Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first?
Question:
Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.
Solution:
When an electron is added to O atom to form O– ion, energy is released. Thus, the first electron gain enthalpy of O is negative.
$\mathrm{O}_{(g)}+\mathrm{e}^{-} \longrightarrow \mathrm{O}_{(g)}^{-}$
On the other hand, when an electron is added to $\mathrm{O}^{-}$ion to form $\mathrm{O}^{2-}$ ion, energy has to be given out in order to overcome the strong electronic repulsions. Thus, the second electron gain enthalpy of $O$ is positive.
$\mathrm{O}_{(g)}^{-}+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{2-}(g)$