What Are Standard State Conditions? - Standard Temperature and Pressure

Values of thermodynamic quantities are commonly expressed for standard state conditions, so it is a good idea to understand what the standard state conditions are.

A superscript circle is used to denote a thermodynamic quantity that is under standard state conditions:

ΔH = ΔH°
ΔS = ΔS°
ΔG = ΔG°

Standard State Conditions

Certain assumptions apply to standard state conditions. Standard temperature and pressure commonly is abbreviated as STP.

• The standard state temperature is 25°C (298 K). It is possible to calculate standard state values for other temperatures.
• All liquids are pure.
• The concentration of all solutions is 1 M (1 molar).
• All gases are pure.
• All gases are at 1 atm pressure.
• The energy of formation of an element in its normal state is defined as zero.

Sources

• International Union of Pure and Applied Chemistry (1982). "Notation for states and processes, significance of the word standard in chemical thermodynamics, and remarks on commonly tabulated forms of thermodynamic functions". Pure Appl. Chem. 54 (6): 1239–50. doi:10.1351/pac198254061239
• UPAC–IUB–IUPAB Interunion Commission of Biothermodynamics (1976). "Recommendations for measurement and presentation of biochemical equilibrium data". J. Biol. Chem. 251 (22): 6879–85.