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Q.
The bond energy of an O - H bond is 109 k cal/mole. When a mole of water is formed then
Thermodynamics
Solution:
Bond energy of O-H is 109 k cal/mol means that when one mole of O-H bond is formed 109 k cal energy is released (because dissociation of bond absorbs energy). Formation of one mole of water is equivalent to formation of 2 moles of O-H bonds (H-O-H)
Hence energy released = 2 $\times$109 = 218 k cal
(Strictly speaking we calculate $\Delta_f$H
$^\circ$4 for the reaction
$H_2(g) + 1/2 O_2 (g) \rightarrow H_2O (l) (H-O-H) $
$\Delta_f H^\circ$$ = BE(H - H) + 1/2 B E(O = O) -2BE$(O - H)