What is Equilibrium

A state in which the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant. Equilibrium is a dynamic process – the conversions of reactants to products and products to reactants are still going on, although there is no net change in the number of reactant and product molecules.

Take a reaction as follows:
\mathrm{pP + qQ \rightleftharpoons rR + sS}

Equilibrium Constant is given by the following equation:
 \mathrm{Kc\, =\, \frac{ [P]^{p}[Q]^{q}}{[R]^{r}[S]^{s}}}

Chemical equilibrium occurs when both the reactions(forward and backward) are occurring at equal rates.

Chemical equilibrium


The chemical equilibrium and its laws have various real-life applications as mentioned below:

  • Hemoglobin and oxygen exist in equilibrium in the blood by the reaction:
    Hb(aq) + 4O2 Hb(O2)4(aq).
    As a result, a person tends to feel light-headed at higher altitudes.

    Hemoglobin and oxygen exist in equilibrium in the blood by the reaction

Prepare Online for JEE Main/NEET

Crack JEE 2021 with JEE/NEET Online Preparation Program

Start Now


  • Nitrogen and other gases are dissolved in our blood, now if the diver comes up too fast from the deep ocean, suddenly there is an equilibrium shift takes place thus develops nitrogen bubbles in the blood which causes severe pain and death.




Some of the phase processes of equilibrium are as follows:

  • Solid-liquid equilibrium:
    For example, ice and water are equilibrium at only one temperature is called NORMAL MELTING POINT.

  • Liquid-gas equilibrium:
    For example, rate of evaporation = rate of condensation
    H2O(l) ? H2O(vap)

  • Solid-gas equilibrium:
    This process is also called as sublimation
    e.g. sublimation of camphor

Solid-liquid equilibrium:
Law of mass action: The equilibrium concentrations of reactants and products may vary, but the value Kc remains constant.


Characteristics of Kc

  1. The reaction equilibrium can be obtained from both directions.

  2. Kc is a function of temperature.

  3. Kc is independent of the initial concentration of reactants and products.

  4. If the Kc value is 

  • << 1, the equilibrium lies to the right and the reaction mixture contains mostly products.

  • 0.10<Kc<10, the mixture contains proportionate amount reactants and products.

  • >> 1, the equilibrium lies to the left and the reaction mixture contains mostly reactants.


Homogeneous Equilibria
When all the reactants and the products in the equilibrium are in the same phase, then this equilibrium is known as homogeneous equilibrium. For example:
\mathrm{N_{2}(g)\, +\, 3H_{2}(g)\rightleftharpoons 2NH_{3}(g)}
In this reaction, all these reactants are in the gaseous phase, thus it is homogeneous equilibrium

Heterogeneous Equilibria
When the reactants are in different phases, then this system of equilibrium is known as the heterogeneous equilibria. For example, 
\mathrm{Ca(OH)_{2}(s)\,\rightleftharpoons Ca^{2+}(aq)\, +\, 2OH^{-}(aq)}
In this reaction, all these reagents are in different phases, thus it is heterogeneous equilibrium.


Applications of Equilibrium Constant:

  • Predicts the extent of reaction, which gives the degree of the disappearance of reactants.

  • Predicts the direction of the reaction.

  • Calculating the equilibrium constant, which gives the relative amount of reactants and products.


Reaction Quotient (Q):
The reaction value obtained when we substitute reactant and product concentrations into the equilibrium expression.

  1. If Q>K the reaction shifts towards the reactants side.

  2. If Q<K the reaction shifts towards the products side.

  3. If Q+K no shift occurs (equilibrium achieved).


Factors affecting chemical equilibrium:

  • Lechatlier ’s Principle: It states that changes in temperature, pressure, volume or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.It can be used to select the optimum conditions to form a substance.
  • Changes in temperature:
    Endothermic(H>0): R+ Heat Products.
    Exothermic(h<0): R Products+ Heat
  • Effect of catalyst:
    The catalyst increases the rate of reaction for both forward and reverse reactions.The catalyst does not affect the equilibrium concentration of both reactants and products. So the value Kdoesn’t change.


Ionic Equilibrium in Solution

Ionic equilibrium is the study of equilibrium between the ions. All those substances that are converted into ions in solution are classified into categories i.e, strong electrolytes and weak electrolytes. Strong electrolytes are those that are completely dissociated into ions and conduct greater electricity, while the weak electrolytes are those that are partially ionized in solutions and conduct smaller electricity. Acids, bases, and salts are either the weak electrolytes or strong electrolytes. In the case of weak electrolytes, the equilibrium established between the ions and non-ionized molecules.


Ionization of Acids and Bases

As already mentioned, some acids and bases are strong in nature and some are weak in nature. When a strong acid dissociates in the aqueous medium then it forms its conjugate base, these conjugate bases are always weak in nature. Similarly, with strong bases, their conjugate acids in solutions are always weak. For weak acids and bases, their conjugates bases and acids, respectively are always strong in nature.

The acid-base dissociation equilibrium of an acid HA is given as below:

\mathrm{HA(aq)\, +\, H_{2}O\rightleftharpoons H_{3}O^{+}\, +\, A^{-}(aq)}
an acid-base conjugate acid conjugate base


Ionic Product of Water

In pure water, one water molecule donates a proton and acts as an acid and another water molecule accepts a proton and acts as a base. The equilibrium reaction goes as follows:

\mathrm{H_{2}O(l)\, +\,H_{2}O(l)\rightleftharpoons H_{3}O^{+}(aq)\, +\, OH^{-}(aq) }

The dissociation constant for this equilibrium reaction is given as follows:

\mathrm{K\, =\, [H_{3}O^{+}][OH^{-}]\, /\, [H_{2}O] }

\mathrm{[H_{2}O] } can be eliminated since it is liquid, thus the ionic product of water or Kw is given as:

\mathrm{K\, =\, [H_{3}O^{+}][OH^{-}]\,}

After calculations, the value of Kw is equal to = 1 x 10-14M2

Thus any aqueous solution is acidic, basic or neutral is determined by the conditions below:

  • For acidic: \mathrm{[H_{3}O^{+}]>[OH^{-}]\,}
  • For neutral: \mathrm{[H_{3}O^{+}]=[OH^{-}]\,}
  • For basic: \mathrm{[H_{3}O^{+}]<[OH^{-}]\,}


The pH Scale

The hydronium ion concentration of any solution is expressed in logarithmic scale, then it is known as pH scale. Thus, the pH of any solution is defined as the negative logarithm value of hydrogen ion concentration. Mathematically, it can be expressed as follows:

\mathrm{pH\, =\, -log\, a_{H^{+}}}

Now as we know, the pure water has hydrogen ion concentration = 10-7, therefore, pH of water = 7  
Thus we can conclude as follows:

  • Acidic solution has pH < 7
  • Basic solution has pH > 7
  • Neutral solution has pH = 7

pH Scale

From the above calculations, it has been found that:
\mathrm{pK_{w}\, =\,pH\, +\, pOH\, =\, 14}


The relation between Ka and Kb

Kand Kb are the strengths of acidic and basic solutions respectively. Now, for every net reaction, the equilibrium constant is equal to the product of the added reactions.
Thus, Ka x Kb = Kw
Therefore, pKa + pKb = pKw = 14


Buffer Solutions

These are the solutions that resist the change in pH if the solution is diluted or some small amounts of alkali or acid are added. For example, a mixture of acetic acid and sodium acetate is a buffer solution with pH equal to 4.75.


How to prepare for Equilibrium?

  • This chapter is a part of Physical chemistry. This chapter is one of the most important chapters of the complete chemistry syllabus. Its concepts, laws, numerical and graphs all are important both for the basic foundation of chemistry and for scoring good marks in the examination.

  • Before reading this chapter, first, you must have the basic knowledge of the mole concept.

  • In this chapter, all the equations and formulae are very simple and easy to remember.

  • Rest this chapter is very simple, just be regular and be consistent in your numerical practice. 


Prescribed Books for Equilibrium

First, you must finish the class XI NCERT textbook and solve each and every example and unsolved question given in it. Then for advanced level preparation like JEE and NEET, you must follow R.C. Mukherjee and O.P. Tandon. You must definitely solve the previous year papers. Meanwhile, in the preparation, you must continuously write the mock tests for the depth of knowledge. Our platform will help you to provide with the variety of questions for deeper knowledge with the help of videos, articles and mock tests.


Chemistry Chapter- wise Notes for Engineering and Medical Exams

Chapters No.

Chapters Name

Chapter 1

Some basic concepts in chemistry

Chapter 2

States of matter

Chapter 3

Atomic Structure

Chapter 4


Chapter 5

Chemical Thermodynamics

Chapter 7

Redox Reaction and Electrochemistry

Chapter 8

Chemical kinetics

Chapter 9

Surface Chemistry

Chapter 10

General Principle and processes of Isolation of metals

Chapter 11

Classification of Elements and Periodic table

Chapter 12 


Chapter 13

p- block Elements

Chapter 14 

s-block Elements (Alkali and Alkaline Earth Metals)

Chapter 15 

Chemical Bonding and Molecular Structure

Chapter 16


Chapter 17

Coordination Compounds

Chapter  18 

Environmental Chemistry

Chapter 19

Purification and Characterisation of Organic Compounds

Chapter 20

Some Basic Principles of Organic Chemistry

Chapter 21


Chapter 22

Organic Compounds containing Halogens

Chapter 23

Organic Compounds containing Oxygen

Chapter 24

Organic Compounds Containing Nitrogen

Chapter 25


Chapter 26


Chapter 27

Chemistry in Everyday Life

Chapter 28

Principles Related to Practical Chemistry

Topics from Equilibrium

  • Meaning of equilibrium ( JEE Main ) (10 concepts)
  • pressure, temperature, effect of catalyst ( JEE Main ) (2 concepts)
  • factors affecting equilibrium concentration ( JEE Main ) (11 concepts)
  • Equilibria involving physical processes: Solid -liquid, liquid - gas and solid - gas equilibria ( JEE Main ) (2 concepts)
  • Henrys law ( JEE Main ) (1 concepts)
  • Law of chemical equilibrium, ( JEE Main ) (6 concepts)
  • equilibrium constants (Kp and Kc) and their significance ( JEE Main ) (9 concepts)
  • hydrolysis of salts and pH of their solutions ( JEE Main ) (3 concepts)
  • significance of Gibbs free energy in chemical equilibrium ( JEE Main ) (4 concepts)
  • Le Chateliers principle ( JEE Main ) (1 concepts)
  • Ionic equilibrium ( JEE Main ) (1 concepts)
  • ionization of water, pH scale ( JEE Main ) (8 concepts)
  • acid - base equilibria (including multistage ionization) and ionization constants ( JEE Main ) (12 concepts)
  • various concepts of acids and bases (Arrhenius, Bronsted - Lowry and Lewis) and their ionization ( JEE Main ) (2 concepts)
  • IONIC EQUILIBRIUM ( JEE Main ) (1 concepts)
  • ACIDS, BASES AND SALTS ( JEE Main ) (1 concepts)
  • IONIZATION OF ACIDS AND BASES ( JEE Main ) (9 concepts)
  • BUFFER SOLUTIONS ( JEE Main ) (7 concepts)

Important Books for Equilibrium

  • Equilibrium Book
  • Equilibrium Book
  • Equilibrium Book