Rate of reaction: sodium thiosulfate and hydrochloric acid — required practical

AQA GCSE Chemistry

Investigate how concentration affects the rate of reaction between sodium thiosulfate and hydrochloric acid.

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Method

  1. Draw a cross on paper and place a conical flask of sodium thiosulfate solution on top.
  2. Add hydrochloric acid, start a timer, and look down through the flask.
  3. Stop the timer when the cross can no longer be seen (the mixture turns cloudy with sulfur).
  4. Repeat with different concentrations of sodium thiosulfate.

Variables

  • Independent variable: Concentration of sodium thiosulfate (or temperature)
  • Dependent variable: Time for the cross to disappear (rate ∝ 1/time)
  • Control variables: Volume of solutions, temperature, same person judging the cross, same cross/flask

Results & analysis

Rate = 1 ÷ time. As concentration increases there are more particles per unit volume, so more frequent collisions and a faster rate — the time to obscure the cross decreases.

Risk assessment

  • The reaction produces sulfur dioxide (toxic) — work in a ventilated room; wear eye protection.

Improving accuracy & reliability

  • Use the same person to judge when the cross disappears.
  • Keep temperature and total volume constant; repeat and take a mean.
Exam tip: This is a "disappearing cross" turbidity method — the DV is the time for the cross to vanish, and rate = 1/time.

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