ABSTRACT
The enthalpy change 6H for any chemical reaction aA + bB A cC + dD depends on the temperature, the pressure, and the purity of the reactants. For convenience, thermodynamic data usually are reported under a standard set of conditions called the Standard State. The standard state at any given temperature T is taken as the pure substance at one bar pressure1 at the temperature T. Thus, 6Ho298 is the standard enthalpy change at one bar pressure and 25°C. The superscript “ o ” indicates that the pressure is at one bar. Although most thermodynamic data are reported at 25°C, strictly speaking, the standard state is not confined to this temperature. Therefore, 6Ho500 would be the standard enthalpy change at one bar and 500 K.
