Quick answer: An enthalpy change ΔH is the heat transferred at constant pressure; it is negative for exothermic reactions. This Chemistry 11-12 worksheet on enthalpy changes gives 20 problems from easy to hard: name the enthalpy change, exothermic or endothermic? and heat in a water bath. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These 20 problems cover enthalpy definitions, calorimetry with q = m c ΔT, Hess cycles from combustion and formation data, and ΔH estimated from bond enthalpies. They begin with definitions and single-step calculations and end with multi-step cycles and critical evaluation of methods.
The level suits IB Chemistry, A Level, CBSE Class 11-12 and WAEC/NECO senior students. Always write the unit and the sign of ΔH.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R1.1-R1.2
Problem 1 Name the enthalpy change ★★★
Mei in Singapore revises definitions.1. Write the equation, with state symbols, for the standard enthalpy of formation of ethanol, C₂H₅OH(l).
2. Then write the equation for the standard enthalpy of combustion of hydrogen.
3. Finally, state the standard pressure and the usual standard temperature.
Problem 2 Exothermic or endothermic? ★★★
Aarav in Pune studies two reactions.1. Classify X and Y as exothermic or endothermic.
2. Then calculate ΔH for each reaction.
3. Finally, calculate the forward activation energy of X and the activation energy of the reverse reaction of X.
Problem 3 Heat in a water bath ★★★
Chiamaka in Lagos heats 150 g of water from 18.0 °C to 27.5 °C (c = 4.18 J/(g K)).1. Calculate ΔT.
2. Then calculate the heat absorbed, in J and in kJ.
3. Finally, calculate the temperature rise if the same energy heated 300 g of water.
Problem 4 Dissolving sodium hydroxide ★★★
Kofi in Accra dissolves 2.00 g of NaOH (M = 40.0 g/mol) in 100 g of water. The temperature rises by 5.3 °C.1. Calculate the moles of NaOH.
2. Then calculate the heat absorbed by the water.
3. Finally, calculate ΔH of solution in kJ/mol.
Problem 5 Neutralisation ★★★
25.0 cm³ of 1.00 mol/dm³ hydrochloric acid is mixed with 25.0 cm³ of 1.00 mol/dm³ sodium hydroxide. The temperature rises by 6.8 K. Assume density 1.00 g/cm³ and c = 4.18 J/(g K).1. Calculate the heat absorbed by the solution.
2. Then calculate the moles of water formed.
3. Finally, calculate ΔH of neutralisation in kJ/mol.
Problem 6 Energy per gram and in kilocalories ★★★
Bilal in Lahore reads that the enthalpy of combustion of glucose, C₆H₁₂O₆ (M = 180.0 g/mol), is −2803 kJ/mol.1. Calculate the energy released per gram.
2. Then calculate the energy released by 25.0 g.
3. Finally, convert your answer to part 2 to kilocalories (1 kcal = 4.184 kJ).
Problem 7 Carbon, carbon monoxide and Hess ★★★
Emma in Leeds wants ΔH for C(s) + ½O₂(g) → CO(g), which cannot be measured directly.1. State Hess’s law.
2. Then calculate ΔH₁ using the cycle.
3. Finally, write the equation for the second step and state its type of enthalpy change.
Problem 8 Combining equations ★★★
N₂(g) + O₂(g) → 2NO(g) has ΔH = +180.5 kJ. 2NO(g) + O₂(g) → 2NO₂(g) has ΔH = −114.1 kJ.1. Write the overall equation for the two steps.
2. Then calculate ΔH for the overall equation.
3. Finally, calculate ΔH per mole of NO₂ formed.
Problem 9 Formation of butane ★★★
Diego in Texas has ΔHc values: C(s) −393.5, H₂(g) −285.8, C₄H₁₀(g) −2877 kJ/mol.1. Write the equation for the formation of butane.
2. Then calculate ΔHf of butane using combustion data.
Problem 10 Reducing iron ore ★★★
Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g). ΔHf values (kJ/mol): Fe₂O₃ −824.2, CO −110.5, CO₂ −393.5.1. Calculate the total ΔHf of the products.
2. Then calculate the total ΔHf of the reactants.
3. Finally, calculate ΔH for the reaction.
Problem 11 Hydrogen chloride ★★★
Use bond enthalpies: H–H 436, Cl–Cl 243, H–Cl 432 kJ/mol for H₂(g) + Cl₂(g) → 2HCl(g).1. Calculate the energy needed to break the bonds.
2. Then calculate the energy released when the bonds form.
3. Finally, calculate ΔH and ΔH per mole of HCl.
Problem 12 Hydrogenation of ethene ★★★
C₂H₄ + H₂ → C₂H₆. Bond enthalpies (kJ/mol): C=C 612, H–H 436, C–C 347, C–H 413.1. List the bonds broken and formed.
2. Then calculate ΔH.
3. Finally, state whether the reaction is exothermic.
Problem 13 Spirit burner ★★★
Wanjiru in Nairobi burns propan-1-ol (C₃H₈O, M = 60.0 g/mol) under 200 g of water. The burner loses 0.80 g and the water warms by 24.0 K. The data-book value is −2021 kJ/mol.1. Calculate the heat absorbed by the water.
2. Then calculate the experimental ΔHc in kJ/mol.
3. Finally, calculate the percentage of the data-book energy not measured and suggest two reasons.
Problem 14 Bond enthalpy versus data ★★★
Methanol vapour burns: CH₃OH(g) + 1½O₂(g) → CO₂(g) + 2H₂O(g). Bond enthalpies (kJ/mol): C–H 413, C–O 358, O–H 464, O=O 498, C=O 805.1. Calculate ΔH using bond enthalpies.
2. ΔHf values are CH₃OH(g) −201.0, CO₂ −393.5, H₂O(g) −241.8 kJ/mol. Calculate ΔH from these.
3. Finally, calculate the difference and explain it.
Problem 15 Zinc and copper sulfate ★★★
Excess zinc is added to 50.0 cm³ of 0.500 mol/dm³ copper(II) sulfate at 4 min. The graph shows readings.1. Extrapolate to 4 min and state ΔT.
2. Then calculate q.
3. Finally, calculate ΔH per mole of copper sulfate.
Problem 16 Hydration of copper sulfate ★★★
Anhydrous CuSO₄(s) dissolves in water with ΔH = −66.1 kJ/mol. CuSO₄·5H₂O(s) dissolves with ΔH = +11.7 kJ/mol.1. Draw the Hess cycle in words, linking the solid hydrate, the anhydrous solid and the solution.
2. Then calculate ΔH for CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s).
3. Finally, state why this value cannot be measured directly.
Problem 17 Formation of ethyne ★★★
ΔHc (kJ/mol): C(s) −393.5, H₂(g) −285.8, C₂H₂(g) −1300.1. Write the formation equation of ethyne.
2. Then calculate ΔHf of ethyne.
3. Finally, explain what the sign of your answer tells you about the stability of ethyne relative to its elements.
Problem 18 Mean C–H bond enthalpy ★★★
ΔHf of CH₄(g) = −74.8 kJ/mol. ΔHat (atomisation) of C(s) = +716.7 kJ/mol and of H₂(g) per mole of H atoms = +218.0 kJ/mol.1. Write the equation for CH₄(g) → C(g) + 4H(g) and state which data combine to give its ΔH.
2. Then calculate ΔH for the equation.
3. Finally, calculate the mean C–H bond enthalpy.
Problem 19 Hydration of ethene ★★★
C₂H₄(g) + H₂O(g) → C₂H₅OH(g). Bond enthalpies (kJ/mol): C=C 612, C–C 347, C–H 413, C–O 358, O–H 464. ΔHf (kJ/mol): C₂H₄ +52.3, H₂O(g) −241.8, C₂H₅OH(g) −235.3.1. Calculate ΔH from bond enthalpies.
2. Then calculate ΔH from formation data.
3. Finally, calculate the percentage difference relative to the formation-data value and suggest one reason.
Problem 20 Spot the errors ★★★
Find and correct the error in each calculation or statement.1. A student uses q = 2.00 × 4.18 × 5.3, where 2.00 g is the mass of solid NaOH.
2. A student states that the temperature rose, so ΔH is +44.3 kJ/mol.
3. A student writes q = 2215 J and gives ΔH = −2215 / 0.0500 = −44.3 J/mol.
4. A student calculates ΔH = Σ(bonds formed) − Σ(bonds broken).
