ExpertMedium1 markMultiple Choice
AQA GCSE · Question 07.2 · The rate and extent of chemical change
One stage in the industrial production of sulfuric acid is the reaction of sulfur dioxide with oxygen to produce sulfur trioxide. This reversible reaction reaches dynamic equilibrium. Figure 7 shows the percentage yield of sulfur trioxide in this reaction at different temperatures.
Which statement about the forward reaction is correct? Use Figure 7.
One stage in the industrial production of sulfuric acid is the reaction of sulfur dioxide with oxygen to produce sulfur trioxide. This reversible reaction reaches dynamic equilibrium. Figure 7 shows the percentage yield of sulfur trioxide in this reaction at different temperatures.
Which statement about the forward reaction is correct? Use Figure 7.
Answer options:
A.
The yield is greater at higher temperatures because the reaction is exothermic.
B.
The yield is greater at higher temperatures because the reaction is endothermic.
C.
The yield is smaller at higher temperatures because the reaction is exothermic.
D.
The yield is smaller at higher temperatures because the reaction is endothermic.
How to approach this question
1. **Analyse the graph:** Look at the relationship between temperature (x-axis) and percentage yield (y-axis). As temperature increases, what happens to the yield? This will eliminate two of the options.
2. **Apply Le Chatelier's Principle:** The principle states that if a change is made to a system at equilibrium, the system will shift to counteract the change.
- If a forward reaction is **exothermic** (releases heat), increasing the temperature will push the equilibrium to the left (the endothermic direction) to "use up" the extra heat, thus *decreasing* the yield.
- If a forward reaction is **endothermic** (absorbs heat), increasing the temperature will push the equilibrium to the right to "use up" the extra heat, thus *increasing* the yield.
3. **Combine your findings:** Match what you see in the graph with the correct explanation from Le Chatelier's principle.
Full Answer
C.The yield is smaller at higher temperatures because the reaction is exothermic.✓ Correct
The correct answer is "The yield is smaller at higher temperatures because the reaction is exothermic." The graph shows that as temperature increases, the percentage yield decreases. According to Le Chatelier's principle, if increasing temperature decreases the yield, the system must be trying to counteract the temperature increase by favouring the endothermic direction. This means the forward reaction must be exothermic.
The question requires interpreting a graph in the context of Le Chatelier's principle.
1. **Graph Interpretation:** The graph in Figure 7 clearly shows a downward trend. As the temperature on the x-axis increases (from 300°C to 800°C), the percentage yield of sulfur trioxide on the y-axis decreases (from nearly 100% to about 30%). So, the yield is smaller at higher temperatures.
2. **Le Chatelier's Principle and Temperature:** For a reversible reaction, changing the temperature will shift the position of equilibrium. The equilibrium will shift in the direction that counteracts the change.
- Increasing the temperature favours the endothermic direction (the one that absorbs heat).
- Decreasing the temperature favours the exothermic direction (the one that releases heat).
3. **Conclusion:** Since increasing the temperature causes the yield of the forward reaction to decrease, the equilibrium must be shifting to the left (the reverse direction). This means the system is trying to cool down by favouring the endothermic direction. Therefore, the reverse reaction is endothermic, and the forward reaction must be **exothermic**.
Common mistakes
✗ Misreading the graph.
✗ Confusing the rules for exothermic and endothermic reactions in Le Chatelier's principle.
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
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