Aim : How can equilibrium be shifted? Do Now: 1.Take out a calculator and reference tables. 2.What can change the equilibrium of a phase change?

What is the effect of any change on an equilibrium reaction? Le Chatlier’s Principle – explains how any system at equilibrium responds to relieve a stress on the system. States if a system is subjected to a stress, the equilibrium will shift in the direction that relieves that stress. A stress is any change in temperature, concentration, or pressure.

Concentration Changes What happens if the concentration of the reactants is changed? Given the equation A + B C + D What will happen if the concentration of A or B is increased? An increase of anything on the left side causes the reaction to shift to the right side. What will happen if the concentration of A or B is decreased? An decrease of anything on the right side causes the reaction to shift to the right.

Practice Problems Human blood contains dissolved carbonic acid in equilibrium with carbon dioxide and water according to the equilibrium system: H 2 CO 3(aq) CO 2(aq) + H 2 O (l) Explain why decreasing the concentration of CO 2 decreases the concentration of H 2 CO 3.

Concentration Changes What happens if the concentration of the products is changed? Given the equation A + B C + D What will happen if the concentration of C or D is increased? An increase of anything on the right side will cause the reaction to shift to the left. What will happen if the concentration of C or D is decreased? A decrease of anything on the right will cause the reaction to shift to the right.

Change in Pressure A change in pressure will affect the equilibrium only if a gas is involved. If the pressure on an equilibrium reaction is increased, the reaction will be driven in the direction of less gas molecules. Example: Given the reaction N 2(g) + 3H 2(g) 2NH 3(g) How many gas molecules are on each side? If pressure is increased, the reaction will favor the side with less molecules. In this case the reaction will shift to the right.

Practice Problem What will happen if pressure is added to the following reaction? H 2(g) + Cl 2(g) 2HCl (g)

Change in Temperature What happens to an equilibrium reaction when temperature is changed? The addition of heat shifts equilibrium so that heat will be absorbed. This will favor the endothermic side of the reaction. Example: What will happen if heat is added to the following reaction? N 2 + 3H 2 2NH 3 + energy Increasing heat will shift the reaction to the left. If heat is decreased, the reaction will shift to the right.

What will happen if a catalyst is added? The addition of a catalyst will change the rate of the forward and reverse reactions equally. A catalyst may cause equilibrium to be established more quickly but does not change any of the equilibrium concentrations.

Activity/Homework Review Book pages Copy and answer #18-35.