Changing the pressure
4 marks · NormalConsider the following data collected for this equilibrium reaction:
| Condition | % M(g) | % N(g) | % Q(g) |
|---|---|---|---|
| Low pressure | 40 | 40 | 20 |
| High pressure | 45 | 45 | 10 |
What is the total number of moles of gas on each side of the equation?
Describe the effect that increasing pressure has on the amounts of M(g), N(g), and Q(g).
Explain your answer to (b) in terms of the number of gas molecules.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Left: 2 moles of gas (M and N), Right: 1 mole of gas (Q).
(b) Increasing pressure increases % of M and N, decreases % of Q.
(c) The equilibrium shifts to the left (side with more gas molecules) when pressure increases.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states number of moles of gas on each side.
B2 (b) · 1 mark · States percentage of Q decreases when pressure increases.
B3 (b) · 1 mark · States percentage of M and N increase when pressure increases.
B4 (c) · 1 mark · Links change to the side with more gas molecules.
Worked solution
a) Left side: 1 (M) + 1 (N) = 2 moles of gas; right side: 1 (Q) = 1 mole.
b) When pressure increases from low to high, percentage of Q decreases (from 20% to 10%). Percentages of M and N increase (40% to 45%).
c) Increasing pressure favours the side with fewer gas molecules; here, the right side has fewer (1), so equilibrium shifts towards M and N on the left.