Changing the pressure
4 marks · NormalThe following reaction takes place in a closed container:
State the total number of moles of gas on each side of this equation.
The pressure of the system is increased. Predict and explain the direction in which the equilibrium will shift.
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(a) Left side: 2 moles of gas; Right side: 4 moles of gas · must show Gaseous moles counted for both sides
(b) The equilibrium will shift to the left (towards \ce{PCl5}). This is because increasing the pressure favours the side with fewer molecules of gas, which is the left side. · must show Shift to left (towards reactants), Explains in terms of number of gaseous molecules, States that increased pressure favours fewer molecules
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Marking points
B1 (a) · 1 mark · States correct number of moles of gas on each side (2 on left, 4 on right).
M1 (b) · 1 mark · Identifies the side with fewer molecules (left/reactants).
M2 (b) · 1 mark · Explains that increased pressure favours the side with fewer gaseous molecules.
A1 (b) · 1 mark · Correctly predicts equilibrium shifts to the left.
Worked solution
a) Left: 2 moles of gas (from 2PCl5), Right: 2 + 2 = 4 moles of gas (2PCl3 + 2Cl2).
b) Increasing pressure will shift the equilibrium to the side with fewer gas molecules, which is the left side. This reduces the number of gas molecules and helps counteract the increase in pressure.