At the same time, the acid molecule is dissociating, a certain percentage of the
ions are reassociating to reform the acid. When the acid is first placed in water, a high
degree of dissociation occurs, and the rate of the forward reaction is greater. Gradually,
as more ions are formed, the rate of the reverse reaction increases. Eventually, a state
of equilibrium is reached in which molecules are being ionized and reassociated at a
given rate. Equilibrium exists when the rates, not the number of molecules or
concentrations, of the opposing reactions are equal. This is a dynamic, constant
process and continues until a force is added to change this equilibrium. For example, if
the temperature is increased, the rates of the reaction are increased, and a new
equilibrium state is reached. Removing one of the ions or tying them up with another
reaction will also shift the equilibrium.
LAW OF MASS ACTION
a. Law. The law of mass action states that the rate of reaction is proportional to
the product of the molar concentrations of the reactants.
A + B <=====> C + D
For this reaction, A and B are the reactants, and C and D are the products.
The rate at which C and D are formed is proportional to the concentration of A and B.
rate = K1 [A] [B]
where K1 is a proportionality constant, and the brackets denote mol/L
concentration of the enclosed substance.
The rate for the reverse reaction is:
rate = K2 [C] [D]
b. Equilibrium Constant (Keq).
The constants K1 and K2 are different. The ratio of the constants is known as
the equilibrium constant (Keq), dissociation constant, or ionization constant.
Keq = ----