Definition
The activity of a chemical entity is a dimensionless quantity derived by multiplying a correction factor (called an activity coefficient ) by some measure of its molar abundance, such as molar concentration (molarity), molality, partial pressure, or mole fraction. See Equation 1, where (a i ) is the activity of a species or component i, γ i is the activity coefficient, and x i is a measure of molar abundance.
Activities are thus used as “effective concentrations” and are necessary to the extent that chemical behavior is not ideal – i.e., strictly proportional to molar abundance – and so γ i ≠ 1. Formally, a i is defined by Equation 2, where it described as a function of the difference between the chemical potential of i (μ i ) and its chemical potential in some standard state \( \left({\mu}_i^{\ominus}\right) \), temperature (T), and the gas constant (R).
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Bickmore, B.R., Wander, M.C.F. (2018). Activity and Activity Coefficients. In: White, W. (eds) Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-319-39193-9_2-1
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