Generally, we think of chemical reactions as proceeding in the FORWARDDIRECTION REACTANTSPRODUCTS But; Some reactions are reversible and the products can react to produce the original reactants. FORWARDDIRECTIONREVERSE REACTANTSPRODUCTS

When forward reaction rate equals reverse reaction rate FORWARDEQUALSREVERSERATE We say the system is in a state of chemical EQUILIBRIUM

It is possible to place a chemical system under a STRESS Which could be a change in the system’s TEMPERATURE PRESSURE CONCENTRATION

LeChatelier’s Principle Says that whenever a stress is applied to a chemical system that is in a state of equilibrium, the equilibrium will shift in the direction that will relieve the applied stress. We can use this principle to predict the direction of the equilibrium shift when a stress is applied to a system.

LeChatelier’s Principle To use this principle, you should ask yourself two questions: #1 What is the stress which has been applied? #2 In which direction must the equilibrium shift to relieve the applied stress.

3 Factors affect equilibrium Concentration, typically measured in Molarity. Temperature of the system. Pressure of the system, only important when gases are involved.

Concentration Increasing the concentration of a substance will shift the reaction away from that substance. Consider the reaction A+B  C If you increase the concentration of A, the reaction will shift to the products.(to the right) This would cause the production of more C, but would decrease the concentration of B since it is reacting with the extra A.

What about decreasing the concentration? A + B  C Using the same reaction, if the concentration of A is decreased, the reaction shift to the reactants (to the left) to make more of A. This will cause the concentration of C to decrease but the concentration of B will increase.

Temperature An increase in temperature will shift the reaction away from the heat. An exothermic reaction is when heat is produced, therefore heat is a product. A + B  C + heat (  H) An endothermic reaction is when heat is taken in, therefore heat is a reactant. A + B + heat (  H)  C

Increasing the Temperature When the temperature is increased, the reaction shifts away from the heat. If the reaction is exothermic; A + B  C + heat (  the reaction will shift to the reactants (to the left). Think of yourself, when you get hot, you try to get out of the heat.

Change the pressure An increase in pressure will shift the reaction towards the side with the least number of molecules. Consider: 2A + 3B  2C + 2D The left side of the The right side of the equation (reactants)equation (products) has 5 molecules.has only 4 molecules. (2A + 3B)(2C + 2D) Therefore, increasing the pressure will cause this reaction to shift to the products. Decreasing the pressure will do the opposite, shift the reaction to the side with the most number of particles. Remember, pressure only affects gas particles.

N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H The above reaction is the production of Ammonia. Notice that it is an exothermic reaction. Notice that all of the particles are gases. What will happen when more nitrogen is added to this reaction?

N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H  Yes, the reaction will shift to the products to relieve the stress of excess nitrogen. N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H Hydrogen atoms react with the nitrogen, making more ammonia and more heat.

N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H Which way will the reaction shift if the pressure of this system is increased? Notice there are 4 molecules on the just 2 molecules on reactant side. the product side. 1 N 2 + 3H 2 2NH 3 Therefore, the reaction will shift to the products because there are fewer molecules.

N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H What happens to this reaction if the temperature of the system is increased? N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H  The reaction shifts away from the heat and towards the reactants. How will this change the concentration of N 2, H 2 and NH 3 ?

N 2 (g) + 3H 2 (g)  2NH 3 (g) +  H Remember: If the temperature decreases, the reaction shifts TOWARDS the heat. If the reaction endothermic, the heat would be on the reactants side.