A student mixes citric acid solution with sodium hydrogencarbonate in a polystyrene cup. The experiment is repeated using the same quantities of chemicals.
The starting temperature is 20.0 ∘C20.0\ ^{\circ}\text{C}20.0 ∘C in both experiments, which is also the room temperature.
| Lid used | Lowest temperature (∘C^{\circ}\mathrm{C}∘C) |
|---|---|
| Yes | 13.5 |
| No | 15.0 |
Which conclusion is correct?
The lidded result is closer to the true temperature change; the reaction is endothermic, and the lid reduces energy transfer from the room into the mixture.
The uncovered result is closer to the true temperature change; the reaction is endothermic, and removing the lid allows the mixture to take in more energy from the room.
The lidded result is closer to the true temperature change; the reaction is exothermic, and the lid reduces energy transfer from the mixture to the room.
The uncovered result is closer to the true temperature change; the reaction is exothermic, and its higher minimum temperature shows energy was transferred to the mixture.
106 exam-style questions on OCR GCSE Chemistry 3.2 Energetics, covering 3.2.1 Exothermic and endothermic reactions and reaction profiles and 3.2.2 Calculating energy change from bond energies. Each one has a worked solution and a mark scheme showing where the marks go.