# Explain why finely powdered calcium carbonate reacts faster with excess hydrochloric acid than an equal mass of calcium carbonate in large lumps.

> IB DP Chemistry Standard Level (2023 syllabus) — R2.2 How fast? The rate of chemical change · Explain · 4 marks

## Mark scheme (4 marks)

1. Powdered calcium carbonate has a greater total surface area than lumps of equal mass.
2. A greater surface area means more reactant particles are exposed/accessible to acid particles (at the surface of the solid).
3. This leads to a greater frequency of (successful) collisions between reactant particles per unit time.
4. A higher collision frequency increases the rate of reaction.

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