Enthalpy of bond dissociation:
Chemical reactions involve the breaking and making of chemical bonds. When a bond is formed between the atoms, energy is released. Obviously, the same amount of energy will be needed to break the bond. This is called the bond dissociation energy or bond dissociation enthalpy and is measure of the bond strength.
The amount of energy required to break one mole of bond of a particular type between the atoms in the gaseous state is called bond dissociation enthalpy or bond energy.
It is generally expressed in terms of kJ/mol. For example, 435 kJ of energy is required to break or dissociate the H—H bonds in one mole of H2 molecules, therefore, the bond dissociation enthalpy of H—H bond is 435 kJ/mol at 298 K.
H2 ——→ H (g) + H(g) ∆H= 435.0 kJ/mol
Since in these reactions H atoms are formed, so this is also known as enthalpy of atomization. It maybe defined as the enthalpy change in breaking one mole of a substance completely into atoms in the gaseous state.
Similarly, the bond dissociation enthalpies of Cl2 is 242.5 kJ/mol, in I2 is 15.1kJ/mol, HI is 299 kJ/mol, etc.