Untitled - Kelly Walsh High School

Chemical Equilibria 207
14-3 Le Châtelier’s Principle
Don’t Forget!
Le Châtelier discovered that if a chemical system at equilibrium is stressed (disturbed)
it will reestablish equilibrium by shifting the rates of the reactions involved.
This means that the amounts of the reactants and products will change, but the ratio
will remain the same. One can stress the equilibrium in a number of ways: changes
in concentration, pressure, and temperature. However, a catalyst will have no effect
on the equilibrium amounts since it affects both the forward and reverse reactions
equally. It will simply allow the reaction to reach equilibrium faster.
Changes in Concentration
If the stress to the equilibrium system is a change in concentration of one of the
reactants or products, then the equilibrium will react to remove that stress. If, for
example, we decrease the concentration of a chemical species, the equilibrium
will shift to produce more of it. In doing so, the concentration of chemical species
on the other side of the reaction arrows will be decreased. If the concentration of
a chemical species is increased, the equilibrium will shift to consume it, increasing
the concentration of chemical species on the other side of the reaction arrows.
For example, again consider the Haber process: N 2(g) 3 H 2(g) L 2 NH 3(g).
If one increases the concentration of hydrogen gas then the equilibrium shifts
to the right in order to consume some of the added hydrogen. In doing so, the
concentration of ammonia (NH 3) will increase and the concentration of nitrogen
gas will decrease.
The concentrations may change, but the value of K c or K p would remain the
same.
Changes in Pressure
Changes in pressure are only significant if there are gases involved. The pressure
may be changed by changing the volume of the container or by changing
the concentration of a gaseous species. If the container becomes smaller, the
pressure increases because there are an increased number of collisions on the
inside walls of the container. This stresses the equilibrium system and it will
shift in order to reduce the pressure. A shift towards the side of the equation
that has the least number of moles of gas will accomplish this. If the container