The effect of concentration
2 marks · HardThe following reversible reaction is at equilibrium in a sealed container:
State and explain the effect on the position of equilibrium if additional Y(g) is added to the system.
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The position of equilibrium shifts to the right (towards producing more Z). Adding more Y(g) increases the concentration of a reactant, so the system responds by forming more products to reduce the added Y(g).
Also accept: Equilibrium shifts towards products; system forms more Z to reduce Y
Common mistakes: Shifts left; No change
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Marking points
M1 · 1 mark · States correct direction of shift (to right/towards Z/products).
M2 · 1 mark · Explains that adding Y increases reactant concentration, so reaction forms more products to reduce Y.
Worked solution
Adding more Y increases the concentration of a reactant. According to Le Chatelier's principle, the system will try to oppose this change by reducing the concentration of Y.
It does this by shifting the equilibrium to the right, forming more Z (product).
Therefore, the equilibrium position shifts towards the products (right).