Dinitrogen tetroxide, N2O4\mathrm{N_{2}O_{4}}N2O4, is a colourless gas. It breaks down into nitrogen dioxide, NO2\mathrm{NO_{2}}NO2, which is a brown gas.
N2O4(g)⇌2NO2(g)\mathrm{N_{2}O_{4}(g)} \rightleftharpoons \mathrm{2NO_{2}(g)}N2O4(g)⇌2NO2(g)
A scientist puts 0.100 mol of N2O4\mathrm{N_{2}O_{4}}N2O4 into a sealed flask kept at 60 °C. The table shows the amounts of each gas over time.
| Time / s | Amount of N2O4\mathrm{N_{2}O_{4}}N2O4 / mol | Amount of NO2\mathrm{NO_{2}}NO2 / mol |
|---|---|---|
| 0 | 0.100 | 0.000 |
| 20 | 0.070 | 0.060 |
| 40 | 0.055 | 0.090 |
| 60 | 0.048 | 0.104 |
| 80 | 0.045 | 0.110 |
| 100 | 0.045 | 0.110 |
| 120 | 0.045 | 0.110 |
Describe how the colour of the gas in the flask changes during the 120 seconds.
Use the results at 20 s to show that the data are consistent with the balanced equation.
The experiment is repeated with 0.100 mol of N2O4\mathrm{N_{2}O_{4}}N2O4 in the same flask at 100 °C. At equilibrium the flask now contains 0.150 mol of NO2\mathrm{NO_{2}}NO2. Calculate the amount of N2O4\mathrm{N_{2}O_{4}}N2O4 in the flask at equilibrium at 100 °C. Deduce whether the forward reaction is exothermic or endothermic, and explain your answer.
Explain, in terms of the rates of the forward and backward reactions, why the amounts change quickly at first and then stop changing.
At 120 s the volume of the flask is halved at the same temperature. Predict how the amount of NO2\mathrm{NO_{2}}NO2 changes when equilibrium is re-established. Explain your answer.
45 exam-style questions on Edexcel GCSE Chemistry 5.2 Reversible reactions and equilibria, covering 5.2.1 Reversible reactions and dynamic equilibrium, 5.2.2 The Haber process, and 5.2.3 Changing the position of a dynamic equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.