A student investigates the energy change when zinc powder is added to copper sulfate solution. The temperature of the mixture rises. Explain why the temperature rises and describe how the student could use the results to compare this reaction with a different displacement reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc is more reactive than copper. When zinc powder is added to copper sulfate solution, zinc displaces copper and the temperature of the mixture increases.
Model answer (5 marks)
The reaction is exothermic, so energy is released.
When zinc displaces copper, bonds in the products (ZnSO₄ and Cu) are formed. The energy released in forming these bonds is greater than the energy required to break the bonds in the reactants (Zn and CuSO₄), so the net energy change is negative.
The released energy appears as heat, raising the temperature of the mixture. The temperature rise (ΔT) is calculated by subtracting the initial temperature from the highest temperature reached.
To compare this reaction with another displacement reaction, the student should keep the same volume of copper‑sulphate solution and the same mass of zinc (i.e. control the variables). The reaction that gives the larger ΔT is the more exothermic one and releases more energy.
When zinc displaces copper, bonds in the products (ZnSO₄ and Cu) are formed. The energy released in forming these bonds is greater than the energy required to break the bonds in the reactants (Zn and CuSO₄), so the net energy change is negative.
The released energy appears as heat, raising the temperature of the mixture. The temperature rise (ΔT) is calculated by subtracting the initial temperature from the highest temperature reached.
To compare this reaction with another displacement reaction, the student should keep the same volume of copper‑sulphate solution and the same mass of zinc (i.e. control the variables). The reaction that gives the larger ΔT is the more exothermic one and releases more energy.
Examiner tips
- Use the term "exothermic" and explain bond energy difference. Show the ΔT calculation. State the need to control volume and mass for a fair comparison. Mention that a larger ΔT indicates a more exothermic reaction.
Common mistakes
- Saying the reaction is endothermic. Failing to explain bond energy difference. Not controlling variables when comparing reactions.
Mark scheme (5 marks)
- The reaction is exothermic
- Energy is released when new bonds are formed (in the products) / the energy released forming bonds in the products is greater than the energy taken in breaking bonds in the reactants
- The temperature rise (ΔT) is measured by subtracting the initial temperature from the highest temperature reached
- Use the same volume / mass of copper sulfate solution and the same mass of zinc (i.e. keep quantities the same / control variables) so that comparisons are valid
- The reaction with the greater temperature rise is more exothermic / releases more energy
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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