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)?
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