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AQA GCSE · Question 07.3 · The rate and extent of chemical change
The equation for the reaction is:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
Explain why the percentage yield of sulfur trioxide in this reaction is greater if the pressure is higher.
The equation for the reaction is:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
Explain why the percentage yield of sulfur trioxide in this reaction is greater if the pressure is higher.
How to approach this question
This question is about applying Le Chatelier's principle to pressure changes.
1. Count the total number of moles of gas on the reactant side (left side) of the equation.
2. Count the total number of moles of gas on the product side (right side) of the equation.
3. State the rule from Le Chatelier's principle about how an increase in pressure affects the equilibrium position based on the number of moles of gas.
4. Apply this rule to the specific reaction to explain why the yield of SO₃ increases.
Full Answer
There are fewer moles of gas on the product side (2 moles) than on the reactant side (2 + 1 = 3 moles). According to Le Chatelier's principle, increasing the pressure will cause the equilibrium to shift to the side with fewer moles of gas to reduce the pressure. This favours the forward reaction, increasing the yield of sulfur trioxide.
This question relates to Le Chatelier's principle and the effect of pressure on gaseous equilibria.
1. **Count the Moles of Gas:** We look at the balanced symbol equation: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).
- On the reactant (left) side, we have 2 moles of SO₂ and 1 mole of O₂, making a total of 2 + 1 = 3 moles of gas.
- On the product (right) side, we have 2 moles of SO₃ gas.
2. **Apply Le Chatelier's Principle:** The principle states that if the pressure of a system at equilibrium is increased, the equilibrium will shift to the side with the fewer number of moles of gas, as this will help to reduce the overall pressure.
3. **Conclusion:** In this reaction, the product side has fewer moles of gas (2) than the reactant side (3). Therefore, increasing the pressure will shift the equilibrium position to the right, favouring the forward reaction and increasing the percentage yield of the product, sulfur trioxide (SO₃).
Common mistakes
✗ Miscounting the number of moles on each side.
✗ Stating the rule for pressure incorrectly (e.g., saying it shifts to the side with more moles).
✗ Just saying "it shifts to the right" without explaining *why* in terms of the number of moles.
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