Question:

Buffer solutions have constant acidity and alkalinity because

Updated On: May 5, 2024

these give unionised acid or base on reaction with added acid or alkali
acids and alkalies in these solutions are shielded from attack by other ions
they have large excess of $ H^+ $ or $ OH^- $ ions
they have fixed value of pH

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Approach Solution - 1

If small amount of an acid or alkali is added to a buffer solution, it converts them into unionised acid or base. Thus, its pH remains unaffected or in other words its acidity/alkalinity remains constant, e.g.
$\hspace15mm H_3O^+ + A^- \rightleftharpoons H_2O + HA $
$ \hspace 18 mm ^-OH + HA \rightarrow H_2O + A^- $
If acid is added, it reacts with $ A^- $ to form undissociated H A. Similarly, if base/alkali is added, $ OH^- $ combines with HA to give $ H_2O $ and $ A^- $ and thus, maintains the acidity/ alkalinity of buffer solution.

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Approach Solution -2

If small amount of an acid or alkali added to a buffer solution, it converts them into unionised acid or base. Thus, its pH remains unaffected or in other words its acidity/alkalinity remains constant. e.g.
H₃O++A−$\rightleftharpoons$H₂O+HA
.⁻OH+HA→H₂O+A⁻
If acid is added, it reacts with A⁻ to form undissociated H A. Similarly, if base/alkali is added, OH− combines with H A to give H₂O and A⁻ and thus, maintains the acidity/alkalinity of buffer solution.

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Concepts Used:

Equilibrium

An equilibrium represents a state in a process when the observable properties such as color, temperature, pressure, concentration etc do not show any change.

The word equilibrium means ‘balance’ which indicates that a chemical reaction represents a balance between the reactants and products taking part in the reaction. The equilibrium state is also noticed in certain physical processes such as the melting point of ice at 0℃, both ice and water are present at equilibrium.

In the case of physical processes such as the melting of solid, dissolution of salt in water etc., the equilibrium is called physical equilibrium while the equilibrium associated with chemical reaction is known as chemical equilibrium.

Equilibrium in Chemical changes

The chemical equilibrium in a reversible reaction is the state at which both forward and backward reactions occur at the same speed.

The stage of the reversible reaction at which the concentration of the reactants and products do not change with time is called the equilibrium state.

Types of Chemical Equilibrium

There are two types of chemical equilibrium:

Homogeneous Equilibrium
Heterogeneous Equilibrium

Homogenous Chemical Equilibrium

In this type, the reactants and the products of chemical equilibrium are all in the same phase. Homogenous equilibrium can be further divided into two types: Reactions in which the number of molecules of the products is equal to the number of molecules of the reactants. For example,

H₂ (g) + I₂ (g) ⇌ 2HI (g)
N₂ (g) + O₂ (g) ⇌ 2NO (g)

Reactions in which the number of molecules of the products is not equal to the total number of reactant molecules. For example,

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)
COCl₂ (g) ⇌ CO (g) + Cl₂ (g)

Heterogeneous Chemical Equilibrium

In this type, the reactants and the products of chemical equilibrium are present in different phases. A few examples of heterogeneous equilibrium are listed below.

CO₂ (g) + C (s) ⇌ 2CO (g)
CaCO₃ (s) ⇌ CaO (s) + CO₂ (g)

Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.

Check Out: Equilibrium Important Questions