Acids, Bases and Salts

Question 1

(a) What to you understand by the term acid?

(b) Name the positive ion formed when an acid is dissolved in water.

(c) Draw the structure of this ion.

Answer

(a) Acids are defined as compounds which contain one or more hydrogen atoms, and when dissolved in water, they produce hydronium ions (H₃O⁺) as the only positively charged ions.

(b) Hydronium ion (H₃O⁺)

(c) The structure of hydronium ion is shown below:

Question 2

Write the ionisation of sulphuric acid showing the formation of hydronium ion.

Answer

H₂SO₄ + H₂O ⇌ H₃O⁺ + HSO₄^–

HSO₄^– + H₂O ⇌ H₃O⁺ + SO₄^2-

Question 3

Water is never added to acid in order to dilute it. Why?

Answer

Water is not added to acid as it is an exothermic process and in this process so much heat is produced that splashing of acidic acid solution may occur, also the container may break which can be fatal to the person.

Question 4

Define the term ‘basicity’ of an acid. Give the basicity of: nitric acid, sulphuric acid and phosphoric acid.

Answer

The basicity of an acid is defined as the number of hydronium ions (H₃O⁺) that can be produced by the ionization of one molecule of that acid in aqueous solution.

The basicity of following compounds are:

Basicity of Nitric acid = 1

Basicity of Sulphuric acid = 2

Basicity of Phosphoric acid = 3

Question 5

Give two examples of each of the following :

(a) oxy-acid

(b) hydracid

(c) tribasic acid

(d) dibasic acid

Answer

(a) Oxyacids — Nitric acid [HNO₃] and Sulphuric acid [H₂SO₄]

(b) Hydracid — Hydrochloric acid [HCl] and Hydrobromic acid [HBr]

(c) Tribasic acid — Phosphoric acid [H₃PO₄] and Citric acid [C₆H₈O₇]

(d) Dibasic acid — Sulphuric acid [H₂SO₄] and Carbonic acid [H₂CO₃]

Question 6

Name the :

(a) acidic anhydride of the following acids:

1. sulphurous acid
2. nitric acid
3. phosphoric acid
4. carbonic acid

(b) acid present in vinegar, grapes and lemons.

Answer

(a) Below are the acidic anhydride of the given acids:

1. Sulphurous acid — SO₂
2. Nitric acid — N₂O₅
3. Phosphoric acid — P₂O₅
4. Carbonic acid — CO₂

(b) Acids present in following are:

1. Vinegar — Acetic acid
2. Grapes — Tartaric acid
3. Lemon — Citric acid

Question 7

What do you understand by the statement ‘acetic acid is a monobasic acid’?

Answer

Acetic acid is a monobasic acid because it’s molecule ionises by liberating only one hydronium ion.

CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO^–

Question 8

Give a balanced equation for :

(i) reaction of nitrogen dioxide with water

(ii) preparation of a non volatile acid from a volatile acid.

Answer

(i) 2NO₂ + H₂O ⟶ HNO₂ + HNO₃

(ii) S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

Question 9

What do you understand by the strength of an acid? On which factor does the strength of an acid depend.

Answer

The strength of an acid is the measure of concentration of hydronium ions [H₃O⁺] produced by that acid in aqueous solution.

Factors on which the strength of an acid depends are:

1. Degree of ionisation (α).
2. Concentration of hydronium ions [H₃O⁺] produced by that acid in aqueous solution.

Question 10

Explain the following:

(a) Carbonic acid gives an acid salt but hydrochloric acid does not.

(b) Dil. HCl acid is stronger than highly concentrated acetic acid.

(c) H₃PO₃ is not a tribasic acid.

(d) Lead carbonate does not react with dil. HCl

(e) Nitrogen dioxide is a double acid anhydride.

Answer

(a) Carbonic acid is a dibasic acid i.e., it has two replaceable hydrogen ions. Hence, it forms one acid salt or one normal salt. On the other hand, hydrochloric acid is a monobasic acid with one replaceable hydrogen ion. Hence, it forms only one normal salt.

(b) Concentration of an acid means the amount of water present in the acid and not at all the strength of an acid. Strength of an acid is the measure of concentration of hydronium ions it produces in its aqueous solution. Thus, dil. HCl is stronger acid than highly concentrated acetic acid.

(c) H₃PO₃ is not a tribasic acid but dibasic because in oxyacids of phosphorus, hydrogen atoms which are attached to oxygen atoms are replaceable. Hydrogen atoms directly bonded to phosphorus atoms are not replaceable.

(d) Generally, acids liberate carbon dioxide on reaction with metallic carbonates and bicarbonates. But if the salt produced is insoluble, then the reaction does not proceed. So, we do not expect lead carbonate to react with hydrochloric acid.

(e) Nitrogen dioxide is called double acid anhydride because two acids – nitrous acid and nitric acid – are formed when it reacts with water.

2NO₂ + H₂O ⟶ HNO₂ + HNO₃

Question 11

How is an acid prepared from a (a) Non-metal (b) salt ? Give an equation for each.

Answer

(a) A non-metal, e.g., sulphur or phosphorous is oxidised by conc. nitric acid to form to form sulphuric acid or phosphoric acid respectively.

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

P + 5HNO₃ ⟶ H₃PO₄ + H₂O + 5NO₂

(b) Normal salts of more volatile acids are displaced by a less or non-volatile acid.

E.g., Both hydrochloric acid and nitric acid are formed by using sulphuric acid [H₂SO₄].

NaCl + H₂SO₄ → Δ  Δ