Exothermic: definition in chemistry
Exothermic: Describes a process or reaction that releases energy, usually as heat, to its surroundings, so the surroundings get warmer. Its enthalpy change (ΔH) is negative.
In an exothermic reaction, forming the bonds in the products releases more energy than was needed to break the bonds in the reactants. Burning, respiration and neutralization are everyday examples, and so are physical changes such as freezing and condensation. Even strongly exothermic reactions often need a push to start, such as a spark, because the reactants must first overcome an activation energy.
Examples
- Burning natural gas on a stove
- Hand warmers, in which iron powder slowly rusts
- Thermite: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, producing molten iron
- Water freezing into ice releases heat
On the periodic table
The alkali metals of group 1 all react exothermically with water, and the reaction becomes more violent down the group: sodium melts into a ball, and potassium ignites the hydrogen released.