Seal it, start it, collect every bubble. Then draw the tangent.
Drawing the apparatus to collect and measure a gas, and finding an initial rate from the curve, comes up again and again in 9701 Paper 5. Here hydrogen peroxide decomposes on a manganese(IV) oxide catalyst at 30.0 °C and the oxygen is collected in a 100 cm³ gas syringe. 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: 9701/51 October/November 2023, (drawing the apparatus to collect nitrogen at 50 °C, tangent at t = 0) · official past papers
Practise next: 9701/52 October/November 2023, (completing the diagram to collect hydrogen from zinc and acid) · official past papers
Practise next: 9701/54 May/June 2026, (collecting ammonia, and why not over water) · official past papers
Examiner reports used: 9701 November 2023 (Papers 51, 52 and 53) and November 2025 (Paper 51), with the November 2023 and June 2026 mark schemes.
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What a full-mark diagram shows
Sealed vessel
A conical flask with a bung and a delivery tube that ends above the liquid. No gaps, no tube dipping into the mixture.
Collect and measure
A gas syringe (give its capacity), or an inverted measuring cylinder or burette full of water in a trough, for a gas that does not dissolve much.
Start without losing gas
Reactant in a small tube or weighing boat on a thread inside the sealed flask, or a divided flask, tipped to start the reaction and the stopwatch together.
Constant temperature
A thermostatically controlled water bath, with a thermometer to monitor it.
Habits the examiner reports keep asking for
Keep the reactants apart until the bung is in. This was the mark fewest students scored.
Say “thermostatically controlled water bath”, not just “heat it” or “a beaker of water”.
Draw the tangent at t = 0, straight and touching the curve only at the origin; most wrong tangents were too steep.
Total volume ÷ total time is the mean rate, not the initial rate.
Higher pressure gives a smaller gas volume, not a faster reaction, when no reactant is a gas.