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Equilibrium I 37
constant (Figure 4.1). A state of equilibrium is established when the
concentrations of reactants (in this example, [A] and [B]) and of
products ([C] and PI) remain constant. At this point, the rate of the
forward reaction equals the rate of the reverse reaction.
Once the state of equilibrium is established, it will persist indefi-
nitely, if the system is undisturbed.

Definition of Equilibrium: Equilibrium is defined as a state of
dynamic balance when two opposing reactions occur at the same
time and the same rate.
i.e. Rate of forward reaction = rate of backward reaction.

THE LAW OF CHEMICAL EQUILIBRIUM
Every chemical reaction has its own state of equilibrium, in which
there is a definite relationship between the concentrations of reactants
and products in the reaction. If the conditions of the reaction are
varied (e.g. if different initial concentrations are used), the concentra-
tions at equilibrium will be changed, but will still assume a constant
value. If the concentrations at equilibrium are expressed in mol dm-3
(M), there is a single expression which holds for all experimental
conditions for a given reaction.
For example, for the reaction, A + B * C + D, at equilibrium:

Kc = - where Kc is termed the equilibrium constant and is
[cl[D1
[A1 PI ’
characteristic of a given reaction.
In general, for the reaction: vAA + VBB + VCC + VDD, where
VA, VB, vc and VD now represent the stoichiometry factors,

i.e. of the form ‘products/reactants’.
For the reaction