A student investigates how the concentration of sodium thiosulfate solution affects the rate of its reaction with dilute hydrochloric acid. The reaction makes a precipitate of sulfur, which makes the mixture cloudy.
The student puts a conical flask on a piece of paper marked with a cross. The student adds sodium thiosulfate solution and water to the flask, then adds 10 cm3\mathrm{cm^{3}}cm3 of dilute hydrochloric acid and starts a stop clock.
The student records the time for the cross to disappear from view.
The table shows the results.
| Volume of sodium thiosulfate solution (cm3\mathrm{cm^{3}}cm3) | Volume of water (cm3\mathrm{cm^{3}}cm3) | Time for cross to disappear (s) |
|---|---|---|
| 50 | 0 | 21 |
| 40 | 10 | 26 |
| 30 | 20 | 35 |
| 20 | 30 | 72 |
| 10 | 40 | 105 |
The student concludes: "The rate of reaction is directly proportional to the concentration of sodium thiosulfate."
Explain why the student adds water so that the total volume of sodium thiosulfate solution and water is always 50 cm3\mathrm{cm^{3}}cm3.
Identify the anomalous result.
Evaluate the student's conclusion.
Use the results and the reacting particle model. Suggest how the experiment could be improved.
122 exam-style questions on OCR GCSE Chemistry 5.2 Controlling reactions, covering 5.2.1 Measuring and interpreting rate of reaction, 5.2.2 Factors affecting rate and collision theory, and 5.2.3 Catalysts, activation energy and enzymes. Each one has a worked solution and a mark scheme showing where the marks go.