1. Le Châtelier Principle: restoring equilibrium

2. Lesson Objectives
• Explain the factors that stress a system at equilibrium, and use Le Châtelier’s principle to predict how the system will respond to each stress.

3. Lesson Vocabulary
Le Chatelier’s principle
System Stress
Exothermic and endothermic reaction

4. Factors Affecting an Equilibrium System
Equilibrium represents a balance between the reactants and the products of a chemical reaction.
Changes to the conditions of the system can disturb that equilibrium.
When this occurs, the system reacts in such a way as to restore the equilibrium.
Chemical equilibrium was studied by French chemist Henri Le Châtelier ( ), and his description of how a system at equilibrium responds to a change in conditions 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 by attempting to counteract that stress until a new equilibrium is established”.

5. Factors Affecting an Equilibrium System
Stresses to a chemical system include changes in:
the concentrations of reactants or products,
the temperature of the system,
the pressure of the system.
STRESS
RESPONSE
addition of reactant
forward reaction favored
addition of product
reverse reaction favored
removal of reactant
removal of product
temperature increase
endothermic reaction favored
temperature decrease
exothermic reaction favored
pressure increase
reaction that produces fewer gas molecules favored
pressure decrease
reaction that produces more gas molecules favored

6. Using a catalyst
A catalyst (for example iridium) speeds up the rate of a reaction, and have equal effects on the forward and reverse rates, so for a system at equilibrium, these two rates remain equal. Hence the equilibrium constant is unaffected.

7. Examples [N2] =1,3 M [H2] = 3,0 M [NH3] = 4,9 M [N2] =2,0 M

8. Problems

9. Lesson Summary
• A system at equilibrium can be disrupted by a change in concentration of one of the substances or by a change in temperature or pressure.
Le Châtelier’s principle states that such a system will respond by attempting to counteract the stress: ither the forward or reverse reaction will temporarily be favored until equilibrium is reestablished.
• A change in concentration or pressure does not change the value of the equilibrium constant for a reaction.
Only a temperature change alters the equilibrium constant.