standardtilstand

Standardtilstand, literally "standard state," is a reference condition used in thermodynamics and related disciplines to define and compare the properties of a system. It establishes a baseline against which changes in temperature, pressure, and composition are measured.

In chemistry, the conventional standard state is defined at a specified temperature, commonly 298.15 K, and at a reference pressure of 1 bar. For gases, the standard state is the pure gas at 1 bar; for solutions, solutes are treated with unit activity, approximated by a concentration of 1 mole per liter. Thermodynamic quantities such as the standard Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) are defined with respect to these conditions.

The standard state provides a reference for calculating actual conditions: ΔG = ΔG° + RT ln Q, where Q is the reaction quotient under the current conditions. Because the standard state is a convention, the numerical values of ΔG°, ΔH°, and ΔS° depend on the chosen standard state and may vary with temperature and phase definitions.

In biology and biochemistry, a transformed standard state is sometimes used, notably standard state at pH 7, to reflect physiological conditions. The term standardtilstand can also appear in fields outside thermodynamics to indicate a system’s default or baseline configuration, such as the initial conditions in an experiment or the default settings in software.

See also: standard state, standard conditions, Gibbs free energy, thermodynamics, 1 bar, 298.15 K.