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AS · Practice questions · Calorimetry

From ΔT to ΔH.

Six original Cambridge-style questions on calorimetry: calculations for solutions and fuels, limiting reagents, heat loss and improvements.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

q = mcΔT, then ΔH = −q ÷ n.

01
Calculation
[3 marks]

50.0 cm3 of 2.00 mol dm−3 HNO3 is added to 50.0 cm3 of 2.00 mol dm−3 KOH. The temperature rises from 21.0 °C to 34.6 °C. Calculate the enthalpy change of neutralisation.

  • q = 100 × 4.18 × 13.6 = 5685 J. ✓
  • n(H2O) = 2.00 × 0.0500 = 0.100 mol. ✓
  • ΔH = −5685 ÷ 0.100 = −56 850 J mol−1 = −56.8 kJ mol−1. ✓
02
Calculation
[3 marks]

0.320 g of methanol, CH3OH (Mr 32.0), is burned and heats 100 g of water from 20.0 °C to 32.5 °C. Calculate the experimental enthalpy change of combustion of methanol.

  • q = 100 × 4.18 × 12.5 = 5225 J = 5.225 kJ. ✓
  • n = 0.320 ÷ 32.0 = 0.0100 mol. ✓
  • ΔHc = −5.225 ÷ 0.0100 = −523 kJ mol−1. ✓
03
Calculation
[3 marks]

5.00 g of ammonium nitrate is dissolved in 50.0 g of water; the temperature falls by 7.2 K. Calculate the enthalpy change of solution of NH4NO3 (Mr 80.0).

  • q = 50.0 × 4.18 × 7.2 = 1505 J (use the mass of water). ✓
  • n = 5.00 ÷ 80.0 = 0.0625 mol. ✓
  • The temperature fell, so the process is endothermic: ΔH = +1505 ÷ 0.0625 = +24.1 kJ mol−1. ✓
04
Calculation
[2 marks]

Excess iron powder is added to 50.0 cm3 of 0.500 mol dm−3 CuSO4(aq); ΔT = +20.4 K. Calculate ΔH for Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s).

  • q = 50.0 × 4.18 × 20.4 = 4264 J; n(CuSO4) = 0.0250 mol. ✓
  • ΔH = −4264 ÷ 0.0250 = −171 kJ mol−1. ✓
05
Explain
[3 marks]

A student's value for ΔHc of propan-1-ol is −1250 kJ mol−1; the data book value is −2021 kJ mol−1. Give two reasons for the difference and one improvement.

  • Heat lost to the surroundings (air, can, clamp) instead of heating the water. ✓
  • Incomplete combustion, shown by soot on the can; or evaporation of fuel from the wick. ✓
  • Improvement: a draught shield and lid, or a flame calorimeter with an oxygen supply. ✓
06
Explain
[2 marks]

Explain why a temperature against time graph, extrapolated back to the time of mixing, gives a more accurate ΔT than simply reading the highest temperature.

  • While the reaction mixture is heating up it is also losing heat, so the highest reading is lower than the true value. ✓
  • Extrapolating the cooling line back to the moment of mixing estimates the temperature reached if no heat were lost. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.