Define ionization enthalpy with an example.

Explanation

If different polyatomic molecules are not same, For example, Bond enthalpy of `C - Cl` bond in `CH_(3) - Cl and CH_(3) - CH_(2) - Cl` etc. is not the same.
In the same polyatomic molecules like `CH_(3)` the bond enthalpy of all the four C- H bonds are not same.
`CH_(4) (g) to CH_(3) (g) + H(g) , Delta_("bond")H^("ϴ") = 427 kj//mol`
`CH_(3) (g) to CH_(2) (g) + H (g), Delta "(bond")H^("ϴ") = + 439 kj//mol`
`CH_(2) (g) to CH (g) + H (g) , Delta_("bond")H^("ϴ") = + 452 kj//mol`
`CH (g) to C (g) + H (g), Delta_("bond")H^("ϴ") = + 347 kj//mol`
We know that none of C - H bond dissociation enthalpy is same. Therefore, average value of bond dissocialation enthalpy is taken.
Now by adding all the above equations, we get
`CH_(4) (g) to C (g) + 4 H (g) , Delta_(a)H^("ϴ") = 1665 kj//mol`
Therefore , average bond energy of C - H bond in `CH_(4) = (1)/(4) (1665) = 416 kj//mol`