CHEMICAL EQUILIBRIUM
All natural systems tend towards a state of dynamic equilibrium, which involves
continuous change at the microscopic (particle) level, but no change at the macroscopic
(visible) level. However, in order for the system to actually attain a state of dynamic
equilibrium, a few key conditions have to be met:



the system must be closed (no input or output of any reactants/products, including
energy- therefore the temperature must also remain constant)
opposing changes must occur at the same rate. This is the fundamental
requirement for equilibrium
the system must have constant observable properties (dynamic equilibrium)
It should be noted that equilibrium can be achieved (assuming the above conditions are
true) starting with either the products or reactants in the case of a chemical reaction, and
also with a physical process (such as liquid water in dynamic equilibrium with water
vapour).
The Equilibrium Constant (K):
The law of chemical equilibrium states that there is a constant ratio between the
concentrations of the products and reactants in any change. In other words, a constant
expression can be written for a given chemical reaction at equilibrium, as follows:
For the general reaction: a A + b B
cC+dD
[C]c [D]d
KC =
[A]a [B]b
Note that it is a ratio of the concentrations of the products over the concentrations of the
reactants (this is a convention to have products over reactants), and that the molar
coefficients of the chemical equation are exponents for their respective chemical species.
In addition, only concentrations of solutes and gases, not pure liquids and solids, are
included in the expression, and the expression, despite appearances, is unit less.
Practically speaking, the equilibrium constant gives a quantitative measure of the extent
of a reaction. A large K indicates that the reaction lies to the right (more products), a
small K indicates the reaction lies to the left (reactants) and a K of 1 shows that the
reaction has approximately equal amounts of products and reactants.
Solving Equilibrium Problems:
Equilibrium problems generally come in one of 2 forms: 1) solve for K, given
concentration value(s), or 2) use a given K to find concentration value(s). In all cases, a
recommended general solution strategy is to use what’s known as an “ice” table: i =
initial concentration, c = change in concentration, and e = equilibrium concentration.
(example)
There are a few problem solving tips that can also be used to make the solution easier:
1) look for perfect squares in order to avoid having to use the quadratic equation
(example)
2) use the approximation method whenever possible – if K is significantly small,
then the amount of product formed is very small, therefore the amount of
reactants used up is also very small. This means that if the ratio of the initial
concentration (i) is compared to K, and the result is greater than 500 (or an
approximation of 5 %), then the amount of change (c) or x can be considered
negligible (the amount used is very small).
(examples)