Insulate, stir, read, extrapolate. The answer is where two lines meet.
Enthalpy changes measured in a polystyrene cup come up again and again in 9701 Paper 5. Here you add 2.00 mol dm⁻³ nitric acid from a burette to 25.00 cm³ of potassium hydroxide, reading the temperature after each portion. The two straight lines of best fit meet at the end point and give both the concentration and the enthalpy change of neutralisation. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Chemistry 9701 Paper 5. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9701/52 October/November 2023, (thermometric titration and enthalpy change of neutralisation) · official past papers
Practise next: 9701/52 May/June 2024, (enthalpy change by two experiments and extrapolation to the time of mixing) · official past papers
Practise next: 9701/52 May/June 2025, (enthalpy change of combustion and q = mcΔT) · official past papers
Examiner reports used: 9701 November 2023 and June 2024.
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What a full-mark diagram shows
The vessel
A polystyrene cup with a lid, standing in a beaker for stability, to cut heat loss: the main weakness of the method.
Measuring
25.0 cm³ volumetric pipette for the alkali, 50.00 cm³ burette for the acid, thermometer with 0.2 °C divisions, stirrer and stopwatch.
The procedure
Steady starting temperature over 3 minutes; add 3.00 cm³ portions, stir and read each time; carry on well past the maximum.
The graph
Two straight lines of best fit, ignoring anomalies, extended until they meet. ΔT = intersection temperature minus the starting temperature.
Habits the examiner reports keep asking for
Subtract the starting temperature. The intersection temperature is not the temperature change.
Do not force the lines through the highest point, and leave anomalous points out of the line of best fit.
m is the mass of both solutions, and never add 273 to a temperature change.
A temperature rise uses two readings, so its error is twice ± half a division.
Safety needs a specific hazard, such as corrosive, not just “harmful” or “dangerous”.