When the first electron gain enthalpy $(\Delta_{\mathrm{eg}} \mathrm{H})$ of oxygen is $-141 \mathrm{~kJ} / \mathrm{mol}$, its second electron gain enthalpy is

(A) a more negative value than the first

(B) almost the same as that of the first

(C) negative, but less negative than the first

JEE Main · 2019 · 09 Jan 2019 · Shift 2 · Classification of Elements and Periodicity

When the first electron gain enthalpy

(Δ_{eg} H)

of oxygen is

- 141 kJ / mol

, its second electron gain enthalpy is

Answer: (D)

The first electron slips onto a neutral oxygen atom with energy released,

O + e X^{-} O X^{-}

, giving

Δ_{e g} H_{1} = - 141 kJ/mol

. The second electron, though, must be forced onto an already negative

O X^{-}

, where electron--electron repulsion opposes it, so energy has to be supplied:

O X^{-} + e X^{-} O X^{2 -}

is endothermic, and the second electron gain enthalpy is a positive value.

JEE Main · 2019 · 09 Jan 2019 · Shift 2 · Single correct option