The effect of concentration
4 marks · NormalThe following reaction is at equilibrium in a closed container: A chemist adds more C to the equilibrium mixture.
Predict the effect of adding more C on the position of equilibrium. (1)
Explain your answer to part (a). (2)
Suggest one way the chemist could adjust the conditions to increase the yield of D. (1)
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(a) The equilibrium shifts to the left (towards reactants).
(b) According to Le Chatelier’s principle, increasing the concentration of a product (C) causes the system to oppose the change by shifting towards the reactants, using up some C.
(c) Remove D from the mixture as it forms, or increase the concentration of A or B.
Common mistakes: Saying the equilibrium does not move; Giving an incorrect direction of shift
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that the equilibrium shifts to the left/towards reactants.
M1 (b) · 1 mark · Correctly refers to increase in concentration of product as the cause.
M2 (b) · 1 mark · Explains the system opposes the change by shifting equilibrium to use up added C (towards reactants).
B2 (c) · 1 mark · Suggests removal of D or increasing A/B to increase yield of D.
Worked solution
(a) The equilibrium shifts to the left towards the reactants.
(b) Adding C increases the concentration of a product. The equilibrium will oppose this change by shifting to remove some of the added C. Therefore, more A and B will form from C and D.
(c) To increase the yield of D, the chemist can remove D as it forms or increase the amount of A or B.