AQA GCSE Combined Science: Chemistry: Energy Changes
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Chemical reactions can be exothermic or endothermic. When a fuel such as methane burns, the surroundings get hotter. When some salts dissolve in water in a cold pack, the surroundings get colder.
Compare exothermic and endothermic reactions. In your answer you should refer to the transfer of energy to or from the surroundings, the change in temperature observed, what happens to the energy stored in the chemicals, and the shape of the reaction profile for each type. (6 marks)
Hydrogen reacts with chlorine to form hydrogen chloride:
H₂(g) + Cl₂(g) → 2HCl(g)
The bond energies are given in the table.
| Bond | Bond energy / kJ per mole |
|---|---|
| H–H | 436 |
| Cl–Cl | 242 |
| H–Cl | 431 |
(a) Calculate the overall energy change for this reaction, in kJ per mole. Show your working. (3 marks)
(b) State whether the reaction is exothermic or endothermic, and justify your answer using your value from part (a). (1 mark)
A reaction profile shows how the energy of the reactants and products changes during a reaction.
(a) For an exothermic reaction, state whether the products are at a higher or a lower energy than the reactants on the reaction profile. (1 mark)
(b) On a reaction profile, what is meant by the activation energy? (2 marks)
A student investigated the temperature change when different volumes of dilute sodium hydroxide were added to a fixed volume of dilute hydrochloric acid in a polystyrene cup. The neutralisation reaction is exothermic. The maximum temperatures recorded are shown.
| Volume of sodium hydroxide added / cm³ | Maximum temperature reached / °C |
|---|---|
| 0 | 20.0 |
| 10 | 23.5 |
| 20 | 27.0 |
| 30 | 25.5 |
| 40 | 24.0 |
(a) State the volume of sodium hydroxide that produced the greatest temperature rise, and give the temperature rise at that volume. (2 marks)
(b) Suggest one reason why a polystyrene cup is used as the reaction container in this practical, rather than a glass beaker. (1 mark)
A hydrogen fuel cell produces electricity from a reaction between hydrogen and oxygen. The overall reaction is:
2H₂(g) + O₂(g) → 2H₂O(l)
Give two advantages of using a hydrogen fuel cell rather than burning a fossil fuel such as petrol to power a vehicle. (2 marks)
When solid ammonium chloride is dissolved in water, the temperature of the water falls, showing that energy is taken in from the surroundings.
Name the type of reaction (in terms of energy) that takes in energy from the surroundings and causes the temperature to fall. (1 mark)