Standard State

a conventionally adopted thermodynamic state for individual substances and components of solutions. The concept of standard states was introduced in connection with the inability of simple thermodynamic principles to describe the actual behavior of substances with precision when pressure c or concentration c serves as the quantitative characteristic. The thermodynamic equations for ideal gases and solutions can be applied to real systems if the fugacity f is used instead of the pressure p and the activity a is used instead of the concentration c. The values of a and f for all substances in the standard state serve as reference values.

The standard state of gas at each temperature is the hypothetical state of an ideal gas when f = p = 1 and the gas has the properties inherent in a real gas at an infinitely small pressure. For individual liquid and crystalline substances, the state at normal pressure is adopted as the standard state at each temperature. In the case of solutions, the state of the pure solvent is usually adopted as the standard state for the solvent; for the solute, the standard state is the solute’s state in an infinitely dilute solution, where a = c (with c usually the molar fraction or molarity). The changes in thermodynamic parameters, such as the Gibbs free energy and entropy, for a certain process calculated by using f or a are independent of the choice of standard state provided that the state is unchanging in this calculation for all initial and final states. The degree sign is added as a superscript to the symbols designating the properties of a substance (characteristics of a process) in the standard state; for example AH°_formation would be standard enthalpy of formation of a particular substance.

Standard state concepts are widely used in physical chemistry. In comparing thermodynamic functions and making thermo-chemical calculations, for example, on the basis of Hess’s law, all heat effects of reactions must be referred to identical conditions since they are temperature-dependent and, to a lesser degree, pressure-dependent; for reactions in solutions, the heat effects are dependent on concentration. In thermochemistry the state of a substance at 298.15°K and p = 1 atmosphere (760 mm Hg) is adopted as the standard state. It should be noted that it is not required that the substance actually exist in the standard state. For example, the heat of formation of gaseous H₂0 in the standard state may appear in calculations even though a similar state is thermodynamically impossible for water vapor at p = 1 atmosphere and 25°C.