Energetics
Physical Chemistry · 3 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
Calorimetry calculations and energy profile diagrams appear in nearly every series.
Conservation of energy
- Energy cannot be created or destroyed during a chemical reaction — only transferred between the reaction mixture (the system) and its surroundings
- When energy leaves the system, the surroundings warm up by the same amount
- When energy enters the system, the surroundings cool down by the same amount
- A thermometer placed in the reaction mixture is the simplest way to detect this transfer
Exothermic reactions
- An exothermic reaction transfers energy out of the system to the surroundings
- The temperature of the reaction mixture, together with any solvent it sits in, rises
- The chemical energy stored in the products is lower than in the reactants — the difference is what escapes as heat
- The enthalpy change is negative: ΔH < 0
- Reactions that are always exothermic include:
- of fuels
- of an acid by a base
- The reaction of a reactive metal with water or acid
- Many displacement and oxidation reactions
Endothermic reactions
- An endothermic reaction transfers energy into the system from the surroundings
- The temperature of the reaction mixture falls
- The products store more chemical energy than the reactants
- The enthalpy change is positive: ΔH > 0
- Reactions that are commonly endothermic include:
- of metal carbonates
- The reaction between citric acid and sodium hydrogencarbonate
- The dissolving of some ionic salts (e.g. ammonium nitrate) — useful in cold packs
- Photosynthesis (energy supplied by sunlight)
Reading the experimental data
| Reaction | Initial T / °C | Final T / °C | ΔT / °C | Type |
|---|---|---|---|---|
| 20 cm³ HCl + 20 cm³ NaOH | 18.0 | 24.5 | +6.5 | Exothermic (T rose) |
| 5 g KNO₃ stirred into 50 cm³ water | 21.0 | 17.2 | −3.8 | Endothermic (T fell) |
| Zn powder + 30 cm³ CuSO₄(aq) | 20.0 | 28.6 | +8.6 | Exothermic |
| 5 g NH₄NO₃ stirred into 50 cm³ water | 20.0 | 13.4 | −6.6 | Endothermic |
- A rise in temperature = exothermic; a fall = endothermic. Mark schemes credit the direction of the temperature change plus the named conclusion
Classifying a reaction as exothermic or endothermic
What comes up: given a reaction or an experiment, state whether it is exothermic or endothermic and justify the answer.
Write (two marks): (1) the temperature of the mixture/surroundings increases (for exothermic) or decreases (for endothermic); (2) so the reaction is exothermic/endothermic. Both the temperature change direction and the correct classification are needed.
Watch out: naming the type without stating the temperature change direction drops the justification mark. Simply writing "heat is released" without tying it to a temperature rise or fall is not enough.