A reaction mixture changes colour as a product forms. A student carries out the reaction twice, using the same quantities of reactants at the same temperature.
For the second experiment, the student adds 0.50 g of solid W. After the reaction, W is filtered from the mixture, washed and dried.
| Result | Without W | With W |
|---|---|---|
| Time to reach the same chosen stage of the colour change in s | 140 | 50 |
| Final mass of product in g | 3.6 | 3.6 |
| Mass of W recovered in g | not added | 0.50 |
Which conclusion is correct?
W is a catalyst, because it gives the reacting particles more energy when they collide.
W is a catalyst, because the same final amount of product forms sooner and W is recovered unchanged.
W is not a catalyst, because a substance that takes part in the reaction cannot be re-formed.
W is not a catalyst, because a catalyst would make the colour change take longer to reach this stage.
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.