A student adds a piece of marble (calcium carbonate) to hydrochloric acid. The student notices that the reaction slows down over time and eventually stops. Explain why the rate of reaction decreases as the reaction proceeds.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Marble chips react with hydrochloric acid to produce carbon dioxide gas. A student observes that bubbles are produced rapidly at first but the rate of bubble production slows noticeably over time.
Model answer (4 marks)
The reaction is:
CaCO₃(s)+2HCl(aq)→CaCl₂(aq)+CO₂(g)+H₂O(l)
As the reaction proceeds the HCl is consumed, so its concentration falls.
With a lower concentration the reacting particles are more widely spaced and fewer collisions occur.
Fewer collisions, and fewer successful collisions per second, mean the rate of reaction falls.
Therefore the rate decreases as the reaction proceeds.
CaCO₃(s)+2HCl(aq)→CaCl₂(aq)+CO₂(g)+H₂O(l)
As the reaction proceeds the HCl is consumed, so its concentration falls.
With a lower concentration the reacting particles are more widely spaced and fewer collisions occur.
Fewer collisions, and fewer successful collisions per second, mean the rate of reaction falls.
Therefore the rate decreases as the reaction proceeds.
Examiner tips
- Use the reaction equation to show HCl is consumed. Explain how a lower concentration leads to fewer collisions. Link fewer collisions to a lower rate.
- Use the exact terminology: concentration, collisions, successful collisions, rate of reaction.
Common mistakes
- Saying the reaction stops because the marble is used up, ignoring HCl. Claiming the rate slows because the marble becomes less reactive. Using vague terms like "less active" instead of "lower concentration".
Mark scheme (4 marks)
- The concentration of hydrochloric acid decreases as the reaction proceeds
- The reacting particles are further apart / less crowded in solution
- Collisions between reacting particles become less frequent / fewer successful collisions per second
- Therefore the rate of reaction decreases
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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