Explain why finely powdered calcium carbonate reacts faster with excess hydrochloric acid than an equal mass of calcium carbonate in large lumps.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Finely powdered calcium carbonate has a much larger total surface area than lumps of the same mass.
Because the surface area is greater, more calcium carbonate particles are exposed to the hydrochloric acid at any one time.
This increases the number of successful collisions between CaCO₃ and HCl molecules per unit time.
A higher collision frequency raises the rate of the reaction, so the powdered form reacts faster than the lumped form.
Because the surface area is greater, more calcium carbonate particles are exposed to the hydrochloric acid at any one time.
This increases the number of successful collisions between CaCO₃ and HCl molecules per unit time.
A higher collision frequency raises the rate of the reaction, so the powdered form reacts faster than the lumped form.
Examiner tips
- Mention surface area first, then link to collision frequency and rate; keep each point brief and use exam terminology.
- Use the word ‘exposed’ or ‘accessible’ to show understanding of surface area effects.
Common mistakes
- Saying ‘more acid is used’ – the acid amount is the same. Forgetting to connect surface area to collision frequency. Using vague terms like ‘faster’ without explaining the mechanism.
Mark scheme (4 marks)
- Powdered calcium carbonate has a greater total surface area than lumps of equal mass.
- A greater surface area means more reactant particles are exposed/accessible to acid particles (at the surface of the solid).
- This leads to a greater frequency of (successful) collisions between reactant particles per unit time.
- A higher collision frequency increases the rate of reaction.
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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