NEET Chemistry Notes Chemical Equilibrium – Le-Chatelier’s Principle

NEET Chemistry Notes Chemical Equilibrium – Le-Chatelier’s Principle

Le-Chatelier’s Principle

Le-Chatelier’s Principle

It states that change in any of the factors that determine the equilibrium conditions of a system, will cause the system to change in such a manner so as to reduce or to counteract the effect of the change.
Different factors affecting equilibrium are discussed below

Effect of Concentration Change

• The concentration stress of an added reactant or product is relieved by net reaction in the direction that consumes the added substance, e.g.
  
•  If we increase the concentration of either A or B (reactants), the equilibrium goes in the direction that consumes A or B, i.e. forward side.
•  If we increase the concentration of C (product), the equilibrium goes in the direction that consumes C, i.e. backward side.
•  If we remove C (product), the equilibrium goes in the direction in which its concentration increases, i.e. forward side.
•  If any of the species is in solid or in liquid state, its addition does not alter the original equilibrium.

Effect of Pressure

• At high pressure, reaction goes from higher moles to lower moles or from higher volume to lower volume and vice versa.
•  If =0, no effect on equilibrium due to pressure change.
• If > 0, the increase in pressure favours backward reaction.
• If < 0, the increase in pressure favours forward reaction.
  (= number of moles of gaseous products-number of moles of gaseous reactants).
  Flash evaporation technique is used for concentrating some aqueous solutions which cannot be concentrated by normal boiling. Concentration of this type of solution is carried out under reduced pressure below 100°C.

Effect of Temperature

At high temperature, reaction goes to endothermic direction while at low temperature reaction goes to exothermic direction. The equilibrium constant for an endothermic reaction  increases as the temperature increases.

Freeze drying technique is used for drying heat sensitive substances. In this technique, water is made to sublime off at a temperature below 0°C.

Effect of Catalyst

A catalyst increases the rate of forward reaction as well as the rate of backward reaction, so it does not affect the equilibrium and equilibrium constant.

Effect of Inert Gas

At constant volume, there is no effect of addition of inert gas. At constant pressure, when inert gas is added, reaction goes from lower moles to higher moles.

Relation between Degree of Dissociation and Density

Degree of dissociation  of a gaseous compound is related to its vapour density by

D = molar density before dissociation/initial density
d = density after dissociation/density of the gaseous mixture
y = number of moles of products

Molecular weight = Density of gas x 2
M₀= Dx 2 or M_c =d x 2

Here, M₀ = Observed molecular weight (Abnormal) Mc = Calculated molecular weight (Theoretical)

Ionic Equilibrium

The weak electrolytes are only partially ionised and a dynamic equilibrium is maintained after sometime between the ions and unionised molecules. This is known as ionic equilibrium. Hence, ionic equilibrium can be defined as equilibrium which is established between the unionised molecules and the ions in the solution of weak electrolytes.

Weak and Strong Electrolytes and their Ionisation

Weak electrolytes dissociate partially in the solutions and such solutions are poor conductor of electricity,
Strong electrolytes dissociate completely into their ions in solution and such solutions are very good conductor of electricity,

Separation of an electrolyte into their ions either on fusion or dissolution is called ionisation or dissociation.

(Usually the term dissociation is used for weak electrolyte and ionisation for strong electrolyte).
The solution of weak electrolytes contain ions, which are in equilibrium with unionised molecules.

This equilibrium is known as ionic equilibrium and is dynamic in nature.

Degree of Ionisation

The fraction of total number of moles undergoing ionisation is called degree of ionisation or dissociation
Alternately, the fraction of the amount of an electrolyte present in the solution as free ions is called degree of ionisation

Ostwald’s Dilution Law

It states that degree of dissociation of weak electrolyte is inversely proportional to the square root of concentration.

where, = degree of dissociation
V = volume containing 1 mole of weak electrolyte
K = dissociation constant.
Ostwald’s dilution law is used to calculate the degree of dissociation, for weak acids and bases from the known value of K.

Limitations of Ostwald’s Law

•  This law holds good only in case of weak electrolytes. Even weak electrolytes do not obey this law in concentrated solutions.
• The value of K can be calculated only in dilute solutions of weak electrolytes.

Factors Influencing Degree of Ionisation/Dissociation

•  For strong electrolyte,  = 1 at normal dilution while for most of the polar covalent compounds i.e. weak electrolytes, <<< 1.
•  Degree of ionisation of an electrolyte increases with polarity of the solvent.
•  The degree of ionisation of an electrolyte decreases with increase in concentration of the electrolyte.
•  The degree of ionisation rises with raise in temperature of the solution.
•  The addition of species possessing a common ion to that of weak electrolyte causes a decrease in the degree of dissociation of weak electrolyte.

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