the above reaction, the net reaction would produce more D and concurrently more C. Likewise,
using an excess of the least expensive reactant helps to drive the reaction forward.
CHANGES IN PRESSURE OR VOLUME
In a system at constant temperature, a change in pressure may cause a change in volume, and vice
versa. Since liquids and solids are practically incompressible, a change in the pressure or volume of
systems involving only these phases has little or no effect on their equilibrium. Reactions involving
gases, however, may be greatly affected by changes in pressure or volume, since gases are highly
compressible.
BASIC CONCEPT
Pressure and volume are inversely related from the ideal gas law (or Boyle’s law to be more precise).
An increase in the pressure (or decrease in the volume) of a system will shift the equilibrium so as to
decrease the number of moles of gas present. This reduces the total pressure of the system and
relieves the stress. Consider the following reaction:
3H 2 (g) + N 2 (g) → 2NH 3 (g)The left side of the reaction has 4 moles of gas, whereas the right side has only 2 moles. When the
pressure of this system is increased, the equilibrium will shift so that the side of the reaction with
fewer moles is favored. Since there are fewer moles on the right, the equilibrium will shift toward
the right. Conversely, if the volume of the same system is increased, its pressure immediately
decreases, which, according to Le Châtelier’s principle, leads to a shift in the equilibrium to the left.
CHANGES IN TEMPERATURE
Higher pressure: favors side with fewer moles of gases
Large volume: favors side with more moles of gases