Question

Assertion (A): In the formation of oxide ion, $O^{2-}\left(g\right)$, first an exothermic reaction and then an endothermic reaction take place.

Reason (R): $\left(O^{-}\right)$ ion has comparatively large size than oxygen atom.

The correct answer is:

• A. The Assertion is not true but Reason is true
• B. The Assertion is true, but Reason is not true
• C. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• D. Both Assertion and Reason are true, Reason is the correct explanation of Assertion

Answer 1

The correct option is C Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion

First electron gain enthalpy for oxygen is negative (-141 kJ/mol) and second electron gain enthalpy is positive (+780 kJ/mol) as energy is required to overcome the force of repulsion between the electron and uninegative ion.

Hence, in the formation of oxide ion, $O^{2-}\left(g\right)$, first an exothermic reaction and then an endothermic reaction take place. Thus assertion (A) is true.

$O^{-}$ ion has a comparatively large size than an oxygen atom. An anion (formed by the gain of one or more electrons to the atom) is always larger than the corresponding atom.

Thus, reason (R) is true. But (R) is not the correct explanation of (A).