Changing the pressure
3 marks · NormalThe reaction below is allowed to reach equilibrium in a sealed container:
Explain, using the number of moles of gas, what will happen to the position of equilibrium if the pressure is decreased.
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
The position of equilibrium will shift to the left (towards reactants). With a decrease in pressure, equilibrium favours the side with more gas molecules. Left side has 1 + 3 = 4 moles, right side has 2 moles, so equilibrium shifts to left to produce more gas molecules.
Must show: Calculates more/less gaseous molecules, Links shift to side with more molecules
Also accept: State left/reactants as answer and explain in terms of more gas molecules
Common mistakes: Saying shift to right; Not explaining moles of gas
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Identifies which side has more gaseous molecules (left/reactants: 4; right/products: 2).
M2 · 1 mark · States that decreasing pressure favours the side with more gaseous molecules.
A1 · 1 mark · Correctly predicts that equilibrium shifts to the left.
Worked solution
Left: 1 (X2) + 3 (Y2) = 4 moles gas. Right: 2 (XY3) = 2 moles gas.
If pressure is decreased, the equilibrium shifts to the side with more gas molecules. Therefore, equilibrium shifts left, making more X2 and Y2.