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19.5: Le Châtelier's Principle

There are people who enjoy going up in an airplane, strapping on a parachute, and diving out the door to free-fall and then open the chute and drop to the ground. This stressful activity (so they say) relieves the stress of everyday life. The release of adrenaline caused by this stressful behavior is said to promote a mood enhancement that helps you deal better with other stresses in your daily life.

Le Châtelier's Principle

Chemical equilibrium was studied by French chemist Henri Le Châtelier (1850 - 1936) and his description of how a system responds to a stress to equilibrium has become known as Le Châtelier's principle: When a chemical system that is at equilibrium is disturbed by a stress, the system will respond in order to relieve the stress. Stresses to a chemical system involve changes in the concentration of reactants or products, changes in the temperature of the system, or changes in the pressure of the system. We will discuss each of these stresses separately. The change to the equilibrium position in every case is either a favoring of the forward reaction or a favoring of the reverse reaction. When the forward reaction is favored, the concentrations of products increase, while the concentrations of reactants decrease. When the reverse reaction is favored, the concentrations of products decrease, while the concentrations of reactants increase.

 Table 19.5.1 Original Equilibrium Favored Reaction Result $$\ce{A} \rightleftharpoons \ce{B}$$ Forward: $$\ce{A} \rightarrow \ce{B}$$ $$\left[ \ce{A} \right]$$ decreases; $$\left[ \ce{B} \right]$$ increases $$\ce{A} \rightleftharpoons \ce{B}$$ Reverse: $$\ce{A} \leftarrow \ce{B}$$ $$\left[ \ce{A} \right]$$ increases; $$\left[ \ce{B} \right]$$ decreases

Summary

• Le Châtelier's principle describes how a reaction system at equilibrium is influenced by stress.

Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.