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AQA GCSE · Question 04.2 · Energy changes
Explain the results shown in Figure 3.
Do not refer to anomalous points.
Use data from Figure 3.
Explain the results shown in Figure 3.
Do not refer to anomalous points.
Use data from Figure 3.
How to approach this question
Your explanation needs to cover both parts of the graph.
1. **The rising section:** Describe the trend (as mass of zinc increases, temperature...). Quote data from the graph to support this. Explain this trend in terms of limiting reactants.
2. **The plateau section:** Describe the trend (after a certain mass of zinc, temperature...). Quote data from the graph. Explain this trend in terms of limiting reactants.
Full Answer
As the mass of zinc increases from 0.0 g to 0.8 g, the highest temperature reached increases. For example, at 0.2 g of zinc the temperature is 28°C and at 0.6 g it is 41°C. This is because zinc is the limiting reactant, so more zinc leads to more reaction and more energy released.
After approximately 0.8 g of zinc, the temperature stops increasing and stays constant at around 47°C. This is because all the copper sulfate has reacted, so the copper sulfate is now the limiting reactant. Adding more zinc does not produce any more energy.
The graph shows that the reaction between zinc and copper sulfate is exothermic, as the temperature increases.
- **Part 1 (Rising slope):** From 0.00 g to approximately 0.80 g of zinc, the temperature increases as more zinc is added. For instance, adding 0.20 g of zinc causes a temperature rise to 28°C, while adding 0.60 g causes a rise to 41°C. In this region, the zinc is the limiting reactant. This means there is an excess of copper sulfate, so adding more zinc allows more reaction to happen, releasing more thermal energy.
- **Part 2 (Plateau):** After about 0.80 g of zinc, the temperature remains constant at approximately 47°C. At this point, the copper sulfate has become the limiting reactant. All of the copper sulfate has been used up, so even though more zinc is added, no further reaction can occur. Therefore, no more heat is produced, and the temperature does not increase further.
Common mistakes
✗ Not using data from the graph to support the explanation.
✗ Only explaining one part of the graph (e.g., only the rising section).
✗ Not using the term "limiting reactant" or explaining the concept correctly.
Practice the full AQA GCSE Chemistry Higher Tier Paper 1
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