In chemical reactions bonds are formed and broken. The breaking of bonds requires the absorption of heat, which is the lowest energy applied to reactants before bond breaking occurs. This energy is called activation energy (AE) of the reaction. Formation of the bonds results in the release of energy to the surroundings.

Exothermic Reaction

There is greater heat released during bond formation than the heat absorb for the bond breaking, this is described as an exothermic reaction.

This means that heat is release during the reaction; as a result the heat of the reaction ( Δ  H) will be negative.

ΔH =Hp-Hr (heat of product – heat of reactant)

Since more heat is evolved during bond formation over bond breaking, the energy of the reactants is greater than the energy of the products.

Energy profile Diagram

Endothermic reaction

The heat absorbed during bond breaking exceeds the heat released during bond formation. So there is basically absorption of heat during this reaction.       H is positive for this reaction, since more energy is supplied than is released.

Energy of reactants < energy of the products

Energy profile diagram