Question

Knowing the electron gain enthalpy values for O → O^– and O → O^2- as -141 and 702 kJmol^-1 respectively. How can you account for the formation of a large number of oxides having O^2- species and not O^– ?

Answer 1

When oxygen reacts with metal then it from following types of oxides.
(i) M₂O
(ii) MO
(iii) MO₂
The formation of these oxides involves the following steps.

We know that the value of ∆_iH₂ is much higher than the value of ∆_iH₁ and the value of ∆_egH₂ is positive as compared to A_egH, but when MO and MO₂ compounds are formed then the lattice energy released, in case of MO it is very much higher than the lattice energy released during the formation of MO₂. It is due to the presence of higher charge on both ions in case of MO. Hence the formation of MO is thermodynamically favourable. Due to above reason a large number of oxides having O^2- species are formed.