Explain why formation of ${C l}^{-}$ is exothermic but from $O^{-}$ formation of $O^{2 -}$ is endothermic?

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Solution

$\cdot$ First electron affinity of Cl (Negative exothermic energy because energy released)
$C l + e^{-} \to C l^{-} \to$ exothermic

$\cdot$ Second electron affinity of O (supply energy to overcome force of electronic repulsion)

$O^{-} + e^{⊝} \to O^{2 -} \to$ endothermic

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Formation of Cl^-is exothermic but formation of O^2- is endothermic . Explain

O^{-}

O^{2 -}

N_{2} H_{4}

Is it necessary that all oxidation reactions are exothermic? If ues then why the reaction for formation of nitrogen oxide an endothermic reaction?

N²+ O²+ heat ----> NO

O^{2 -}

O + e^{-} \to O^{-}; Δ H = - 142 k J m o l^{- 1}

O^{-} + e^{-} \to O^{2 -}; Δ H = 844 k J m o l^{- 1}

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