A student sets up a sealed flask containing the following reversible reaction at constant temperature: H₂(g) + I₂(g) ⇌ 2HI(g). The system reaches dynamic equilibrium. The student then increases the pressure by reducing the volume of the flask. Explain what happens to the position of equilibrium and the concentration of HI when the pressure is increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
H₂(g) + I₂(g) ⇌ 2HI(g)
Model answer (5 marks)
The pressure increase does not shift the equilibrium.
Because the reaction involves an equal number of moles of gas on each side (1 H₂ + 1 I₂ ⇌ 2 HI), the total number of gas moles is the same whether the system is on the left or the right.
Le Chatelier’s principle says the system will try to oppose the change, but shifting in either direction would not change the total moles of gas and therefore would not reduce the pressure. Consequently, the position of equilibrium remains unchanged.
With the volume reduced, the same amount of HI occupies a smaller space, so its concentration increases.
Because the reaction involves an equal number of moles of gas on each side (1 H₂ + 1 I₂ ⇌ 2 HI), the total number of gas moles is the same whether the system is on the left or the right.
Le Chatelier’s principle says the system will try to oppose the change, but shifting in either direction would not change the total moles of gas and therefore would not reduce the pressure. Consequently, the position of equilibrium remains unchanged.
With the volume reduced, the same amount of HI occupies a smaller space, so its concentration increases.
Examiner tips
- State that the total moles of gas are equal on both sides before applying Le Chatelier’s principle.
- Explain that a pressure change cannot be offset by a shift because moles are equal.
- Mention that concentration increases simply because volume decreases.
- Use the exact wording from the mark scheme to gain full marks.
Common mistakes
- Claiming the equilibrium shifts to the side with fewer moles of gas, ignoring that the moles are equal.
- Saying the concentration of HI decreases, confusing pressure with concentration.
- Using the wrong units or not showing the reasoning for no shift.
Mark scheme (5 marks)
- The position of equilibrium does not shift (stays the same / no change in position)
- Because there are equal numbers of moles of gas on both sides of the equation (1 + 1 = 2)
- Le Chatelier's principle states the system opposes / minimises the change applied
- Because shifting in either direction would not reduce pressure (as moles of gas are equal either way), no shift occurs
- The concentration of HI increases (because the same amount of HI is now in a smaller volume)
Key terms in this question
dynamic equilibrium · position of equilibrium · concentration
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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