A student adds a small piece of magnesium ribbon to dilute sulfuric acid at room temperature. The magnesium dissolves and bubbles of gas are produced. Explain why increasing the concentration of the sulfuric acid would increase the rate of this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Increasing the concentration of the acid increases the number of H⁺ ions in the same volume.
This raises the frequency of collisions between H⁺ ions and magnesium atoms.
With more collisions per unit time, the number of effective collisions that have the energy equal to or greater than the activation energy also increases.
Hence the rate of the reaction rises when the acid concentration is increased.
This raises the frequency of collisions between H⁺ ions and magnesium atoms.
With more collisions per unit time, the number of effective collisions that have the energy equal to or greater than the activation energy also increases.
Hence the rate of the reaction rises when the acid concentration is increased.
Examiner tips
- Use the word ‘increasing’ to link concentration and collision frequency. Mention H⁺ ions explicitly. Show the chain: more ions → more collisions → more effective collisions → higher rate. Include a brief reference to activation energy to demonstrate understanding of collision theory.
Common mistakes
- Failing to specify H⁺ ions and using vague terms like ‘acid particles’. Not linking collision frequency to activation energy. Omitting the step that effective collisions are the ones that actually lead to reaction.
Mark scheme (5 marks)
- Increasing concentration means there are more acid particles (hydrogen ions / H⁺ ions) in the same volume
- This leads to a greater frequency of collisions between acid particles and magnesium atoms
- More collisions per unit time means more successful/effective collisions per unit time
- Therefore the rate of reaction increases
- Reference to activation energy: only collisions that have energy equal to or greater than the activation energy result in a reaction
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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