Unit 9: Applications of Thermodynamics
AP Chemistry: 79 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 9 unifies energy and disorder: ΔG decides whether a process is thermodynamically favored, and electrochemistry converts that favorability into voltage.
Entropy
Entropy (S) measures dispersal of energy; it increases with temperature, phase change to gas, more particles, and dissolution. Absolute entropies are tabulated (S° of elements > 0, unlike ΔH°_f). ΔS° = ΣS°(products) − ΣS°(reactants); more gas moles on the product side → positive ΔS°.
Gibbs free energy
ΔG° = ΔH° − TΔS°. Negative ΔG° = thermodynamically favored (spontaneous). The sign of ΔS° combined with ΔH° sets temperature dependence: exothermic + entropy increase favors at all T; endothermic + entropy decrease never favors; otherwise a crossover temperature exists (T = ΔH°/ΔS° at ΔG° = 0).
Free energy and equilibrium
ΔG° = −RT ln K. K > 1 ↔ ΔG° < 0. A thermodynamically favored reaction can still be slow — thermodynamic vs. kinetic control. Coupled reactions sum ΔG: an unfavorable reaction runs when paired with a favorable one.
Electrochemistry
A galvanic (voltaic) cell produces electricity from a spontaneous redox reaction; electrolytic cells consume electricity to drive nonspontaneous reactions. Anode = oxidation, cathode = reduction (in both cell types). E°cell = E°cathode − E°anode; a positive E°cell means ΔG° < 0.
Cell potential and free energy
ΔG° = −nFE° with F = 96,485 C/mol e⁻ and n = moles of electrons transferred. The Nernst equation corrects for nonstandard conditions: E = E° − (0.0592/n) log Q at 25 °C.
Electrolysis and Faraday's law
Charge (coulombs) = current (A) × time (s). Moles of electrons = charge / 96,485. Use the half-reaction stoichiometry to convert electrons to moles of metal plated or gas evolved.
Quantitative skill-set
ΔS° and ΔG° summations; crossover-temperature calculations; ΔG° = −RT ln K; E°cell from half-cell potentials; ΔG° = −nFE°; electrolysis stoichiometry via coulombs.
Exam traps
ΔG° uses standard conditions; ΔG (not ΔG°) determines actual direction, and ΔG = ΔG° + RT ln Q. Voltages never multiply by coefficients (E° is intensive) — but ΔG does scale. Spontaneity needs ΔG° < 0, not ΔH° < 0. In electrolysis, mass deposited comes from electrons, which come from charge = current × time.