Chemical equilibrium is an important chapter as it includes many solid concepts to understand. Umair Khan Academy provides notes along with video lectures. For all notes visit Umair Khan Academy.

Notes including all Sort questions, are given below. Long questions can be answered by combining a few short questions.


Q1. Define chemical reaction. Give its types.

A process in which substances undergo a change in chemical composition
Example: Na + H2O → NaOH + H2
Types of Chemical Reactions:

  1. Irreversible chemical reaction:
    A chemical reaction which proceeds in one direction only
    Example: Na + H2O → NaOH + H2
  2. Reversible Chemical Reaction:
    A chemical reaction that proceeds in both directions i.e. forward & reverse
    Reversible reaction


Q2. Explain the state of Chemical Equilibrium.


A state during a reversible chemical reaction at which

  1. Rates of two opposing reactions become equal
  2. The concentration of reactants and products that become constant is called a chemical equilibrium.
state of Chemical Equilibrium
  • Chemical Equilibrium is a Dynamic Equilibrium:
    In which two opposing processes are proceeding at an equal rate
  • Characteristics of Chemical Equilibrium:
    At the state of equilibrium
  1. The rate of forward and backward reactions becomes equal.
  2. The concentration of reactants and products becomes constant.
  3. The reaction does not cease but proceeds in both directions at the same rate.
  4. This state is attained for the reversible reaction no matter in which direction the reaction is
  5. At equilibrium, both reactants and products are present in a definite ratio.


state of Chemical Equilibrium in HI

Q3. State the Law of Mass Action.

The rate of reaction is directly proportional to the product of active masses of reacting species.

Law of Mass Action

Q4. Write down the Units of Equilibrium Constant (Kc).

  1. Kc has no units when the number of moles of reactant & product are equal.
    The unit of Kc depends upon the number of moles of reactants and products.
    Example: Chemical Equilibrium in ester formation
  2. KC have certain units when no. of moles of reactants and products are not equal.
    Example: Kc has units when no of moles of reactant and products are different.

Q5. What are the units of Kc for a given reaction?

formation of NO2

According to the law of mass action.

Units of Kc

Q6. Write equilibrium constant expression for some important reactions.

1- Formation of an Ester: (Aqueous phase reaction)

formation of Ester

2- Dissociation of PCl5: (Gaseous phase reaction)

Kc for PCl5

Q7. Write down relationships between equilibrium constants (Kp And Kc).

Let us consider the following general reaction.
aA + bB ⇌ cC + dD
The equilibrium constant is called Kc

 relationships between equilibrium constants

The equilibrium constant is called Kp

 relationships between equilibrium constants

Relationship between Kp and Kc

 relationships between equilibrium constants

Δn = no. of moles of products – no. of moles of reactants

Q8. Write the applications of the equilibrium constant.

1- Direction of Reaction:

Kc for general reaction
  • a) If [Products]/[Reactants] < Kc then the reaction will proceed (forward)
  • b) If [Products]/[Reactants] > Kc then the reaction will move backwards (reverse)
  • c) If [Products]/[Reactants] = Kc then it means the reaction is already at equilibrium.

2- Extent of Reaction:

  • a) If the value Kc is very large: the reaction is almost complete
  • b) If the Kc value is small: the reaction does not proceed (moves forward) appreciably
  • c) If the value of Kc is very small this shows a very little forward reaction.

3- The Effect of Conditions on The Position of Equilibrium:

  • Kc is equilibrium constant, it remains constant for a particular reaction at a constant temperature
  • It can be varied if external conditions like temperature, pressure and concentrations are altered.


Q9. The Le-Chatelier’s Principle.

If stress is applied to a system at equilibrium then the system acts in such a way that it nullifies the effect of the stress as far as possible.
Effect of Change in Different Parameters:

Effect of Change in Different Parameters

Q10. Write down the applications of chemical equilibrium in industry.

1- Synthesis of Ammonia by Haber’s Process:

Ammonia formation and Kc

Maximum yield of ammonia may be obtained by

  1. continual removal of ammonia
  2. increase in pressure
  3. decrease in temperature

2- Preparation of Sulphur Trioxide:

SO3 formation and Kc

Maximum yield of sulphur Trioxide may be obtained by

  1. continual removal of SO3
  2. decreasing temperature
  3. increasing pressure


Q11. Write the equilibrium constant expression for the ionic product of water.

ionic product of water

Q12. Write Relation between Kw and Temperature:

By increasing the temperature Kw also increased. (Kw ∝ T)


Q13. Describe the ionization constant of acid (Ka)

 ionization constant of acid (Ka)

Ka for Acidic Strength:

Ka < 10-3 acid is weak
Ka = 1 to 10-3 acid is moderately strong,
Ka > 1 acid is strong

Percentage of Ionization of Acids:

% ionization


Q14. Describe ionization constant of bases (Kb)

ionization constant of bases (Kb)


Q15. Write Lowry Bronsted concept of acids and bases

  1. The species which release a proton or have a tendency to release a proton in water are called
  2. The species which accept a proton or have a tendency to accept proton in water are called
Lowry–Bronsted concept of acids and bases

Q16. Write relations of pKa, pKb, and pKw

pKa + pKb = pKw
As pKw at 25 oC is 14,
So, pKa + pKb = 14

Q17. Calculate the pH of 10-4 moles/dm3 of HCl.

Calculate the pH of 10-4 moles/dm3 of HCl


Q18. Define the Common Ion Effect.

The suppression of ionization of weak electrolytes by strong electrolytes having common ion

Common Ion Effect.


Q19. Write a brief note on Buffers.


The solution which resists the change in its pH when a small amount of an acid or base is added is called a Buffer solution.


How do Buffers Act?

Buffer action can be explained by keeping in view the concepts of the common ion effect and Le-Chatelier’s principle.

Henderson’s Equation for the preparation of Buffer Solution of Required pH Value:

(Calculation of pH of a Buffer)

Henderson’s Equation

Buffer Capacity

resistance offered by the buffer to change in its pH


Q20. Define Solubility Product. Give its applications.

The product of equilibrium concentrations of ions of sparingly soluble salt is called solubility product.

Solubility Product.

Applications of Solubility Product:
Following are the applications of solubility products.

  1. Determination of Ksp from solubility.
  2. Determination of solubility from Ksp
  3. Effect of common ion on solubility.

Ksp > Ionic Product ———— Unsaturated solution
Ksp = Ionic Product ———— Saturated solution
Ksp < Ionic Product ———— Super Saturated solution

Q21. What are buffer solutions? How acidic buffer can be prepared?

Ans: The solutions which show resistance to change in their pH when a small amount of an
acid or base is added are called Buffer solutions.
There are two types of Buffer solutions

  1. Acidic Buffers
  2. Basic Buffers
    Acidic buffer can be prepared by mixing weak acid and its salt with a strong base.
    Example:CH3COOH + CH3COONa

Q22. What is the effect of common ions on solubility? Give example

When HCl gas is passed through the saturated solution of NaCl. Sodium chloride is crystallized out due to the common ion effect because HCl is more soluble in water than NaCl, so it will suppress the solubility of NaCl in the solution.

Common ion effect


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