Q. 44

The enthalpy of reaction for the reaction :

2H2(g) + O2(g) 2H2O(l) is ∆HrΘ = – 572 kJ mol–1

What will be standard enthalpy of formation of H2O (l)?

Answer :

The standard enthalpy of formation of H2O (l ) can be obtained by considering the following reaction :



As ΔfHΘ is related with one mole of a substance at standard conditions.


For the given reaction : 2H2(g) + O2(g) 2H2O(l) the standard enthalpy of reaction is ∆HrΘ = – 572 kJ mol–1 , so the half of ∆HrΘ will be the standard molar enthalpy of formation ;


ΔfHΘ = 1/2 × ∆HrΘ = (-572/2) = -286 kJ mol–1 .


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