Reversibility and dynamic equilibria
4 marks · NormalThe table shows the concentration of hydrogen, iodine and hydrogen iodide in a sealed container at different times during a reversible reaction.
| Time (min) | [H2] (mol dm⁻³) | [I2] (mol dm⁻³) | [HI] (mol dm⁻³) |
|---|---|---|---|
| 0 | 0.50 | 0.50 | 0.00 |
| 1 | 0.30 | 0.30 | 0.40 |
| 2 | 0.21 | 0.19 | 0.60 |
| 3 | 0.19 | 0.18 | 0.64 |
| 4 | 0.18 | 0.17 | 0.65 |
| 5 | 0.18 | 0.17 | 0.65 |
Use the data to answer the following questions.
At what time does the reaction reach dynamic equilibrium? Explain how you know.
State one characteristic of the reaction mixture at equilibrium based on the data.
Suggest what would happen if more H2 is added at equilibrium.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) At 4 minutes (or between 4 and 5 min), because the concentrations remain constant after this time.
(b) The concentrations of all substances are constant at equilibrium.
(c) The position of equilibrium will shift to produce more HI (products).
Common mistakes: Not linking constant concentration to equilibrium; Incorrectly reading the data table
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies correct time (4 min or between 4 and 5 min)
M1 (a) · 1 mark · Gives correct reason: concentrations remain constant after this time
B2 (b) · 1 mark · States a characteristic of equilibrium (e.g., constant concentration)
B3 (c) · 1 mark · Predicts increase in product (HI) when more H2 is added
Worked solution
a) At 4 minutes, the concentrations of H2, I2, and HI stop changing, so dynamic equilibrium is reached.
b) At equilibrium, concentrations of all substances remain constant.
c) If more H2 is added, the equilibrium will shift to make more HI to re-establish equilibrium.