ACID, BASES AND SALTS I

Introduction – Acid, Bases and Salts I

We have so far studied matter in terms of constituents they are made of, types of forces binding them
together, how they behave under heat and pressure, and the physical states they exist naturally.
In this chapter, we shall continue our discussion on matter with respect to taste, touch, reaction with
water, effect on other substances, etc. Chemists have been able to classify some substances under three
distinct categories.
These are Acid, Bases and Salts.

Acid
Acids have long been associated with the sour taste of some fruits such as lime and lemon. However, the
definition of an acid has now been modified to express and reflect the behaviour of acid in solution.
An Acid is defined as a substance which when dissolved in water produces hydrogen or hydroxonium
ion as the only positive ion.
(i) HCl(aq) → H+(aq)) + OH–(aq)
(ii) HCl(aq) → H3O+ (aq) + OH–(aq)
Definitions
Arrhenius definition of an Acid:
An Acid is a substance that dissociates in water to produce hydrogen ions (H+) or protons.
HCl(aq) → H+(aq) + OH–(aq)
Brønsted–Lowry Concept of Acids:
An acid is a proton donor e.g
HCl + NH3 → NH4+ + Cl–
HCl donates its proton to form ammonium ion
Lewis Concept of Acid:
An acid is an electron-pair acceptor.
Lewis acids do not have hydrogen atoms in their molecules but have empty valence orbital on their
central atoms e.g. BF3, AlCl3, FeCl3 etc.
AlCl3 + Cl– → AlCl4–
In the above reaction, the empty valence orbital on the aluminium atom in AlCl3 accepts a pair of
electrons from Cl–.
Types of Acid:
Two types of Acids are:
1. Organic Acid.
2. Inorganic Acid or Mineral Acids.

1. Organic Acid:
Organic Acids occur naturally in plants and animals. Most of the fruits we eat are acidic, for example,
ethanoic acid is found in lemon and orange juice. The most common organic acids are the carboxylic
acids, whose acidity is associated with their carboxyl group −COOH.

Citric and malic acids are the primary organic acids found in fruit juices.
A list of common organic acids and their sources are in the table below.
2. Inorganic Acids:
Inorganic Acidsare called mineral acids since they can be prepared from mineral ores. e.g Hydrochloric
acid (HCl)
Table: List of Common Organic Acids
Organic Acid Source
Ethanoic acid Vinegar
Lactic acid Milk
Tartaric acid Grape
Citric acid Lime, Lemon
Ascorbic acid (vitamin C) Orange
Palmitic acid Palm oil
Amino acid Protein
Fatty acid Fats and Oils
Methanoic acid Bees and ants’ stings
Malic acid Grapes, Watermelons,
Cherries

Table: List of Organic Acids

The most common inorganic acids are;
Inorganic Acid Formula
Hydrochloric Acid HCl
Tetraoxosulphate (VI) Acid H2SO4
Trioxonitrate (V) Acid HNO3
Strengths of Acids (Strong & Weak Acids)
Acids are classified into strong and weak acids.
1. Strong Acids:
A strong acid ionizes or dissociates completely in water to give hydrogen ions and anions. The
concentration of hydrogen ions is very high in such acid solutions.

Examples of Strong Acids:

2. Weak Acids:
A weak acid ionizes partially in water to give hydrogen ions and anions.
Examples of Weak Acids:

When a large amount of water is added to a small amount of acid, the resulting acid solution is diluted.
But when little water is added to a large amount of acid, the resulting acid solution is concentrated.
A solution made by adding 8 moles of a pure acid to 1500 cm3 (or 1.5 dm3) of water is more
concentrated than another made by adding 2 moles of the same acid to the same amount of water.
Basicity of an Acid
Basicity of an acid is the number of replaceable hydrogen ions (H+) in one molecule of the acid.
Basicity of some Common Acids:
Acid Basicity
HCl ⇋ H+ + Cl– 1 or monobasic
H2SO4 ⇋ 2H+ + SO42- 2 or dibasic
H3PO4 ⇋ 3H+ + PO43- 3 or tribasic
CH3COOH ⇋ H+ + CH3COO– 1 or monobasic
In organic acids like Ethanoic (CH3COOH), it is only the hydrogen atom in the carboxylic group (COOH)
that is replaced. Hence, ethanoic Acid CH3COOH and methanoic acid, HCOOH that have four and two
hydrogen atoms per molecule respectively, are both monobasic.
Methanoic Acid:
HCOOH ⇋H+ + HCOO–
Basicity = 1 i.e monobasic.

Physical & Chemical Properties of Acids

Physical Properties of Acid:
1. Acids have a sour taste e.g Lemon, Grape fruits and sour milk.
2. Acids are electrolytes i.e their aqueous solution conducts electricity.
3. Concentrated forms of acid are corrosive e.g trioxonitrate (V) acid, hydrochloric acid and
tetraoxosulphate (VI) acid.

Caution
Always add acid to water. Not water to acid, because the hot acid solution may splatter onto your body
and face and may cause serious burns.

Chemical Properties of Acid:

1. Their aqueous solution turns blue litmus paper to red due to the formation of hydroxonium H3O+ in
hot water
HCl + H2O(l) → H3O(aq)+ + Cl–(aq)
In chemistry, hydronium is the common name for the aqueous cation H₃O⁺, the type of oxonium ion
produced by protonation of water.
2. Reaction with Metals: Metals which are higher than hydrogen in the activity series (e.g Potassium,
Calcium, Iron, Zinc, Magnesium etc.) react with acid to liberate hydrogen gas.
Examples:
(i) 2HCl(aq) + Ca(s) → CaCl2(aq) + H2(g)
(ii) H2SO4(aq) + Zn(s) → ZnSO4(aq) + H2(g)
(iii) H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g)

In the above reaction, hydrogen ion in the acid has been displaced by metal.
Trioxonitrate (V) acid HNO3 will not react with acid because it is a strong oxidising agent.

3. Reaction with Bases: Acid reacts with bases and alkalis to form salt and water only. This is
a neutralization reaction.
Acid + Base → Salt + H2O(l)
Examples:

4. Reaction with Trioxocarbonate (IV)

Acids react with trioxocarbonate (IV) salt and hydrogen trioxocarbonate (IV) salt to liberate carbon (IV)
oxide.
Examples:
(i) H2SO4(aq) + ZnCO3(g) → ZnSO4(aq) + H2O(l) + CO2(g)
(ii) HCl(aq) + NaHCO3(g) → NaCl(aq) + H2O(l) + CO2(g)
Preparation of Acids
In this topic, we will outline the general methods for the preparation of acids.
1. Action of an Acid Anhydride on Water:
An acid anhydride is an oxide of a non-metal that dissolves in water to produce acid.
Example: CO2, SO2, SO3, NO2 etc
(i) CO2(g) + H2O(l) → H2CO3(aq) (Trioxocarbonate (IV) acid)
(ii) SO2(g) + H2O(l) → H2SO3(aq) (Trioxosulphate (IV) acid)
(iii) SO3(g) + H2O(l) → H2SO4(aq) (Tetraoxosulphate(VI) acid)

Nitrogen (IV) oxide (NO2) is referred to as mixed anhydride because when NO2 dissolves in water, it
forms two acids HNO3 and HNO2.

2. Combination of Constituent Elements

This method is suitable for the preparation of hydrochloric acid and other hydrogen halides.
(i) By bubbling hydrogen gas over hot chlorine in the presence of activated charcoal as a catalyst, yields
hydrogen chloride gas.

(ii) By heating hydrogen gas and bromine vapour in the presence of a platinum catalyst, produces
hydrogen bromide.

The gas dissolves readily in water to produce hydrobromic acid

3. Displacement Reaction
A stronger or less volatile acid displaces a weaker or more volatile acid from its salt. The order of
decreasing strengths of acids are as follows
Concentrated H2SO4 is a non-volatile acid, it will displace other more volatile acids like
trioxonitrate(V)acid, hydrochloric acid, etc, from their salts.
(i) H2SO4(aq) + NaNO3(aq) → Na2SO4(aq) + HNO3(aq)
(ii) H2SO4(aq) + 2KCl(aq) → K2SO4(aq) + 2HCl(aq)
Reaction (i) and (ii) above show that H2SO4(aq) is less volatile acid and a stronger acid than HNO3(aq) and
HCl(aq)
(iii) On addition of dilute hydrochloric acid to crystals of Iron(II) sulphide, hydrogen sulphide gas is
liberated.
FeS(s) + 2HCl(aq) → FeCl(aq) + H2S(g)
(iv) Dilute hydrochloric acid which is a strong acid displaces the weak trioxocarbonate (IV) acid from
trioxocarbonate(IV)
On addition of HCl to CaCO3, there is effervescence, the colourless, odourless gas produced is carbon
(IV) oxide.
2HCl(aq) + CaCO3(s) → CaCl2 + H2O + CO2(g)
A solution of carbon (IV) oxide in water gives a weak acid trioxocarbonate (IV) acid
H2O(l) + CO2(g) → H2CO3(aq)
Reactions (iii) and (iv) above show that HCl(aq) is less volatile and a stronger acid than H2S(aq) and
H2CO3(aq).

4. Precipitation Reaction:
This method is used when preparing ethanoic acid from its salt. If hydrogen sulphide gas for example is
bubbled into a solution of Lead(II)Oxide ethanoate, black Lead (II) sulphide will be precipitated while
ethanoic acid solution is formed.
(CH3COO)2F(aq) + H2S(g) → PbS(s) + CH3COOH(aq)
General Uses of Acids
1. In the production of salts.
2. In the dissolution of metals.
3. As electrolytes.
4. As Redox reagents.

Uses of some Common Acids

1. (i) Tetraoxosulphate (VI) acid is used as a heavy chemical in chemical industries.
(ii) Used in detergents as well as fertilizer industries
(iii) Used as electrolyte in Lead-acid accumulators
(iv) It is also used as a drying agent

2. Trioxonitrate (V) acid is used in making fertilizers and also in the production of explosives.
3. Hydrochloric acid is used by industries to make chemicals.
4. Ethanoic acid (acetic acid) used in the cosmetic industry i.e making dyes pigment, the printing of
fabrics, etc.
5. Fatty acid: Used in the manufacture of soap. The process is known as saponification.
6. Boric acid is used as a mild antiseptic or germicide. It is also used in the production of baking powder.
7. Citric acid used in making fruit juice.
8. Tartaric acid used in making baking soda, soft drinks and health salts.