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The formation enthalpies, Hf for H(g) and O(g) are 220.0 and 250.0 kJ mol-1, respectively, at 298.15 K, and Hf for H2O(g) is -242.0 kJ mol-1 at the same temperature. The average bond enthalpy of the O-H bond in water at 298.15 K is kJ mol-1 (nearest integer).
Answer
466
Solution
Breaking both O-H bonds of water vapour:
H2O(g) 2H(g) + O(g); H = 2 O-H
H = [2 Hf (H) + Hf (O)] - Hf (H2O)
= [2(220.0) + 250.0] - (-242.0)
= 440.0 + 250.0 + 242.0 = 932.0 kJ mol-1
Average O-H bond enthalpy = 932.02 = 466 kJ mol-1
Answer: 466
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