Le Chatelier’s Principle

? What controls the position of chemical equilibrium? We can change certain aspects of an equilibrium reaction to force the equilibrium to shift to one side of the reaction (reactant or product side) This is called Le Chatelier’s Principle.

LeChatelier’s Principle Le Chatelier’s Principle: A change (stress) imposed on a system at equilibrium shifts in a direction that tends to reduce the effect of that change. ???

What does it mean? When a reaction is at equilibrium, we can add a stress (change) to the system. The reaction shifts its equilibrium position to reduce that stress. What can we do to cause the equilibrium stress?

Stresses Concentration Pressure (for gases) Volume (for gases) Temperature

Effect of Change in Concentration Look again at the following reaction: N2(g) + 3H2(g)↔2NH3(g) Let’s say we have the following concentrations at equilibrium: [N2]=0.399 M, [H2]=1.197 M, [NH3]=0.203 M What is the equilibrium constant?

What is going to happen if we all of a sudden add 1.00 mol/L of N2? Remember, the system was at equilibrium right before the extra N2 was added. Also, when you increase concentration, what happens to the rate of the reaction?

The rate of the forward reaction has increased, so what is going to happen to the concentration of [NH3]? The system wants to reestablish equilibrium, but the equilibrium has shifted now to the right, or more products.

Take a look at the new equilibrium concentrations: [N2]=1 Take a look at the new equilibrium concentrations: [N2]=1.348 M, [H2]=1.044 M, [NH3]=0.304 M What is the equilibrium constant? The equilibrium constant does not change when reactant or product is added or removed.

TO SUMMARIZE When a reactant or product is added to a system at equilibrium, the system shifts away from the side with the added component. When a reactant or product is removed from a system at equilibrium, the system shifts toward the side with the removed component.

As4O6(s) + 6C(s) ↔As4(g) + 6CO(g) Predict the shift in the equilibrium position for this reaction in response to the following changes: A) Addition of CO B) Addition of C(s) C) Removal of As4(g)

Effect of a Change in Volume For equilibrium containing a gas only When the volume of a gas decreases, what happens to the pressure? This is a sudden stress on equilibrium. What can the reaction do to reduce this stress?

Consider the following reaction: CaCO3(s) ↔CaO(s) + CO2(g) -If the volume containing the above reaction is suddenly deceased, the pressure of CO2 suddenly increases. To reduce this stress, the reaction will shift away from the gas, or to the left. Again, no change in the equilibrium constant.

To SUMMARIZE When the volume of a gaseous reaction system at equilibrium is decreased (thus increasing the pressure), the system shifts in the direction that gives the smaller number of gas particles. Reverse is true…increasing volume shifts to the side with more gas molecules

Predict the shift in equilibrium position that occurs in each of the following reactions when the volume is reduced: A) P4(s) + 6Cl2(g) ↔4PCl3(l) B) PCl3(g) + Cl2(g)↔ PCl5(g) C) PCl3(g) + 3NH3(g) ↔P(NH2)3(g) + 3 HCl(g)

Effect of a Change in Temperature **When the temperature of reaction changes, the equilibrium constant changes. Reactions can either absorb heat or release heat. A reaction that releases heat (heat is a product) is called an exothermic reaction. A reaction that absorbs heat (heat is a reactant) is called an endothermic reaction.

N2(g) + 3H2(g)↔2NH3(g) + Energy What you do is you put energy into the chemical equation. For example, the synthesis of ammonia is an exothermic reaction, so we would write the reaction with energy included like: N2(g) + 3H2(g)↔2NH3(g) + Energy

CaCO3(s) + Energy ↔CaO(s) + CO2(g) The decomposition of calcium carbonate is an endothermic reaction, written like this: CaCO3(s) + Energy ↔CaO(s) + CO2(g) Once you have energy written as a reactant or product, treat it like any other substance.

In Summary Treat energy as a reactant or product. If energy is added (increasing temperature), the equilibrium will shift away from the position of energy. Reverse is true

Examples For each of the following reactions, predict how the equilibrium will shift if the temperature is increased. A) N2(g) + O2(g)↔2NO(g) B) 2SO2(g) + O2(g) ↔2SO3(g) C) C2H2(g) + 2Br2↔C2H2Br4(g)