Equilibrium: Class 11 Chemistry NCERT Chapter 7

Key Features of NCERT Material for Class 11 Chemistry Chapter 7  – Equilibrium

In the last chapter 6, you learned about Thermodynamics. In this chapter: Equilibrium, you will learn about, why does everything taste so unexpected? Why are lemon tart and mango sweet? This is a result of various levels of acids, bases, and salts in their chemical synthesis. We figure out how these can be portrayed, understanding the ideas by Arrhenius, Bronsted-Lowry, and Lewis, and so forth. 

Test Definitions 

Prior, acids, bases, and salts were described by the trial testing of their fluid arrangements. An acid is characterized as a substance whose water arrangement tastes sharp, turns blue litmus red, and kills bases. A substance is called base if its fluid arrangement feels severe, turns red litmus blue, or kills acids. 

Quick Revision notes :

Chemical Equilibrium 

In a chemical reaction, chemical equilibrium is characterized as the state at which there is no further change in the convergence of reactants and items. 

For instance,

At equilibrium, the pace of forward reaction is equivalent to the pace of inverse reaction. Equilibrium mixtures.: The blend of reactants and items in the equilibrium state is called an equilibrium mixture. 

They may be classified into three categories:

(i) The reactions which proceed almost to completion while the concentrations of the left reactants are practically negligible.

(ii) The reactions in which almost every reactant remains unaltered

(iii) The reactions in which the concentrations of both the reactants and products are comparable when the system is in equilibrium.

  • Equilibrium in Physical Processes

Rate of liquefying of ice = Rate of freezing of water. 

The framework here is in unique equilibriums, and the following can be derived. 

(a) Both the restricting procedures happen at the same time 

(b) Both the procedures happen at a similar rate with the goal that the measure of ice and water – stays consistent. 

(ii) Liquid-Vapor Equilibrium 

The equilibrium can be spoken to as 

Rate of evaporation = Rate of condensation 

When there is an equilibrium among fluid and fumes, it is called fluid fume equilibrium. 

Rate of liquefying of ice = Rate of freezing of water. 

The framework here is in unique equilibriums, and the following can be derived. 

(a) Both the restricting procedures happen at the same time 

(b) Both the procedures happen at a similar rate with the goal that the measure of ice and water – stays consistent. 

(ii) Liquid-Vapor Equilibrium 

The equilibrium can be spoken to as 

Rate of evaporation = Rate of condensation 

When there is an equilibrium among fluid and fumes, it is called fluid fume equilibrium. 

  • Equilibrium involving Dissolution of Solid in Liquid


  At the point when a restricted measure of salt or sugar or any solute dissolves in a given measure of water solution is framed. 

At a given temperature state is arrived at when no more solute can be dissolved, then the solution is called a saturated solution. 

The equilibrium betwixt a solid and its solution is demonstrated by the saturated solution and might be represented as 

Here dissolution and precipitation take place at the same speed. 

On including a small measure of radioactive sugar to the saturated solution, it will be discovered that the sugar present in the solution and the solid-state is radioactive. 

  • Equilibrium between a Gas and its Solution in Liquid
  • Equilibrium between a Gas and its Solution in Liquid 

The accompanying model can see this kind of equilibrium: 

Let us consider a sealed soda water bottle in which C02 gas is dissolved under high tension. A state of equilibrium is achieved between CO2 present in the solution and vapors of the gas. 

Henry’s law: Henry’s law administers the solubility of a gas in a liquid at a specific temperature. It says that the mass of a gas dissolving in a given mass of a solvent at any temperature corresponds to the gas’s pressure over the surface of the solvent.

  • Characteristics of Equilibria Involving Physical Processes

(I) At a given temperature, the equilibrium can be achieved distinctly in closed systems. 

(ii) At the equilibrium, the measurable properties are constant. 

(iii) The equilibrium is dynamic as both the forward and reverse processes happen at the same pace. 

(iv) At equilibrium, when a constant temperature arrives, the concentrations of substances become constant.

(v) The estimation of equilibrium constant represents the degree to which the process proceeds before equilibrium is accomplished. 

  • Equilibrium in Chemical Processes 

Just as equilibria in physical systems, it can also be accomplished in a chemical process, including reversible chemical reactions conveyed in closed compartments. 

The dynamic idea of chemical equilibrium can be demonstrated in the synthesis of alkali by Haber’s process. Haber started his examination with the known amounts of N2 and H2 at high temperature and pressure. At normal intervals of time, he decided the measure of smelling salts present. He also discovered grouping of unreacted N2 and H2. 

After a specific time, he found that the blend’s composition remains the same even though some reactants are still present. This constancy indicates the achievement of equilibrium. When all is said in done, for a reversible reaction the chemical equilibria can be shown by 

After a specific time, the two reactions happen at the same rate, and the system reaches a state of equilibrium. This can be shown in the given figure

  • Equilibrium in Homogeneous System

When in a system including reversible reaction, reactants and products are in the same phase, at that point, the system is called a homogeneous system. 

For Example

After some time, it tends to be observed that equilibrium is shaped.

  • Law of Chemical Equilibrium

At a constant temperature, the rate at which a chemical reaction occurs is legitimately relative to the result of the reactants’ molar concentrations, each raised to a force equivalent to the corresponding stoichiometric coefficients as represented by the fair chemical condition. Let us consider the below example

  • Relationship between Gibbs vitality G, reaction Quotient Q, and Equilibrium constant K 

A scientific expression of the thermodynamic perspective on equilibrium can be described by prong condition. 

  • Factors Affecting Equilibria

Le Chatelier’s principle: Under equilibrium, If a system tends to an adjustment in temperature, pressure, or fixation, at that point, the equilibrium shifts in such a way as to diminish or to balance the impact of progress. 

Impact of Change of Concentration: When the grouping of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium changes so as to limit the impact. 

Impact of Pressure Change : There is no impact of pressure on the off chance that the number of moles of gaseous reactants and products is equivalent. 

When the all outnumber of moles of gaseous reactants, the complete number of moles of gaseous products is unique. 

On increasing pressure, the absolute number of moles per unit volume increases, thus the equilibrium will shift in heading in which the number of moles per unit volume will be less. 

On the off chance that all outnumber of moles of products are more than the absolute number of moles of reactants, low pressure will support forward reaction. 

On the off chance that the absolute number of moles of reactants are more than all outnumber of moles of products, high pressure is favorable to advance reaction. 


Impact of Inert Gas : If the volume’s value is kept constant, there is no impact on equilibrium after the expansion of inert gas. 

Reason: This is because the expansion of inert gas at constant volume does not change the incomplete pressure or the molar focus. 

The reaction remainder changes just if the additional gas is engaged with the reaction. 

Impact of Temperature Change: When the system’s temperature is either increased or decreased, the equilibrium shifts the opposite way to neutralize the effect of progress. In exothermic reaction low temperature favors forward reaction, e.g., 

however, low temperature slows down the response, and thus a catalyst is used. In the case of an endothermic reaction, the increase in temperature will shift the equilibrium toward the endothermic reaction. 

Impact of a Catalyst 

Catalyst has no impact on the equilibrium composition of a reaction blend. 

Reason: Since the catalyst increases the speed of both the forward and in reverse reactions to the same degree in a reversible reaction. 

  • Ionic Equilibrium in Solution 

Electrolytes: Substances which produces ions when dissolved in water are known as Electrolytes. 

Strong Electrolytes: Those electrolytes which in water are ionized almost are entirely called strong electrolytes. 

Powerless electrolyte: Those electrolytes which on dissolution in water incompletely dissociated are called frail electrolytes. 

Ionic Equilibrium: The equilibrium shaped among ions and unionized substance is called ionic equilibrium, e.g., 

Acids: Acids are the substances that turn blue litmus paper to red and liberate dihydrogen on responding with some metals. 

Bases: Base turns red, litmus paper blue. It is severe in taste. Basic Example: NaOH, Na2C03. 

  • Arrhenius Concept of Acids and Bases 

Acids: According to the Arrhenius hypothesis, acids are substances dissociated in water to give hydrogen ions H+(aq). 

Bases: Bases produce OH–(aq) after dissociation in water.

  • Limitations of the Arrhenius Concept

(I) According to the Arrhenius idea, an acid gives H+ ions in water; however, the H+ ions do not exist freely because of its extremely small size (~H-18 m radius) and intense electric field. 

(ii) It does not represent the basicity of substances like smelling salts, which does not possess a hydroxyl gathering. 

  • The Bronsted-Lowry Acids and Bases 

As indicated by Bronsted-Lowry, an acid is a substance that is capable of donating a hydrogen particle H+ and bases are substances capable of accepting a hydrogen particle H+. 

Acids donate protons and are known as proton donors, and bases are proton acceptors. The accompanying example can clarify this.

  • Acid and Base as Conjugate Pairs

The acid-base pair that differs just by one proton is called a conjugate acid-base pair. 

Let us consider the example of the HCL ionization. 

Here water acts as a base because it accepts the proton. 

Cl is a conjugate base of HCl and HCl is the conjugate acid of base CL. Similarly, H20 is a conjugate base of an acid H30+, and H30+ is the conjugate acid of base H2O. 

  • Lewis Acids and Bases 

As per Lewis, acid is a substance that accepts electron pairs, and the base is a substance that donates an electron pair. 

Electrons lacking species like AlCl3, BH3, H+, and so forth can go about as Lewis acids while species like H20, NH3, and so on. Can give a pair of electrons, can go nearly as Lewis bases. 

  • Ionization of Acids and Bases 

The strength of acid or base is resolved with the assistance of the degree of ionization in aqueous solution. 

pH Scale: Hydrogen-particle fixation is measured as the number of gram ions of hydrogen ions present per liter of solution. Since these concentrations are usually small, the focus is commonly expressed as the pH of the solution. pH is the logarithm of the corresponding hydrogen particle focus. 

  • Di and Polybasic Acids

Acids more than one ionizable proton per molecule are called Dibasic acids or polybasic acids or polyprotic acids. 

Regular examples are oxalic acid, sulphuric acid, phosphoric acid, and so on. 

Factors Affecting Acid Strength

At the point when the strength of the H-A bond decreases 

The vitality required to break the bond decreases; H-A becomes a stronger acid. 

As the size of An increases down the gathering, H-A bond strength decreases; thus, the acid strength increases. 

In a period, as the electronegativity of An increases, the strength of the acid increases.

  • Common Ion Effect

On the off chance that in an aqueous solution of a frail electrolyte, a strong electrolyte is included having a particle normal with the feeble electrolyte, at that point, the dissociation of the powerless electrolyte is decreased or suppressed. The impact by which the dissociation of weak electrolyte is suppressed is known as the common ion effect.

  • Hydrolysis of Salts and the pH of their Solutions 
  • Solubility Products

It is applicable to a sparingly soluble salt. There is equilibrium among ions and solid unionized substance. 

  • Equilibrium: It can be established for both physical and chemical processes. At the state of the equilibrium rate of forward and in reverse reactions are equivalent. 
  • Equilibrium constant: Kc is expressed as the grouping of products separated by reactants each term raised to the stoichiometric coefficients. 
  • Le Chatelier’s principle: It states that the adjustment in any factor such as temperature, pressure, fixation and so on., will cause the equilibrium to shift such a way to diminish the impact of the change. 
  • Electrolytes: Substances that lead to electricity in aqueous solutions are called electrolytes. 
  • Arrhenius Concept: According to Arrhenius, acids give heterogeneous while bases produce hydroxyl ions in their aqueous solution. 
  • Bronsted-Lowry idea: Bronsted-Lowry characterized acid as proton giver and a base as a proton acceptor. 
  • Conjugate base and Conjugate acid: Whenever a Bronsted-Lowry acid undergoes a reaction with a base, it produces its conjugate base and conjugate acid. 
  • Conjugate pairs of acid and base: Such pairs of base and acid differs just by one proton. 
  • Lewis acids: It defines an acid as an electron pair acceptor and a base as an electron pair giver. 
  • pH Scale: Hydronium particle focus in molarity is all the more advantageously expressed on a logarithmic scale known as the pH scale. The pH of unadulterated water is 7. 
  • Buffer solution: The solution whose pH does not change by expanding a small measure of strong acid or base. 

For example, CH3COOH + CH3COONa. 

  • Solubility ion: For a sparingly soluble salt, it is characterized as the molar centralization of the atoms raised to the force equivalent to the times every particle occurs in the condition for solubility. 


Q:Whether the accompanying ions or molecules can go about as Lewis acid or a Lewis base? 




A silver cation is Lewis acid. 

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