Changing the temperature
5 marks · HardThe following reversible reaction is at equilibrium:
State whether the forward reaction is exothermic or endothermic. Explain how you know.
If the position of equilibrium shifts to the right when the temperature is changed, is the temperature being increased or decreased? Explain your reasoning using the ΔH value.
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) Exothermic; because ΔH is negative for the forward reaction.
(b) Temperature is decreased; since the forward reaction is exothermic (ΔH negative), lowering temperature favours the exothermic direction, shifting equilibrium right.
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Marking points
B1 (a) · 1 mark · States the forward reaction is exothermic.
B2 (a) · 1 mark · Explains this using ΔH being negative for the forward reaction.
M1 (b) · 1 mark · Identifies that a rightward shift means more products are formed.
M2 (b) · 1 mark · Links the rightward equilibrium shift to the exothermic direction using ΔH value.
A1 (b) · 1 mark · Concludes that the temperature is being decreased, with correct reasoning.
Worked solution
a) The forward reaction is exothermic because ΔH is negative (ΔH = -150 kJ/mol for the forward reaction signals heat released in the forward direction).
b) If equilibrium shifts right (towards products V), this means the system favours the exothermic direction. Lowering the temperature favours exothermic reactions (releases heat to raise the temperature again), so the temperature has been decreased.