The effect of concentration
3 marks · NormalConsider the equilibrium: A sealed vessel contains 0.20 mol of X and 0.15 mol of Y at equilibrium. If 0.10 mol of Y is removed from the vessel, predict the change in the amount of X at equilibrium. Explain your answer with a calculation showing the direction of shift.
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(a) Amount of X will decrease because the equilibrium shifts right to replace Y.
(b) Calculation: Initial at equilibrium: X = 0.20 mol, Y = 0.15 mol After removing 0.10 mol Y: Y = 0.05 mol Reaction shifts right, so more X is used up to make more Y. For every 3 moles of Y made, 2 moles of X are used up.
(c) So the amount of X at the new equilibrium will be less than 0.20 mol.
Common mistakes: Forgetting direction or not showing calculation
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Marking points
M1 · 1 mark · States equilibrium shifts right to replace Y/removed product.
M2 · 1 mark · Shows using calculation that more X will be used up as more Y is made (with correct stoichiometry: 2X → 3Y).
A1 · 1 mark · Concludes that new amount of X will be less than 0.20 mol (or comments on a decrease).
Worked solution
When 0.10 mol Y is removed, Y becomes 0.05 mol. Equilibrium shifts right to replace Y by using more X. From the equation: 2X ⇌ 3Y, so for every 3 moles of Y made, 2 moles of X are used. Therefore, as Y increases back towards equilibrium, more X is consumed, so the amount of X drops below 0.20 mol.