Thermochemical equations and standard states

In order to define the thermochemical properties of a process, it is first necessary to write a thermochemical equation that defines the actual change taking place, both in terms of the formulas of the substances involved and their physical states (temperature, pressure, and whether solid, liquid, or gaseous. 

To take a very simple example, here is the complete thermochemical equation for the vaporization of water at its normal boiling point  

The quantity 40.7 is known as the enthalpy of vaporization ( heat of vaporization ) of liquid water. The following points should be kept in mind when writing thermochemical equations  

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