The dissociation energies of H 2 and O 2 are 104 and 118 kcal m o l - 1 respectively. The heat of reaction 1 2 H 2 ( g ) + 1 2 O 2 ( g ) → O - H ( g ) is 10 kcal. The bond energy of O - H bond is

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Δ H r x n = Σ B E (reactants) - Σ B E (Products) Δ H r x n = 1 2 . B E ( H - H ) + 1 2 B E ( O = O ) - 1 B E ( O - H ) So 10 kcal/mol = 1 2 × 104 kcal/mol + 1 2 × 118 kcal/mol - B E ( O - H ) or B E ( O - H ) = 101 kcal/mol

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