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1. Symbol a. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction

A ⇌ B + C

the true equilibrium constant is given by

K = aBaC/aA

where aA, aB, and aC are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ) are defined for gases by γ = a/p (where p is pressure) and for solutions by γ = aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form

Kp = γBpBγCpCApA

The equilibrium constant of a reaction in solution is


The activity coefficients thus act as correction factors for the pressures or concentrations. See fugacity.

2. Symbol A. The number of atoms of a radioactive substance that disintegrate per unit time. The specific activity (a) is the activity per unit mass of a pure radioisotope. See radiation units.

Subjects: Chemistry — Physics.

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