Class 11th science chemistry (equilibrium)

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Equilibrium is attained when molecules leaving the liquid to vapor equals the molecules returning to the liquid from vapor. Kc is the equilibrium constant. It is defined as the concentration of products divided by the concentration of reactants where each term is raised to stoichiometric coefficient.
Kc = [C]c[D]d/[A]a[B]b

Le Chatelier’s Principle

It states that change in any factor such as concentration, pressure, temperature, etc. causes the equilibrium to shift in such a direction to counteract or reduce the effect of a change.

Electrolytes

Electrolytes are substances which conduct electricity in aqueous solution. Some examples of electrolytes are bases salts and acids. The electricity conducted in aqueous solution is due to cations and anions produced by the ionization or dissociation of electrolytes in the solution.

Lewis Acid and Base

A Lewis acid is a species containing an empty orbital which can accept an electron pair (an electrophile) from a Lewis base and forms a Lewis adduct.
A Lewis base is a species having a filled orbital which contains an electron pair that will not be involved in bonding but might form a dative bond with Lewis acid.


Le Chatelier’s principles, also known as the equilibrium law, are used to predict the effect of some changes on a system in chemical equilibrium (such as the change in temperature or pressure). The principle is named after the French chemist Henry Louis Le Chatelier.
Le Chatelier said that equilibrium adjusts the forward and backward reactions in such a way to accept the changes affecting the equilibrium conditions.



When factors like concentration, pressure, temperature, inert gasesthat affect equilibrium are changed, the equilibrium will shift in that direction where the effects caused by these changes are nullified.
Le Chatelier’s principles are often used to manipulate rein order to obtain suitable outcomes (such as an improvement in yield).

As per Le Chatelier’s principles, the only way of equilibrium to accept more reactant is to increase product formation.
Similarly, the addition of product (concentration/pressure) shall increase the backward reaction to decrease the product concentration.

Kp=Kc(RT)Δn=Kc(pv)Δn
Change of volume, pressure, or inert gases has no effect on reactions of liquids and solids. They may have an effect in gaseous reactions and that too only when the difference in the sum of the number of reactant and product molecules (∆n) is not zero.
When ∆n = 0:
As per the Le Chatelier’s principles, there will be no effect on Equilibrium and Product Formation on changing the volume, pressure or inert gas.
When ∆n = +ve:
Increase of pressure or decrease in volume will decrease the formation of the product. Decrease of pressure or increase of volume shall have the opposite effect of increasing the product formation.
Inert gases do not take part in the reaction and shall increase the volume or pressure only.
  • At constant pressure, the addition of inert gas increases the volume, so increase the product formation.
  • At constant volume, the addition of inert gas increases the pressure, so decreases the product formation.
PCl5 ⇌ PCl3 + PCl2
In the decomposition of phosphorus pentachloride ∆n = +1.
Increase of pressure or decrease in volume decreases the decomposition of PCl5
  • At constant pressure, the addition of inert gas increases the PCl5 formation.
  • At constant volume, the addition of inert gas decreases the PCl5formation.
When ∆n = -ve:
As per Le Chatelier’s principle, an increase of pressure or decrease in volume will increase the formation of the product.
  • At constant pressure, the addition of inert gas increases the volume, so decrease the product formation.
  • At constant volume, the addition of inert gas increases the pressure, so increases the product formation
N2 + 3H2 ⇌ 2NH3
In the formation of ammonia ∆n = -2. The increase of pressure or decrease in volume increases the formation of ammonia.
  • At constant pressure, the addition of inert gas decreases ammonia formation.
  • At constant volume, the addition of inert gas increases ammonia formation.

The individual reaction in the equilibrium can be either endothermic and exothermic. Likewise, at equilibrium net energy involved may make the reversible reactions either endothermic or exothermic.
According to Le Chatelier’s Principles,
  • In exothermic equilibrium, an increase in temperature decreases the product formation and decrease in temperature increases product formation.
  • In endothermic reactions, an increase in temperature increases the product formation and decrease in temperature decreases the product formation
Le Chatelier’s Principles
Le Chatelier’s Principle on Change of Temperature
As per the ban't hoff, for an exothermic equilibrium, ∆H will be negative. Increase of temperature shall decrease K2 or decrease in temperature increases K2. The opposite is true for an endothermic reaction.


A catalyst is a substance that changes the rate of reactions (increase or decrease) without quantitatively taking part in the reaction.
In a reversible reaction, the change of reaction rate is the same for both forward and backward reactions.
The ratio of the reaction rates remains the same and so the equilibrium constant. According to Le Chatelier’s principles, the presence of the catalyst may speed up or delay the attainment of equilibrium but will not affect the equilibrium concentration.


A few important examples of dynamic equilibrium in our everyday life are listed below.
  • A new bottle of an aerated drink has a specific value for the concentration of the carbon dioxide present in the liquid phase in it. When the bottle is opened and half of the drink is poured out of it, the liquid carbon dioxide is slowly converted into gaseous carbon dioxide until a new point of equilibrium is reached, and the rate of conversion of CO2 from gas to liquid is equal to the rate of conversion of CO2 from liquid to the gaseous phase.
  • The single-phase system in which acetic acid's undergoes dissociation, leading to an acid-base equilibrium. This state of dynamic equilibrium can be described by the following reaction. CH3COOH ⇌ CH3COO– + H+
  • In the gaseous phase can be observed in the dimerization of nitrogen dioxide. Reaction: 2NO2 ⇌ N2O4
  • Henry's law is applicable in the first example of dynamic equilibrium provided above, wherein the equilibrium concentration of carbon dioxide in the liquid phase is proportional to the partial pressure of the CO2 gas in the bottle.
  • Industrial synthesis of ammonia via Haber’s process. Reaction: N2 (g) + 3H2 (g) ⇌ 2NH3 (g).


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