Heat and chemical change
Thermochemistry studies the heat change that happens when bonds break and form during a reaction. When reactant bonds break, energy is absorbed; when product bonds form, energy is released. The overall heat change decides whether the surroundings get hotter or colder.
Exothermic and endothermic reactions
An exothermic reaction releases heat to the surroundings, so the temperature of the mixture rises. Examples include combustion, neutralisation and the reaction of reactive metals with acid. An endothermic reaction absorbs heat from the surroundings, so the temperature falls. Examples include the thermal decomposition of carbonates and dissolving ammonium salts such as NH4NO3 in water.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Heat | released | absorbed |
| Temperature | rises | falls |
| ΔH sign | negative (−) | positive (+) |
Key idea / 要点
Heat of reaction ΔH compares the energy of products with reactants. Exothermic: products are lower in energy, so ΔH is negative. Endothermic: products are higher in energy, so ΔH is positive.
Energy level diagrams
An energy level diagram plots the energy of reactants and products. For an exothermic reaction the products sit below the reactants and the arrow points down. For an endothermic reaction the products sit above the reactants and the arrow points up. The vertical gap represents the size of ΔH.
Worked example / 例题
Burning methane releases 890 kJ per mole: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔH = −890 kJ mol⁻¹. The negative sign shows the reaction is exothermic, so on the diagram the products lie 890 kJ below the reactants.
Remember / 记住
- Exothermic: heat out, temperature up, ΔH negative.
- Endothermic: heat in, temperature down, ΔH positive.
- Total energy is conserved; heat is transferred between system and surroundings.