Equilibrium and Le Chatelier Principle

Chemical Equilibrium operates in reversible reactions Reversible Reactions are those in which products revert back to give products. Here the reaction takes place in both directions. In reversible reactions, we have three possibilities: Rate of the forward is greater than the rate of the backward reaction The rate of the backward reaction is greater than the rate of the forward reaction The rate of the forward reaction is equal to the rate of the backward reaction. The third possibility, wherein the rate of forward is equal to the rate of the backward is called Chemical Equilibrium. The general representation of a chemical equilibrium is as follows: R P where R represents Reactants; P represents Products; is the equilibrium symbol. A chemical equilibrium is a dynamic equilibrium as there is movement on either side.

A chemical equilibrium is like a seesaw Law of Conservation of Mass Operates

All chemical equilibria have an Equilibrium Constant, K Equilibrium constant is of two kinds: Kc and Kp where the subscripts c and p represent concentration and pressure. Note that Kp operates only in gaseous reactions. Let us consider a chemical reaction in which a moles of a reactant A and b moles of a reactant B are reacting to form c moles of a product C and d moles of another Product D. Kc and Kp are related:

Le Cahtelier or Le Chatelier-Braun Principle When a chemical system at equilibrium is disturbed, the system responds in such way to nullify the disturbance and restore the equilibrium. Generally, a chemical system is subjected to the following disturbances: Addition of reactant or Removal of Reactant Addition of Product or Removal of Product Increasing Temperature of Decreasing Temperature Increasing Pressure or Decreasing Pressure Catalyst

1. Addition of Reactant Disturbance: Addition of 10 g of Reactant Let us consider a chemical reaction in equilibrium with 20 g of product and 20 g of reactant; Now let us cause a disturbance by add ing10 g of the reactant. The system now responds as follows: 5 gram of the added reactant will quickly turn into product As a result , the equilibrium is restored. The schematic representation of the disturbance-response-restoration of equilibrium is shown below: 20 g 20 g Original Equilibrium: 20 g reactant and 20 g of product R P + 10 g 20 g 20 g Disturbance: Addition of 10 g of Reactant R P 20 g 30 g Response: 5 g of the added reactant turns into product 5 g R P Restoration of Equilibrium: A new equilibrium is established with 25 g of reactant and 25 g of the product. 25 g 25 g R P Effect: Addition of Reactant favors the Forward Reaction

2. Addition of Product Disturbance: Addition of 10 g of Product Let us consider a chemical reaction in equilibrium with 20 g of product and 20 g of reactant; Now let us cause a disturbance by add ing10 g of the product. The system now responds as follows: 5 gram of the added product will quickly turn into reactant As a result , the equilibrium is restored. The schematic representation of the disturbance-response-restoration of equilibrium is shown below: 20 g 20 g Original Equilibrium: 20 g reactant and 20 g of product R P 20 g 20 g Disturbance: Addition of 10 g of Product + 10 g R P 20 g Response: 5 g of the added product turns into reactant 5 g 30 g R P Restoration of Equilibrium: A new equilibrium is established with 25 g of the reactant and 25 g of the product 25 g 25 g R P Effect: Addition of Product favors the Backward Reaction

3. Temperature Change: Increasing the Temperature Let us consider a chemical reaction in equilibrium with the forward as endothermic and the backward as exothermic process Forward: Endothermic Original Equilibrium R P Backward: Exothermic Disturbance: Heated Response: Endothermic side of the reaction responds as it is the one that requires temperature Restoration of Equilibrium: The system nullifies the added heat by consuming the heat via the forward reaction (here as endothermic reaction is the forward reaction) Effect: Heating favors the Endothermic Reaction

4. Pressure Change: Increasing the Pressure Increasing the Pressure results in decreasing the Volume. Volume is equivalent of number of moles. This means that increasing the Pressure will favor the side that leads to less number of moles. Let us consider three cases N2 + 3H2 2NH3 Disturbance: Increase of Pressure Response: Forward reaction will be favored (I + 3) moles = 2 moles 4 moles = 2 moles Forward: Reduction in number of moles 2SO3 2SO2 + O2 Disturbance: Increase of Pressure Response: Backward reaction will be favored 2 moles = 3 moles Backward: Reduction in number of moles 2HI H2 + I2 Disturbance: Increase of Pressure Response: No effect of Pressure 2 moles = 1 + 1 moles 2 moles = 2 moles No change in number of moles on both sides

5. Effect of Catalyst Catalyst generally favors both the forward and backward reactions

Your task (42 Pts) Use your own paper Question 1 Discuss (follow model given), with neat diagrams and explanation, the effect of the following on a chemical equilibrium (1) Removal of Reactant ( 8 Pts) (2) Removal of Product (8 Pts) (3) Decreasing Temperature (8 Pts) (4) Decreasing Pressure (12 Pts) Question 2 (6 Pts) Discuss Equilibrium constants, Kc and Kp and the relationship between them.