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Wednesday, 23 December 2015

FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 5

FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 5



Applications of Le- Chatelier's Principle : (Effect of Change in Conditions upon Equilibrium)

Le – Chatelier's principle is used to predict, how the variable like concentration. Temp, pressure etc, affect the position of equilibrium.
This principle also has a valuable application in prediction the condition for maximum yield of a particular in a reversible reaction.
1) Effect of Concentration:
According to Le – Chatelier's principle an increase in the concentration of any the reactants, shifts the equilibrium to the right i.e forward reaction increases similarly an increase in the concentration of any of the products shifts the equilibrium to the left i.e reverse reaction increases Same examples are as follow

N2 + 3H2--------------------2NH3
The addition of H2 and N2 would shift the equilibrium to the right. Thus the use of excess of N2 yield of NH3

FeCl3+n3NH4CNS --------------------Fe ( CNS)3 + 3NH4C ff2
Yellow colorless blood red colorless The addition of Fecl3 , shifts the equilibrium to the right & thus more Fe (CNS)3 (blood red) is formed , forward reaction increases. Similarly the addition of NH4Cl , shift the equilibrium to the left resulting in the formation of more NH4CNs & FeCl3 i.e reverse reaction is increased.

2) Effect OF Pressure:
In case of reactions ( reversible ) in which the total volume of reactants is equal to the total volume of the products , the equilibrium is not affected by change in pressure.
e.g

1. CO(g) + H2O(g) ---------------- CO2(g) + H2(g)
    1 vol       1 vol                                 1vol        1vol
Total     2 vol                                             2 vol

2. N2 (g) + O2(g) --------------- 2NO(g)
     1vol      1 vol                             2 vol
Total   2 vol                                    2 vol
Change of pressure has no effect upon the equilibrium in the above mentioned equilibrium system between total volume of reactants is equal to total volume of products. In case of reactions ( reversible ) where the total volume of reactants is not equal to the volume of the products, with an increase of pressure the equilibrium system shifts towards the smaller volume.
In case of these reversible reactions in which the volume of reactants is greater than that of the products, the equilibrium shifts to the right side i.e more products are formed by increasing the pressure of the system.
In case of those reversible reactions in which the volume of products is greater than that of reactants increases with increase of pressure.

3) Effect of Temperature:
There are two types of reversible reactions i.e endothermic and exothermic.
In case of endothermic reversible reactions, heat is absorbed in the forward direction and the same is evolved in the reverse direction and the same is absorbed in reverse direction. With increase of temperature of an reversible reaction ( endothermic or exothermic ) at equilibrium, the equilibrium always shifts in that direction where the temperature decreases. This in case of endothermic reaction (reversible) the equilibrium with crease of temperature . shifts to the forward direction
Examples :
Reversible endothermic Reactions
N2+O2+Heat ………………..2NO
H2S+O2+Heat ………………….H2 + S
As these are endothermic breakfronts thus in decease of temperature Shifts The equilibrium in forward direction . As the reaction are exothermic thus an increase in temperature will shift the equilibrium in
reverse direction. Thus more amount of NH3& SO2 can be formed by lowering the temperature.

4) Effect of Catalyst:
According to Le – Chatelier's principle, catalyst has no effect upon the equilibrium except the equilibrium reaches earlier in the presence of a catalyst. Actually the catalyst increases the speed of both forward & reverse reactions of a reversible reaction to the some extent, therefore the equilibrium remains unchanged.  However it is reached earlier.

Written by: Asad Hussain

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