Exothermic
Definition and meaning of Exothermic in chemistry.
Exothermic describes a chemical reaction or physical process that releases energy, usually as heat, to its surroundings. This corresponds to a negative enthalpy change (ΔH < 0), since the products end up at lower energy than the reactants.
In more detail
In an exothermic process, the energy released when new bonds form in the products exceeds the energy required to break bonds in the reactants, so the surplus is given off as heat (and sometimes light). Because energy leaves the system, the surroundings typically warm up, which is why exothermic reactions feel hot to the touch. Exothermic behavior is central to combustion, many neutralization and precipitation reactions, and processes like condensation. Whether an exothermic reaction proceeds spontaneously still depends on entropy changes as well, since overall spontaneity is governed by Gibbs free energy, not enthalpy alone.
Key facts
| Enthalpy Change | ΔH < 0 |
|---|---|
| Energy Flow | System releases energy to surroundings |
| Example Reaction | CH4 + 2O2 → CO2 + 2H2O |
| Field | Physical Chemistry |
The combustion of methane, CH4 + 2O2 → CO2 + 2H2O, releases about 890 kJ per mole of methane burned, making it strongly exothermic (ΔH = -890 kJ/mol).
Frequently asked questions
Does exothermic mean a reaction is spontaneous?
Not necessarily. Spontaneity depends on the Gibbs free energy change (ΔG = ΔH - TΔS), which includes entropy as well as enthalpy, though many exothermic reactions are spontaneous at room temperature.
Is exothermic the opposite of endothermic?
Yes. Endothermic processes absorb energy from the surroundings, cooling them, while exothermic processes release energy, warming them.