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AP Chemistry Unit 6, Thermochemistry, covers heat capacity, calorimetry, and energy changes in chemical and physical processes across 9 topics, making up 7-9% of the AP exam. You'll work through endothermic and exothermic processes, energy diagrams, enthalpy of reaction, bond enthalpies, and Hess's Law. AP Chem Unit 6 connects thermal equilibrium and phase changes to predicting how energy moves in real reactions.
AP Chemistry Unit 6, Thermochemistry, is all about tracking where energy goes when matter changes, whether ice melts in your drink or methane burns in a stove. The single biggest idea is conservation of energy: breaking bonds costs energy, forming bonds releases energy, and every joule that leaves a system shows up in the surroundings. Unit 6 makes up 7-9% of the AP exam and gives you the tools (calorimetry, bond enthalpies, formation enthalpies, Hess's Law) to calculate exactly how much heat a process absorbs or releases.
| Topic | Core idea | Key equation or tool | Watch out for |
|---|---|---|---|
| Endo/exothermic processes | Temperature changes signal energy flow between system and surroundings | Sign of ΔH (negative = exo, positive = endo) | "Feels hot" means the system is releasing energy |
| Energy diagrams | Picture the energy of reactants vs. products | Products below reactants = exothermic | Diagram shows the system's energy, not the surroundings |
| Heat transfer and equilibrium | Collisions transfer kinetic energy until temperatures match | Particle-level reasoning | Heat flows hot to cold, never the reverse spontaneously |
| Heat capacity and calorimetry | Quantify heat with mass, c, and ΔT | q = mcΔT; q(sys) = -q(surr) | Different substances need different heat for the same ΔT |
| Energy of phase changes | Temperature holds constant during a phase change | q = n × ΔH(fus or vap) | No mcΔT during the flat part of a heating curve |
| Enthalpy of reaction | ΔH scales with moles reacted | q = n × ΔH(rxn) | ΔH is per mole of reaction as written |
| Bond enthalpies | Breaking costs energy, forming releases it | ΔH ≈ Σ broken - Σ formed | These are averages, so it is an estimate |
| Enthalpy of formation | Build ΔH from tabulated formation values | ΔH° = ΣΔH°f(prod) - ΣΔH°f(react) | Elements in standard state have ΔH°f = 0 |
| Hess's Law | Enthalpy is a state function, so steps add up | Sum the ΔH of each step | Flip the sign when you reverse a reaction |
Unit 6 is where AP Chem starts answering the question "will this reaction happen, and what do we get out of it energetically?" Everything before this unit described matter and how it reacts; thermochemistry attaches an energy price tag to those changes, which is the first half of predicting reaction favorability.
Thermochemistry is 7-9% of the exam, showing up in both multiple choice and free response. Expect to do real calculations, not just identify "endo vs. exo."
AP Chem Unit 6 covers 9 topics in thermochemistry: Endothermic and Exothermic Processes, Energy Diagrams, Heat Transfer and Thermal Equilibrium, Heat Capacity and Calorimetry, Energy of Phase Changes, Introduction to Enthalpy of Reaction, Bond Enthalpies, Enthalpy of Formation, and Hess's Law. Together they build a complete picture of how energy moves in chemical and physical processes. See AP Chem Unit 6 for matched practice on each topic.
AP Chem Unit 6 makes up 7-9% of the AP exam. That slice covers thermochemistry concepts including heat capacity, calorimetry, enthalpy of reaction, bond enthalpies, enthalpy of formation, and Hess's Law. It's a focused unit, so strong performance here is very achievable with targeted practice.
The AP Chem Unit 6 progress check includes both MCQ and FRQ parts drawn from all 9 thermochemistry topics. MCQ questions test concepts like endothermic vs. exothermic processes, heat capacity, calorimetry calculations, and energy diagrams. FRQ questions typically ask you to calculate enthalpy changes using Hess's Law, bond enthalpies, or enthalpy of formation data. Practicing these topics before the progress check is the best prep. Find matched questions at AP Chem Unit 6.
AP Chem Unit 6 FRQs most often ask you to calculate enthalpy changes using Hess's Law, enthalpy of formation tables, or bond enthalpies, and to interpret calorimetry data using heat capacity equations. To practice, work through multi-step calculation problems where you show each step clearly, since College Board awards points for process, not just the final answer. Start with topic-level practice at AP Chem Unit 6, focusing on Topics 6.6 through 6.9 where FRQ prompts are most common.
For AP Chem Unit 6 practice questions, including multiple-choice and practice test style problems, head to AP Chem Unit 6. You'll find MCQ sets covering heat capacity, calorimetry, energy diagrams, and enthalpy calculations, plus FRQ practice for Hess's Law and enthalpy of formation. Working through both question types gives you the best coverage of the 7-9% exam weight this unit carries.
Start AP Chem Unit 6 by making sure you can identify endothermic and exothermic processes and read energy diagrams before moving to calculations. Then build your heat capacity and calorimetry skills, since those equations show up in both MCQ and FRQ. Once calculations feel solid, work through enthalpy of reaction, bond enthalpies, enthalpy of formation, and Hess's Law in order, because each topic builds on the last. A few concrete steps that help: - Write out Hess's Law problems by hand until flipping and scaling equations feels automatic. - Practice calorimetry problems with real data sets, not just plug-and-chug examples. - Review energy diagrams for both phase changes and reactions side by side. Find topic-by-topic practice at AP Chem Unit 6.