Standard enthalpy of reaction (ΔrH0)

To compare the enthalpy changes of different reactions, they have to be reported under a similar set of conditions.

Thermodynamic standard state : The standard state of a substance is the form in which the substance is most stable at a pressure of 1 bar and at temperature 298 K. If the reaction involves species in solution, its standard state refers to 1 M concentration.

Standard states of certain elements and compounds are H$_2$(g), Hg($l$), Na(s) or C(graphite), C$_2$H$_5$OH($l$), H$_2$O($l$), CaCO$_3$(s) and CO$_2$(g) — each referring to 1 bar and 25 $^0$C. …