species that can donate or accept a proton, such as HCO₃⁻.

344. Carte 344

     Question

     What amounts remain after a limited amount of strong base partially neutralizes weak acid HA?

     Réponse

     Subtract the reacted moles from HA and form the same number of moles of A⁻. The result gives the remaining HA and formed A⁻ amounts before any equilibrium or buffer-pH calculation.

345. Carte 345

     Question

     How are Ka, Kb, pKa, and pKb related for a conjugate pair at 25°C?

     Réponse

     KaKb = Kw = 1.0 × 10^-14, and pKa + pKb = pKw = 14.00.

346. Carte 346

     Question

     When does pH have little effect on a salt's solubility?

     Réponse

     When neither dissolved ion reacts appreciably with H₃O⁺ or OH⁻.

347. Carte 347

     Question

     How does H₂O act in HCl + H₂O → H₃O⁺ + Cl⁻ and in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻?

     Réponse

     It acts as a base in the first reaction by accepting H⁺, and as an acid in the second by donating H⁺.

348. Carte 348

     Question

     After mixing weak base B with strong acid, what controls the final solution in the three stoichiometric regimes?

     Réponse

     Excess B leaves a B/BH⁺ buffer; equimolar amounts leave BH⁺, so the solution is acidic; excess strong acid sets the pH from the remaining H₃O⁺.

349. Carte 349

     Question

     What two components make a typical weak-acid buffer?

     Réponse

     A weak acid and a significant amount of its conjugate base.

350. Carte 350

     Question

     What do the successive half-equivalence pH values approximate in a diprotic weak-acid titration?

     Réponse

     The first approximates pKa₁ and the second approximates pKa₂ because each conjugate pair has equal concentrations at its half-equivalence point.

351. Carte 351

     Question

     Which acid is stronger, one with pKa 2 or pKa 5?

     Réponse

     The acid with pKa 2; lower pKa means larger Ka.

352. Carte 352

     Question

     What is the Henderson–Hasselbalch equation?

     Réponse

     pH = pKa + log([A⁻]/[HA]).

353. Carte 353

     Question

     Why are larger binary hydrides down a group often stronger acids?

     Réponse

     The H–A bond becomes weaker as the central atom grows, so proton release is easier.

354. Carte 354

     Question

     What mainly determines buffer capacity?

     Réponse

     The concentrations of both members of the conjugate acid–base pair. Increasing both concentrations at a fixed ratio increases capacity without changing pH; capacity is best balanced for added acid and base when their concentrations are similar.

355. Carte 355

     Question

     Why does acid increase CaCO₃ solubility?

     Réponse

     H₃O⁺ converts CO₃²⁻ to HCO₃⁻ or carbonic acid species, reducing free carbonate and driving more CaCO₃ to dissolve.

356. Carte 356

     Question

     What does pH < pKa imply for a weak-acid pair?

     Réponse

     The protonated form HA predominates over A⁻.

357. Carte 357

     Question

     What happens when stoichiometrically equal amounts of a monoprotic weak acid and strong base are mixed?

     Réponse

     The weak acid is consumed to its conjugate base; at equivalence, the solution isn't a buffer containing both forms.

358. Carte 358

     Question

     What is [H₃O⁺] when pH = 4.50?

     Réponse

     3.2 × 10^-5 M, from [H₃O⁺] = 10^-pH.

359. Carte 359

     Question

     What is the pH of 0.010 M Ba(OH)₂ at 25°C?

     Réponse

     About 12.30. Complete dissociation gives [OH⁻] = 0.020 M, so pOH = 1.70. At 25°C, pH + pOH = 14.00, so pH = 12.30.

360. Carte 360

     Question

     How does a buffer respond to a small amount of added strong acid?

     Réponse

     Its conjugate base consumes H⁺, converting to the weak acid and limiting the pH change.

361. Carte 361

     Question

     Why is the equivalence-point solution basic in a monoprotic weak-acid–strong-base titration?

     Réponse

     The conjugate base produced at equivalence reacts with water to form OH⁻, so the pH is above neutral—above 7.00 at 25°C.

362. Carte 362

     Question

     How is percent ionization calculated for a weak acid or weak base?

     Réponse

     For HA, use ([H₃O⁺]equilibrium ÷ [HA]initial) × 100%. For B, use ([BH⁺]equilibrium ÷ [B]initial) × 100%, under the usual monoprotic setup.

363. Carte 363

     Question

     When is Henderson–Hasselbalch useful for an initial buffer-pH calculation?

     Réponse

     Use it when both members of a conjugate acid–base pair are present in meaningful amounts, including after in-scope stoichiometry creates a buffer. Calculating the pH change after acid or base is added to an existing buffer is outside this deck’s scope.

364. Carte 364

     Question

     Why does adding oxygen atoms usually strengthen oxyacids with the same central atom?

     Réponse

     Extra oxygens withdraw electron density and delocalize negative charge in the conjugate base.

365. Carte 365

     Question

     A prepared buffer is accidentally diluted to twice its intended volume; what happens to its pH and capacity?

     Réponse

     Its pH stays nearly the same, and its capacity per liter is halved because both component concentrations halve. The total neutralizing moles in the sample remain unchanged.

366. Carte 366

     Question

     How does adding OH⁻ affect Mg(OH)₂ solubility?

     Réponse

     It decreases solubility through the common-ion effect, shifting Mg(OH)₂(s) ⇌ Mg²⁺ + 2OH⁻ toward the solid.

367. Carte 367

     Question

     A buffer has equal [A⁻] and [HA]; what is its pH?

     Réponse

     pH = pKa because log(1) = 0.

368. Carte 368

     Question

     How should a weak acid–strong base mixture be solved before equivalence?

     Réponse

     First use mole stoichiometry; if both HA and A⁻ remain, use the resulting buffer relation.

369. Carte 369

     Question

     Why can pure neutral water have a pH other than 7.00?

     Réponse

     Kw changes with temperature. Neutrality means [H₃O⁺] = [OH⁻], while pH = 7.00 only when Kw = 1.0 × 10^-14 at 25°C.

370. Carte 370

     Question

     25.0 mL of 0.200 M HCl is diluted to 100.0 mL; what is the pH?

     Réponse

     1.301. Dilution gives [H₃O⁺] = (0.200 M)(25.0 mL)/(100.0 mL) = 0.0500 M, so pH = -log(0.0500).

371. Carte 371

     Question

     How does a buffer respond to a small amount of added strong base?

     Réponse

     The weak acid consumes OH⁻, forming conjugate base and water.

372. Carte 372

     Question

     How do you find the final pH after mixing a strong acid and strong base at 25°C?

     Réponse

     Use H₃O⁺ + OH⁻ → 2H₂O and compare their moles. Divide excess H₃O⁺ or OH⁻ by the total volume, then calculate pH or pOH from that excess concentration. Equal moles give pH 7.00 at 25°C.

373. Carte 373

     Question

     What distinguishes acid strength from acid concentration?

     Réponse

     Strength is the equilibrium tendency to donate H⁺, reflected by Ka or pKa; concentration is the amount of acid per solution volume.

374. Carte 374

     Question

     If [A⁻]/[HA] = 10, how does pH compare with pKa?

     Réponse

     pH = pKa + 1 because log 10 = 1.

375. Carte 375

     Question

     Which conjugate base is more stable, one with localized or resonance-delocalized charge?

     Réponse

     The resonance-delocalized conjugate base, which generally corresponds to the stronger acid.

376. Carte 376

     Question

     Which 1.0 L buffer has greater capacity: 1.0 mol each of HA/A⁻ or 0.10 mol each at the same ratio?

     Réponse

     The 1.0 mol pair; both have the same initial pH, but the larger amounts neutralize more added acid or base.

377. Carte 377

     Question

     For BHX(s) ⇌ BH⁺ + X⁻, why can raising pH increase the salt's solubility?

     Réponse

     OH⁻ consumes BH⁺ to form B and H₂O, so dissolution shifts right to replace BH⁺. This is a qualitative prediction, not a pH-dependent solubility calculation.

378. Carte 378

     Question

     What does pH > pKa imply for a weak-acid pair?

     Réponse

     The deprotonated form A⁻ predominates over HA.

379. Carte 379

     Question

     For HA + B ⇌ A⁻ + BH⁺, which side is favored when pKa(HA) = 4 and pKa(BH⁺) = 9?

     Réponse

     Products are favored. Proton transfer moves toward the weaker acid–base pair, and K ≈ 10^(9 − 4) = 10^5.

380. Carte 380

     Question

     What is the pOH when [OH⁻] = 2.5 × 10^-4 M?

     Réponse

     3.60, from -log(2.5 × 10^-4).

381. Carte 381

     Question

     What is the pH of 0.100 M HA when Ka = 1.0 × 10^-5?

     Réponse

     About 3.00. The ICE setup gives Ka = x²/(0.100 − x); x ≈ 1.0 × 10^-3 M, and the 1.0% change validates the approximation.

382. Carte 382

     Question

     Why does a buffer fail after too much strong acid is added?

     Réponse

     Its conjugate base is depleted, so added H⁺ is no longer consumed effectively.

383. Carte 383

     Question

     What do two clear equivalence regions on an acid titration curve suggest?

     Réponse

     At least two distinguishable titratable protons; on a clean ideal curve with exactly two equivalence regions, this is consistent with a diprotic acid.

384. Carte 384

     Question

     A buffer has pKa 4.8 and [A⁻]/[HA] = 0.10; what is pH?

     Réponse

     3.8, from 4.8 + log(0.10).

385. Carte 385

     Question

     Why is HCl stronger than HF in water despite F being more electronegative?

     Réponse

     The H–F bond is much stronger; bond strength dominates this down-group binary-acid comparison.

386. Carte 386

     Question

     Why does percent ionization increase when a weak acid is diluted?

     Réponse

     Dilution shifts ionization toward more particles, so a larger fraction ionizes even though [H₃O⁺] decreases.

387. Carte 387

     Question

     A buffer contains more HA than A⁻. Which addition can it neutralize in greater amount: strong acid or strong base?

     Réponse

     Strong base. The larger HA reserve consumes more added OH⁻; a buffer with more A⁻ than HA instead has greater capacity for added strong acid.

388. Carte 388

     Question

     Why can removing a basic anion increase a salt's molar solubility without changing Ksp?

     Réponse

     The equilibrium shifts to replace the consumed ion; Ksp remains fixed at the same temperature.

389. Carte 389

     Question

     Why can an acid–base indicator change color as pH changes?

     Réponse

     Its protonated and deprotonated forms have different colors or other observable properties, and their relative amounts change with pH.

390. Carte 390

     Question

     What buffer results from mixing 1.0 mol HA with 0.40 mol OH⁻?

     Réponse

     0.60 mol HA and 0.40 mol A⁻ remain, forming a buffer before any equilibrium calculation.

391. Carte 391

     Question

     What is the pH of 0.200 M weak base B when Kb = 2.0 × 10^-5 at 25°C?

     Réponse

     About 11.30. The ICE setup gives Kb = x²/(0.200 − x); x ≈ 2.0 × 10^-3 M OH⁻, and the 1.0% change validates the approximation.

392. Carte 392

     Question

     Why does a weak acid alone not make an effective buffer?

     Réponse

     It lacks a substantial conjugate-base reserve to consume added strong acid.

393. Carte 393

     Question

     What controls pH after excess strong base passes equivalence?

     Réponse

     The concentration of excess OH⁻ after accounting for reaction stoichiometry and total volume.

394. Carte 394

     Question

     How should an indicator be chosen for a titration?

     Réponse

     Its color-change range should fall within the steep pH change near the equivalence point.

395. Carte 395

     Question

     How can a measured pH and known pKa give a conjugate-base/acid ratio?

     Réponse

     Rearrange Henderson–Hasselbalch: [A⁻]/[HA] = 10^(pH − pKa).

396. Carte 396

     Question

     Can a weak base and its conjugate acid form a buffer?

     Réponse

     Yes, when both are present in significant amounts.

397. Carte 397

     Question

     For equal-volume buffers with the same conjugate-base/acid ratio, how does adding the same amount of strong acid affect a more concentrated versus less concentrated buffer?

     Réponse

     The concentrated buffer changes pH less because it has greater capacity.

398. Carte 398

     Question

     How does equivalence-point pH compare for strong acid–strong base, weak acid–strong base, and weak base–strong acid titrations at 25°C?

     Réponse

     Strong acid–strong base: pH 7.00. Weak acid–strong base: above 7.00 because the conjugate base reacts with water. Weak base–strong acid: below 7.00 because the conjugate acid reacts with water.

399. Carte 399

     Question

     Why should mole ratios replace concentration ratios after mixing buffer solutions?

     Réponse

     Both components share the same final volume, so that volume cancels in [A⁻]/[HA].

400. Carte 400

     Question

     How does adding a little strong acid change a buffer's conjugate-base and conjugate-acid amounts?

     Réponse

     The conjugate base decreases and its conjugate acid increases by the amount of strong acid consumed.

401. Carte 401

     Question

     What does entropy measure qualitatively?

     Réponse

     The dispersal of matter and energy among available microstates.

402. Carte 402

     Question

     How is standard reaction entropy calculated?

     Réponse

     ΔS°rxn = ΣνS°(products) − ΣνS°(reactants).

403. Carte 403

     Question

     What equation gives ΔG° from ΔH° and ΔS°, and what standard states do the degree symbols assume?

     Réponse

     ΔG° = ΔH° − TΔS°. The standard states are pure substances, 1.0 M solutions, and gases at 1 atm or 1 bar; T is in kelvins and energy units must match.

404. Carte 404

     Question

     Does thermodynamic favorability guarantee a fast reaction?

     Réponse

     No. A favorable reaction can be slow when its activation barrier is large.

405. Carte 405

     Question

     What is ΔG at equilibrium?

     Réponse

     Zero under the current conditions because there is no net driving force.

406. Carte 406

     Question

     Why can an endothermic dissolution still be thermodynamically favorable?

     Réponse

     A sufficiently positive entropy change can make TΔS exceed positive ΔH, giving negative ΔG.

407. Carte 407

     Question

     How can an unfavorable reaction be driven by a favorable one?

     Réponse

     Couple them so their equations and ΔG values add to a negative overall ΔG.

408. Carte 408

     Question

     Where does oxidation occur in every electrochemical cell?

     Réponse

     At the anode.

409. Carte 409

     Question

     How are standard cell potential and standard free energy related?

     Réponse

     ΔG° = -nFE°cell.

410. Carte 410

     Question

     What equation gives cell potential under nonstandard conditions?

     Réponse

     E = E° − (RT/nF) ln Q. When Q = 1, ln Q = 0, so E = E°.

411. Carte 411

     Question

     How is electrical charge related to current and time?

     Réponse

     q = It.

412. Carte 412

     Question

     Which phase has greater molar entropy, liquid water or ice at the same temperature?

     Réponse

     Liquid water because its molecules have more accessible arrangements and motion.

413. Carte 413

     Question

     Do elements in their standard states have zero standard molar entropy?

     Réponse

     No. Their ΔHf° is zero, but their absolute S° values are positive above 0 K.

414. Carte 414

     Question

     How do the four ΔH° and ΔS° sign combinations determine thermodynamic favorability across temperature?

     Réponse

     ΔH° < 0 and ΔS° > 0 is favorable at every temperature; ΔH° > 0 and ΔS° < 0 is thermodynamically unfavored at every temperature. If both are positive, favorability requires high temperature; if both are negative, it requires low temperature.

415. Carte 415

     Question

     What does it indicate when a thermodynamically favored process does not occur at a measurable rate?

     Réponse

     It is under kinetic control, commonly because of a high activation energy; no measurable reaction does not mean the system is at equilibrium.

416. Carte 416

     Question

     How are ΔG° and K related?

     Réponse

     ΔG° = -RT ln K.

417. Carte 417

     Question

     What two contributions compete in dissolving an ionic solid?

     Réponse

     Enthalpy changes from separating and solvating particles, and entropy changes from their new dispersal and solvent organization.

418. Carte 418

     Question

     What must cancel when coupled reactions are added?

     Réponse

     Shared intermediates, leaving the desired net reaction.

419. Carte 419

     Question

     Where does reduction occur in every electrochemical cell?

     Réponse

     At the cathode.

420. Carte 420

     Question

     What sign of E°cell indicates a favorable standard galvanic reaction?

     Réponse

     Positive E°cell, corresponding to negative ΔG°.

421. Carte 421

     Question

     If Q increases for a galvanic reaction, how does E change at fixed temperature?

     Réponse

     E decreases according to the Nernst equation. Le Châtelier's principle does not apply to an operating cell away from equilibrium; use Q and Nernst reasoning instead.

422. Carte 422

     Question

     How are moles of electrons found from charge?

     Réponse

     Moles e⁻ = q/F, where F ≈ 96485 C mol^-1 e⁻.

423. Carte 423

     Question

     How does producing more gas particles usually affect system entropy?

     Réponse

     It increases entropy because the particles have more positional microstates.

424. Carte 424

     Question

     Can a dissolution with negative ΔH be unfavorable?

     Réponse

     Yes. A sufficiently negative entropy change at the stated temperature can make ΔG positive.

425. Carte 425

     Question

     When can a process with ΔH > 0 and ΔS > 0 become favorable?

     Réponse

     At sufficiently high temperature, when TΔS exceeds ΔH.

426. Carte 426

     Question

     How does a catalyst affect ΔG?

     Réponse

     It does not change ΔG; it lowers the activation barrier for both directions.

427. Carte 427

     Question

     For A → B, ΔGf°(A) = -50 kJ mol^-1 and ΔGf°(B) = -80 kJ mol^-1. What is ΔG°rxn?

     Réponse

     -30 kJ mol^-1. Use ΣνΔGf°(products) − ΣνΔGf°(reactants) = -80 − (-50).

428. Carte 428

     Question

     Why can dissolving a gas in a liquid have a negative entropy change?

     Réponse

     Gas particles lose much of their translational freedom when confined and solvated in the liquid.

429. Carte 429

     Question

     If coupled steps have ΔG values +20 kJ and -35 kJ, what is overall ΔG?

     Réponse

     -15 kJ, so the combined process is thermodynamically favorable under those conditions.

430. Carte 430

     Question

     What role does each half-cell solution play in an electrochemical cell?

     Réponse

     It supplies dissolved redox species at an electrode interface and carries ions within its compartment. Separate compartments prevent direct mixing while the external circuit and salt bridge connect the half-cells.

431. Carte 431

     Question

     How is E°cell found from standard reduction potentials?

     Réponse

     E°cell = E°cathode − E°anode, using both tabulated values as reductions.

432. Carte 432

     Question

     How does a cell's potential magnitude change as Q approaches or moves away from K, and what is E at equilibrium?

     Réponse

     |E| falls toward zero as Q approaches K and grows as the system moves farther from equilibrium. At equilibrium, Q = K and E = 0.

433. Carte 433

     Question

     How many moles of electrons pass when 1.93 × 10^5 C flows?

     Réponse

     2.00 mol e⁻, from q/F.

434. Carte 434

     Question

     How does a salt bridge maintain charge balance in a galvanic cell?

     Réponse

     Anions migrate toward the anode compartment and cations toward the cathode compartment, countering the net charge imbalances created by the two half-reactions.

435. Carte 435

     Question

     Why does raising a substance's temperature generally increase its entropy?

     Réponse

     Energy spreads across more accessible particle energy states, increasing the number of possible microscopic arrangements.

436. Carte 436

     Question

     When can a process with ΔH < 0 and ΔS < 0 be favorable?

     Réponse

     At sufficiently low temperature, where the unfavorable -TΔS term is small.

437. Carte 437

     Question

     Why can diamond persist even though graphite is more stable at standard conditions?

     Réponse

     Conversion has a large activation barrier, so diamond is kinetically persistent.

438. Carte 438

     Question

     What do the external circuit and measuring device do in an electrochemical cell?

     Réponse

     The circuit carries electrons from anode to cathode; a voltmeter measures potential difference, while an ammeter in series measures current.

439. Carte 439

     Question

     At constant temperature, how does increasing the volume available to a gas affect its entropy?

     Réponse

     Entropy increases because the gas particles can occupy more positions in the larger space, so more microstates are accessible.

440. Carte 440

     Question

     How does reversing one coupled reaction affect its ΔG?

     Réponse

     It reverses the sign of that reaction's ΔG.

441. Carte 441

     Question

     Why is n required in ΔG° = -nFE°?

     Réponse

     It is the moles of electrons transferred per balanced reaction, linking charge flow to reaction extent.

442. Carte 442

     Question

     What makes an electrolytic cell operate?

     Réponse

     An external power source drives a thermodynamically unfavorable redox reaction; oxidation still occurs at the anode and reduction at the cathode.

443. Carte 443

     Question

     In an Mⁿ⁺/M concentration cell, which half-cell is the anode: the dilute or concentrated ion solution?

     Réponse

     The dilute half-cell. Oxidation produces Mⁿ⁺ there, while reduction consumes Mⁿ⁺ in the concentrated half-cell, so electrons flow from dilute to concentrated as the concentrations move toward equality.

444. Carte 444

     Question

     How is deposited metal mass found from current and time?

     Réponse

     Find q = It, convert q/F to moles e⁻, use the half-reaction ratio to moles metal, then multiply by molar mass.

445. Carte 445

     Question

     Given product S° total 500 J mol^-1 K^-1 and reactant total 420 J mol^-1 K^-1, what is ΔS°?

     Réponse

     +80 J mol^-1 K^-1.

446. Carte 446

     Question

     How do electrode masses change in a Zn–Cu galvanic cell?

     Réponse

     The Zn anode loses mass as Zn → Zn²⁺ + 2e⁻, while the Cu cathode gains mass as Cu²⁺ + 2e⁻ → Cu.

447. Carte 447

     Question

     What is ΔG° when ΔH° = 50 kJ mol^-1, ΔS° = 0.200 kJ mol^-1 K^-1, and T = 300 K?

     Réponse

     -10 kJ mol^-1, from ΔG° = 50 − (300)(0.200).

448. Carte 448

     Question

     Why can temperature change a solid's solubility?

     Réponse

     Temperature changes the balance of ΔH and TΔS, so it changes the free energy of dissolution and the equilibrium constant.

449. Carte 449

     Question

     What does the size of ΔG° relative to RT imply about K?

     Réponse

     ΔG° near zero gives K near 1. When |ΔG°| is much larger than RT, K is far from 1: negative ΔG° gives K ≫ 1, while positive ΔG° gives K ≪ 1.

450. Carte 450

     Question

     Bubbles form at an inert cathode in acidic solution; which half-reaction can explain them?

     Réponse

     2H⁺ + 2e⁻ → H₂(g). Gas evolution at the cathode can be direct evidence of reduction.

450 cartes

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