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2H2 + O2 → 2H2OEach H2O molecule contains two O-H bonds, so two moles of H2O contains four moles of O-H bonds.2 moles of H2 contain a total of 4 moles of H-H bonds and 1 mole of O2 contains 1 mole of  O-O bonds.Therefore, heat of reaction using bond energy form of Hess's law,∆H°rxn=[∆HO-O + 2∆HH-H] - [4∆HO-H]             =[ 496 + 2×436]-[4×464] kJ/mol             = 1368-1856 kJ/mol             = -488 kJ/molThe correct option is (1)

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for H2 molecule, the bond energy is 436 kJ/ mol since there is only 1 H-H bond.
Hence,for 2 molecules of H2, bond energy will be 872
For O2 molecule, bond energy = 496 kJ/ mol since there is only one O-O bond
Hence, for L.H.S. of reaction, bond energy = 872 + 496 kJ/ mol = 1368 kJ/ mol
For H2O, there are two O-H bonds. Hence, for 2 molecules of H2O, there will be 4 O-H bonds.
Hence, bond energy = 4*464 = 1856 kJ/mol = R.H.S. bond energy
​Hence, approximate heat of reaction = ΔHpro - ΔH reac.
= 1856 - 1368 = 488 kJ/mol
Hence, the correct option is (4) = 488 kJ/mol
 
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