pBlock ElementsII - Result Question 121
####6. When the first electron gain enthalpy $\left(\Delta _{e _g} H\right)$ of oxygen is $-141 kJ / mol$, its second electron gain enthalpy is
(2019 Main, 9 Jan II)
(a) a positive value
(b) a more negative value than the first
(c) almost the same as that of the first
(d) negative, but less negative than the first
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Solution:
- As given, the first electron gain enthalpy of oxygen can be shown as,
$$ O(g)+e^{-} \longrightarrow O^{-}(g) \Delta e _g H _1=-141 kJ / mol $$
The expression of second electron gain enthalpy of oxygen will be,
$$ O^{-}(g)+e^{-} \longrightarrow O^{2-}(g) \quad \Delta e _g H _2=+ \text { ve } $$
$\Delta e _g H _2$ of oxygen is positive, i.e. endothermic, because a strong electrostatic repulsion will be observed between highy negative $O^{-}$ and the incoming electron $\left(e^{-}\right)$. A very high amount of energy will be consumed (endothermic) by the system to overcome the electrostatic repulsion.
