Copper(II) sulfate solution is a deep blue colour. When excess zinc powder is added to copper(II) sulfate solution, the blue colour disappears and a reddish-brown solid forms. A student notices that the reaction happens much faster when the zinc is ground into a fine powder rather than used as large lumps. Explain why the reaction is faster when fine zinc powder is used, and describe the role of a catalyst as an alternative way to increase the rate of this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Transition elements such as copper and zinc are common in laboratory reactions. Changing the physical form of a reactant or adding a catalyst are two ways to increase the rate of a chemical reaction.
Model answer (5 marks)
Fine zinc powder has a greater surface‑area‑to‑volume ratio than large lumps, so more zinc atoms are exposed to the copper(II) sulfate solution. This increases the number of reacting particles that can collide with the solution, giving more frequent successful collisions and therefore a higher reaction rate.
A catalyst increases the rate of this reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is not consumed in the reaction and can be recovered after the reaction has finished.
A catalyst increases the rate of this reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is not consumed in the reaction and can be recovered after the reaction has finished.
Examiner tips
- Mention surface area and collision frequency for fine powder; explain lower activation energy and catalyst recovery for catalyst.
- Use exact terms: surface‑area‑to‑volume ratio, activation energy, catalyst not used up.
- Keep answer concise – 5 marks only.
Common mistakes
- Failing to mention surface area or collision frequency; writing only "more zinc" without explaining the effect on collisions.
- Confusing catalyst with reactant – stating it is used up or not providing a lower activation energy.
- Using vague phrases like "faster" without linking to surface area or activation energy.
Mark scheme (5 marks)
- Fine powder has a greater surface area (to volume ratio) than large lumps
- More reacting particles are exposed / available to collide with the copper(II) sulfate solution
- There are more frequent successful collisions (so the rate of reaction increases)
- A catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy
- The catalyst is not used up during the reaction
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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