Gases A and B react reversibly to form gas C in a closed container.
2A(g)+B(g)⇌2C(g) \mathrm{2A(g) + B(g) \rightleftharpoons 2C(g)} 2A(g)+B(g)⇌2C(g)The same initial amounts of A and B, and no C, are used in each experiment. The mixture is allowed to reach equilibrium under the conditions shown.
| Experiment | Temperature in °C | Pressure in kPa | Catalyst | Percentage yield of C at equilibrium |
|---|---|---|---|---|
| 1 | 350 | 100 | absent | 42 |
| 2 | 350 | 300 | absent | 58 |
| 3 | 500 | 100 | absent | 25 |
| 4 | 350 | 100 | present | 42 |
Experiment 1 reaches equilibrium after 180 s. Experiment 4 reaches equilibrium after 60 s.
The rates of the forward and reverse reactions are measured during experiment 2.
| Time in s | Rate of forward reaction in mmol s−1^{-1}−1 | Rate of reverse reaction in mmol s−1^{-1}−1 |
|---|---|---|
| 0 | 1.20 | 0.00 |
| 40 | 0.86 | 0.18 |
| 80 | 0.61 | 0.39 |
| 120 | 0.48 | 0.48 |
| 160 | 0.48 | 0.48 |
At which recorded time has dynamic equilibrium first been established?
Explain your answer.
A student says that the forward reaction is endothermic.
Use the results to explain whether the student is correct.
The student also says that increasing the pressure decreases the percentage yield of C at equilibrium.
Use the equation and the results to explain whether the student is correct.
The student's last statement is:
'The catalyst increases the percentage yield of C and decreases the time taken to reach equilibrium.'
Evaluate this statement using the results, and explain the effects of the catalyst.
124 exam-style questions on OCR GCSE Chemistry 5.3 Equilibria, covering 5.3.1 Reversible reactions and dynamic equilibrium and 5.3.2 Changing the position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.