A reversible gas reaction takes place in an airtight steel vessel. The forward and reverse reaction rates are recorded at different times.
| Time in s | Forward reaction rate in mol/s | Reverse reaction rate in mol/s |
|---|---|---|
| 0 | 0.080 | 0.000 |
| 15 | 0.063 | 0.018 |
| 30 | 0.049 | 0.034 |
| 45 | 0.041 | 0.041 |
| 60 | 0.041 | 0.041 |
| 75 | 0.041 | 0.041 |
When was the mixture in the vessel first at equilibrium?
At 45 s45\ \text{s}45 s, because the forward and reverse reaction rates are equal
At 15 s15\ \text{s}15 s, because the reverse reaction has started to take place
At 60 s60\ \text{s}60 s, because neither rate has changed since the previous reading
At 75 s75\ \text{s}75 s, because both reactions have stopped by this time
109 exam-style questions on AQA GCSE Chemistry 6.2 Reversible reactions and dynamic equilibrium, covering 6.2.1 Reversible reactions, 6.2.2 Energy changes and reversible reactions, 6.2.3 Equilibrium, 6.2.4 The effect of changing conditions on equilibrium, 6.2.5 The effect of changing concentration, 6.2.6 The effect of temperature changes on equilibrium, and 6.2.7 The effect of pressure changes on equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.