Le Chatelier's Principle is the principle when a stress is applied to a chemical system at equilibrium, the equilibrium will shift to relieve the stress. In other words, it can be used to predict the direction of a chemical reaction in response to a change in conditions of temperature, concentration, volume, or pressure. While Le Chatelier's principle can be used to predict the response to a change in equilibrium, it does not explain (at a molecular level), why the system responds as it does.
Chatalier's Principle or the Equilibrium Law
The principle is named for Henry Louis Le Chatelier. Le Chatelier and Karl Ferdinand Braun independently proposed the principle, which is also known as Chatelier's principle or the equilibrium law. The law may be stated:
When a system at equilibrium is subjected to a change in temperature, volume, concentration, or pressure, the system readjusts to partially counter the effect of the change, resulting in a new equilibrium.
While chemical equations are typically written with reactants on the left, an arrow pointing from left to right, and products on the right, the reality is that a chemical reaction is at equilibrium. In other words, a reaction may proceed in both the forward and backward direction or be reversible. At equilibrium, both the forward and back reactions occur. One may proceed much more quickly than the other.
In addition to chemistry, the principle also applies, in slightly different forms, to the fields of pharmacology and economics.
How to Use Le Chatelier's Principle in Chemistry
Concentration: An increase in the amount of reactants (their concentration) will shift the equilibrium to produce more products (product-favored). Increasing the number of products will shift the reaction to make more reactants (reactant-favored). Decreasing reactants favors reactants. Decreasing product favors products.
Temperature: Temperature may be added to a system either externally or as a result of the chemical reaction. If a chemical reaction is exothermic (ΔH is negative or heat is released), heat is considered a product of the reaction. If the reaction is endothermic (ΔH is positive or heat is absorbed), heat is considered a reactant. So, increasing or decreasing temperature can be considered the same as increasing or decreasing the concentration of reactants or products. In the temperature is increased, the heat of the system increases, causing the equilibrium to shift to the left (reactants). If the temperature is decreased, the equilibrium shifts to the right (products). In other words, the system compensates for the reduction in temperature by favoring the reaction that generates heat.
Pressure/Volume: Pressure and volume can change if one or more of the participants in a chemical reaction is a gas. Changing the partial pressure or volume of a gas acts the same as changing its concentration. If the volume of gas increases, pressure decreases (and vice versa). If the pressure or volume increase, the reaction shifts toward the side with lower pressure. If the pressure is increased or volume decreases, equilibrium shifts toward the higher pressure side of the equation. Note, however, that adding an inert gas (e.g., argon or neon) increases the overall pressure of the system, yet does not change the partial pressure of the reactants or products, so no equilibrium shift occurs.