AP Chem · Unit 9 Progress Check

AP Chemistry Unit 9 Progress Check Applications of Thermodynamics walkthrough.

AP Chemistry Unit 9 pulls thermodynamics, equilibrium, and electrochemistry together, and the Unit 9 Progress Check MCQ is where those threads finally connect. This walkthrough covers what the questions test, the traps, and how to reason about spontaneity. Explanations only — no AP Classroom answer keys.

Updated August 2026Written by Mahmudul HasanFree · No signup

What Unit 9 covers

Unit 9 is Applications of Thermodynamics. Applications of thermodynamics is roughly 7–9% of the exam and the course’s capstone unit.

Entropy

ΔS and what increases disorder — phase changes, more gas particles, dissolution.

Gibbs free energy

ΔG = ΔH − TΔS, and what makes a process thermodynamically favourable.

Temperature dependence

When a reaction switches from unfavourable to favourable as temperature changes.

ΔG and K

The link between free energy and the equilibrium constant.

Electrochemistry

Galvanic and electrolytic cells, cell potential, and its relationship to ΔG.

Coupled reactions

Driving an unfavourable process with a favourable one.

The Progress Check MCQ: what each question type tests

These are the question patterns that recur on this Progress Check, and what each one is really asking.

Predict the sign of ΔS

Count gas moles on each side. More gas means higher entropy. Phase changes dominate.

Favourability from ΔH and ΔS

Work through the four sign combinations and which are temperature-dependent.

ΔG and K reasoning

Negative ΔG means K greater than 1. Usually conceptual rather than computational.

Cell potential

A positive cell potential means a favourable galvanic reaction; negative means electrolysis is required.

Identify oxidation and reduction

Oxidation is loss at the anode, reduction is gain at the cathode.

The Progress Check FRQ

Unit 9 free response asks you to justify whether a process is thermodynamically favourable and explain why, often across a temperature range. Points come from stating signs of ΔH and ΔS with reasoning, using ΔG = ΔH − TΔS explicitly, and connecting the result to K or to a cell potential.

Where students lose the most points

Saying “spontaneous” means fast

Thermodynamics says whether, kinetics says how quickly. A favourable reaction can be immeasurably slow.

Sign errors in ΔG = ΔH − TΔS

The subtraction and the units trip people up — ΔH in kJ, ΔS usually in J/K.

Forgetting temperature in kelvin

Every thermodynamics calculation uses K, not °C.

Mixing up anode and cathode

Oxidation always happens at the anode, in both cell types.

Treating ΔS as intuition

Justify entropy changes by counting particles and phases, not by vibes.

How to work through this unit

Before calculating anything, predict the signs of ΔH and ΔS from the reaction itself. Half of Unit 9 is answerable from signs alone.

Once you have finished the Progress Check, put your raw score into our AP Chemistry Calculator to see roughly where that pace puts you on the 1–5 scale, and use the AP Chemistry Review for the full exam format and study plan.

Frequently asked questions

Quick answers — written by humans, not a chatbot.

What does the AP Chemistry Unit 9 Progress Check cover?

Entropy, Gibbs free energy, thermodynamic favourability and its temperature dependence, the relationship between ΔG and K, and electrochemistry including galvanic and electrolytic cells.

Does a negative ΔG mean the reaction is fast?

No. A negative ΔG means the reaction is thermodynamically favourable, but the rate is a kinetics question — a favourable reaction can still be extremely slow.

How do I predict the sign of ΔS?

Count gas particles on each side and look at phase changes. More gas or a solid turning to liquid or gas means entropy increases.

Do you post AP Chemistry Unit 9 answer keys?

No. We publish walkthroughs explaining the reasoning and the common mistakes instead.