Quick answer: An enthalpy change ΔH is the heat transferred at constant pressure; it is negative for exothermic reactions. This Chemistry 11-12 test on enthalpy changes has 8 questions worth 20 marks, to do in 45 minutes: combustion enthalpy, dissolving ammonium nitrate and energy cycle. The mark scheme is on a separate page; free printable PDF.
This test covers enthalpy definitions, calorimetry, Hess’s law and bond enthalpies. Show your working, give the sign and unit of every ΔH, and use the mark values as a guide to how much detail is needed.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R1.1-R1.2
Question 1 Combustion enthalpy (2 marks)
- Define the standard enthalpy of combustion. (1 mark)
- Write the equation for the standard enthalpy of combustion of propane, C₃H₈(g). (1 mark)
Question 2 Dissolving ammonium nitrate (3 marks)
4.00 g of ammonium nitrate (M = 80.0 g/mol) dissolves in 100 g of water and the temperature falls by 3.1 °C. Calculate ΔH of solution in kJ/mol (c = 4.18 J/(g K)).Question 3 Energy cycle (3 marks)
- State Hess’s law. (1 mark)
- Use the cycle to calculate the enthalpy of formation of liquid water. (2 marks)
Question 4 Bromine addition (3 marks)
Use the bond enthalpies C=C 612, Br–Br 193, C–C 347, C–Br 285 (all kJ/mol) to calculate ΔH for C₂H₄ + Br₂ → C₂H₄Br₂.Question 5 Why only an estimate (2 marks)
Explain why ΔH calculated from mean bond enthalpies differs slightly from values found by calorimetry.Question 6 Decomposition of calcium carbonate (3 marks)
ΔHf (kJ/mol): CaCO₃(s) −1207.0, CaO(s) −634.9, CO₂(g) −393.5. Calculate ΔH for CaCO₃(s) → CaO(s) + CO₂(g) and state whether it is exothermic.Question 7 Per mole (2 marks)
A reaction releases 5.6 kJ of heat when 0.020 mol of reactant reacts. Calculate ΔH in kJ/mol.Question 8 Experiment versus data book (2 marks)
A student finds ΔHc of a fuel to be −1450 kJ/mol but the data book gives −1780 kJ/mol. Explain the difference.
