Experiments to measure the rate of reaction
3 marks · NormalExplain why the rate of the reaction between marble chips and hydrochloric acid is fastest at the start and slows down as the reaction continues. Refer to changes in concentration and particle collisions in your answer.
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Common mistakes: only mentions one reason; does not relate to particles
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Marking points
M1 · 1 mark · Mentions that rate is fastest at start because concentration of acid is highest.
M2 · 1 mark · Describes rate slows as concentration of acid decreases.
M3 · 1 mark · Links change in rate to fewer/faster/slower collisions between reactant particles.
Worked solution
At the start of the reaction, the concentration of hydrochloric acid is highest, so there are more acid particles available to collide with the marble chips. This means there are more frequent successful collisions, so the rate is fastest. As the reaction continues, the acid is used up, its concentration decreases, and there are fewer collisions per second, so the reaction rate slows down.