AP Chemistry — Unit 9: Applications of Thermodynamics
Practice questions, answers, and key terms for Unit 9, aligned to the College Board CED.
Exam weighting: Unit 9 is 7-9% of the AP Chemistry exam.
What AP Chemistry Unit 9 covers
The College Board Course and Exam Description breaks Unit 9 into 8 topics:
- 9.1 Introduction to Entropy
- 9.2 Absolute Entropy and Entropy Change
- 9.3 Gibbs Free Energy and Thermodynamic Favorability
- 9.4 Thermodynamic and Kinetic Control
- 9.5 Free Energy and Equilibrium
- 9.6 Coupled Reactions
- 9.7 Galvanic (Voltaic) Cells
- 9.8 Electrolysis and Faraday's Law
AP Chemistry Unit 9 practice questions
What does entropy measure in a chemical or physical system?
Entropy (S) measures the dispersal of matter and energy. Increasing dispersal = increasing entropy.
A reaction converts 1 mol of solid into 3 mol of gas. What happens to entropy, and why?
Entropy increases (ΔS > 0): more moles of gas disperse matter more widely. Gas molecules spread into a much larger accessible volume.
As temperature increases, what happens to the entropy of a gas sample? What particulate-level model explains this?
Entropy increases as higher T spreads energy over more microstates. Higher T broadens the Maxwell-Boltzmann distribution, dispersing energy.
A reaction takes 2 mol of solid reactants and produces 3 mol of gas. Before plugging into any table, can you predict the sign of ΔS°rxn — and what's the equation that confirms it?
ΔS°rxn = ΣS°products − ΣS°reactants. Predict ΔS° > 0 here: gas forms where a solid was, so entropy rises.
How does using absolute (standard molar) entropies differ from using standard enthalpies of formation when calculating ΔS°rxn?
Absolute entropies of elements are NOT zero, unlike ΔH°f for elements. Every species has a nonzero S° value.
What does it mean for a process to be 'thermodynamically favored'? What is the sign of ΔG°?
A thermodynamically favored process has ΔG° < 0; products win. Products dominate at equilibrium under standard conditions.
Ice melting is unfavorable in a freezer but happens on its own at room temperature — same ΔH° and ΔS° the whole time. Which equation explains how temperature alone flips its favorability, and which term takes over when T climbs?
ΔG° = ΔH° − TΔS°. At low T (in kelvin) the ΔH° term dominates (T is small, so −TΔS° barely matters). At high T the −TΔS° term takes over because multiplying by a big T magnifies the entropy contribution. For melting, ΔS° > 0, so cranking T makes −TΔS° large and negative — eventually dragging ΔG° below zero.
A reaction has ΔH° > 0 and ΔS° < 0. At what temperatures, if any, is it thermodynamically favored?
No temperature works: ΔG° = ΔH° − TΔS° is always positive here. Positive ΔH° minus a negative TΔS° is always positive, so it's never favored.
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Key terms in Unit 9
These 12 terms show up in the Unit 9 cards. Each one links to its definition in the AP Chemistry key-term reference.
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