It is important to understand what the law of mass action is saying. It is always possible to write an
expression like the right-hand side in the example above; in fact, this expression involving the
concentrations of the species is known generally as the reaction quotient. Its value changes as the
reaction progresses: It will be very small in the beginning since the system is composed only of
reactants (small numerator, large denominator), but will increase as products accumulate
(numerator gets bigger as denominator decreases). When its value has reached that of Kc , the
system or the reaction has reached equilibrium and the concentrations of each species will no
longer change. The concentrations that appear in the law of mass action are therefore the
equilibrium concentrations of the species. A large Kc (Kc >> 1) means that the reaction goes almost
to completion: at equilibrium, there is a high concentration of products (high [C] and [D]), and we
say that the equilibrium “lies to the right.” A small Kc (Kc << 1), on the other hand, means that the
reaction is not very favorable: At equilibrium there are still a lot of reactants around (high [A] and
[B]); we say that the equilibrium “lies to the left.”
Equilibrium can be reached from either direction. In other words, in the generic equation written
above, we can start with a container filled with C and D. At equilibrium, the relative concentrations
of the species would be the same. The reaction would now technically be written as
cC + dD → aA + bB,and the mass action expression would be
Note that the equilibrium constant for this reverse reaction, Kc, is simply the reciprocal of the
equilibrium constant for the forward reaction. That is:
The more favorable a reaction is, the less favorable its reverse will be, and vice versa.