A student adds marble chips (calcium carbonate) to two separate acid solutions of equal concentration and volume. One acid is hydrochloric acid (a strong acid) and the other is ethanoic acid (a weak acid). The student observes that the marble chips react more vigorously with the hydrochloric acid. Explain why the reaction with hydrochloric acid is more vigorous than the reaction with ethanoic acid.

Edexcel GCSE Chemistry (1CH0) — 3.1 Acids · Explain · 4 marks · View as Markdown

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

Marble chips react with acids to produce carbon dioxide gas. A student investigates the reaction of marble chips with two different acids of equal concentration.

Model answer (4 marks)

Hydrochloric acid is a strong acid and is completely ionised in aqueous solution, giving a high concentration of H⁺ ions. Ethanoic acid is a weak acid and is only partially ionised, so it supplies fewer H⁺ ions. The higher concentration of H⁺ ions in the hydrochloric acid solution therefore reacts faster with the calcium carbonate, producing a more vigorous reaction.

Examiner tips

  • State the ionisation difference between strong and weak acids.
  • Explain that more H⁺ ions give a faster reaction.
  • Use the word ‘vigorous’ to describe the rate difference.
  • Link the higher H⁺ concentration directly to the reaction rate.

Common mistakes

  • Confusing the acid strength with the amount of acid added.
  • Failing to mention that H⁺ concentration is the key factor.
  • Using vague terms like ‘more acid’ instead of ‘more H⁺ ions’.

Mark scheme (4 marks)

  1. Hydrochloric acid is a strong acid that is completely ionised in aqueous solution
  2. Ethanoic acid is a weak acid that is only partially ionised in aqueous solution
  3. Hydrochloric acid therefore has a higher concentration of hydrogen ions (H⁺) in solution
  4. A higher concentration of H⁺ ions causes a faster/more vigorous reaction with the marble chips

Key terms in this question

strong acid · weak acid

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