Exothermic and endothermic reactions
In any chemical reaction, bonds break (needs energy in) and new bonds form (releases energy out). The net energy change determines whether the reaction is exothermic or endothermic.
Exothermic reactions
An exothermic reaction transfers energy to the surroundings, usually as heat — temperature rises.
Common examples:
- Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O (releases lots of heat).
- Neutralisation: HCl + NaOH → NaCl + H₂O (warms slightly).
- Most oxidation reactions: rusting, respiration.
- Hand warmers (oxidation of iron, or crystallisation of supersaturated salt).
In an exothermic reaction, the products have less energy than the reactants — the difference is released to the surroundings.
Endothermic reactions
An endothermic reaction takes in energy from the surroundings — temperature drops.
Common examples:
- Thermal decomposition: CaCO₃ → CaO + CO₂ (only happens if heated).
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (energy from sunlight).
- Citric acid + sodium hydrogencarbonate (a school staple).
- Sports cold packs (often ammonium nitrate dissolving).
The products have more energy than the reactants — the difference came from the surroundings.
Recognising exothermic vs endothermic experimentally
Take starting temperature, mix reactants in an insulated container, take final temperature.
- ΔT positive → exothermic.
- ΔT negative → endothermic.
A polystyrene cup with a lid is a simple GCSE calorimeter — minimises heat loss to the surroundings.
Energy changes are reversible
Reverse reactions have the opposite energy change of the same magnitude. If A → B releases 50 kJ (exothermic), then B → A absorbs 50 kJ (endothermic).
Practical applications
- Exothermic: hand warmers, self-heating cans, combustion engines, most chemical industry.
- Endothermic: cool packs, instant cold compresses, photosynthesis (chemical energy storage in plants).
✦Worked example
When 50 cm³ of HCl reacts with 50 cm³ of NaOH, the temperature rises from 20 °C to 26 °C. State whether the reaction is exo- or endothermic and explain.
ΔT = +6 °C → temperature rose → reaction is exothermic. Energy is released to the surroundings (the solution).
⚠Common mistakes
- "Energy is created" in exothermic reactions. No — it's transferred from the chemicals to the surroundings (conservation of energy).
- Confusing absorbed vs released. Endo = energy IN. Exo = energy OUT.
- Saying combustion can be endothermic. Combustion is always exothermic at GCSE.
- Forgetting the temperature change is in the surroundings (e.g. the solution), not the chemicals.
Links
Sets up C5.2 (reaction profiles), C5.3 (bond energies HT), and links to C6 (rates: increasing T speeds reaction).
AI-generated · claude-opus-4-7 · v3-deep-chemistry