Why does the reaction release so much heat?

The slaking of lime is a highly exothermic reaction— it releases a significant amount of energy as heat. This occurs because the product, calcium hydroxide, has stronger, more stable bonds (lower potential energy) than the reactants calcium oxide and water.

CaO(s) + H₂O(l) → Ca(OH)₂(s) + heat

Step-by-step Energy Picture

1. Reactant energy (high): Calcium oxide (quicklime) is relatively high in internal energy. It is produced by heating calcium carbonate, CaCO₃, in a kiln (calcination), which requires substantial energy input to break strong carbonate bonds. Much of this input is effectively stored in CaO.
2. Product energy (low): Calcium hydroxide, Ca(OH)₂, is a more stable, lower-energy compound with strong O–H and Ca–O interactions in its crystal lattice.
3. Energy release: When water is added, atoms rearrange from the higher-energy reactants to the lower-energy product lattice. The difference in potential energy is released to the surroundings as heat (reaction enthalpy).

Enthalpy and Safety Note

The reaction enthalpy is approximately ΔH ≈ −65 kJ mol⁻¹ (per mol of CaO), indicating a strong heat release. This can raise the mixture well above 100 °C, flashing water into steam and posing a burn hazard. Always add water carefully, use appropriate PPE, and allow for heat dissipation.