Question

Formation of $O^{-}$ is ________, whereas formation of $O^{-2}$ is __________.

• A. endothermic, exothermic
• B. exothermic, exothermic
• C. endothermic, endothermic
• D. exothermic, endothermic

Answer 1

Answer: (D)

The correct option is A exothermic, endothermic
The formation of an $O^{-}$ ion is exothermic, which means energy is released in the process. This is because adding an electron to $O$ is much easier as it is electropositive and has a high electron affinity. The electron gain enthalpy in this case is $-142{\text{kjmol}}^{-1}$.

$O+e^{-}\to O^{-}$ $[\Delta r^{\circ }=-142{\text{kjmol}}^{-1}]$

To form an $O^{2-}$ ion, another electron has to be added to $O^{-}$ ion. However, since $O^{-}$ ion is already negatively charged, it resists the addition of a negatively charged electron. Thus, because of the repulsive coulumbic forces between the ion and the electron, more energy is required to overcome these forces. So, the formation of $O^{2-}$ is an endothermic process as energy is absorbed. The electron gain enthalpy in this case is $+844{\text{kjmol}}^{-1}$.

$O^{-}+e^{-}\to O^{2-}$ $[\Delta r^{\circ }=+844{\text{kjmol}}^{-1}]$