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.
9-6.
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.
K1
Keq = ----
K2
MD0837
9-5