A dissolved substance can exist as two forms, P and Q.
P(aq)⇌Q(aq) \mathrm{P(aq) \rightleftharpoons Q(aq)} P(aq)⇌Q(aq)P is colourless and Q is dark green.
At 25 °C, the equilibrium concentration of Q is 0.080 mol dm−30.080\,\mathrm{mol\,dm^{-3}}0.080moldm−3.
A solid material, M, can remove some P from the solution but does not remove Q. A student makes one change to each of two separate, identical equilibrium mixtures. Neither P nor Q can escape.
| Change | Concentration of Q at the new equilibrium in mol dm−3\mathrm{mol\,dm^{-3}}moldm−3 |
|---|---|
| M is added at a constant temperature of 25 °C | 0.052 |
| The temperature is increased from 25 °C to 55 °C | 0.113 |
Explain the result when M is added.
Use the results for the change in temperature to explain whether the forward reaction is exothermic or endothermic.
State what happens to the colour of another equilibrium mixture when it is cooled.
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.