ICSE Class 10 Chemistry Chapter 03 Acids Bases and Salts

Acids, Bases And Salts

Syllabus

Study of Acids, Bases and Salts

(i) Simple definitions in terms of the molecules and their characteristic properties. Self-explanatory.

(ii) Ions present in mineral acids, alkalis and salts and their solutions: use of litmus and pH paper to test for acidity and alkalinity. Examples with equation for the ionisation/dissociation of ions of acids, bases and salts: acids form hydronium ions (only positive ions) which turn blue litmus red, alkalis form hydroxyl ions (only negative ions) with water which turns red litmus blue. Salts are formed by partial or complete replacement of the hydrogen ion of an acid by a metal, should be taught with suitable examples. Introduction to pH scale to test for acidity, neutrality and alkalinity by using: pH paper or Universal indicator.

(iii) Definition of salt; types of salts. Types of salts: normal salts, acid salt, basic salt, definition and examples.

(iv) General properties of salts:

Deliquescence, Efflorescence, water of crystallization. Definition and example of each of the above.

Decomposition of hydrogen carbonates, carbonates, chlorides and nitrates by appropriate acids with heating if necessary. (relevant laboratory work must be done). Action of dilute acids on carbonates, hydrogen carbonates and action of concentrated acid equations of formation of acid rain (Sulphates and nitrates, to obtain carbon dioxide, hydrogen chloride and nitric acid respectively should be taught. This will assist the students in their practical work.

(v) Preparation: laboratory preparation of salts (normal and acid salts) - relevant laboratory work is essential. (no apparatus details are required). Laboratory preparation of salts (normal and acid salts): Direct combination; decomposition; displacement; double decomposition; neutralization.

3A. Acids And Bases

Introduction

Lavoisier and Davy studied the terms acids, bases and salts on more scientific lines. Their findings say that

- all acids compulsorily have hydrogen as their constituent element in the form of proton (H+). Examples: Hydrochloric acid, sulphuric acid and nitric acid.

- all bases are either metallic oxide (O2-) or metallic hydroxide (OH-) or ammonia solution. Examples: Copper oxide, sodium hydroxide and ammonium hydroxide.

- all salts are chemical compounds. They are formed by the interaction of acid and base replacing hydrogen ions present in an acid, by metallic ion [or non-metallic ammonium ions (NH4+)].

Examples: Sodium chloride, calcium carbonate and ammonium phosphate.

Acids

The word 'acid' comes from the Latin word acidus, meaning 'sour'. In fact, the sour taste of some foods is due to the acids they contain.

Classification Of Acids

Acids can be classified in different ways.

1. Depending on their sources

(i) Organic acids: Acids which are obtained usually from plants are called organic acids. They contain carbon atom also along with hydrogen atom.

Examples: Oxalic acid (COOH)2, Acetic acid (CH3COOH).

They are weak acids and they do not ionise completely in solution. So they contain ions as well as molecules.

(ii) Inorganic acids (mineral acids): Acids which are obtained usually from minerals are known as inorganic acids. They do not contain carbon (except carbonic acid H2CO3).

Examples: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3).

They are strong acids. They ionise completely in solution producing a high concentration of hydronium ions [H3O]+.

Carbonic acid (H2CO3) is a weak mineral acid. It turns blue litmus pink. It is non-corrosive, and so used in soft drinks.

Acids which contain oxygen along with hydrogen and some other element are oxy-acids. Examples: Nitric acid (HNO3), sulphuric acid (H2SO4).

Hydracids contain hydrogen and a non-metallic element, and no oxygen. Examples: Hydrochloric acid (HCl), hydrobromic acid (HBr).

2. Depending on their concentration

Concentration of an acid means the amount of acid present in a definite amount of its aqueous solution.

Concentrated acid: An acid which contains a very small amount of water or no water is called concentrated acid.

Dilute acid: An acid which contains far more amount of water than its own mass is known as dilute acid.

In order to dilute an acid, pour acid into water in small amounts and stir constantly. 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 solution may occur, also the container may break which can be fatal to the person.

Note: Concentration of an acid simply tells us the amount of water present in the acid and not at all the strength of the acid. Strength of an acid is the measure of concentration of hydronium ions [H3O]+ it produces in its aqueous solution. Thus dil. HCl is stronger acid than highly concentrated acetic acid.

Note: The strength of an acid depends on the degree of ionisation (α) and concentration of hydronium ions [H3O]+ produced by that acid in aqueous solution.

1. Degree of ionisation (α)

No. of acid molecules ionised / Total no. of acid molecules present in aqueous solution × 100

2. If the degree of ionisation (α) for an acid, base or salt in aqueous solution is greater than 30%, it is strong and if it is less than 30%, it is weak.

3. Depending on their basicity

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

Therefore we have,

Examples: Hydrochloric acid (HCl),

(i) Monobasic acids: Acids which on ionisation in water produce one hydronium ion per molecule of the acid are known as monobasic acids.

For example: Hydrochloric acid

HCl + H2O - H3O+ + Cl-

Other similar examples are: HBr, HNO3, HI, CH3COOH, etc.

It must be emphasized here that the basicity of an acid depends not on the number of hydrogen atoms in one molecule of that acid, but on the number of ionisable hydrogen atoms that it has per molecule.

For example, acetic acid (CH3COOH or C2H4O2) contains four hydrogen atoms in its molecule, but it is a monobasic acid because its molecule ionises by liberating only one hydronium ion.

CH3COOH + H2O - H3O+ + CH3COO-

Monobasic acids ionise in one step and so form only one normal salt.

NaOH + HCl - NaCl + H2O

(ii) Dibasic acids: Acids which, on ionisation in water, produce two hydronium ions (H3O)+ per molecule of the acid, are known as dibasic acids.

Examples: Sulphuric acid (H2SO4)

Oxalic acid (H2C2O4)

Sulphurous acid (H2SO3)

Carbonic acid (H2CO3)

Dibasic acids ionise in two steps, as shown below:

(1) H2SO4 + H2O \(\rightleftharpoons\) H3O+ + HSO4-

HSO4- + H2O \(\rightleftharpoons\) H3O+ + SO42-

(2) H2CO3 + H2O \(\rightleftharpoons\) H3O+ + HCO3-

HCO3- + H2O \(\rightleftharpoons\) H3O+ + CO32-

Dibasic acids have two replaceable hydrogen ions, therefore they form one acid salt or one normal salt.

NaOH + H2SO4 - NaHSO4 + H2O

2NaOH + H2SO4 - Na2SO4 + 2H2O

(iii) Tribasic acids: Acids which, on ionisation in water, produce three hydronium ions per molecule of the acid, are called tribasic acids.

Examples: Phosphoric acid (H3PO4)

Citric acid (C6H8O7)

They ionise in three steps, as shown below:

(a) H3PO4 + H2O \(\rightleftharpoons\) H3O+ + H2PO4-

H2PO4- + H2O \(\rightleftharpoons\) H3O+ + HPO42-

HPO42- + H2O \(\rightleftharpoons\) H3O+ + PO43-

H3PO4 is a dibasic acid 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.

Tribasic acids can form two different acid salts and one normal salt, because they have three replaceable hydrogen ions.

NaOH + H3PO4 - NaH2PO4 + H2O (monosodium dihydrogen phosphate)

2NaOH + H3PO4 - Na2HPO4 + 2H2O (disodium hydrogen phosphate)

3NaOH + H3PO4 - Na3PO4 + 3H2O (trisodium phosphate)

Silicic acid H2SiO4 has basicity four.

Preparation Of Acids

(1) By synthesis

Binary acids (acids containing two elements) are prepared by this method.

(2) By the action of water on non-metallic or acidic oxides

Acidic oxides (also called acidic anhydrides) dissolve in water to give acids.

Note: (i) If the salt produced is insoluble, then the reaction does not proceed. So, we do not expect lead carbonate to react with hydrochloric acid or sulphuric acid. Nor does calcium carbonate react with sulphuric acid.

(ii) The hydrogen carbonates of sodium and potassium are the only common ones, which exist in the solid state.

(4) By displacement

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

Examples: Both hydrochloric and nitric acids are formed

Properties Of Acids

Physical properties

(1) Taste: Acids have a sour taste. Mineral acids like H2SO4 and HNO3 are highly corrosive in nature. Therefore, they should not be tasted.

(2) Physical state: Some acids are solids and some are liquids at room temperature.

For example:

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