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.

$\mathrm{O}_{(g)}^{-}+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{2-}(g)$

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