making ammonia. The reaction is
N 2 (g) + 3H 2 (g) 2NH 3 (g) + heat (at equilibrium)If the concentrations of the nitrogen and hydrogen are increased, the forward
reaction is increased. At the same time, if the ammonia produced is removed by
dissolving it into water, the forward reaction is again favored.
Because the reaction is exothermic, the addition of heat must be considered
with care. Increasing the temperature causes an increase in molecular motion and
collisions, thus allowing the product to form more readily. At the same time, the
equilibrium equation shows that the reverse reaction is favored by the increased
temperature, so a compromise temperature of about 500°C is used to get the best
yield.
An increase in pressure will cause the forward reaction to be favored since
the equation shows that four molecules of reactants are forming two molecules of
products. This effect tends to reduce the increase in pressure by the formation of
more ammonia.
EQUILIBRIA IN HETEROGENEOUS SYSTEMS
The examples so far have involved systems made up of only gaseous substances.
Expression of the K values of systems changes when other phases are present.
Equilibrium Constant for Systems Involving Solids
If the experimental data for this reaction are studied:
CaCO 3 (s) CaO(s) + CO 2 (g)it is found that at a given temperature an equilibrium is established in which the
concentration of CO 2 is constant. It is also true that the concentrations of the
solids have no effect on the CO 2 concentration as long as both solids are present.
Therefore, the Keq, which would conventionally be written like this:
can be modified by incorporating the concentrations of the two solids. This can be
done since the concentration of solids is fixed. It becomes a new constant K,
known as:
K = [CO 2 ]Any heterogeneous reaction involving gases does not include the