AP Chemistry Unit 9 Cheatsheet: Thermodynamics and Electrochemistry

Matter dispersal: Entropy increases when particles occupy larger volume or move more freely.

• Phase changes: solid → liquid → gas (ΔS > 0)
• Gas expansion at constant T: larger volume = higher S
• Moles of gas: more moles of gaseous products than reactants → ΔS > 0

Energy dispersal: Entropy increases with temperature. Higher T broadens kinetic energy distribution among particles (KMT).

Dissolution factors: Breaking ionic bonds (ΔH > 0), hydrating ions (ΔH < 0), dispersing solute (ΔS > 0). These often cancel—predictions difficult without data.

Coupled reactions: Link unfavored reaction (ΔG° > 0) to highly favored one via shared intermediate.

• Overall: ΔG°_total = ΔG°₁ + ΔG°₂ < 0
• External energy: electrical (electrolysis), light (photosynthesis)
• Biological: ATP → ADP couples to biosynthesis

Qualitative reasoning:

• Q = 1 (standard): E = E°
• Q < 1: E > E°; Q = 1: E = E°; Q > 1: E < E° (for the reaction as written).
• Q = K (equilibrium): E = 0

Important: Le Châtelier’s principle applies to electrochemical cell equilibria; concentration changes shift the equilibrium, while the Nernst equation quantifies the resulting change in cell potential.