Hydrogen iodide, HI, is formed when hydrogen and iodine react together in a closed container. The reaction reaches a dynamic equilibrium. Explain what is meant by the term 'dynamic equilibrium' and describe what would happen to the position of this equilibrium if the concentration of hydrogen gas were increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrogen and iodine react together reversibly according to the following equation: H₂(g) + I₂(g) ⇌ 2HI(g)
Model answer (4 marks)
At dynamic equilibrium the forward and reverse reactions are still occurring, but the rate of the forward reaction equals the rate of the reverse reaction.
If the concentration of hydrogen gas is increased, the system will shift the equilibrium to the right, towards the products.
This shift occurs because the system acts to reduce the increased concentration of hydrogen, thereby opposing the change.
If the concentration of hydrogen gas is increased, the system will shift the equilibrium to the right, towards the products.
This shift occurs because the system acts to reduce the increased concentration of hydrogen, thereby opposing the change.
Examiner tips
- Use the exact wording from the mark scheme – ‘forward and reverse reactions are still occurring’ and ‘rate of forward equals rate of reverse’.
- State the direction of the shift (to the right) and explain that it is to reduce the added hydrogen.
- Mention that the shift is a response to the change in concentration, not to temperature or pressure unless specified.
Common mistakes
- Writing that the reactions have stopped at equilibrium.
- Saying the equilibrium shifts to the left or not specifying the direction.
- Failing to explain that the shift is to reduce the increased concentration of hydrogen.
Mark scheme (4 marks)
- At dynamic equilibrium, the forward and reverse reactions are both still occurring (reactions have not stopped)
- At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction
- The position of equilibrium shifts to the right / towards the products
- This is because the system acts to reduce the increased concentration of hydrogen / opposes the change
Key terms in this question
dynamic equilibrium · concentration
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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