Le Chatelier’s Principle  A reaction at equilibrium, when “stressed,” will react to relieve the stress.  (If you mess with it, it will work to return to its equilibrium ratio.)

Le Chatelier’s Principle  “Stress” includes changes to:  concentration  pressure  temperature

Le Chatelier’s Principle  Questions will tell you HOW equilibrium was stressed.  You answer with what the reaction does to FIX itself.  The choices are: It will shift LEFT, RIGHT, or nothing will happen.

Le Chatelier: Concentration  If the concentration of a reactant is increased the equilibrium will shift in the direction that uses the reactants, so that the reactant concentration decreases.  (The forward reaction is favored– more products will be made as a result.)

Le Chatelier: Concentration 2SO 2 (g) + O 2 (g) ⇌ 2 SO 3 (g)  If the SO 2 or O 2 concentration was increased: (i.e., we added more reactants)  Equilibrium will shift right to decrease the concentration of reactants.  (Some of the reactant we added gets used up and more SO 3 (product) is produced.)

Le Chatelier: Concentration  “Equilibrium will shift right”  Let’s define that:  When a new equilibrium is reached (when the rate of forward and reverse reactions become equal again), there will be more product than there was before.  The equilibrium RATIO did not change.

Le Chatelier: Concentration  The forward reaction is also favored if the concentration of the product is decreased.  If product is removed, equilibrium shifts to the right in order to:  Decrease the amount of reactant (by using some up)  Increase the amount of product (by making some more)

Le Chatelier: Concentration  Equilibrium shifts to the right when  Concentration of reactants increases  Concentration of products decreases  The reverse is also true:  Equilibrium shifts to the LEFT when  Concentration of reactants decreases  Concentration of products increases

Le Chatelier: Concentration 2SO 2 (g) + O 2 (g) ⇌ 2 SO 3 (g)  Adding more SO 2 (g)  shifts it to the right  The question might say “increasing [SO 2 ]” which implies that we’re measuring concentration.  The question might say “increasing P SO2 ” which implies that we’re measuring concentration of a GAS.  (Both mean the same thing.)

Le Chatelier: Concentration Be careful!  Solids (s) and liquids (l) cannot become more “concentrated.”  There are only two options.  You have some (concentration = 100%)  Or you don’t (concentration = 0%) 2SO 2 (g) + O 2 (l) ⇌ 2 SO 3 (g)  Adding more SO 2 (g) shifts it to the right  But Adding more O 2 (l) makes no difference! (because it is liquid)

Le Chatelier: Concentration  Increasing or decreasing the amount of solids (s) or liquids (l) DOES NOT AFFECT equilibrium  That means the answer is “no change”  Increasing or decreasing the amount of gases (g) or aqueous solutions (aq) DOES AFFECT equilibrium.  That means the answer is either “shifts left” or “shifts right.”

Le Chatelier: Pressure  Changing the pressure of the system  This is indicated either by “pressure of the system increased/decreased”  OR “volume of the container is increased/decreased”  Making a container larger makes pressure DECREASE.  Pressure and volume are opposites!

Le Chatelier: Pressure  Changing the pressure of the system  If pressure is increased the equilibrium will shift to favor a decrease in pressure.  If the pressure is decreased the equilibrium will shift to favor an increase in pressure.

Le Chatelier: Pressure  The reaction will “shift to favor a decrease/increase in pressure” 2SO 2 (g) + O 2 (g) ⇌ 2 SO 3 (g)  Count the moles of gas on each side of the equilibrium arrow.  Reactants: 3 moles of gas  Products: 2 moles of gas  (Coefficients MATTER!)

Le Chatelier: Pressure 2SO 2 (g) + O 2 (g) ⇌ 2 SO 3 (g) (3 moles of gas > 2 moles of gas)  Shifting left will increase pressure  Shifting right will decrease pressure (If there aren’t any gases, it’s a tie, and nothing happens.)

Le Chatelier: Pressure 2SO 2 (g) + O 2 (l) ⇌ 2 SO 3 (g) (2 moles of gas = 2 moles of gas)  It’s a tie.  Nothing happens.

Le Chatelier: Temperature  Changing the temperature of the system  If the temperature is increased the equilibrium will shift to favor the reaction which will absorb heat.  (The endothermic reaction is favored.)

Le Chatelier: Temperature  Changing the temperature of the system  If the temperature is decreased the equilibrium will shift to favor the reaction which will release heat.  (The exothermic reaction is favored.)

Le Chatelier: Temperature N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g) ; ΔH= −92 kJ  The reaction is exothermic.  Energy is a product.  An increase in temperature shifts this reaction to the left.  A decrease in temperature shifts this reaction to the right.

Exothermic vs Endothermic  ΔH=  N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g); ΔH= −92 kJ  2 NH 3 (g) ⇌ N 2 (g) + 3 H 2 (g); ΔH= 92 kJ  Energy as part of the reaction:  N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g) + energy  N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g) + 92 kJ  2 NH 3 (g) + energy ⇌ N 2 (g) + 3 H 2 (g)  2 NH 3 (g) + 92 kJ ⇌ N 2 (g) + 3 H 2 (g)