Page 10 - Pharmaceutical analytical chemistry |

• As the reaction proceeds the concentration of the reactants decreases and

so does the rate at which they react and the concentration of the products

increases and so does the rate at which they react.

• A state of equilibrium is reached wherein

A+B C+D

• The rate of the forward reaction (between the reactants A&B) is
balanced by the rate of the backward reaction (between products C&D).

• This equilibrium is dynamic as each substance is constantly used up and

reformed, in other words the equilibrium consists of reactions that
constantly and simultaneously proceed in opposite directions but with

equal speed.

• At equilibrium the rate of formation of each substance equals its rate of
consumption and there is no net change in concentration.
It is necessary here to mention Le Chatelier’s Principle which states

that:

If a stress is applied to a system in equilibrium, the equilibrium tends

to shift in a direction to neutralize the stress.

For example, in the above equation any increase in concentration of C or

D or any decrease in the concentration of A or B results in shifting the

equilibrium to the left and vice versa.
It is important to mention that Le Chatelier’s principle tells only in what

direction the equilibrium is shifted by a stress but not how far i.e. it
allows a qualitative prediction of the effect of stress on equilibrium. The

law of mass action, on the other hand, enables a quantitative treatment
through the calculation of the equilibrium constant.

9