Question

The formation of $O ^{2-}( g )$ starting from $O ( g )$ is endothermic by $603\, \, \, kJ \, mol ^{-1}$. If electron affinity of $O ( g )$ is $-141 \, kJ\, mol ^{-1}$, the second electron affinity of oxygen would be

• A. $603 \, kJ \, mol ^{-1}$
• B. $-603 \, kJ\, mol ^{-1}$
• C. $-744\, kJ \, mol ^{-1}$
• D. $+744 \, kJ \, mol ^{-1}$

Answer 1

Answer: (D)

$O ( g )+2 e ^{-} \rightarrow O ^{2-}, \Delta H=603\, kJ \, mol ^{-1} $....... (i);
$O ( g )+ e ^{-} \rightarrow O ^{-}, \Delta H=-141 \, kJ\, mol ^{-1}$.......(ii)
(i) -(ii) gives:
$ O ^{-}( g )+ e ^{-} \rightarrow O ^{2-}( g ) $
$\Delta H=603-(-141)=744 \, kJ \, mol ^{-1}$