AP Chemistry Unit 9 review: Thermodynamics and Electrochemistry
Unit 9 connects entropy and enthalpy to free energy, equilibrium and electrochemical cells. It is 7–9% of multiple choice. Expect to justify thermodynamic favorability, distinguish it from rate and calculate cell potentials, free-energy changes or electrolysis amounts.
Unit 9 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 7–9% of the multiple-choice section |
| Exam date | Thursday, May 6, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 7 questions in 105 minutes, worth 50%: three long questions (10 points each) and four short questions (4 points each). |
| Calculator | Both sections allow scientific or approved graphing calculators. Four-function calculators are allowed but not recommended. Bluebook includes both Desmos scientific and graphing calculators. |
| Class time | About 10–13 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 76% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 9 covers
College Board's course description splits the unit into 11 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 9.1 Introduction to Entropy | Explain entropy in terms of the dispersal of energy and accessible particle arrangements. |
| 9.2 Absolute Entropy and Entropy Change | Calculate reaction entropy from standard molar entropies and coefficients. |
| 9.3 Gibbs Free Energy and Thermodynamic Favorability | Use ΔG = ΔH − TΔS to predict thermodynamic favorability and temperature effects. |
| 9.4 Thermodynamic and Kinetic Control | Distinguish a favorable reaction from one that proceeds rapidly. |
| 9.5 Free Energy and Equilibrium | Relate standard free-energy change to the equilibrium constant. |
| 9.6 Free Energy of Dissolution | Explain dissolution from the balance of enthalpy and entropy contributions. |
| 9.7 Coupled Reactions | Add free-energy changes for coupled reactions. |
| 9.8 Galvanic (Voltaic) and Electrolytic Cells | Compare galvanic and electrolytic cells, electrode reactions and charge flow. |
| 9.9 Cell Potential and Free Energy | Calculate standard cell potential and connect it to ΔG° = −nFE°. |
| 9.10 Cell Potential Under Nonstandard Conditions | Predict changes in cell potential from reaction quotient and concentration changes. |
| 9.11 Electrolysis and Faraday’s Law | Use current, time and electron stoichiometry to calculate electrolysis products. |
Keep ΔG, ΔG°, K and cell potential consistent
Standard quantities describe standard-state conditions. For a nonstandard mixture, use Q and the actual driving force.
| Condition | Free energy and equilibrium | Cell interpretation |
|---|---|---|
| K > 1 | ΔG° < 0 because ΔG° = −RT ln K | E° > 0 for the reaction as written. |
| K = 1 | ΔG° = 0 | E° = 0. |
| K < 1 | ΔG° > 0 | E° < 0 for the reaction as written. |
| Actual equilibrium: Q = K | ΔG = 0 even when ΔG° is nonzero | Actual cell potential E = 0. |
| Q < K | ΔG < 0; net forward reaction is favorable | E > 0 for the forward cell reaction. |
| ΔH < 0, ΔS > 0 | ΔG < 0 at all positive temperatures in the constant-ΔH, ΔS model | Both terms favor the reaction. |
| ΔH > 0, ΔS > 0 | Favorable when T > ΔH/ΔS | Convert ΔS to the same energy units as ΔH. |
| Electrolysis | Moles e⁻ = It/F | Divide by electrons per deposited atom, then multiply by molar mass. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Saying both enthalpy and entropy favor every spontaneous reaction. Inspect each sign and the magnitude of TΔS. The CED identifies this reasoning error on printed p. 163.
- Treating favorable as fast. A negative ΔG says nothing by itself about the activation barrier. CED topic 9.4.
- Multiplying E° when multiplying a half-reaction. Potential is intensive; use electron counts in ΔG° and balance the overall equation. CED topic 9.9.
- Calling the anode the positive electrode in every cell. Oxidation always occurs at the anode, but its sign differs between galvanic and electrolytic cells. CED topic 9.8.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 9.3
A reaction has ΔH = +24.0 kJ mol⁻¹ and ΔS = +100 J mol⁻¹ K⁻¹. What is ΔG at 300 K?
- A+54.0 kJ mol⁻¹
- B+23.97 kJ mol⁻¹
- C−6.0 kJ mol⁻¹
- D−30.0 kJ mol⁻¹
Show the answer
C. Convert entropy to 0.100 kJ mol⁻¹ K⁻¹. ΔG = 24.0 − 300(0.100) = −6.0 kJ mol⁻¹. The entropy term outweighs the unfavorable positive enthalpy.
Question 2 · 9.9
Standard reduction potentials are +0.80 V for the cathode couple and −0.40 V for the anode couple. What is E°cell?
- A1.20 V
- B0.40 V
- C−0.40 V
- D−1.20 V
Show the answer
A. E°cell = E°reduction,cathode − E°reduction,anode = 0.80 − (−0.40) = 1.20 V. Both given values are reduction potentials, so subtract the anode value.
Question 3 · 9.9
A cell reaction transfers 2 mol electrons per mole of reaction and has E° = 0.500 V. With F = 96,485 C mol⁻¹, what is ΔG°?
- A+96.5 kJ mol⁻¹
- B−48.2 kJ mol⁻¹
- C+48.2 kJ mol⁻¹
- D−96.5 kJ mol⁻¹
Show the answer
D. ΔG° = −nFE° = −2(96,485)(0.500) = −96,485 J mol⁻¹ = −96.5 kJ mol⁻¹. Volt times coulomb is a joule.
Question 4 · 9.5
At 298 K, K = 100 for a reaction. Using R = 8.314 J mol⁻¹ K⁻¹, what is ΔG°?
- A+11.4 kJ mol⁻¹
- B−11.4 kJ mol⁻¹
- C−2.48 kJ mol⁻¹
- D−247.8 kJ mol⁻¹
Show the answer
B. ΔG° = −RT ln K = −8.314(298)ln(100) ≈ −11,410 J mol⁻¹ = −11.4 kJ mol⁻¹. Natural logarithm is required in this form of the equation.
Question 5 · 9.11
A current of 1.93 A runs for 1,000 s through a Cu²⁺ solution. Assume 100% current efficiency. With F = 96,500 C mol⁻¹ and Cu = 63.5 g mol⁻¹, what mass of Cu deposits?
- A1.27 g
- B0.318 g
- C0.635 g
- D2.54 g
Show the answer
C. Charge is It = 1,930 C, giving 1,930/96,500 = 0.0200 mol e⁻. Cu²⁺ + 2e⁻ → Cu requires two electrons per atom, so 0.0100 mol Cu deposits, with mass 0.635 g.
Question 6 · 9.7
Reaction A has ΔG = +18 kJ mol⁻¹ and reaction B has ΔG = −31 kJ mol⁻¹ under the stated conditions. If one occurrence of each is coupled, what is the combined ΔG?
- A−13 kJ mol⁻¹
- B+13 kJ mol⁻¹
- C−49 kJ mol⁻¹
- D+49 kJ mol⁻¹
Show the answer
A. Free-energy changes add for the coupled process: +18 + (−31) = −13 kJ mol⁻¹. The negative total makes the combined process thermodynamically favorable under those conditions.
How to study Unit 9
Create a sign table for ΔH and ΔS, then test it with actual temperatures. Explain which term favors or opposes the reaction. Keep the standard free-energy change separate from ΔG for an arbitrary mixture, especially when interpreting equilibrium.
For a cell problem, write oxidation and reduction half-reactions before assigning electrodes or calculating E°. Balance electrons without multiplying potentials. For electrolysis, carry units through amperes × seconds → coulombs → moles of electrons → moles and grams of product.
Ashore's Chemistry course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
- AP Chemistry: all 9 units, exam format and scores
- AP Chemistry Unit 1: Atomic Structure and Properties
- AP Chemistry Unit 2: Compound Structure and Properties
- AP Chemistry Unit 3: Properties of Substances and Mixtures
- AP Chemistry Unit 4: Chemical Reactions
- AP Chemistry Unit 5: Kinetics
- AP Chemistry Unit 6: Thermochemistry
- AP Chemistry Unit 7: Equilibrium
- AP Chemistry Unit 8: Acids and Bases
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