Reversibility of Reactions at Equilibrium
Once a system has reached equilibrium, any factor which causes a change in the rate of either the forward or reverse reaction will disturb or shift the equilibrium. The system will readjust itself so that a new equilibrium is reached and the rates will again become equal.
Le Châtelier's Principle
When a stress is applied to a system at Equilibrium, the system readjusts so as to relieve or offset the stress. Stress is any imposed factor which upsets the balance in rates between the forward and reverse reactions. Stress factors may be changes in concentrations [], total pressure with gases only, volume changes (which cause the pressure changes), and temperature.
Stress and Changes in Concentration [ ]
Increasing the concentration of the reactants will cause a stress which is relieved by an increase in the concentration of product, [product]. If the [product] is increased the [reactants] will increase to offset the stress.
Stress and Changes in total Volume (or related pressure)
eg.     N2(g) + 3 H2(g) <=====>  2 NH3(g) + 92.00 kJ
A stress imposed by a decrease in volume is actually a stress caused by the increase in the concentration. The stress is relieved when the system reduces the number of molecules.
ie.    4 molecules ------> 2 molecules
A decrease in total pressure (which is caused by an increase in volume) will shift the reaction to the reverse. A decrease in to total volume of a gaseous system (or the accompanying increase in pressure) shifts an equilibrium in the direction of the fewer molecules as shown by the equation for the reaction.
Stress and Changes in Temperature
In any reaction a temperature increase favours the reaction that absorbs heat. i.e. the endothermic reaction.
eg. N2(g) + 3 H2(g) <=====>  2 NH3(g) + 92.00 kJ
This reaction is exothermic as written. That means the reverse reaction or backward reaction is endothermic. Adding heat (demonstrated as an increase in temperature) will shift the reaction left and more reactants will be formed as the products get used up. A decease in temperature will favour the reaction that produces heat. i.e. the exothermic reaction. The reaction about would switch to the forward direction to produce more heat and product.
Effect of a Catalyst
A catalyst equally favours both the forward and reverse reactions. Therefore a catalyst does not shift the Equilibrium [ ]'s. It simply causes the reaction system to reach Equilibrium in a shorter period of time.
Example Problem
For the reaction below:

     2 SO2(g) + O2(g) <=====>  2 SO3(g) + heat

Indicate the favoured reaction either forward or reverse under the stress applied below.
a) increase the [SO2]
The reaction will shift to the forward direction and more SO3will be produced.
b) Partial pressure of SO3 is decreased (Some of the SO3 is removed fro the system).
The system will shift to the right. As the SO3 is removed its concentration drops. The reactants will react to produce more product.
c) Decrease in temperature.
The reaction will shift right. As written the reaction is exothermic. As heat is removed the reaction proceeds to produce more heat.
d) He gas is added at constant volume so that the total pressure is increased.
No change takes place. Dalton Law of Partial Pressures comes into play here. Each gas is treated separately from it's neighbours. Because the volume stays constant then the concentrations don't change. Therefore no changes take place.
e) He gas is added, and the container is allowed to expand so that the total pressure is kept constant.
To the left or the reactants will be favoured. The volume was allowed to increase to maintain the total pressure. Since the volume increased the concentrations must have dropped for all reactants and products. The reaction will react do produce the maximum number of moles of gas to fill the available space. Hence the reactants are favoured.
f) A catalyst is added.
No change. The catalyst favours both reactions, forward and reverse equally.
Follow-up Problem:
For the reaction below:

        heat + 2 H2O(g) <=====>  2 H2(g) + O2(g)

Indicate the direction of the reaction most favoured by each of the following conditions.
a) increase the [H2]
b) the partial pressure of [H2O] is increased
c) the [O2] is increased
d) increase the temperature
e) decrease the volume of the container
f) He gas is added