Standard States

The superscript "o" or degree sign on ΔH designates a standard enthalpy change, that is, an enthalpy change when the reactants start out in standard states and the products end up in standard states. Standard states are defined as follows:

1. For a solid, liquid, gas or the solvent in a solution: The pure material at 1 atm pressure and 25°C.

2. For a solute in a solution: The ideal 1 molar solution (1 mol solute per liter of solution) at 1 atm and 25°C.

The 1-atm pressure specification is usually not very important for liquids and solids, but is vital for gases. Some books define the standard pressure as 1 bar. Fortunately ΔH is not strongly dependent on pressure, and since 1 bar is very close to 1 atm, numerical values of ΔH° show only small differences which usually can be overlooked.