The position of equilibrium
4 marks · NormalConsider the following equilibrium established in a sealed container:
The forward reaction is endothermic.
State and explain what will happen to the position of equilibrium if the temperature is decreased.
State and explain what will happen to the position of equilibrium if the pressure is increased.
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) Equilibrium shifts to the left (towards 2A) when temperature is decreased, because this favours the exothermic (reverse) reaction.
(b) Equilibrium shifts to the left (towards 2A), since increasing the pressure favours the side with fewer moles of gas (2 on left vs 4 on right).
Common mistakes: Mixing up forward/reverse reaction direction; Referring to 'equal concentrations' at equilibrium
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Marking points
M1 (a) · 1 mark · States shift to the left (towards 2A/ reactants) or correctly describes movement towards side with fewer moles of gas (if correct reasoning is given).
A1 (a) · 1 mark · Explains this change is because decreasing temperature favours the exothermic (reverse) reaction.
M2 (b) · 1 mark · States shift to the left (towards 2A/ reactants) when pressure is increased.
A2 (b) · 1 mark · Explains this is because the left side has fewer moles of gas (2 on left, 1+3=4 on right). Accept equivalent wording.
Worked solution
a) Lowering temperature favours the exothermic (reverse) reaction, so equilibrium shifts to the left (towards 2A).
b) Increasing pressure favours the side with fewer gas molecules (2A: 2 moles, B + 3C: 4 moles), so equilibrium shifts to the left (towards 2A).