Enthalpy plays an important role in whether a reaction will happen on its own (spontaneous) or not. But it’s not the only factor.
1. Role of Enthalpy (ΔH):
- If a reaction releases heat (negative enthalpy change, exothermic), it often favors spontaneity.
- If a reaction absorbs heat (positive enthalpy change, endothermic), it is less likely to be spontaneous — unless something else helps it.
2. Other factor – Entropy (ΔS):
- Entropy is a measure of disorder.
- Reactions that increase disorder (like ice melting into water, or solid turning into gas) are more likely to be spontaneous.
3. Gibbs Free Energy (ΔG):
- Spontaneity is actually decided by both enthalpy and entropy together, through a quantity called Gibbs free energy.
- A reaction is spontaneous if ΔG is negative.
- ΔG depends on ΔH (enthalpy change), ΔS (entropy change), and temperature.
Simple Examples:
- Burning wood: Strongly exothermic (releases heat), increases disorder → spontaneous.
- Ice melting at room temperature: Absorbs heat (endothermic) but increases disorder a lot, so it becomes spontaneous above 0°C.
In short:
Enthalpy affects spontaneity because releasing heat usually makes a reaction more favorable, but entropy and temperature also play key roles.