Page 21 - Pharmaceutical analytical chemistry |

At equilibrium, the rate of formation of each substance equals the rate of

consumption and there is no net change in concentration.

Equilibrium constant expressions

The influence of concentration (or pressure) on the position of a

chemical equilibrium is conveniently described in quantitative terms by

means of an equilibrium constant expression. They are of great practical

importance because they permit the chemist to predict the direction and

completeness of a chemical reaction.

The law of mass action states that the rate of chemical reaction at

constant temperature is proportional to the product of the concentration

of the reacting substances, each raised to a power equal to the number of

molecules or ions of the reactant which appears in the balanced equation

of the reaction.

aA + bB ↔ cC + dD

The equilibrium constant is expressed as follows:

Therefore, at equilibrium, the product of concentration of the substances
on the right side of the equation (the products) divided by the product of
the concentration of the substances on the left side of the equation (the
reactants) is constant at constant temperature.
The square-bracketed terms have the following meanings:

1. Molar concentration if the species is a dissolved solute.
2. Partial pressure in atmospheres if the species is a gas.
3. Unity if the species is (a) a pure liquid, (b) a pure solid or (c) the

solvent in a dilute solution.

p-Functions

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