A chemical company produces ammonia using a reversible reaction carried out in a closed system. The engineers increase the concentration of nitrogen gas fed into the reactor. Explain what happens to the position of equilibrium and the concentrations of the reactants and products after this change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nitrogen and hydrogen react together in a reversible reaction to form ammonia. The reaction is carried out in a closed system so that dynamic equilibrium can be established.
Model answer (4 marks)
The added nitrogen shifts the equilibrium to the right, forming more ammonia. This occurs because the system opposes the increase in nitrogen concentration (Le Chatelier’s principle). Consequently the concentration of ammonia rises, while the concentrations of nitrogen and hydrogen fall until a new equilibrium is reached where the concentrations of all species remain constant but differ from the initial values.
Examiner tips
- Use the phrase ‘shifts to the right’ or ‘towards the products’ to show understanding of Le Chatelier’s principle.
- Explain that the system opposes the change in concentration of nitrogen.
- State that ammonia concentration increases and a new equilibrium is established.
- Use correct chemical symbols (N₂, H₂, NH₃) and UK spelling (equilibrium).
Common mistakes
- Writing ‘shifts to the left’ or ‘towards the reactants’ – the equilibrium actually moves to the right.
- Failing to mention Le Chatelier’s principle or the reason for the shift.
- Not specifying that a new equilibrium is reached with different concentrations.
Mark scheme (4 marks)
- The position of equilibrium shifts in the forward direction / towards the products (ammonia)
- This is because the system acts to counteract / oppose the increase in concentration of nitrogen (Le Chatelier's principle)
- The concentration of ammonia (products) increases
- A new equilibrium is reached where the concentrations of reactants and products remain constant (but are different from before)
Key terms in this question
closed system · position of equilibrium · concentration
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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