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Question

The enthalpy of combustion of H2(g) at 298 K to give H2O(g) is 249 kJ mol1 and bond enthalpies of H - H and O = O are 433 kJ mol1 and 492 kJ mol1, respectively. The bond enthalpy of O - H is

A
464 kJ mol1
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B
928 kJ mol1
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C
232 kJ mol1
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D
232 kJ mol1
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Solution

The correct option is A 464 kJ mol1
Combustion of H2 gives H2O(g),
H2(g)+12O2(g)H2O(g)
Enthalpy of combustion of reaction , ΔH=249 kJ mol1
Enthalpy of reaction = Bond enthalpy of reactant - Bond enthalpy of product
ΔH=B.E. of H2+12B.E. of O2B.E. of H2O
H2O has two OH bond hence,
ΔH=B.E. of H2+12B.E. of O22×B.E. of OH
249=433+12×4922×B.E. of OH
928=2×B.E. of OH
Bond ethalpy of OH =9282
Bond enthalpy of OH is 464 kJ mol1

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