Would you expect the second electron gain enthalpy of $O$ to be positive,more negative,or less negative than the first? Justify your answer.
(A) When an electron is added to an $O$ atom to form an $O^{-}$ ion,energy is released. Thus,the first electron gain enthalpy of $O$ is negative.
$O_{(g)} + e^{-} \longrightarrow O_{(g)}^{-} \quad \Delta_{eg}H = -141 \ kJ \ mol^{-1}$
On the other hand,when an electron is added to an $O^{-}$ ion to form an $O^{2-}$ ion,energy must be supplied to overcome the strong electrostatic repulsion between the incoming electron and the negatively charged $O^{-}$ ion. Thus,the second electron gain enthalpy of $O$ is positive.
$O_{(g)}^{-} + e^{-} \longrightarrow O^{2-}_{(g)} \quad \Delta_{eg}H = +780 \ kJ \ mol^{-1}$
$O_{(g)} + e^{-} \longrightarrow O_{(g)}^{-} \quad \Delta_{eg}H = -141 \ kJ \ mol^{-1}$
On the other hand,when an electron is added to an $O^{-}$ ion to form an $O^{2-}$ ion,energy must be supplied to overcome the strong electrostatic repulsion between the incoming electron and the negatively charged $O^{-}$ ion. Thus,the second electron gain enthalpy of $O$ is positive.
$O_{(g)}^{-} + e^{-} \longrightarrow O^{2-}_{(g)} \quad \Delta_{eg}H = +780 \ kJ \ mol^{-1}$
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