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# LeChateliers Principle

## Introduces LeChâtelier’s Principle and how it governs chemical equilibria.

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LeChâtelier's Principle

Credit: Courtesy of the US Marine Corps
Source: http://commons.wikimedia.org/wiki/File:USMC-101116-M-3816G-001.jpg

How far down?

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### 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 increases, while the concentrations of reactants decreases. When the reverse reaction is favored, the concentrations of products decreases, while the concentrations of reactants increases.

 Original Equilibrium Favored Reaction Result A⇄B\begin{align*}A \rightleftarrows B\end{align*} forward: A→B\begin{align*}A \rightarrow B\end{align*} [A] decreases; [B] increases A⇄B\begin{align*}A \rightleftarrows B\end{align*} reverse: A←B\begin{align*}A \leftarrow B\end{align*} [A] increases; [B] decreases

Source: http://commons.wikimedia.org/wiki/File:Lechatelier.jpg

Henri Le Châtelier [Figure2]

#### Summary

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

#### Practice

Questions

1. Who gave him his early training in math and chemistry?
2. What was his first area of investigation?
3. What was another area of interest?
4. When did he first propose his principle?

#### Review

Questions

1. What does Le Châtelier’s principle state?
2. If the forward reaction is favored in the equilibrium, what will increase?
3. If the reverse reaction is favored in the equilibrium, what will increase?