Quick answer: Thermochemistry measures heat changes in reactions. This AP Chemistry worksheet on thermochemistry gives 20 problems from easy to hard: exothermic or endothermic, heat absorbed by water and specific heat of copper. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These twenty problems train the main skills of AP Chemistry Unit 6: classifying heat flow, using q = mcΔT, combining equations with Hess’s law, and calculating ΔH from formation data and bond enthalpies. Early problems check definitions and signs; later ones involve calorimeter constants, graphs and multi-step reasoning. Show every step and keep the sign of ΔH.
Level: AP Chemistry · Curriculum: AP Chem Unit 6
Problem 1 Exothermic or endothermic ★★★
Classify each as exothermic or endothermic.1. A reaction has ΔH = −92 kJ.
2. An instant ice pack gets cold when the seal is broken.
3. The products have more enthalpy than the reactants.
4. The test tube becomes hot to the touch.
Problem 2 Heat absorbed by water ★★★
Chiamaka in Lagos heats 250 g of water from 20.0 °C to 35.0 °C (c = 4.18 J/(g °C)).1. Calculate q in joules.
2. Convert it to kJ.
Problem 3 Specific heat of copper ★★★
A 100.0 g piece of copper absorbs 962 J and warms from 20.0 °C to 45.0 °C.1. Calculate the temperature change.
2. Calculate the specific heat capacity.
Problem 4 Heat from burning methane ★★★
Burning 1 mol of CH₄ releases 890 kJ (M = 16.04 g/mol).1. Write the sign of ΔH.
2. Calculate the heat released when 4.00 g of CH₄ burns.
Problem 5 Adding two steps ★★★
The enthalpy levels are shown.2. Calculate ΔH for A → C.
3. State ΔH for C → A.
Problem 6 Reversing and scaling ★★★
N₂(g) + O₂(g) → 2 NO(g) has ΔH = +180 kJ.1. State ΔH for 2 NO(g) → N₂(g) + O₂(g).
2. State ΔH for ½N₂(g) + ½O₂(g) → NO(g).
3. State ΔH for 3 N₂(g) + 3 O₂(g) → 6 NO(g).
Problem 7 Writing formation equations ★★★
1. Write the equation whose enthalpy is ΔH°f of CO₂(g).2. Write the equation for ΔH°f of ethanol, C₂H₅OH(l).
3. Explain why ΔH°f of Cl₂(g) is 0.
Problem 8 Neutralisation in a cup ★★★
Diego in Texas mixes 50.0 mL of 1.00 M HCl and 50.0 mL of 1.00 M NaOH, both at 21.0 °C. The temperature rises to 27.5 °C. Take 1.00 g/mL and 4.18 J/(g °C).1. Calculate q gained by the solution.
2. Calculate the moles of water formed.
3. Calculate ΔH in kJ/mol.
Problem 9 A cold dissolving ★★★
A student dissolves 8.00 g of NH₄NO₃ (M = 80.04 g/mol) in 100.0 g of water. The temperature falls from 22.0 °C to 16.4 °C. Use 4.18 J/(g °C) and the total mass 108.0 g.1. Calculate q for the solution.
2. Calculate ΔH of solution in kJ/mol and state its sign.
3. Explain the temperature drop.
Problem 10 Hess’s law with combustion data ★★★
Combustion enthalpies: C₂H₄(g) −1411 kJ/mol; C₂H₆(g) −1560 kJ/mol; H₂(g) −286 kJ/mol (all to CO₂(g) and H₂O(l)).1. Write the equation for C₂H₄ + H₂ → C₂H₆.
2. Calculate ΔH using the combustion data.
Problem 11 Formation enthalpies ★★★
Fe₂O₃(s) + 3 CO(g) → 2 Fe(s) + 3 CO₂(g). ΔH°f (kJ/mol): Fe₂O₃ −824.2, CO −110.5, CO₂ −393.5.1. State ΔH°f of Fe(s).
2. Calculate ΔH°.
3. State whether the reaction is exothermic.
Problem 12 Bond enthalpies for HCl ★★★
H₂ + Cl₂ → 2 HCl. Bond enthalpies (kJ/mol): H–H 436, Cl–Cl 243, H–Cl 432.1. Calculate the energy to break the bonds.
2. Calculate the energy released on forming bonds.
3. Calculate ΔH.
Problem 13 Spirit burner ★★★
A burner heats 200.0 g of water from 20.0 °C to 45.0 °C and loses 1.20 g of ethanol (M = 46.07 g/mol).1. Calculate the heat gained by the water.
2. Calculate the experimental molar enthalpy of combustion.
3. The data book value is −1367 kJ/mol; calculate the percentage of the heat that reached the water.
Problem 14 Melting and warming ice ★★★
Mei in Singapore melts 10.0 g of ice at 0 °C and warms the water to 25.0 °C. ΔHfus = 334 J/g; c = 4.18 J/(g °C).2. Calculate the heat to warm the water.
3. Calculate the total.
Problem 15 Hydrogenation by bond enthalpy ★★★
C₂H₄ + H₂ → C₂H₆. Bond enthalpies (kJ/mol): C=C 614, H–H 436, C–C 347, C–H 413.1. Calculate ΔH from bond enthalpies.
2. Compare with the value −137 kJ from Hess’s law and suggest why they differ.
Problem 16 Calorimeter with a heat capacity ★★★
Excess zinc is added to 100.0 mL of 0.100 M CuSO₄ in a calorimeter of heat capacity 25.0 J/°C. The temperature rises from 21.0 °C to 25.9 °C. Take 1.00 g/mL and 4.18 J/(g °C).1. Calculate the heat absorbed by the solution and by the calorimeter.
2. Calculate ΔH per mole of Cu²⁺ reacted.
3. Explain why zinc is used in excess.
Problem 17 Formation enthalpy from combustion ★★★
The enthalpy of combustion of methanol, CH₃OH(l), is −726 kJ/mol to CO₂(g) and H₂O(l). ΔH°f: CO₂ −393.5, H₂O(l) −285.8 kJ/mol.1. Write the balanced combustion equation.
2. Calculate ΔH°f of methanol.
Problem 18 Ethyne from its elements ★★★
Given: (1) C₂H₂(g) + 5/2 O₂(g) → 2 CO₂(g) + H₂O(l), ΔH = −1300 kJ; (2) C(s) + O₂(g) → CO₂(g), ΔH = −393.5 kJ; (3) H₂(g) + ½O₂(g) → H₂O(l), ΔH = −285.8 kJ.1. Combine the equations to give 2 C(s) + H₂(g) → C₂H₂(g).
2. Calculate ΔH.
3. State whether ethyne is more or less stable than its elements.
Problem 19 Ammonia synthesis ★★★
N₂(g) + 3 H₂(g) → 2 NH₃(g). Bond enthalpies (kJ/mol): N≡N 945, H–H 436, N–H 391.1. Calculate ΔH.
2. Calculate the heat released per kilogram of NH₃ formed (M = 17.03 g/mol).
3. Explain why the industrial reactor must remove heat.
Problem 20 Reading a temperature-time graph ★★★
A student mixes 50.0 cm³ of 2.00 M HCl and 50.0 cm³ of 2.00 M NaOH (3 min mark) in a cup.2. Read the extrapolated maximum temperature and ΔT.
3. Calculate ΔH (100.0 g, 4.18 J/(g °C)).
4. Compare with the result if the highest reading (35.4 °C) were used, and explain the difference.
