Changing the pressure
3 marks · NormalThe following reaction is allowed to reach equilibrium in a sealed container:
The number of gaseous molecules on each side of the equation is different.
Explain, with calculations, why increasing the pressure will affect this equilibrium, and state which side the equilibrium will shift towards. Use the number of moles of gas in your answer.
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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Increasing the pressure shifts the equilibrium towards the side with fewer gaseous molecules, in this case towards the left (reactants).
Also accept: The equilibrium will move to decrease the number of gas molecules by favouring the left side.; Equilibrium shifts to the side with fewer moles of gas when pressure is increased.
Common mistakes: Stating that pressure affects all reactions, not just those with different gaseous moles; Incorrectly identifying which side has fewer molecules
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Marking points
M1 · 1 mark · Identifies the number of gaseous molecules on each side: 2 on left (reactants), 3 on right (products)
M2 · 1 mark · Explains that only reactions with different numbers of gas molecules are affected by pressure changes
A1 · 1 mark · Correctly states that increasing pressure favours the side with fewer gaseous molecules (left/reactants in this case)
Worked solution
There are 2 moles of gaseous molecules on the left: 2N2O(g).
There are 3 moles of gaseous molecules on the right: 2N2(g) + 1O2(g).
Changing the pressure only affects equilibria in which the number of moles of gas is different on each side.
Increasing the pressure causes the equilibrium to shift towards the side with fewer moles of gas to counteract the change.
In this example, the side with fewer moles of gas is the left side (reactants). So, increasing pressure shifts equilibrium to the left.