Acids and Bases: Theories, Classification, Properties, Uses

Acids and Bases: In our daily lives, we taste various types of substances. Some are sour, some are bitter, while yet others are salty. Lemon juice, for example, is sour, soap solution is bitter, and saltwater is salty. One method of categorising these substances is to divide them into three groups: acids, bases, and salts.

These chemicals’ characteristics differ from one another. Acids, for example, have different characteristics than bases. Acids react with bases to form salts with characteristics that are fundamentally opposed to those of the interacting acids and bases.

Theories of Acids and Bases

There have been many attempts to define the difference between acids and bases. The three main theories in use today are:

1. Arrhenius concept of acids and bases
2. Brønsted–Lowry theory of acids and bases
3. Lewis theory

CLEAR YOUR CONCEPTUAL DOUBTS ON ACIDS & BASES

Arrhenius Concept of Acids and Bases

Arrhenius defined acids and bases as follows:
An acid is a substance that produces hydrogen ions when dissolved in water.
A base is a substance that produces hydroxyl ions when dissolved in water.

Examples for acids: \( {\text{HCl}},\, {{\text{H}}_2}{\text{S}}{{\text{O}}_4},\,{\text{HN}}{{\text{O}}_3}\)

\( {\text{HCl}} + {{\text{H}}_2}{\text{O}} \leftrightarrow {{\text{H}}_3}{{\text{O}}^ + } + {\text{C}}{{\text{l}}^ – }\)

Examples for bases: \( {\text{NaOH}},\, {\text{KOH}},\) etc.

\( {\text{NaOH + }}{{\text{H}}_2}{\text{O}} \leftrightarrow {\text{N}} {{\text{a}}^ + } + {\text{O}}{{\text{H}}^ – }\)

Limitations of Arrhenius Theory of Acids and Bases

1. It recognises the dissociation of acids and bases in an aqueous medium only.
2. It restricts acids to merely hydrogen-containing compounds and bases to merely hydroxide-containing compounds.

According to Arrhenius concept, carbon dioxide, sulfur dioxide, etc., are not regarded as acids and \( {\text{N}}{{\text{H}}_3},\,{\text{CaO}},\,{\text{MgO}},\) etc. are not regarded as bases.

Bronsted–Lowry’s Concept of Acids and Bases

Bronsted and Lowry proposed a concept of acids and bases, which are independent of solvent. According to this concept, acids and bases are defined as follows:

An acid is a species (a molecule, a cation (or) an anion) capable of donating one or more protons to any other substance called Bronsted-Lowry acid.

A base is a species (a molecule, a cation (or) an anion) capable of accepting one or more protons from an acid; such a base is called the Bronsted-Lowry base.
Therefore, an acid is a proton donor, and a base is a proton acceptor.

Examples for acids: \( {\text{HCl,}}\, {{\text{H}}_2}{\text{O}},\,{{\text{H}}_2}{\text{S}}{{\text{O}}_4},\,{\text{HN}}{{\text{O}}_3}\)

\( {{\text{H}}_2}{\text{O}} \to {{\text{H}}^ + } + {\text{O}}{{\text{H}}^ – }\)

Examples for bases: \( {\text{N}}{{\text{H}}_3},\,{\text{O}}{{\text{H}}^ – },\,{\text{C}}{{\text{N}}^ – }\)
\( {\text{N}}{{\text{H}}_3}{\text{ + }}{{\text{H}}^ + } \to {\text{N}}{{\text{H}}_4}^ + \)

Limitations of Bronsted-Lowry’s Concept

Bronsted-Lowry’s concept of acids and bases cannot explain the acidic nature of carbon dioxide, sulfur dioxide, and sulfur trioxide and the basic nature of calcium oxide, magnesium oxide, and barium oxide.

Conjugate Acids and Bases

A pair of a Bronsted acid and a base that differs by one proton is known as conjugate acid-base pair. A strong acid has a weak conjugate base, and a strong base has a weak conjugate acid.

Lewis Theory of Acids and Bases

In \(1923\), G.N. Lewis proposed a more general theory of acids and bases. According to this theory,

“A substance that can accept an electron pair to form a coordinate covalent bond with the donor” is called acid.

Examples: \({{\rm{H}}^ + },{\mkern 1mu} \,{\rm{B}}{{\rm{F}}_3},{\mkern 1mu} \,{\rm{SnC}}{{\rm{l}}_4},\) etc.

“A substance that can donate a lone pair of electrons to form a coordinate covalent bond with the acceptor” is called a base.

Examples: \({{\text{H}}_2}{\text{O}},\,{\text{N}}{{\text{H}}_3},\,{\text{CO}},\) etc.

Limitation of Lewis Theory

1. This theory cannot explain the strength of acids and bases.
2. Acids like hydrochloric acid, sulphuric acid react with bases such as sodium hydroxide or potassium hydroxide but do not form coordinate covalent bonds.
3. All the acid-base reactions do not involve coordinate covalent bond formation.

Classification of Acids and Bases

The classification of acids and bases are discussed below:

Classification of Acids

The term ‘acid’ has its origin in the Latin word acidus, which means “sour”. Arrhenius defined acids as the substances which dissociate in water to give hydrogen ions or hydronium ions. The chemical substances which have a sour taste are called acids. For example, lemon juice, tomato, vinegar, etc.,

Acids are broadly classified into two types: Organic acids and mineral acids.

1. Organic Acids: Acids found in plants and animals are called organic acids. Example: Citric acid, Lactic acid, Acetic acid, Oxalic acid, etc.
2. Mineral Acids: Acids prepared from the minerals of the earth are called mineral acids. They are human-made and are also called laboratory acids.
   Example: Sulphuric acid, Hydrochloric acid, Nitric acid, etc. Concentrated mineral acids are hazardous. Therefore, they should handle the acids with care. In the laboratory, acids are generally mixed with water to dilute them. Such acids are called dilute acids. These are less harmful to us. For example, carbonic acid is a mineral acid, but it is a weak acid.

Strong Acids and Weak Acids

Based on strength, acids are classified into strong acids and weak acids.

1. Acids that undergo ionization completely in an aqueous solution are called strong acids. For example, hydrochloric acid \(\left({{\text{HCl}}} \right)\), nitric acid \(\left({{\text{HN}}{{\text{O}}_3}} \right)\), and sulphuric acid \(\left( {{{\text{H}}_2}{\text{S}}{{\text{O}}_4}}\right)\).

2. Acids that undergo ionization to a small extent only are called weak acids. For example, carbonic acid \(\left({{{\text{H}}_2}{\text{C}}{{\text{O}}_3}} \right)\) and acetic acid \(\left({{\text{C}}{{\text{H}}_3}{\text{COOH}}} \right)\)

Based on their concentration, acids are classified into concentrated and dilute acids.

Concentrated Acids

Those acids with the maximum amount of acid dissolved in a very less amount of water are called concentrated acids. Generally, acids supplied commercially are concentrated acids. For example, hydrochloric acid and sulphuric acid are supplied as concentrated acids.

Dilute Acids

Those acids with a relatively lesser amount of acid dissolved in a large amount of water are called dilute acids. A dilute solution contains less solute in comparison to a concentrated solution.

Classification of Bases

Bases are substances that are soapy to touch and bitter. A base is a substance, usually the oxide or the hydroxide of a metal, which can react with an acid to produce salt and water. For example, sodium oxide \(\left({{\text{N}}{{\text{a}}_2}{\text{O}}} \right)\) calcium oxide \(\left({{\text{CaO}}} \right),\) cupric oxide \(\left({{\text{CuO}}} \right),\) iron oxides (\({\text{FeO}},\,{\text{F}}{{\text{e}}_2}{{\text{O}}_3},\) etc), sodium hydroxide \(\left({{\text{NaOH}}} \right)\), and calcium hydroxide \(\left({{\text{Ca}}{{\left({{\text{OH}}} \right)}_2}} \right)\) are all bases.

Alkalis

Bases that are soluble in water are called alkalis. For example, sodium hydroxide, potassium hydroxide, calcium hydroxide is soluble in water. Therefore, they are alkalis. But bases like copper hydroxide, ferric hydroxide, aluminium hydroxide do not dissolve in water. They are, therefore, not alkalis. Hence, all alkalis are bases, but all bases are not alkalis.

When the solution of a base is diluted by mixing more water in it, then the concentration of hydroxide ions per unit volume decreases. This gives us a dilute solution of the base. Bases are of two types: strong bases and weak bases.

Strong Bases and Weak Bases

1. A base that completely ionizes in water and thus produces many hydroxide ions is called a strong base. Examples: Sodium hydroxide, potassium hydroxide.
2. A weak base produces a low concentration of hydroxyl ions in an aqueous solution. Examples: Ammonium hydroxide, calcium hydroxide.

Properties of Acids and Bases

The properties of acids and bases are given below:

Properties of Acids

1. Acids are sour.
2. They are soluble in water.
3. All acidic solutions conduct electricity.
4. They turn blue litmus red.
5. They are corrosive.
6. Reaction with metals: Acids react with some reactive metals to form salt and hydrogen gas.
   \({\rm{Acid}} + {\rm{Metal}} \to {\rm{Salt}} + {\rm{Hydrogen}}\,{\rm{gas}}\)
   Example 1:
   \({\rm{Zn}}\left( {\rm{S}} \right) + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\left( {{\rm{aq}}} \right) \to {\rm{ZnS}}{{\rm{O}}_4}\left( {{\rm{aq}}} \right) + {{\rm{H}}_2}\left( {\rm{g}} \right)\)
   Example 2:
   \({\rm{Zn}}\left( {\rm{S}} \right) + 2{\rm{HCl}}\left( {{\rm{aq}}} \right) \to {\rm{ZnC}}{{\rm{l}}_2}\left( {{\rm{aq}}} \right) + {{\rm{H}}_2}\left( {\rm{g}} \right)\)
7. Reaction with metal carbonates and metal hydrogen carbonates: Acids react with metal carbonates and metal hydrogen carbonates to form salt, carbon dioxide, and water. The carbon dioxide gas is observed in the form of brisk effervescence.
   \( {\text{Acid}} + {\text{Metal}}\, {\text{Carbonate}} \to {\text{Salt}} + {\text{Carbon}}\, {\text{dioxide}} + {\text{Water}}\)
   \({\text{Acid}} + {\text{Metal}}\,{\text{hydrogen}}\,{\text{carbonat}}e \to {\text{Salt}} + {\text{Carbon}}\,{\text{dioxide}} + {\text{Water}}\)
8. Reaction with oxides: Acids react with the oxides of metals to form salt and water.
   \({\rm{N}}{{\rm{a}}_2}{\rm{O}} + 2{\rm{HCl}} \to 2{\rm{NaCl}} + {{\rm{H}}_2}\)
   \({\rm{CuO}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{CuS}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}}\)

Properties of Bases

1. The solutions of bases in water give a soapy touch. It is because when dissolved in water, they produce hydroxide ions in the solution.
2. The presence of ions and the aqueous solutions of bases can conduct electricity.
3. They turn red litmus paper blue.
4. Reaction with metals: Bases react with some active metals like zinc on heating to form salt and hydrogen gas.
   \({\rm{Base + Metal}} \to {\rm{Salt + Hydrogen}}\)
   For example,
   \(2{\rm{NaOH}}\left( {{\rm{aq}}} \right) + {\rm{Zn}}\left( {\rm{s}} \right) \to {\rm{N}}{{\rm{a}}_2}{\rm{Zn}}{{\rm{O}}_2}\left( {{\rm{aq}}} \right) \to {\rm{NaCl}}\left( {{\rm{aq}}} \right) + {{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right)\)
5. Reaction with acids: Bases react with acids to form salt and water. This reaction is called the neutralization reaction.
   \({\text{Base}} + {\text{Acid}} \to {\text{Salt}} + {\text{Water}}\)
   For example \({\rm{NaOH}}\left( {{\rm{aq}}} \right) + {\rm{HCl}}\left( {{\rm{aq}}} \right) \to {\rm{NaCl}}\left( {{\rm{aq}}} \right) + {{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right)\)
6. Reaction with non-metal oxides: Non-metallic oxides being acidic, react with bases to form salt and water. This reaction confirms the acidic nature of non-metal oxides.
   \( {\text{Base}} + {\text{Non}} – {\text{metal}}\,{\text{oxide}} \to {\text{Salt}} + {\text{Water}}\)
   For example, \({\rm{Ca}}{\left( {{\rm{OH}}} \right)_2}\left( {{\rm{aq}}} \right) + {\rm{C}}{{\rm{O}}_2}\left( {\rm{g}} \right) \to {\rm{CaC}}{{\rm{O}}_3}\left( {\rm{s}} \right) + {{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right)\)

Uses of Acids and Bases

The uses of acids and bases are given below:

Uses of Acids

1. Hydrochloric Acid: Used in manufacturing of chemicals, tanning of leather, dyeing, aqua regia.
2. Nitric Acid:Used in manufacturing fertilizers, aqua regia, extraction of metals.
3. Sulphuric Acid: Used in manufacturing fertilizers, plasters, textiles, paper, and leather industry.
4. Acetic acid is used as vinegar to preserve food and as a flavoring agent.

Uses of Bases

1. Sodium hydroxide is used in the manufacture of soaps, textiles, paper, medicines, and petroleum refining.
2. Magnesium hydroxide is used as an antacid.
3. Ammonium hydroxide is used in making fertilizers, rayon, plastics, and dyes.
4. Calcium hydroxide is used in making cement and mortar.
5. Sodium carbonate is used as a washing soda and for softening hard water.

Summary

Acids and bases are not only used in laboratories. They are used in different places in our society as well. We use acids and bases every single day for various purposes. Through this article, you must have learnt the theories of acids and bases, properties, and uses of acids and bases.

PRACTICE QUESTIONS RELATED TO ACIDS & BASES

FAQs on Acids & Bases

Q.1. What are five examples of bases?
Ans: Bases are substances that are soapy to touch and bitter. A base is a substance, usually the oxide or the hydroxide of a metal, which can react with an acid to produce salt and water. For example, sodium oxide \(\left( {{\text{N}}{{\text{a}}_2}{\text{O}}} \right)\), calcium oxide \(\left({{\text{CaO}}} \right)\), cupric oxide \(\left({{\text{CuO}}} \right)\), iron oxides (\( {\text{FeO}},\,{\text{F}}{{\text{e}}_2}{{\text{O}}_3},\) etc), sodium hydroxide \(\left({{\text{NaOH}}} \right)\), and calcium hydroxide \(\left({{\text{Ca}}{{\left({{\text{OH}}} \right)}_2}} \right),\) etc.

Q.2. What are acids and bases?
Ans: Acids are those chemical substances that have a sour taste. Acids change the color of blue litmus to red. Bases are those chemical substances that have a bitter taste. Bases change the color of red litmus to blue.

Q.3. How to identify acids and bases?
Ans:  We can identify the acids and bases with the help of litmus paper. When a drop of solution is added to the litmus paper, the litmus paper changes to red to tell that it is an acid. If the litmus paper changes to blue, it suggests it is a base. 

Q.4. what do all acids and bases have in common?
Ans: A solution conducts electricity due to the presence of ions in it. The solutions of acids and bases to conduct electricity. In their aqueous solutions, acids produce \({{\text{H}}^ + }\) ions, whereas bases produce \({\text{O}}{{\text{H}}^ – }\) ions. Substances like ethanol and glucose, which do not furnish ions, fail to conduct electricity. So, the ability to form ions in their aqueous solutions is the common property of acids and bases.

Q.5. Write the chemical equation for the reaction of zinc metal on sodium hydroxide.
Ans: \( {\text{Zn}}\left({\text{s}} \right) + 2{\text{NaOH}}\left({{\text{aq}}} \right) \to {\text{N}}{{\text{a}}_2}{\text{Zn}}{{\text{O}}_2}{\left({{\text{aq}}} \right)_{{\text{Sodium}}\,\,{\text{zincate}}}} + {{\text{H}}_2}\left({\text{g}} \right)\)

Q6. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Ans: When an acid reacts with a metal, hydrogen gas is evolved. For example, zinc reacts with hydrochloric acid to evolve hydrogen gas.
\( {\text{Zn}}\left({\text{s}} \right) + 2{\text{HCl}}\left({{\text{aq}}} \right) \to {\text{ZnC}}{{\text{l}}_2}\left({{\text{aq}}} \right) + {{\text{H}}_2}\left( {\text{g}} \right)\)
The evolved gas burns violently with a pop sound. This is a confirmatory test for hydrogen gas. 

Q.7. How do acids and bases react with each other?
Ans: A reaction in which an acid and base react with each other to give salt and water is termed a neutralization reaction. For example,
(a) Neutralization of \( {\text{HCl}}\) by \({\text{NaOH}}\)
. \( {\text{HCl}}\left({{\text{aq}}} \right) + {\text{NaOH}}\left( {{\text{aq}}} \right) \to {\text{NaCl}}\left({{\text{aq}}} \right) + {{\text{H}}_2}{\text{O}}\left( {\text{l}} \right)\)
(b) Neutralization of \({\rm{HCl}}\) by \({\rm{Mg}}{\left( {{\rm{OH}}} \right)_2}.\)
. \(2 {\text{HCl}} + {\text{Mg}}{\left({{\text{OH}}} \right)_2} \to 2{\text{MgC}}{{\text{l}}_2} + 2{{\text{H}}_2}{\text{O}}\)

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