You cannot measure the enthalpy change of heating sodium hydrogencarbonate directly, but you can measure two reactions that you can do in a cup. By hand, weigh each weighing bottle, pour 30.0 cm³ of acid with the measuring cylinder, tip the solid in a little at a time and stir with the thermometer, catching the lowest and then the highest temperature. Then work through q = mcΔT, the two enthalpy changes and the Hess cycle, and evaluate the errors. Every attempt uses different masses and a different room. Allow about 35 minutes.
Checked: Cambridge 9701 Paper 3, February/March 2023 to May/June 2026. Only the papers in our collection are listed, so the real number may be higher. Question papers are available from Cambridge.
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Why the decomposition cannot be measured directly: it needs continuous strong heating, so its own heat change cannot be separated from the heat supplied.
One table for both experiments with a heading and unit on every row, balance readings to 0.01 g and temperatures and volumes to 0.5, readings that match the apparatus, correct subtractions, and an accuracy mark for each temperature change compared with careful technique.
q = mcΔT with the mass of the acid, the amount of each solid from its mass by difference, then ΔH1 (positive, endothermic) and ΔH2 (negative, exothermic) with signs and 2 to 4 significant figures.
The cycle gives ΔHr = 2ΔH1 − ΔH2, worked out from your own values with its sign.
The percentage error in a temperature change from two readings, the main source of error and the direction it moves each value, an improvement that tackles it, and why the solid goes in a little at a time.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.