A student adds magnesium ribbon to excess dilute hydrochloric acid. The rate of reaction decreases as the reaction goes on.
Which statement explains why?
The activation energy increases as more product is made.
The particles lose energy because the mixture cools down.
The surface area of the magnesium increases as it reacts.
The reactants are used up, so collisions become less frequent.
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.