Unit 9: Thermodynamics and Electrochemistry
Practice AP Chemistry Unit 9 — Gibbs free energy, entropy, cell potential, Nernst equation, and electrolysis. Prep for AP-style FRQs on thermodynamics.
What Unit 9 Covers in AP Chemistry
Unit 9 brings together the most mathematically demanding concepts in AP Chemistry. Thermodynamics and electrochemistry are closely connected — both relate to spontaneity — and AP Chemistry FRQs frequently combine Gibbs free energy with cell potential calculations in a single multi-part question.
Core Topics in Unit 9
- Entropy and the second law — Predicting the sign of delta-S for reactions; understanding entropy at the molecular level (dispersal of energy and matter)
- Gibbs free energy — Calculating delta-G using delta-G equals delta-H minus T times delta-S; interpreting the sign of delta-G for spontaneity under different temperature conditions
- Temperature and spontaneity — Analysing all four combinations of delta-H and delta-S signs to predict spontaneity across temperature ranges
- Standard cell potential — Using standard reduction potentials to calculate E-cell; identifying cathode and anode in galvanic cells
- Relationship between delta-G and E-cell — Applying delta-G equals negative n times F times E-cell; connecting thermodynamics to electrochemistry
- Nernst equation — Calculating cell potential under non-standard conditions; understanding how concentration affects E-cell
- Electrolysis — Distinguishing electrolytic from galvanic cells; predicting products at each electrode; calculating mass deposited or gas produced using Faraday's laws
AP FRQ Approaches for Unit 9
A strong Unit 9 FRQ response connects delta-G, K, and E-cell coherently. Know the relationships: delta-G equals negative RT times ln(K), and delta-G equals negative nFE-cell. These allow you to move between spontaneity, equilibrium position, and cell voltage in a single question. Practice multi-part FRQs that ask you to calculate E-cell, determine spontaneity, and use the Nernst equation in sequence.
Electrochemistry Diagrams
AP Chemistry electrochemistry FRQs often include or ask you to draw a galvanic cell diagram. You should be able to label the anode (oxidation), cathode (reduction), direction of electron flow, direction of ion flow through the salt bridge, and the sign of each electrode. These are frequently awarded individual points.
Common Mistakes in Unit 9
- Flipping the sign of E-cell by accidentally reversing the standard reduction potential for the anode — remember to subtract, not negate
- Using the Nernst equation with incorrect units for R or forgetting to convert temperature to Kelvin
- Confusing the direction of electron flow in electrolytic vs. galvanic cells
- Predicting spontaneity from delta-H alone without accounting for the entropy term in delta-G