Questions of ionic equilibrium- Why does the self-ionisation of water increases on dissolution of a salt and decreases with the dissolution of an acid or base?

1.       What are protophilic solvents? Give two examples of such solvents?

These are the solvents which have greater tendency to accept a proton. Example are water methyl alcohol, liquid ammonia etc.

2.       What are protogenic solvents? Give two examples.

These are the solvents which have the tendency to furnish the protons. Such solvents are glacial acetic acid, liquid HCl, water etc.

3.       Can you tell the influence of solvent on the acid strength?

The strength of the acid depends upon the substances acting as a base. A strong acid in water solution may become weak in acetic acid. The reason is that acetic acid has fewer tendencies to accept a proton as compared to water. Taking acetic acid as the solvent, the relative strengths of the acids has the order:

HCLO₄ > HBr > H₂SO₄ > HCL > HNO₃.

In water, the above acids are equally strong and thus, their relative strength can be compared only in acetic acid.

4.       What are hard acids? Give two examples.

Hard acids are the ions of alkali and alkaline earth metals and also light transition metals with higher oxidation states.

Examples are Na⁺, Mg²⁺, Fe³⁺, H⁺ ions are hard acids.

5.       What are hard bases?  Give two examples.

These are the ligands which form stable complexes with hard acids. Examples are NH₃, OH^-, H₂O etc.

Comment on the statement that the conjugate base of a strong acid is a weak base and vice versa.

According to Bronsted- Lowry concept, a strong acid has great tendency to donate a proton whereas a strong base has a great tendency to accept a proton.

HCL + H₂O D H₂O+ + Cl^-

The equilibrium, lies mostly towards right hand side. It follows, therefore, that Cl^- -ion must have a little tendency to accept a proton. Hence, Cl^- ion is a weak base. On the other hand acetic acid is a weak acid because the equilibrium,

CH₃COOH  D  CH₃COO^- + H⁺

Lies mostly towards left. It follows, therefore, that CH3COO- ion must have a strong tendency to accept a proton. Hence acetate ion (conjugate base of CH3COOH) is a strong base. Thus the statement that conjugate base of a strong acid is a weak base and vice-versa is true.

6.       What are amphiprotic substances?

Substances which act as acids as well as bases are called amphiprotic substances. Examples of some amphiprotic substances are H2O, HCO3-, HSO4-, etc.

7.       Why does the self-ionisation of water increases on dissolution of a salt and decreases with the dissolution of an acid or base?

Pure water is very electrolyte. It is very slightly dissociated into hydrogen and hydroxyl ions.

H₂O(l)D H⁺ (aq) +OH^-(aq) or  2H₂O(l) DH₃O+ + OH-

On adding a salt into water, the cation or anion of the salt combines with OH-or H+ ions produced by the dissociation of water. This leads to the fall in concentration of OH- or H+ ions. According to Le-chatelier’s principle, it results in self-ionisation of water. On the other hand, addition of an acid or base results in increase of H+ or OH- ion concentration which results in decrease in self-ionisation of water.

8.       Can we have solutions having pH more than 14 or less than zero?

We know that pH =-log[H+] so, we can have any value depending  upon the concentration of H+ ions in solution e.g., assuming complete dissociation, the H+ ion concentration of 2M  HCL will be 2M and pH= -0.3010. Similarly, assuming complete dissociation, pH of 2M NaOH should be 14.3010. However, in fairly concentrated solutions the electrolytes are not completely dissociated and pH of solutions varies from zero to 14.

9.       What do you understand by buffer solution? Explain how the pH of a buffer solution does not change on the addition of a drop or two of strong acid or base.

Buffer solution, a buffer solution is defined as solution whose pH value does not change appreciably upon addition of small amounts of acids, bases and water from outside.

(a)    Acidic buffer: An acidic buffer is a mixture of a weak acid and its salt with a strong base. E.g., a mixture containing one mole acetic acid and one mole sodium acetate forms an acidic buffer.

(b)   Basic Buffer:  A basic buffer is a mixture of a weak base and its salt with strong acid. e.g., a mixture containing one mole of ammonium hydroxide and one mole of ammonium chloride forms a basic buffer.

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