AP Chemistry, AP exams, 7–9%

Unit 6: Thermochemistry

Unit 6 covers energy transfer in chemical and physical processes: endothermic and exothermic behavior, heat capacity and calorimetry, phase-change energetics, and enthalpy of reaction. Bond enthalpies, enthalpies of formation, and Hess's law give three routes to the same quantity, and the exam expects you to use whichever the data supports.

Position: 6 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.

Endothermic and exothermic processes: Sign conventions, energy diagrams, and describing energy flow between system and surroundings.
Heat transfer and thermal equilibrium: How particle-level collisions move energy, and what thermal equilibrium looks like macroscopically.
Heat capacity and calorimetry: Using q equals mass times specific heat times temperature change, and interpreting calorimetry data.
Energy of phase changes: Heating curves, plateaus at phase transitions, and the enthalpies of fusion and vaporization.
Enthalpy of reaction and bond enthalpies: Estimating reaction enthalpy from bonds broken and formed, and knowing why that route is approximate.
Enthalpy of formation and Hess's law: Combining known reactions or formation values to obtain an unmeasured enthalpy change.

Study tips

How to practice Thermochemistry.

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

Fix the sign conventions once and test them on a calorimetry problem where the system loses heat.
Practice all three enthalpy routes on the same reaction so you can pick the one the given data supports.
Annotate a heating curve with what is changing (temperature or phase) in each segment and why.

Where this fits

Unit 6: Thermochemistry is 6 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 Thermochemistry 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 H for an exothermic reaction?
1. APositive
2. BNegative
3. CZero
4. DIt depends on the temperature
Show answer
Answer: B. Negative
An exothermic reaction releases energy to the surroundings, so the system ends with less enthalpy than it started with and delta H is negative. A positive delta H marks an endothermic process, in which the system absorbs energy and the surroundings cool. The sign convention is always written from the point of view of the system.
Key insight: Exothermic means the system loses energy, so delta H is negative even though the surroundings warm up.
Question 2Foundational
How much heat is required to raise the temperature of 50.0 g of liquid water by 20.0 degrees Celsius, given a specific heat capacity of 4.18 J/(g degree C)?
1. A209 J
2. B418 J
3. C4180 J
4. D41800 J
Show answer
Answer: C. 4180 J
Heat equals mass times specific heat capacity times temperature change, so q equals 50.0 times 4.18 times 20.0, which is 4180 J. The value 209 J comes from multiplying mass by specific heat capacity but forgetting the 20.0 degree temperature change. Because grams and degrees Celsius cancel, the answer carries units of joules.
Key insight: Water's large specific heat is why it takes so much energy to warm even a small sample.
Question 3Intermediate
While a pure substance is boiling at constant pressure and heat is still being added, what happens to its temperature?
1. AIt increases steadily
2. BIt decreases steadily
3. CIt increases and then decreases
4. DIt remains constant
Show answer
Answer: D. It remains constant
During a phase change the added energy goes into overcoming intermolecular attractions rather than increasing the average kinetic energy of the particles, so the thermometer reading does not move. That is why a heating curve shows a flat plateau at each transition. Temperature rises again only after the last of the liquid has become vapor.
Key insight: Sloped segments of a heating curve change temperature; flat segments change phase.
Question 4Intermediate
How is the enthalpy change of a reaction estimated from average bond enthalpies?
1. ASum of bonds broken minus sum of bonds formed
2. BSum of bonds formed minus sum of bonds broken
3. CSum of every bond enthalpy in the products only
4. DSum of bonds broken divided by sum of bonds formed
Show answer
Answer: A. Sum of bonds broken minus sum of bonds formed
Breaking bonds always costs energy and forming bonds always releases it, so the net enthalpy change is the energy put in minus the energy given back. Reversing the order is the tempting error and flips the sign of every answer. Bond enthalpies are averages taken across many molecules, so this route gives an estimate rather than an exact value.
Key insight: Bonds broken cost energy, bonds formed pay it back, and the difference is delta H.
Question 5Advanced
Given that C(s) + O2(g) -> CO2(g) has delta H = -394 kJ and that 2 CO(g) + O2(g) -> 2 CO2(g) has delta H = -566 kJ, what is delta H for 2 C(s) + O2(g) -> 2 CO(g)?
1. A-960 kJ
2. B-222 kJ
3. C-172 kJ
4. D+222 kJ
Show answer
Answer: B. -222 kJ
Doubling the first reaction gives 2 C + 2 O2 -> 2 CO2 with delta H = -788 kJ, and reversing the second gives 2 CO2 -> 2 CO + O2 with delta H = +566 kJ. Adding the two cancels the carbon dioxide and one oxygen molecule, leaving the target reaction with delta H = -788 plus 566, or -222 kJ. Answering -960 kJ adds the two given values without doubling the first or reversing the second.
Key insight: Enthalpy is a state function, so scaling a reaction scales delta H and reversing it flips the sign.

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Unit 6: Thermochemistry