AP Chemistry, AP exams, 7–9%

Unit 9: Thermodynamics and Electrochemistry

Unit 9 brings entropy and Gibbs free energy together with the electrochemistry that applies them. It covers thermodynamic favorability versus kinetic control, the link between free energy and the equilibrium constant, coupled reactions, and galvanic and electrolytic cells. Faraday's law connects charge passed to moles of substance produced.

Position: 9 of 9 units
Published weighting: 7–9% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Chemistry. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Entropy: Predicting the sign of entropy change from phase, particle count, and dispersal of energy and matter.
Gibbs free energy and thermodynamic favorability: Combining enthalpy and entropy, and identifying the temperature range in which a process is favorable.
Thermodynamic versus kinetic control: Why a favorable reaction can still be immeasurably slow, and what changes that.
Free energy and equilibrium: The relationship between the standard free energy change and K, and free energy of dissolution.
Galvanic and electrolytic cells: Cell diagrams, electron and ion flow, standard cell potential, and the sign of the potential in each cell type.
Nonstandard conditions and electrolysis: How concentration shifts cell potential, and using Faraday's law to relate current and time to product amount.

Study tips

How to practice Thermodynamics and Electrochemistry.

Section-specific guidance that follows from how this material is usually tested.

Build one table linking the signs of enthalpy, entropy, and free energy to temperature dependence, and reproduce it from memory.
Label a galvanic and an electrolytic cell diagram completely; direction-of-flow errors are the common loss here.
Practice Faraday's law problems with careful unit tracking from current and time through to grams.

Where this fits

Unit 9: Thermodynamics and Electrochemistry is 9 of 9 units in the AP Chemistry guide, and it carries a published weighting of 7–9% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Chemistry guide Official exam information

Sample Thermodynamics and Electrochemistry questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What is the sign of delta S when a solid sublimes directly into a gas?
1. ANegative
2. BPositive
3. CZero
4. DIt depends on the substance
Show answer
Answer: B. Positive
Gas particles are far more spread out and have many more available arrangements than particles locked in a crystal lattice, so entropy increases and delta S is positive. Answering negative would describe the reverse process, deposition. Any phase change toward a more disordered state raises entropy regardless of which substance is involved.
Key insight: Entropy rises going solid to liquid to gas, and rises when moles of gas increase.
Question 2Foundational
At which electrode does oxidation occur in an electrochemical cell?
1. AThe cathode
2. BThe salt bridge
3. CThe external wire
4. DThe anode
Show answer
Answer: D. The anode
Oxidation always occurs at the anode and reduction at the cathode, in both galvanic and electrolytic cells. What changes between the two cell types is the electrode charge, not the chemistry: the anode is negative in a galvanic cell and positive in an electrolytic one. The salt bridge is not an electrode at all; it only carries ions to keep the half-cells electrically neutral.
Key insight: Oxidation at the anode is fixed for every cell; only the electrode signs flip between cell types.
Question 3Intermediate
A reaction has delta H = +50.0 kJ/mol and delta S = +100.0 J/(mol K), and both are assumed constant. Above what temperature does the reaction become thermodynamically favorable?
1. A0.500 K
2. B50.0 K
3. C500 K
4. D5000 K
Show answer
Answer: C. 500 K
Free energy becomes negative when the temperature times delta S exceeds delta H, so the crossover is at T equal to delta H divided by delta S. Converting 50.0 kJ/mol to 50000 J/mol and dividing by 100.0 J/(mol K) gives 500 K. The value 0.500 K comes from dividing without converting kilojoules to joules, which is the most common slip in this calculation.
Key insight: Enthalpy is usually tabulated in kilojoules and entropy in joules, so convert before dividing.
Question 4Intermediate
If the standard Gibbs free energy change of a reaction is negative, what does that indicate about its equilibrium constant K?
1. AK is greater than 1
2. BK is less than 1
3. CK equals 1
4. DK equals zero
Show answer
Answer: A. K is greater than 1
The standard free energy change equals the negative of RT times the natural logarithm of K, so a negative value requires the logarithm of K to be positive and therefore K to exceed 1. That means products are favored at equilibrium under standard conditions. A K of exactly 1 corresponds to a standard free energy change of zero.
Key insight: Negative standard free energy means products dominate at equilibrium; it says nothing about how fast that happens.
Question 5Advanced
A constant current of 2.00 A passes for 965 seconds through a solution containing Cu2+ ions. Using 96500 C/mol of electrons and a molar mass of 63.5 g/mol, what mass of copper is deposited?
1. A0.318 g
2. B0.635 g
3. C1.27 g
4. D63.5 g
Show answer
Answer: B. 0.635 g
Charge equals current times time, so 2.00 A for 965 s delivers 1930 C, which is 1930 divided by 96500, or 0.0200 mol of electrons. Reducing Cu2+ takes two electrons per atom, so only 0.0100 mol of copper forms, and multiplying by 63.5 g/mol gives 0.635 g. Answering 1.27 g skips the division by the two-electron requirement.
Key insight: Faraday problems run current and time to charge, charge to moles of electrons, then electrons to moles of substance using the half-reaction.

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Unit 9: Thermodynamics and Electrochemistry