How does an equilibrium react when the conditions of concentration, temperature or pressure are changed?
Once an equilibrium forms, they react to changes in conditions in a very different way from their behaviour as the equilibrium forms. A French scientist named Le Chatelier was the first to succeed in describing a principle that explains this behaviour, and predicts correctly changes that will occur when the equilibrium's conditions are changed.
Le Chatelier's Principle
Le Chatelier's principle states that any equilibrium that is subjected to changing conditions of temperature, concentration or pressure will react in a way to minimize the change.
A number of generalizations can help us apply his principle in chemical systems.
Complete the following concept attainment to help develop some of these generalizations.
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The above concept attainment allows students to develop some simple generalizations about how equilibrium will react under changing conditions of temperature and concentration.
List the generalizations taken from the concept attainment.
Physical Analogy of Le Chatelier's principle
The following physical analogy can be used to develop better understanding of how Le Chatelier's principle works.
 | Orange Pekoe Tea Analogy Le Chatelier's principle click on the teapot and perform the analogy |
How does Le Chatelier's principle work for changes in pressure and volume?
Le Chatelier predicts that the equilibrium will react in a way to minimize these stresses. If volumes are reduced ( and therefore pressure increased ) the equilibrium will attempt to reduce pressure by occupying less space. Fewer gas molecules occupy less space. Therefore the following generalization can be made.
Generalization:
The equilibrium will move to the side of the equation with fewer gas molecules if pressure is increased. The reverse is true if pressure is decreased. The number of total gas molecules present in the reaction is the sum of the balances of the substances in a gas phase. If the number of gas molecules is equal in reactant and product pressure and volume changes will have no effect on the equilibrium
How does Le Chatelier's principle work for the addition of catalysts?
Since catalysts will speed up the rate of both forward and reverse reactions, catalysts have no net affect on the relative amounts of reactant and products at equilibrium . They do however allow the equilibrium state to be reached more quickly.
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