Standard state

In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. A degree sign (°) or a superscript Plimsoll symbol () is used to designate a thermodynamic quantity in the standard state, such as change in enthalpyH°), change in entropyS°), or change in Gibbs free energyG°).[1][2] The degree symbol has become widespread, although the Plimsoll is recommended in standards, see discussion about typesetting below.

In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry (IUPAC) recommends a conventional set of standard states for general use.[3] The standard state should not be confused with standard temperature and pressure (STP) for gases,[4] nor with the standard solutions used in analytical chemistry.[5] STP is commonly used for calculations involving gases that approximate an ideal gas, whereas standard state conditions are used for thermodynamic calculations.[6]

For a given material or substance, the standard state is the reference state for the material's thermodynamic state properties such as enthalpy, entropy, Gibbs free energy, and for many other material standards. The standard enthalpy change of formation for an element in its standard state is zero, and this convention allows a wide range of other thermodynamic quantities to be calculated and tabulated. The standard state of a substance does not have to exist in nature: for example, it is possible to calculate values for steam at 298.15 K and 105 Pa, although steam does not exist (as a gas) under these conditions. The advantage of this practice is that tables of thermodynamic properties prepared in this way are self-consistent.

  1. ^ Toolbox, Engineering (2017). "Standard state and enthalpy of formation, Gibbs free energy of formation, entropy and heat capacity". Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!. www.EngineeringToolBox.com. Retrieved 2019-12-27.
  2. ^ Helmenstine, PhD, Ann Marie (March 8, 2019). "What Are Standard State Conditions? - Standard Temperature and Pressure". Science, Tech, Math > Science. thoughtco.com. Retrieved 2019-12-27.
  3. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard state". doi:10.1351/goldbook.S05925
  4. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard conditions for gases". doi:10.1351/goldbook.S05910
  5. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard solution". doi:10.1351/goldbook.S05924
  6. ^ Helmenstine, PhD, Ann Marie (July 6, 2019). "Standard Conditions Versus Standard State". Science, Tech, Math > Science. thoughtco.com. Retrieved 2020-09-06.