Entalphy of Formation: Why Use Water at Std Cond?

[Est120]

why do we use the entalphy of formation of water at standard conditions to calculate the entalphy of a reaction even if water is not gas at 1bar and 298K

 

[bigfooted]

A lot of water is in gaseous form at room temperature and pressure...
When gaseous water condenses to liquid, thermal energy is released. This is the latent heat of condensation. It is taken into account separately because formation enthalpy and condensation enthalpy are different things.
condensation enthalpy depends on the local saturation conditions, so if you want to know how much additional energy is released due to condensation, you have to compute how much water condenses. The difference in heat release between combustion with and without water condensation is the difference between the Higher Heating Value and the Lower Heating Value.

 

[Borek]

why do we use the entalphy of formation of water at standard conditions to calculate the entalphy of a reaction even if water is not gas at 1bar and 298K

To be honest, I have no idea what you are asking about. Doesn't enthalpy of formation of water at STP assume water to be in a liquid form?

 

[Chestermiller]

In the tables, there are two values for the heat of formation of water. One is for liquid water at 25 C and the standard pressure of 1 bar, and the other for the hypothetical state of water vapor at 25 C and 1 bar. The hypothetical state value is useful in many calculations where water vapor is part of a gaseous product mixture.