A standard state is a specified reference state used to define a species' standard chemical potential . The reference convention, including temperature, must be consistent when calculating thermodynamic activities, reaction quotients, and equilibrium constants.
The reference pressure used to define a standard gas activity; a common convention is one bar. Normalizing pressure by this reference keeps a thermodynamic equilibrium constant dimensionless.
The reference concentration used to make concentration activities dimensionless. Changing this reference changes the numerical dimensionless equilibrium constant when the reaction changes the total number of solute or gas molecules.
The thermodynamic activity is the dimensionless quantity defined by , where is the molar gas constant. For an ideal dilute solution with a specified concentration reference, ; departures from ideality are represented by an activity coefficient multiplying this ratio.
An effective gas pressure defined by in a specified standard-state convention. It approaches the gas pressure at low density and differs from the dimensionless fugacity.
The dimensionless ratio of a gas species' chemical fugacity to its pressure in the pure-fluid convention. Its thermodynamic activity relative to a gas reference pressure is . For mixtures the pressure denominator uses the species' partial pressure. The ideal-gas limit has .
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The term "standard state" refers to a specific set of conditions used as a reference point in thermodynamics, particularly in the fields of chemistry and materials science. It is essential for defining properties of substances and for comparing their thermodynamic data. The standard state of a substance is typically defined under the following conditions: 1. **Temperature**: 25 degrees Celsius (298.15 K). 2. **Pressure**: 1 atmosphere (101.3 kPa).