Le Chatelier’s Principle Chem 12 - Unit 3

Le Chatelier’s Principle The French chemist Henri Le Chatelier ( ) studied how the equilibrium position shifts as a result of changing conditions

LeChatelier’s Principle When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress. When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

Le Chatelier Translated: When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away. When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away. When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added. When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added.

What items did he consider to be stress on the equilibrium? 1.Concentration 2.Temperature 3.Pressure

Examples: Let’s look at some examples, there will be a summary table at the end!

Le Chatelier Example #1 A closed container of ice and water at equilibrium. The temperature is raised. Ice + Energy  Water The equilibrium of the system shifts to the _______ to use up the added energy. right

Le Chatelier Example #2 A closed container of N 2 O 4 and NO 2 at equilibrium. NO 2 is added to the container. N 2 O 4 (g) + Energy  2 NO 2 (g) The equilibrium of the system shifts to the _______ to use up the added NO 2. left

Le Chatelier Example #3 A closed container of water and its vapor at equilibrium. Vapor is removed from the system. water + Energy  vapor The equilibrium of the system shifts to the _______ to replace the vapor. right

Le Chatelier Example #4 A closed container of N 2 O 4 and NO 2 at equilibrium. The pressure is increased. N 2 O 4 (g) + Energy  2 NO 2 (g) The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation. left

Pressure Changes to system: If the volume decreases, the concentration increases, and there will be a shift to the side with the less amount of moles. If the volume increases, the concentration decreases, and there will be a shift to the side with the more amount of moles.

Another Example: 1.If I increase the pressure, where is the shift? (right) 2.If I decrease the pressure, where is the shift? (left) 2SO 2 + O 2 2SO 3 (3moles) (2moles)

Effect of Concentration: 1.If you add more reactant, it shifts to the right increasing the formation of product, using up the reactants. 2.If you add product, it shifts to the left 3.If you remove product, it shifts to the right, increasing the formation of product. 4.If you remove reactant, it shifts to the left

Effect of temperature: Remember: Energy is treated as a reactant if endothermic equation and as a product if exothermic equation. If heating a system, it shifts so extra heat is used up. If cooling a system, then it shifts so more heat is produced.

Example Endothermic Reaction: Heating this reaction causes the system to shift to the right = more products, because you treat energy like a reactant (this reaction needs energy to go forward) 2NaCl +H 2 SO 4 + energy 2HCl + Na 2 SO 4 Cooling this reaction causes the system to shift to the left = less reactants, so need to make up more

Example Exothermic Reaction: Heating this reaction causes the system to shift to the left, to use up the extra heat. 2SO 2 + O 2 2SO 3 + energy Cooling this reaction causes the system to shift to the right, to make up for the lost heat.

ConcentrationDirection of Shift Increase in concentration of a reactantTowards products Increase in concentration of a productTowards reactants Decrease in concentration of a reactantTowards reactants Decrease in concentration of a productTowards products Temperature Endothermic Reaction  H>0 Increase in TTowards products Decrease in TTowards reactants Exothermic Reaction  H<0 Increase in TTowards reactants Decrease in TTowards products Volume More moles of gas on right Decrease in Total VolumeTowards reactants Increase in total volumeTowards products More moles of gas on left Decrease in total volumeTowards products Increase in total volumeTowards reactants

Common Ion Effect: Involves adding of an ion to a solution in which the ion is already present in the solution – Increases concentration of that ion – Eq shifts away from that ion Example: Cu 2+ (aq) + 4Cl -1 CuCl 4 2+ (aq) Adding NaCl would increase the Cl -1 concentration and shift Eq right!

Try it : Handouts Page 529 #33-37