The main difference is that if the water formed is in gaseous state then we lose the heat required to evaporate the water also from the combustion. This is the difference between Gross calorific value and Net calorific value. Net calorific value is the GCV-heat required to evaporate water from liquid state to gas state
The difference is 132.0 kJ/mol (3 times the molar enthalpy of vaporization of water).
You can calculate the enthalpy change of a reaction by using the
enthalpies of formation of reactants and products.
The formula is
#color(blue)(bar(ul(|color(white)(a/a)Δ_text(rxn)H° = Δ_text(f)H^@("products") - Δ_text(f)H^@("reactants")color(white)(a/a)|)))" "#
Forming liquid water
Forming gaseous water
Calculate the enthalpy difference
The only difference is that in one reaction we are forming 3 mol of liquid water, while in the other reaction we are forming 3 mol of gaseous water.
Hence, this difference represents the enthalpy of vaporization of 3 mol of water.