Chemical equilibrium is a state when the reactants, and the products, will undergo no further change. This means that: 

Rate of forwarding reaction = Rate of Backward Reaction

For instance,

Equilibrium Mixture: The reaction or mixture of both the reactants and products involved in a reaction that achieves equilibrium later is known as equilibrium mixture. 

The various types of reactions based on the extent of equilibrium in them are:

• When the concentration of products and reactants are present in a comparable concentration in an equilibrium reaction.
• When a really small amount of products are left behind. This means that the reactants undergo less change.
• When the reactants change completely, and no residue of the same is left behind.

Equilibrium in Physical Processes

These reactions are concerned with the ones wherein the reactants and reactants undergo physical changes.

For instance, solid-liquid

liquid \( \rightleftharpoons \) gas

solid \( \rightleftharpoons \) gas

1. Solid-Liquid Equilibrium 

At a given temperature and pressure, it is said that ice and water exist in a state of equilibrium. This can be obtained when in the reaction: 

• Both the opposing processes take place simultaneously.
• The process takes place at the same rate. This means that the amount remains constant for both

2. Liquid-Vapour Equilibrium 

In this, we see that: 

Rate of evaporation = Rate of condensation

H2O(l) \( \rightleftharpoons \) H2O(vap)

This implies that the water molecules present in the gaseous state increase continuously into the liquid state until the state of equilibrium is reached. 

The evaporation may depend on; 

• Nature of liquid
• Amount of liquid
• Temperature Applied

3. Solid-Vapour Equilibrium 

This occurs when we see a solid change into a vapour state. 

For instance, 

I2(solid)\( \rightleftharpoons \) I2(vapour)

Camphor(solid) \( \rightleftharpoons \) Camphor(vapour)

NH4Cl(solid) \( \rightleftharpoons \) NH4Cl(vapour)

Characteristics of equilibrium involving physical processes: 

• Equilibrium can occur in closed systems only at a given temperature.
• All the measurable components are always constant in a system.
• It is characterized by a constant value of one of the components. 
• If we observe that the opposing processes occur at the same rate, the system is said to be dynamic.

Equilibrium in Chemical Processes - Dynamic Equilibrium

A chemical reaction is dynamic when,

The rate of forwarding reaction = The rate of backward reaction

When two reactants combine to yield two interchangeable products, it is a reversible reaction.

Consider this, 

A+B\( \rightleftharpoons \)C+D

As time passes, there is a decrease in reactants A and B and an increase in the concentration of C and D. This means, the rate of forward reaction decreases and that of backward reaction increases.

When the products can not change back to the reactants, it is known as an irreversible reaction.

For example, A+B\( \rightarrow \)C+D

It is a dynamic and stable reaction.

This can be explained by Harber's process, which is used in the production of ammonia.

In this reaction, the concentration of N2, H2, and NH3 remains constant. The reaction is carried out till it attains equilibrium.

 

• The product and reactant's concentration attains consistency during equilibrium.
• Rate of forwarding reaction = Rate of backward reaction.
• No products escape or separate throughout the reaction. 

Law of Chemical Equilibrium and Equilibrium Constant 

This law states that:

"At a given temperature that is constant, the rate of a chemical reaction is directly proport ional to the product of the molar concentrations of the reactants each raised to a power equal to the corresponding stoichiometric coefficients."

The equilibrium constant can be denoted as: 

aA+bB \( \rightleftharpoons \) cC+dD

Kc=[C]^c[D]² / [A]^a[B]^b

Herein, A, B, C, D are the concentration of the products and reactants in equilibrium.

• It is always constant depending upon the temperature. It is independent of concentration.
• It is inverted in its value when we see a reversible reaction.
• When the reaction is divided by 2, the constant value changes to square root (√K).
• When the reaction is multiplied by 2, the value changes to K2. 
• If we add a catalyst to the reaction, the value of the constant remains unchanged.

Homogeneous Equilibria 

When all the reactants and products of equilibrium are in the same phase, it is termed homogeneous equilibria. 

Example, 

N2(g)+3H2(g)\( \rightleftharpoons \)2NH3(g)

We see that all the reactants and products in the above reaction are in a gaseous state.

Heterogeneous Equilibria 

In this type of reaction, the products and reactants may have more than one type of phase.

The most common example is changing water into vapour in a closed container.

For example, H2O(l) \( \rightleftharpoons \) H2O(vap)

Applications of Equilibrium Constant

1. Predicting the extent of reactions.
   The equilibrium constant's magnitude gives a brief idea about the relative amount of the reactants and products.
   
   
2. Predicting the direction of the reaction.
   The concentration quotient is represented by:
   
   
   
   If Q equals K_c, then the reaction is at equilibrium.
   If Q>K_c, the reaction proceeds in the backward direction.
   If Q<K_c, the reaction proceeds in the forwarding direction.
   
   
3. Calculating Equilibrium Concentration
   If you want to determine the equilibrium concentration, then:
   Step 1: Write the balanced reaction.
   Step 2: Now, list the: initial concentration, change in concentration for equilibrium, and equilibrium concentration.
   Step 3. Fill the values for equilibrium concentration into the equation and solve for x.
   Step 4: Calculate the equilibrium concentration from it.
   Step 5: Now, substitute into the equilibrium equation and check.
   

When we calculate Gibbs Energy, we can find the spontaneity of the reaction.

Factors Affecting Equilibria

1. Le Chatelier's Principle 

If any change occurs in terms of temperature, concentration, or pressure in equilibrium, it shifts in a way that the effect is counteracted. 

2. Effect of change of concentration

When the concentration in an equilibrium of the products or reactants changes, the composition also changes.

3. Effect of Pressure Change

When there's a change in the number of moles of a gaseous product, a change in the pressure can affect the product's yield.

4. Effect of Catalyst

A catalyst does not influence the equilibrium reaction.

5. Effect of Temperature Change 

If the temperature increases or decreases in the reaction, the equilibrium starts shifting in the opposite direction to normalise the change.

Ionic Equilibrium in Solution 

This is mainly defined as the equilibrium that can form between ions and unionised substances. 

For example,

Electrolytes are substances that conduct electricity in the water. 

• Strong Electrolytes: They are ionised completely in water. 
• Weak Electrolytes: They do not ionise in water. If they do, it is partial. 

Acids, Bases, and Salts 

Acids: In simple terms, acids turn blue litmus paper to red.
Bases: They turn red litmus paper to blue. 

According to Arrhenius, acids dissociate in water to give out H+ ions. 

However, bases dissociate into OH- ions when they are in the water.

According to him, acids are those substances that can donate an H+ ion, and bases can accept the Hydrogen ion.

According to him, a base can donate an electron pair. Whereas an acid can accept an electron pair. 

The ionisation of Acid and Bases 

The strength of a particular acid or base is determined by how well it can ionize itself in an aqueous solution.

pH Scale:

It is the log of the reciprocal of Hydrogen ion concentration.

Buffer Solutions

The solutions that show no change in their pH when mixed with acid or alkali are known as buffer solutions.

• Acidic Buffer: A weak acid and its salt with a strong base is known as an acidic buffer.
• Basic Buffer: A weak base and its salt with a strong acid is called a basic buffer.

Solubility Equilibria of Sparingly Soluble Salts 

Each salt has a different solubility depending on the temperature. They are classified as:

• Soluble Salts: Solubility>0.1M
• Slightly Soluble Salts: 0.01M<solubility<0.1M
• Sparingly Soluble Salts: Solubility<0.01M

1. What is equilibrium according to NCERT?
   NCERT defines chemical equilibrium as a state when the rates of forwarding and backward reactions become equal. The concentration of the reactants and products remain constant too.
   
   
2. What are the different types of equilibrium Class 11?
   They are of 3 types: stable, unstable, and neutral.
   
   
3. What is the use of equilibrium?
   It can help in determining the direction of the reaction, the extent of a reaction, and so on. 

PS. The plagiarism was not found in the corresponding website upon re-checking. Attaching screenshot for confirmation.