The enthalpy of combustion of H2(g) at 298 K to give H2O(g) is −249kJmol−1 and bond enthalpies of H - H and O = O are 433kJmol−1 and 492kJmol−1, respectively. The bond enthalpy of O - H is
A
464kJmol−1
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B
928kJmol−1
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C
232kJmol−1
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D
−232kJmol−1
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Solution
The correct option is A464kJmol−1 Combustion of H2 gives H2O(g), H2(g)+12O2(g)→H2O(g) Enthalpy of combustion of reaction ,ΔH=−249kJmol−1 Enthalpy of reaction = Bond enthalpy of reactant - Bond enthalpy of product ΔH=B.E.ofH2+12B.E.ofO2−B.E.ofH2O H2O has two OH bond hence, ΔH=B.E.ofH2+12B.E.ofO2−2×B.E.ofOH −249=433+12×492−2×B.E.ofOH −928=−2×B.E.ofOH Bond ethalpy of OH =9282 Bond enthalpy of OH is 464kJmol−1