Changing the surface area of the reactants
2 marks · NormalA student reacts powdered calcium carbonate with dilute hydrochloric acid. Explain why the reaction happens faster than if the same mass of calcium carbonate is in large lumps, not powder.
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Also accept: Powdered calcium carbonate has a greater surface area, so more particles are exposed to react with the acid.; With powder, there are more collisions per second between acid and calcium carbonate.
Common mistakes: Saying only 'it is smaller' with no mention of surface area or collisions.
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Marking points
M1 · 1 mark · Mentions that powder has greater surface area (or more particles exposed).
M2 · 1 mark · Explains that more particles are available to collide/react with acid (or collision frequency increases).
Worked solution
The powdered calcium carbonate has a greater surface area than large lumps.
This means more particles are exposed and available to react with the acid, so collisions between particles happen more often and the reaction is faster.