Page 11 - Pharmaceutical analytical chemistry |

The law of Mass Action

“The rate of a chemical reaction at constant temperature is directly

proportional to the product of the molar concentration of the reacting

substances each raised to a power equal to the number of ions or
molecules appearing in the balanced equation for the reaction”.

aA + bB cC + dD

In this equation a, b, c and d signify the stoichiometric numbers of

particles of A, B, C and D, respectively, involved in the completely

balanced reaction.

The equilibrium constant may be expressed by the relation.

[C]c [D]d
K = ________________

[A]a [B]b
Regardless of the detailed mechanism of the reaction.

The value of the equilibrium constant K depends on:

• The specific reaction

• The specific substances involved

• Temperature

Different reactions have different equilibrium constants.

In dilute solutions the concentration of water remains constant and is

therefore omitted in the formulation of the equilibrium constant.

The same is true for a solid substance e.g. a precipitate.

Equilibrium constants for acid-base reactions
Ionization constant

When a weak electrolyte BA dissolves in water the chemical equation is
BA B+ + A-

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