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)

218 k cal is released

Answer 2

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)