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Chemical Equilibrium Class 11 Chemistry Notes

CHEMICAL EQUILIBRIUM

REVERSIBLE REACTIONS

3Fe + 4H2O Fe3O4 + 4H2

H2 + I2 2HI

N2O4 2NO2

N2 + 3H2  2NH3

IRREVERSIBLE REACTIONS

       

       

CHEMICAL EQUILIBRIUM

CHARACTERISTICS OF CHEMICAL EQUILIBRIUM

1. Equilibrium can be attained from either side.
2. Equilibrium is dynamic in nature i.e. at equilibrium, reaction does not stop.
3. At equilibrium there is no change in the concentration of various species.
4. The equilibrium state remains unaffected by the presence of catalyst. Catalyst helps to attain the equilibrium state rapidly.
5. It can be achieved in a closed container.
6. The observable properties of the process become constant and remain unchanged.

EQUILIBRIUM STATE AND FREE ENERGY CHANGE

At equilibrium G is equal to zero and we have

G = H – TS

H = TS

LAW OF MASS ACTION

aA + bB   cC + dD

Rate of forward reaction

Rate of backward reaction

FACTORS INFLUENCING EQUILIBRIUM CONSTANT

EQUILIBRIUM CONSTANT IS NOT INFLUENCED BY

1. Concentration of reactants and products.
2. Presence of a catalyst.
3. Pressure.
4. Presence of inert materials.
5. The direction from which the equilibrium state is reached.

EQUILIBRIUM CONSTANT IS INFLUENCED BY

1. Temperature : The variation of equilibrium constant is given by Van't Hoff  equation.

where ,   (Kp)1 and (Kp)2 = Equilibrium Constant at temperature T1 & T2

2. The mode of representing the reaction :

The reaction A + B C + D may be written as

   C + D  A + B

            

 

3. Stoichiometric representation of equation :

N2 + 3H2   2NH3

 NH3

          

USE OF PARTIAL PRESSURE INSTEAD OF CONCENTRATIONS

aA + bB   cC + dD

RELATION BETWEEN KC AND KP

n = [moles of products – moles of reactants] gaseous only.

RELATION BETWEEN KC AND KP FOR DIFFERENT TYPES OF REACTIONS

1. When n = 0, Kp = Kc e.g. for reaction A B.

2. When n = +ve, Kp > Kc e.g. for reaction A  2B.  

3. When n = –ve, Kp < Kc e.g. for reaction 2A  B.  

UNITS OF Kp AND Kc

• Unit of Kp = (atm) n
• Unit of Kc = (mol lit–1) n

CHARACTERISTICS OF EQUILIBRIUM CONSTANT

1. It has definite value for every chemical reaction at a particular temperature
2. The more is the value of Kc or Kp, the more is the completion of reaction or the more is the concentration of products.
3. When the reaction can be expressed as sum of two other reactions, the Kc of overall reaction is equal to the product of equilibrium constants of individual reactions.

HOMOGENEOUS EQUILIBRIUM

2SO2 (g) + O2 (g)  2SO3 (g)

HETEROGENEOUS EQUILIBRIUM

3Fe(s) + 4H2O(g)  Fe3O4 (s) + 4H2 (g)

CHEMICAL EQUILIBRIUM APPLIED TO HOMOGENEOUS SYSTEM

GASEOUS SYSTEM

1. Gaseous reactions in which the number of moles of products remain the same as that of reactants

           

Initial molar conc.            0

Eqb. molar conc.         

..........(i)

2. Gaseous reactions in which the number of moles of products and reactants are different.

        

              

Initial molar Conc.        0         0

Eqb. molar Conc.                   

.......(i)

If the value of x is small then   ;

LIQUID SYSTEM

1. Esterification of acetic acid

2. Reaction between amylene and tricholoroacetic acid

CHEMICAL EQUILIBRIUM APPLIED TO HETEROGENEOUS SYSTEM

1. Dissociation of calcium carbonate

The active mass of a solid reactant and product is assumed to have a constant value and is taken as unity. The equilibrium  constant is determined by gaseous substances only. Therefore,

2. Reaction of steam on heated iron

3. Water gas reaction

VAN'T HOFF ISOCHORE

A relationship between the equilibrium constant Kp, at  any temperature T and constant pressure P, and heat of reaction H°.

1. when H = 0 no heat is evolved or absorbed

2. when H = +ve i.e. heat is absorbed

3. when H = –ve i.e. heat is evolved

  ;   Equilibrium constant

4. when n = 0 i.e. there is no change in volume during a reaction KP = Kc. The variation of equilibrium constant with temperature is given by

, E heat of reaction at constant volume.

VAN'T HOFF REACTION ISOTHERM

G = G° + RT ln J

At equilibrium G = 0 then G° = –RT ln Jeq

viz.

G° = –RT ln

HENRY'S LAW

• The volume of the gas dissolved remains the same inspite of increase in pressure.
• The dissolution of a gas in a liquid is spontaneous process (G = 0), accompanied by decrease in entropy (S = –ve). Since G = H – TS, G can only be negative if H is –ve. Therefore dissolution of a gas in a liquid is always exothermic in nature.

FACTORS ALTERING THE STATE OF EQUILIBRIUM - LE CHATELIER'S PRINCIPLE

1. Effect of change of concentration :

 Fe3+ (aq)      + SCN– (aq)         [Fe(SCN)]2+ (aq)

2. Effect of change in pressure :
1. No effect of pressure on equilibria having same moles of reactants and products e.g. N2 + O2 2NO
   H2 + I2  2HI
2. When there is change in the number of moles the equilibrium will shift in the direction having smaller number of moles when the pressure is increased and vice versa e.g.

N2 + 3H2 2NH3

PCl5 PCl3 + Cl2

3. Effect of temperature :
1. When process is exothermic - Low temperature favours the formation of products.
2. When process is endothermic - High temperature favours the formation of products

e.g. N2 + 3H2  2NH3 + 24.0 kcal.

4. Effect of addition of inert gas :
1. Addition of Inert gas at constant volume : The total pressure of the system is increased, but the partial pressure of each reactant and product remains the same. Hence no effect on the state of equilibrium.
2. Addition of Inert gas at constant pressure : The total volume is increased, the number of moles per unit volume of each reactant and product is decreased. Hence equilibrium will shift to the side where number of moles are increased e.g.

PCl5 (g)    PCl3 (g) + Cl2 (g)

5. Effect of catalyst : The presence of catalyst does not change the position of equilibrium. It simply fastens the attainment of equilibrium.

LE CHATELIER'S PRINCIPLE APPLICABLE TO PHYSICAL EQUILIBRIUM

1. Effect of pressure on solubility : The increased pressure, will increase the solubility of a gas and vice versa.
2. Effect of temperature on solubility : The substances which dissolve with the absorption of heat, their solubility will increase with increase of temperature and vice versa e.g. dissolution of NH4Cl, KCl, KNO3 is endothermic which increases with increase of temperature. The dissolution of calcium acetate and Calcium hydroxide is exothermic, their solubility is lowered at higher temperature.
3. Effect of pressure on the melting point of ice :

Ice  liquid water

FAVOURABLE CONDITIONS FOR SOME IMPORTANT REACTIONS

N2 (g) + 3H2 (g)  2NH3 (g) + 22.4 kcal

• Low temperature (500°C)
• High pressure (200 – 1000 atm.)
• Excess of N2 and H2

N2 (g) + O2 (g)  2NO(g)– 43.2 kcal

• High temperature
• Excess of N2 and O2
• No effect of pressure

2SO2 (g) + O2 (g)  2SO3 + 42.0 kcal

• Low temperature
• High pressure
• Excess of SO2 and O2

2NO + O2 2NO2 + 27.8 kcal

• Low temperature
• High pressure
• Excess of NO and O2

N2O4  2NO2 – 14 kcal

• High temperature
• Low pressure
• Excess of N2O4

CO + H2O  CO2 + H2 + x kcal

• Low temperature
• Excess of steam and CO
• No effect of pressure

PCl5 PCl3 + Cl2 – 15 kcal

• High temperature
• Low pressure
• Excess of PCl5

TRIPLE POINT

Ice (s) water (l)  vapour (g) can exist at 0.0098°C and 4.58 mm.

DEGREE OF DISSOCIATION FROM DENSITY MEASUREMENT

D = where M = Mol. wt of gas; P = Total pressure.

PCl5        PCl3   + Cl2

D= Molecular mass