Excess marble chips react with dilute hydrochloric acid in a flask kept in a water bath at 25 ∘C25\,^\circ\mathrm{C}25∘C. The rate of gas production decreases as the reaction goes on.
Which explanation is correct?
The particles move more slowly because the reaction mixture uses up its energy
The activation energy increases as the products build up in the reaction mixture
The acid concentration falls, so there are fewer collisions per second with the marble
A smaller proportion of collisions have energy at least equal to the activation energy
54 exam-style questions on Edexcel GCSE Chemistry 8.1 Rates of reaction, covering 8.1.1 Measuring and interpreting rates of reaction, 8.1.2 Collision theory and the factors affecting rate, and 8.1.3 Catalysts and enzymes. Each one has a worked solution and a mark scheme showing where the marks go.