Exothermicity
Definition and meaning of Exothermicity in chemistry.
Exothermicity is the property of a chemical reaction or physical process that releases net energy, usually as heat, to its surroundings as it proceeds.
In more detail
A process is exothermic when the energy released in forming new bonds (or a more stable arrangement of particles) exceeds the energy required to break bonds or separate particles in the reactants. At constant pressure, this corresponds to a negative enthalpy change, ΔH < 0, and the surroundings gain heat, so their temperature typically rises. Exothermicity says nothing about reaction rate or spontaneity on its own, it simply describes the direction of energy flow, which can be tracked with an energy diagram or measured calorimetrically.
Key facts
| Field | Physical Chemistry |
|---|---|
| Sign convention | ΔH < 0 (exothermic) |
| Opposite process | Endothermic (ΔH > 0) |
| Typical measurement | Calorimetry (heat released to surroundings) |
The combustion of methane, CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), releases about 890 kJ of heat per mole of methane burned, making it strongly exothermic (ΔH = −890 kJ/mol).
Frequently asked questions
Does an exothermic reaction always occur spontaneously and quickly?
No. Exothermicity only describes energy release (ΔH < 0); spontaneity depends on the overall Gibbs free energy change (ΔG), and reaction rate depends on activation energy, which is independent of ΔH.
How can you tell if a reaction is exothermic from an energy diagram?
On an energy (reaction coordinate) diagram, an exothermic reaction shows products at a lower energy level than reactants, with the difference released as heat.