The Le Chatelier's Principle reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Le Chatelier's Principle

In chemistry, Le Chatelier's Principle applies to systems at, or approaching, equilibrium. It states that changing one of the conditions of the equilibrium (applying a "stress" to it) will cause the equilibrium to shift in a direction that compensates for the change or stress. For example, in the equilibrium

A + B <-> C + D

(where the concentrations of ingredients A, B, C, and D are constant) if the equilibrium is "stressed" by adding more ingredient A, the equilibrium will shift to the right, producing more C and D.

The principle applies to heat as well as concentrations of substances. If in the above example, the reaction of A + B is exothermic (A + B --> C + D + heat), then raising the temperature will drive the equilibrium to the left, increasing the concentrations of A and B and reducing the concentrations of C and D.

Similarly, if ingredient B is a gas and all others are solids or liquids, then increasing the overall pressure will drive the equilibrium to the right since gas pressure is equivalent to concentration.

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