Knowing the electron gain enthalpy values for

Question:

Knowing the electron gain enthalpy values for O → O and O → O2− as −141 and 702 kJ mol−1 respectively, how can you account for the formation of a large number of oxides having O2− species and not O?

(Hint: Consider lattice energy factor in the formation of compounds).

Solution:

Stability of an ionic compound depends on its lattice energy. More the lattice energy of a compound, more stable it will be.

Lattice energy is directly proportional to the charge carried by an ion. When a metal combines with oxygen, the lattice energy of the oxide involving O2− ion is much more than the oxide involving O ion. Hence, the oxide having O2− ions are more stable than oxides having O. Hence, we can say that formation of O2− is energetically more favourable than formation of O.

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