## Quick Reference

*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* = *a*_{B}a_{C}/*a*_{A}

where *a*_{A}, *a*_{B}, and *a*_{C} 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

*K*_{p} = γ_{B}*p*_{B}γ_{C}*p*_{C}/γ_{A}*p*_{A}

The equilibrium constant of a reaction in solution is

*K*_{c} = γ_{B}*X*_{B}γ_{C}*X*_{C}/γ_{A}*X*_{A}

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

*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.

**From:**
activity
in
A Dictionary of Physics »

*Subjects:*
Chemistry — Physics.