Changing the temperature

4 marks · Normal

For the following reversible reaction, answer the questions below:

(a)

Is the forward reaction exothermic or endothermic? Explain your reasoning.

[1 mark]

(b)

What will happen to the position of equilibrium if the temperature is increased? Explain your answer.

[2 marks]

(c)

State whether more SO3 or more SO2 will be present at equilibrium if the temperature is decreased. Give a reason for your answer.

[1 mark]

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.

Show answer

(a) The forward reaction is exothermic because ΔH is negative.

(b) Equilibrium shifts to the left (towards reactants); the endothermic direction is favoured as system absorbs heat.

(c) More SO2 is present at equilibrium when temperature is decreased, because exothermic (forward) direction is favoured.

Common mistakes: Confusing direction of equilibrium shift; Incorrect reason for exothermicity

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States reaction is exothermic AND links this to negative ΔH.

M1 (b) · 1 mark · States that equilibrium will shift to the left/reactants/against forwards.

M2 (b) · 1 mark · Explains that this is because endothermic direction is favoured to absorb added heat.

B2 (c) · 1 mark · States more SO3 is produced at lower temperature; gives correct reason: forward/exothermic direction is favoured.

Worked solution

(a) The forward reaction is exothermic because ΔH is negative.

(b) Increasing temperature favours the endothermic direction (reverse), so the equilibrium shifts to the left and more reactants are formed.