AP Chemistry Unit 6 review: Thermochemistry
Unit 6 tracks energy transferred as heat during reactions, temperature changes and phase changes. It is 7–9% of multiple choice. Expect to define the system, assign the sign of heat correctly and calculate enthalpy from calorimetry, bond energies or summed reactions.
Unit 6 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 10–11 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 6 covers
College Board's course description splits the unit into 9 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 6.1 Endothermic and Exothermic Processes | Identify the system and surroundings and the direction of heat transfer. |
| 6.2 Energy Diagrams | Represent endothermic and exothermic processes with relative energy levels. |
| 6.3 Heat Transfer and Thermal Equilibrium | Explain thermal equilibrium and heat flow from warmer to cooler matter. |
| 6.4 Heat Capacity and Calorimetry | Use q = mcΔT or calorimeter heat capacity and relate measured heat to reaction enthalpy. |
| 6.5 Energy of Phase Changes | Calculate heat for phase changes and distinguish them from temperature changes. |
| 6.6 Introduction to Enthalpy of Reaction | Relate a balanced equation and ΔH to the actual amount reacting. |
| 6.7 Bond Enthalpies | Estimate reaction enthalpy from bonds broken minus bonds formed. |
| 6.8 Enthalpy of Formation | Calculate reaction enthalpy from standard enthalpies of formation. |
| 6.9 Hess’s Law | Add, reverse and scale reactions and their enthalpy changes. |
Select the heat calculation and keep its sign
Name what gains or loses heat before inserting numbers. All rows assume the stated energy model and units.
| Evidence | Use | Common sign or unit check |
|---|---|---|
| Temperature change | q = mc(Tfinal − Tinitial) | A warming sample has positive q. |
| Calorimeter constant | qcal = CcalΔT | Do not multiply by mass again if Ccal is for the whole calorimeter. |
| Insulated reaction calorimeter | qreaction = −(qsolution + qcal) | A warming solution usually indicates an exothermic reaction. |
| Phase change | q = nΔHphase | Vaporization and melting absorb heat; reverse changes release it. |
| Bond enthalpies | ΔH ≈ ΣDbroken − ΣDformed | Count every bond and every stoichiometric coefficient. |
| Formation enthalpies | ΔH°rxn = ΣnΔH°f(products) − ΣnΔH°f(reactants) | An element in its standard state has ΔH°f = 0. |
| Hess’s law | Reverse: change sign; scale: multiply ΔH | Cancel species only after adjusting whole equations. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Counting molecules instead of individual bonds. Include the number of each bond per molecule and the equation coefficient. CED printed p. 109 identifies this error.
- Giving the reaction the same heat sign as the warming solution. In the insulated model the reaction loses the heat the solution gains. CED topic 6.4.
- Saying bond breaking releases energy. Breaking a bond requires energy; forming a bond releases energy. CED topic 6.7.
- Using q = mcΔT during a constant-temperature phase change. Use the phase-change enthalpy for that interval. CED topic 6.5.
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 · 6.4
A 75.0 g solution warms by 4.00 °C. Its specific heat is 4.00 J g⁻¹ °C⁻¹. How much heat does the solution absorb?
- A0.300 kJ
- B12.0 kJ
- C3.00 kJ
- D1.20 kJ
Show the answer
D. q = mcΔT = 75.0(4.00)(4.00) = 1,200 J = 1.20 kJ. The positive sign belongs to the warming solution.
Question 2 · 6.4
An insulated calorimeter’s solution absorbs 2.40 kJ while 0.0300 mol of reactant is consumed. Neglect calorimeter heat. What is reaction ΔH per mole consumed?
- A+80.0 kJ mol⁻¹
- B−80.0 kJ mol⁻¹
- C−0.0720 kJ mol⁻¹
- D+0.0720 kJ mol⁻¹
Show the answer
B. The reaction releases 2.40 kJ, so qreaction = −2.40 kJ. Dividing by 0.0300 mol gives −80.0 kJ mol⁻¹. The solution and reaction have opposite heat signs.
Question 3 · 6.7
For H₂ + Cl₂ → 2HCl, use bond enthalpies H–H = 436, Cl–Cl = 243 and H–Cl = 431 kJ mol⁻¹. What is estimated ΔH?
- A−183 kJ mol⁻¹
- B+183 kJ mol⁻¹
- C+248 kJ mol⁻¹
- D−679 kJ mol⁻¹
Show the answer
A. Break one H–H and one Cl–Cl bond: 436 + 243 = 679 kJ. Form two H–Cl bonds: 2(431) = 862 kJ released. ΔH ≈ 679 − 862 = −183 kJ per mole of reaction as written.
Question 4 · 6.9
For A → B, ΔH = +35 kJ mol⁻¹; for B → C, ΔH = −82 kJ mol⁻¹. What is ΔH for C → A?
- A−117 kJ mol⁻¹
- B−47 kJ mol⁻¹
- C+47 kJ mol⁻¹
- D+117 kJ mol⁻¹
Show the answer
C. Adding the original equations gives A → C with ΔH = 35 − 82 = −47 kJ mol⁻¹. Reversing to C → A changes the sign to +47 kJ mol⁻¹.
Question 5 · 6.5
At its boiling point, 0.250 mol of a liquid vaporizes with ΔHvap = 32.0 kJ mol⁻¹. What heat is absorbed?
- A128 kJ
- B32.0 kJ
- C0.00781 kJ
- D8.00 kJ
Show the answer
D. q = nΔHvap = 0.250(32.0) = 8.00 kJ. Temperature remains constant during the idealized phase change, but energy still enters the sample.
Question 6 · 6.8
For CO(g) + ½O₂(g) → CO₂(g), use ΔH°f(CO) = −111 and ΔH°f(CO₂) = −394 kJ mol⁻¹. What is ΔH°rxn?
- A+283 kJ mol⁻¹
- B−283 kJ mol⁻¹
- C−505 kJ mol⁻¹
- D+505 kJ mol⁻¹
Show the answer
B. ΔH°f for O₂ in its standard state is zero. Products minus reactants gives −394 − (−111) = −283 kJ mol⁻¹. Keep the negative sign on the reactant formation enthalpy inside parentheses.
How to study Unit 6
Draw a boundary around the reaction and arrows for heat entering or leaving. Calculate the measured sample’s heat first, then translate it to reaction heat and finally to enthalpy per mole. Keep joules and kilojoules separate until the unit conversion is explicit.
For a bond calculation, draw all reactant and product structures and tally bonds in a table. For Hess’s law, write the target equation first, scale and reverse source equations, and verify species cancellation before adding their enthalpies.
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 7: Equilibrium
- AP Chemistry Unit 8: Acids and Bases
- AP Chemistry Unit 9: Thermodynamics and Electrochemistry
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