Chemistry›Rate of Reaction — Practical Methods
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C6 · RatesAQA GCSE Chemistry · 8462 (Triple)

Rate of Reaction — Practical Methods

Measuring rate using gas volume, mass loss, colour change. Required practical methods and analysis.

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CognitoRate of reaction — measuring methods
Key facts & methods

Methods of measuring rate

  • Method 1 — Gas volume: collect gas produced in a gas syringe or inverted measuring cylinder of water. Plot volume vs time. Rate = gradient of graph.
  • Method 2 — Mass loss: place flask on balance, measure mass decrease as gas escapes. Plot mass vs time.
  • Method 3 — Colour change / turbidity: e.g. sodium thiosulfate + HCl → sulfur precipitate makes solution cloudy. Time until cross under flask disappears. Rate ∝ 1/time.
  • Method 4 — Titration samples: remove aliquots at timed intervals, quench reaction, titrate. Gives concentration vs time.

Interpreting rate graphs

  • Plot of product volume (or mass) vs time: gradient = rate.
  • Steeper gradient at start = fastest rate (highest concentration of reactants).
  • Gradient decreases as reactants are used up.
  • Flat line = reaction complete (all reactant used).
  • Same mass of product in all experiments using same starting mass — only rate changes when you change conditions.

Required practical — HCl and marble chips

  • Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
  • CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
  • Variables: temperature, concentration of HCl, particle size (powder vs chips), catalyst.
  • Measure: volume of CO₂ collected in gas syringe at regular time intervals.
  • Control: use same mass of CaCO₃, same volume of HCl each time.

Required practical — sodium thiosulfate

  • Na₂S₂O₃(aq) + HCl(aq) → NaCl(aq) + SO₂(g) + S(s) + H₂O(l).
  • Sulfur precipitate makes solution cloudy.
  • Place flask on paper with cross. Time until cross disappears (cannot be seen through flask).
  • Rate ∝ 1/time. Plot 1/time vs temperature or concentration.
  • Change temperature by heating Na₂S₂O₃ in water bath before adding HCl.
Exam questions — 4 questions · 11 marks · AQA 8462 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
4 marksGas syringe methodAQA 8462 P2 style

A student investigates the effect of concentration of hydrochloric acid on the rate of reaction with marble chips (CaCO₃). Describe the method and explain how the student would use the results to compare rates at different concentrations. (4 marks)

Hint: Method: marble chips + HCl → CO₂. Measure: gas volume over time. Control variables. Compare: initial gradient = rate.
+40 XP
1 markCross methodAQA 8462 style

In the sodium thiosulfate and HCl experiment, the rate is measured by timing until the cross disappears. How is rate calculated from this timing?

A Rate = time taken
B Rate = 1 ÷ time
C Rate = mass ÷ time
D Rate = time²
3 marksInterpret graphAQA 8462 style

Two experiments produce the same total volume of CO₂. Experiment A uses 1 mol/dm³ HCl; Experiment B uses 2 mol/dm³ HCl, same mass of CaCO₃. Sketch-describe the graph and explain the differences. (3 marks)

Hint: Same total product (same CaCO₃). Different rates (different HCl concentration). Steeper = faster rate. Same endpoint.
+30 XP

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