A state of a system used as a reference value in thermodynamic measurements. Standard states involve a reference value of pressure (usually one atmosphere, 101.325 kPa) or concentration (usually 1 M). Thermodynamic functions are designated as ‘standard’ when they refer to changes in which reactants and products are all in their standard and their normal physical state. For example, the standard molar enthalpy of formation of water at 298 K is the enthalpy change for the reaction
H2(g) + ½O2(g) → H2O(l)
Note the superscript⦵is used to denote standard state and the temperature should be indicated.
Subjects: Chemistry — Physics.