A reversible reaction is investigated 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 forward reaction is exothermic. The same starting amounts of A and B are used in each experiment. The percentage yield of C at equilibrium is measured under different conditions.
| Temperature | Pressure | Percentage yield of C |
|---|---|---|
| 300 ∘C300\,^{\circ}\mathrm{C}300∘C | 1.0 MPa | 42% |
| 300 ∘C300\,^{\circ}\mathrm{C}300∘C | 4.0 MPa | 72% |
| 500 ∘C500\,^{\circ}\mathrm{C}500∘C | 1.0 MPa | 18% |
| 500 ∘C500\,^{\circ}\mathrm{C}500∘C | 4.0 MPa | 39% |
Compare the equilibrium yields at 300 ∘C300\,^{\circ}\mathrm{C}300∘C when the pressure is changed from 1.0 MPa to 4.0 MPa. Explain the difference using the equation.
Compare the equilibrium yields at 1.0 MPa when the temperature is changed from 300 ∘C300\,^{\circ}\mathrm{C}300∘C to 500 ∘C500\,^{\circ}\mathrm{C}500∘C. Explain the difference using the information about the forward reaction.
An equilibrium mixture at 4.0 MPa is cooled from 500 ∘C500\,^{\circ}\mathrm{C}500∘C to 300 ∘C300\,^{\circ}\mathrm{C}300∘C, and the pressure is kept the same.
Use the data to describe how the rates of the forward and reverse reactions compare while the mixture is changing and once the new equilibrium is reached.
A technician says, “Adding a catalyst will increase the percentage yield of C at equilibrium because it makes the reaction faster.” Explain why this statement is incorrect.
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.