Reversible Reactions and Equilibria
6 marks · NormalConsider the equilibrium:
State whether the forward reaction is exothermic or endothermic. Give a reason based on the value of ΔH.
Predict and explain what happens to the amount of PCl₃(g) at equilibrium if the temperature is increased.
State whether adding a catalyst affects the amount of PCl₃(g) present at equilibrium. Explain 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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(a) The forward reaction is endothermic because ΔH is positive.
(b) Increasing temperature favours the endothermic direction, which is the forward reaction. This increases the amount of PCl₃(g) at equilibrium.
(c) Adding a catalyst does not affect the amount of PCl₃(g) at equilibrium. A catalyst only speeds up the rates of the forward and reverse reactions equally.
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Marking points
B1 (a) · 1 mark · States 'endothermic' for the forward reaction
B2 (a) · 1 mark · Correct reasoning: ΔH positive, so absorbs heat
B3 (b) · 1 mark · States equilibrium favours forward direction (increasing PCl₃)
B4 (b) · 1 mark · Mentions that increasing temperature favours endothermic direction
B5 (c) · 1 mark · States that a catalyst does not affect the equilibrium amount
B6 (c) · 1 mark · Explains (catalyst increases rates both ways equally)
Worked solution
(a) ΔH = +85 kJ/mol means the forward reaction absorbs heat, so it is endothermic.
(b) Increasing temperature favours the endothermic (forward) direction.
So, equilibrium shifts right, forming more PCl₃(g) and Cl₂(g).
(c) Adding a catalyst does not change the position of equilibrium, so the amount of PCl₃ at equilibrium remains the same. It only helps to reach equilibrium faster.