A student is investigating the rate at which marble chips (calcium carbonate) react with hydrochloric acid. The student uses large marble chips in one experiment and then repeats the experiment using powdered marble of the same mass. Explain why the powdered marble reacts faster than the large marble chips.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.2 Controlling reactions · Explain · 4 marks · View as Markdown

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

Marble chips and powdered marble are both calcium carbonate. When the same mass of each is reacted with excess hydrochloric acid, the powdered marble produces gas at a faster rate than the large marble chips.

Model answer (4 marks)

Powdered marble has a greater surface area to volume ratio than large chips, so more calcium carbonate particles are exposed to the acid.

Because more surface is available, a larger number of reactant particles can collide with the acid molecules.

The increased number of collisions raises the probability of successful collisions.

A higher rate of successful collisions gives a faster overall reaction rate.

Examiner tips

  • Use the term ‘surface area to volume ratio’ to show understanding of surface area effect.
  • Explain that more exposed particles = more collisions = higher reaction rate.
  • Keep the answer concise – 4 points only.
  • Use correct chemical formula CaCO3 and HCl if needed.

Mark scheme (4 marks)

  1. Powdered marble has a greater surface area (to volume ratio) than large marble chips
  2. More reacting particles are exposed (at the surface)
  3. There are more frequent collisions between reactant particles
  4. This leads to a higher rate of successful collisions and therefore a faster rate of reaction

Related

More Controlling reactions questions

▶ Try answering this question with AI marking (free) →