Energetics
Physical Chemistry · 3 question types
Exothermic and Endothermic Reactions
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
Defining exothermic and classifying a change from results
Question: State what is meant by the term exothermic, or use the temperature readings from an experiment to explain what type of energy change has taken place (1–2 marks).
The definition is asked in 3 of the 23 papers, and the mark is for heat energy (or thermal energy) being given out to the surroundings. "Energy is given out" on its own is ignored: name heat.
Reading the type from results is set in 2 of the 23 papers. The first mark is for the direction of the change, the temperature of the mixture rose (or fell), and the second for the conclusion, so the reaction is exothermic (or endothermic). For an endothermic change, "the temperature decreased" or "heat energy was taken in" is credited. The direction of the temperature change is a mark of its own, so state it rather than jumping straight to the name.