Reactivity

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Chemical Reactivity: A Study Guide

has been defined as

       [C]^c    [D]^d
    ----------------  =  K_{eq
       [A]^a    [B]^b}

where [A], [B], [C], and [D] are stoichiometric concentrations of A, B, C, and D respectively.

However, dilute solutions and concentrated solutions have slight differences, and a more precise method of calculating and defining the equilibrium constant is desirable. For such an approach, the reactivities of A, B, C, and D are used in place of the concentrations in the definition of K. The reactivity of A ({A}) is proportional to [A], and the proportional constant in most text is a gamma, which is called the activity coefficient

{A} = g [A]
{B} = g [B]
{C} = g [C]
etc.

The application of science (engineering) often requires some refinement, and the use of activity is an refinement base on the theory of equilibrium.

The reactivities based on concentrations given above work well for non-electrolytes (or molecular compounds). In dilute solutions, the activity coefficient is unity.

g = 1
or
{A} = [A]

In solutions of electrolytes, the interactions of charges require some special consideration.

Contributors and Attributions

Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)