Question

Electron gain enthalpy is positive when:
A. ${O^ - }$ is formed from O.
B. ${O^{2 - }}$ is formed from ${{O}^{-}}$
C. ${O^ + }$ is formed from O.
D. Electron affinity is always a negative value.

Answer 1

Electron gain enthalpy is described as the quantity of energy released when an electron is added to a neutral atom. When addition of electrons is done to a neutral atom it forms a negative ion. To add more electrons to the ion, high energy is needed which is more than the energy which is gained. Therefore, the electron gain enthalpy will be positive.

Complete answer: When the electron is added to a neutral gaseous atom, the amount of energy released is known as electron gain enthalpy. The energy can either be released or can be absorbed during the addition. When the energy is released during the addition of an electron, then the reaction is known as an exothermic reaction and the sign is negative and when the energy is absorbed during the addition, then the reaction is said to be an endothermic reaction and the sign is positive.
When in a neutral gaseous atom, the electron is added then the atom becomes negative and the energy is released. When more electrons are added to the negative ion, extra energy is needed. This energy is the more than the energy which is gained. Thus, the energy is absorbed and the electron gain enthalpy becomes positive.
First electron gain enthalpy: $O(g) + {e^ - } \to {O^ - }(g)$
Second electron electron gain enthalpy: ${O^ - }(g) + {e^ - } \to {O^{2 - }}(g)$
Thus, when ${O^{2 - }}$ is formed from ${O^ - }$the electron gain enthalpy is positive.
Therefore, the correct option is B.

Note: Electron gain enthalpy is sometimes named as electron affinity. In metals the electron gain enthalpy is positive as metals lose their electron to form a stable configuration so the energy required for the addition of electrons is very high.