Changing the pressure
2 marks · HardConsider the following reversible reaction, which takes place in a sealed vessel at constant temperature:
Explain why changing the pressure will or will not affect the position of equilibrium for this reaction.
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Also accept: Pressure changes only affect equilibria involving gases with a different number of gaseous molecules on each side; this reaction has 3 moles of gas on the left and 2 moles on the right (the solid does not count), so pressure changes will affect the position of equilibrium.; Pressure changes will affect the equilibrium because there are more moles of gas on one side (3 on left, 2 on right; ignore the solid).
Common mistakes: Counting the solid Z as a gas.; Saying pressure changes do not affect equilibria involving gases.; Ignoring the number of gas molecules.
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Marking points
M1 · 1 mark · States or implies that only gaseous molecules count when considering pressure effects (solids/liquids do not count).
A1 · 1 mark · Correctly explains that because the number of gaseous molecules is different on each side (3 on left, 2 on right), changing pressure will affect equilibrium.
Worked solution
Only consider gaseous molecules when predicting the effect of pressure changes on equilibrium; solids do not count.
On the left: 3 moles of X(g); on the right: only 2 moles of Y(g); Z is a solid and does not count.
Because there is a different number of gaseous molecules on each side, changing the pressure will affect the position of equilibrium for this reaction.