Three gases, A, B and C, react in a sealed vessel.
2A(g)+B(g)⇌2C(g) \mathrm{2A(g) + B(g) \rightleftharpoons 2C(g)} 2A(g)+B(g)⇌2C(g)The mixture is allowed to reach equilibrium at stage 1. The conditions are then changed in stages. After each change, the mixture is left until the amount of C becomes constant. When the temperature is changed, the pressure is kept constant.
| Stage | Temperature (°C) | Pressure (kPa) | Change made at the start of the stage | Amount of C when constant (mol) |
|---|---|---|---|---|
| 1 | 350 | 200 | none | 0.84 |
| 2 | 250 | 200 | temperature decreased | 1.08 |
| 3 | 250 | 600 | pressure increased | 1.27 |
| 4 | 250 | 600 | catalyst added | 1.27 |
A student concludes that the unchanged amount of C at stage 4 shows that the forward and reverse reactions have stopped.
Give the meaning of the symbol ⇌ \rightleftharpoons\,⇌ in the equation.
Describe and explain the results in the table.
Use ideas about changes in equilibrium and dynamic equilibrium to explain whether the student's conclusion is correct.
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.