A student adds sodium carbonate powder to two separate beakers, one containing dilute hydrochloric acid and one containing dilute ethanoic acid of the same concentration. Both reactions produce carbon dioxide gas. Explain why the rate of fizzing is faster in the hydrochloric acid, even though both acids have the same concentration.

Eduqas A-Level Chemistry — 3.9 Acid-base equilibria · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Hydrochloric acid is a strong acid and therefore fully dissociates in water, giving a high concentration of H⁺ ions. Ethanoic acid is a weak acid and only partially dissociates, so at the same concentration it supplies far fewer H⁺ ions. The higher H⁺ concentration in the HCl solution leads to more frequent collisions between H⁺ and the carbonate ions, so the reaction with sodium carbonate proceeds more rapidly and the fizzing is faster.

Examiner tips

  • State that HCl is a strong acid and ethanoic acid is weak
  • Explain that full dissociation gives more H⁺ ions
  • Link higher H⁺ concentration to faster reaction rate

Common mistakes

  • Confusing the role of Na₂CO₃ with the acid strength
  • Failing to mention that HCl is fully dissociated
  • Using vague terms like "more acid" instead of "more H⁺ ions"

Mark scheme (5 marks)

  1. Hydrochloric acid is a strong acid
  2. Hydrochloric acid fully/completely dissociates into ions in solution
  3. Ethanoic acid is a weak acid and only partially dissociates
  4. Hydrochloric acid produces a higher concentration of H⁺ ions than ethanoic acid at the same concentration
  5. The higher H⁺ ion concentration means more frequent collisions with sodium carbonate particles, so the reaction is faster

Key terms in this question

concentration

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