Question

The bond energy of an O - H bond is 109 k cal/mole. When a mole of water is formed then

• A. 109 k cal is released
• B. 218 k cal is released
• C. 109 k cal is absorbed
• D. 218 k cal is absorbed

Answer 1

Answer: (B)

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
$H_2(g)+1/2O_2(g)\to H_{2}O(l)(H-O-H)$
${\Delta }_f{H}^{\circ }$$=BE(H-H)+1/2BE(O=O)-2BE$(O - H)