A chemist is investigating the following reversible reaction, which takes place in a closed system at a fixed temperature: SO₂(g) + NO₂(g) ⇌ SO₃(g) + NO(g). The chemist adds extra NO₂ gas to the system once dynamic equilibrium has been established. Explain what happens to the position of equilibrium and why, and state what happens to the concentration of NO once the system reaches equilibrium again.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The reaction SO₂(g) + NO₂(g) ⇌ SO₃(g) + NO(g) reaches dynamic equilibrium in a closed system. Extra NO₂ is then added to the system.
Model answer (4 marks)
Adding NO₂ increases the concentration of a reactant. According to Le Chatelier’s principle the equilibrium shifts to the right, towards the products, to counteract the change. The forward reaction rate therefore increases until it equals the reverse rate again. As a result the concentration of NO increases when the system reaches a new equilibrium.
Examiner tips
- Use the phrase ‘Le Chatelier’s principle’ to show understanding of the cause of the shift.
- State the direction of the shift (right, towards products) explicitly.
- Explain that the forward rate rises until the rates are equal again.
- Mention the increase in NO concentration as the final consequence.
Common mistakes
- Saying the equilibrium shifts to the left or to the reactants.
- Failing to mention the increase in the forward reaction rate.
- Not specifying that NO concentration rises at the new equilibrium.
Mark scheme (4 marks)
- Adding extra NO₂ increases the concentration of a reactant
- The position of equilibrium shifts to the right / towards the products (to counteract the change / according to Le Chatelier's principle)
- The rate of the forward reaction increases (until it equals the rate of the backward reaction again)
- The concentration of NO increases (as more NO is produced)
Key terms in this question
dynamic equilibrium · closed system · reversible reaction · concentration
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Equilibria questions
- Ammonia is produced in the Haber process using the following reversible reaction…
- The Contact Process is used in industry to produce sulfur trioxide, SO₃, which i…
- Nitrogen dioxide, NO₂, and dinitrogen tetroxide, N₂O₄, exist in a reversible rea…
- A chemical engineer is studying the following reversible reaction used in the pr…