A student adds a small piece of marble (calcium carbonate) to dilute hydrochloric acid. The student then repeats the experiment using the same mass of marble, but crushed into a powder instead. Explain why the reaction with powdered marble is faster than the reaction with the small piece of marble.

AQA A-Level Chemistry (7405) — 3.1.5 Kinetics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Powdering the marble increases its surface area, so more calcium carbonate particles are exposed to the acid.
This gives more contact between the acid and the reactant.
Consequently, there are more frequent collisions between HCl molecules and CaCO₃ particles.
More collisions are successful, giving a higher reaction rate per unit time.
Therefore the reaction with powdered marble is faster than with a small piece.

Examiner tips

  • Use the term ‘surface area’ and ‘contact’ to show understanding of the rate law. Explain the link between surface area, collision frequency and reaction rate. Show the logical sequence: surface area → more collisions → more successful collisions → higher rate.
  • common_mistakes
  • :
  • Saying ‘powdering makes the marble smaller’ without mentioning surface area. Confusing ‘more collisions’ with ‘more acid’ – the amount of acid is unchanged. Using vague phrases like ‘the reaction is quicker’ without explaining why.

Mark scheme (5 marks)

  1. Powdering the marble increases the surface area (of the marble / reactant)
  2. More (marble) particles are exposed to / in contact with the acid
  3. There are more frequent collisions between (acid / hydrochloric acid) particles and (marble / calcium carbonate) particles
  4. More collisions are successful / result in a reaction (per unit time)
  5. Therefore the rate of reaction increases

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