Enthalpy is a measure of the total heat content of a system.
In simple terms:
Enthalpy (H) = the internal energy of the system + the energy needed to make space for it (pressure × volume).
It is useful because it helps us understand how much heat is absorbed or released during chemical reactions, especially at constant pressure.
- If enthalpy increases (ΔH > 0), the reaction absorbs heat → endothermic reaction.
- If enthalpy decreases (ΔH < 0), the reaction releases heat → exothermic reaction.
Example:
- Burning fuel releases heat (exothermic, negative ΔH).
- Melting ice absorbs heat (endothermic, positive ΔH).