Acids, Bases and Salts

1. Acids

Introduction

The word 'acid' is derived from the Latin word Acidus, meaning sour-tasting. The theories of acids and bases have been central to chemistry for centuries, shaping our comprehension of chemical reactions and their effects on the world around us. The historical journey of acid-base theories can be traced back to ancient civilizations. However, the formalization of acid-base theories began in the late 19^th Century with Svante Arrhenius.

 

THEORIES

Antoine Lavoisier

Antoine Lavoisier introduced one of the earliest modern acid-base theories in the late 18^th century. He proposed acids as substances that contained oxygen and believed that the presence of oxygen was responsible for their acidic properties.

Humphry Davy (1810)

Additional research in 1810 revealed that muriatic acid was a compound of hydrogen and chlorine (HCl), and contained no oxygen, and Davy’s research on chlorine led to a rejection of Lavoisier’s theory that oxygen was an essential constituent of acids.

Arrhenius Theory (1884)

The first comprehensive and widely accepted acid-base theory was proposed by Svante Arrhenius in 1884. Arrhenius defined acids as any substances that release or produce hydrogen ions (H⁺) in water, and bases as substances that release hydroxide ions (OH^-),  e.g.

This groundbreaking theory provided a clear definition of acids and bases in aqueous solutions, and the process demonstrated above is known as ionization. The characteristic properties of an acid in solution are due to the presence of these hydrogen ions.

 

Modification of Arrhenius' definition of an acid

The existence of hydronium (oxonium) ion, H₃O⁺, modifies the earlier definition of an acid. Thus, an acid is also defined as a substance which in aqueous solution produces hydronium (oxonium) ions, H₃O⁺ as the only positive ions, e.g.

HCl + H₂O → H₃O⁺ + Cl^-

HNO₃ + H₂O → H₃O⁺ + NO3-

H₂SO₄ + H₂O → H₃O⁺ + SO₄^2-

 

Bronsted- Lowry Theory (1923)

The Arrhenius Theory makes use of hydroxide ions, which may not exist in non-aqueous solvents, and does not cover weak bases, this prompted Bronsted and Lowry to put forward a more general theory of acids and bases which incorporates all protonic solvents, and not just water.

They defined an acids as substances that can donate protons (H⁺ ions), and bases are substances that can accept protons (H⁺). E.g.

HCl + H₂O ↔ H₃O⁺ + Cl^-

HCl + NH₃ ↔ NH₄⁺ + Cl^-

H₂O + NH₃ ↔ H₃O⁺ + NH₄⁺

In the forward reactions above a proton is being donated by (HCl in example 1, HCl in example 2 and H₂O in example 3) and accepted by (H₂O in example 1, NH₃ in example 2 and 3). Any substance which donates a proton to the other have the potential of being an acid while the other which accept the proton is the base.

If you study the forward reaction of equation 1, HCl donates a proton to H₂O which accepts it. HCl behaves as an acid while H₂O behaves as a base. In the backward reaction H₃O⁺ donates a proton behaving like an acid and Cl^- accepts a proton behaving like a base. From the above explanation, HCl and Cl- are a conjugate acid-base pair. Similarly, H₃O⁺ and H₂O are a conjugate acid-base pair.

Lewis Theory

In 1923, Gilbert N. Lewis proposed a generalized theory of acids and bases, expanding on the existing Arrhenius and Brønsted-Lowry definitions. Through the use of the Lewis definition of acids and bases, chemists are now able to predict a wider variety of acid-base reactions. Lewis' theory used electrons instead of proton transfer and specifically stated that:

An acid is a species that accepts an electron pair, while a base donates an electron pair.

The reaction of a Lewis acid and a Lewis base will produce a coordinate covalent bond as seen above. A coordinate covalent bond is just a type of covalent bond in which one reactant gives it electron pair to another reactant. In this case the lewis base donates its electrons to the Lewis acid. When they do react this way the resulting product is called an addition compound, or more commonly an adduct.

• Lewis Acid: a species that accepts an electron pair (i.e., an electrophile) and will have vacant orbitals
• Lewis Base: a species that donates an electron pair (i.e., a nucleophile) and will have lone-pair electrons

 

Several categories of substances can be considered Lewis acids:

1) All cations, i.e. positive ions (e.g., Cu²⁺, Fe²⁺, Fe³⁺)

2) An atom, ion, or molecule with an incomplete octet of electrons (e.g., BF₃, AlF₃).

3) Molecules that have multiple bonds between two atoms of different electronegativities (e.g., CO₂, SO₂)

4) Molecules where the central atom can have more than 8 valence shell electrons can be electron acceptors (e.g., SiBr₄, SiF₄).

 

Several categories of substances can be considered Lewis bases:

1) All anions, i.e. negative ions (e.g. OH⁻, CN⁻, CH3COO⁻⁾

2) Atom of one of more unshared pairs in the valence shell (e.g. NH₃, H₂O)

3) The presence of a double bond e.g. CO.

 

Amphoterism

As of now you should know that acids and bases are distinguished as two separate things however some substances can be both an acid and a base. You may have noticed this with water, which can act as both an acid or a base. This ability of water to do this makes it an amphoteric molecule. Water can act as an acid by donating its proton to the base and thus becoming its conjugate acid, OH-. However, water can also act as a base by accepting a proton from an acid to become its conjugate base, H₃O⁺.

• Water acting as an Acid:

H₂O + NH₃ → NH₄⁺ + OH⁻

• Water acting as a Base:

H₂O + HCl → Cl⁻ + H₃O⁺

You may have noticed that the degree to which a molecule acts depends on the medium in which the molecule has been placed in. Water does not act as an acid in an acid medium and does not act as a base in a basic medium. Thus, the medium which a molecule is placed in has an effect on the properties of that molecule. Other molecules can also act as either an acid or a base. For example,

Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O

• where Al(OH)₃ is acting as a Lewis Base.

Al(OH)₃ + OH⁻ → Al(OH)₄^-

• where Al(OH)₃ is acting as an Lewis Acid.

Note how the amphoteric properties of the Al(OH)₃ depends on what type of environment that molecule has been placed in.

 

Classification of Acids

Acids can be categorized into different types based on various criteria, including their composition, strength, and occurrence. Here are some common types of acids:

A. Based on their Origin (Source)

1. Organic Acids: Organic acids are a type of weak acid that contains a carbon atom bonded to hydrogen and a carboxyl group (COOH). They are found in various natural sources, including fruits and other organic matter. Examples include acetic acid (CH₃COOH) found in vinegar and citric acid (C₆H₈O₇) found in citrus fruits.

2. Mineral Acids: Mineral acids are strong acids that are typically derived from minerals and inorganic compounds. They are highly corrosive and can cause severe chemical burns.

Examples include sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and nitric acid (HNO₃)

 

B. Based on their Strength

1. Weak Acids: Weak acids are acids that only partially dissociate in water, releasing a limited number of hydrogen ions (H⁺). Their dissociation equilibrium lies more to the left.

Examples include acetic acid (CH₃COOH) and carbonic acid (H₂CO₃).

2. Strong Acids: Strong acids completely dissociate in water, releasing all their hydrogen ions (H⁺). Their dissociation equilibrium lies almost entirely to the right. Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).

 

C. Based on elements present in the Acids 

1. Hydracids (Binary acids): are acids that consist of hydrogen and a non-metal element other than oxygen. 

Examples:

• Hydrochloric acid (HCl)
• Hydrogen sulphide (H₂S)
• Hydrobromic acid (HBr)
• Hydroiodic acid (HI) 

Oxyacids (Ternary acids):  These acids contain hydrogen, non-metal elements, and hydrogen. 

Examples:

• Sulphurous acid (H₂SO₃)
• Chloric acid (HClO₃)
• Perchloric acid (HClO₄)
• Sulphuric acid (H₂SO₄) 

 D. Based on Basicity of Acids 

Basicity is defined as the number of hydronium ions [H⁺ (aq)] released by one acid molecule to complete ionization. 

Based on basicity, acids are divided into three types. 

1. Monobasic acid: When one acid molecule on complete ionisation gives one hydronium ion [H⁺ (aq)], the acid is called monobasic acid. 

Examples: 

• HCl
• Hydrogen fluoride acid (HF)
• HBr
• Acetic acid (CH₃COOH)
• Formic acid (HCOOH)

Note:

        I.            A monobasic acid always ionizes in one step in an aqueous solution. 

      II.            A monobasic acid forms only a single salt. 

  

2. Dibasic acid: One molecule of dibasic acid on complete ionisation gives two hydronium ions [H⁺ (aq)]. 

Examples:

• These acids ionise in two steps. 

 

3. Tribasic acid: Here, one molecule of an acid gives three hydronium ions [H⁺ (aq)] on complete ionization. 

Complete ionisation is taking place in three steps for tribasic acids 

 

E. Based on Concentration (water content) of Acid

Concentration: It measures the amount of water present in a given sample of acid. 

Based on this, the acids are classified into two types as follows: 

1. Concentrated acid: A sample of an acid that contains only very little or no amount of water. 

2. Dilute acid:  For dilute acids, they contain a lot more amount of water than their weight 

 

  

General Properties of Acids

1. Taste

• Generally, acids taste sour. 

 

2. Effect on Skin

• Strong inorganic acids are corrosive action on the skin, and they can even damage other substances. 

 

 

3. Effect of Indicators

• Indicator: Indicators are the substances used to distinguish between acids and bases. They change their color when dissolved in the acids or bases; that is why they are known as indicators. 
• Effect Of Litmus Paper: Litmus is a natural indicator, and when the blue litmus paper is dipped in the acid solution, they turn blue litmus paper into the red. 

4. Effect of Electric Current

• Acids are good conductors of electricity when dissolved in water. This is because, in water, they get dissolved to release Hydrogen (H⁺ as Hydronium) ions. These free ions help in the conduction of electricity.  

 

Chemical Properties of Acid

 

Uses of Acids

1. Oxalic Acid

• Oxalic acid is used for removing food and ink stains. 
• It is also used in wastewater treatment to remove deposits of calcium. 

2. Sulphuric Acid

• It is very important industrial chemical and used in the preparation of many chemicals.  
• In the car batteries, sulphuric acid is used. 
• It is also used in the preparation of paints, dyes, and fertilizers. 

 

3. Acetic Acid

• The common name of acetic acid is vinegar and it used in the food industry to enhance the flavours of food. 
• Apart from that, it is also used to clean the utensil, floors. 
• It also used to remove stain of wooden furniture. 
• It is used as preservatives in many food items like ketchups, pickels, sauces. 

4. Boric Acid

• Boric acid is used as grain preservatives and used in the manufacture of glasses and adhesives. 

5. Hydrochloric Acid

• It is responsible for the digestion of the food we eat. 
• It is also used as a bathroom cleaner. 

6. Carbonic Acid

• In the preparation of soft drinks 

7. Tartaric Acid

• In the baking powder. 

8. Citric Acid

• In food prevention 

9. Ascorbic Acid

• In the treatment of scurvy disease and bone marrow. 

  

Methods of Preparation of Acids

Acids can be prepared in many ways; we will study the three most important methods for the preparation of acids:

1. Synthetic Method  

In the synthetic method, acids are prepared by a direct combination of elements, mostly non-metals. 

For example, when hydrogen gas and chlorine react with each other under the action of electricity, the hydrogen chloride gas is absorbed in water to produce hydrochloric acid. 

The chemical reaction can be written as below: 

Another is the formation of hydrogen sulphide; when the non-metals like sulphur and hydrogen gas are boiled, it forms hydrogen sulphide gas. 

  

2. By Dissolving Acidic Oxides in Water 

Acidic oxides: some of the oxides of non-metals are dissolved in the water. These oxides are known as acidic oxides. 

When acidic oxides of carbon, i.e., carbon dioxide, react with water, it gives carbonic acid. 

Similarly, sulphur trioxide dissolves in the water forms sulphuric acid. 

Detailed examples include:

 

3. The Reaction of an Acid with Salt of Another Acid

The principle of this preparation method is that the acid which has a higher boiling point reacts with the salt of an acid having the lower boiling point to produce an acid.  

For example, common salt, i.e., sodium chloride is salt and salt of hydrochloric acid (HCl).   

The boiling point of hydrochloric acid is lesser than sulphuric acid (H₂SO₄), and therefore, when sodium chloride (NaCl) reacts with sulphuric acid, it gives the product as hydrochloric acid (HCl). 

 

Other examples include:

 

2. Bases

Bases

Definition:  Bases are those substances, when dissolved in water, that give hydroxyl ions (OH^–). 

Generally, bases are bitter in taste and feel soapy to touch. 

Examples:

• Sodium hydroxide (NaOH) or caustic soda 
• Potassium hydroxide (KOH) 
• Calcium hydroxide: Ca(OH)₂ 

When a base like sodium hydroxide dissociates in water, it gives (Na⁺) and hydroxide(OH^–) ions. 

Similarly, calcium hydroxide dissociates in water to give (Ca⁺) and hydroxide(OH^–) ions. 

  

Alkalis

Alkalis are the bases that are soluble in water. 

 

Classification of Bases

Bases are classified into three categories based on various factors as follows. 

1. Based on Strength (Ionising Capability)

• Strong Bases 
• Weak Bases 

Strong Bases: 

The bases which completely dissociate in water and therefore produce a large number of hydroxyl ions (OH^–) are known as strong bases. 

Examples:

• Hydroxide of all reactive metals
• Sodium Hydroxide (NaOH)
• Potassium hydroxide(KOH)
• Aluminium hydroxides (Al(OH)₃) 

 

Weak Bases: 

The bases that do not completely dissociate in the water and give very few hydroxyl ions are known as weak bases. 

Examples:

• Ammonium hydroxides (Al(OH)₃), 
• Copper hydroxide (Cu(OH)₂),
• Zinc hydroxide (Zn(OH)₂),
• Ferric hydroxide (Fe(OH)₃) 

 

2. Based on Concentrations  

Based on the amount of water present in the given base sample, they are classified into two types. 

a. Concentrated Base: A given sample of a base that contains very little or no amount of water is called a concentrated base. 

b. Dilute base: If a given sample contains an amount of water very much than its own weight, then the given base is called the weak base. 

Based on Acidity of Base: 

 

3. Acidity

Acidity is defined as the number of hydroxyl ions [OH^– ion] provided by one base molecule on complete dissociation in water. 

Types of Bases

i. Moacidic Bases

• Acidity: Dissociates to give one OH^– ion per molecule of base 
• Examples: NaOH, KOH

ii. Diacidic Bases

• Acidity: Dissociates to give two OH^– ion per molecule of base 
• Examples: Ca(OH)_2, Mg(OH)

 

iii. Triacidic Bases

• Acidity: Dissociates to give three OH^– ion per molecule of base 
• Examples: Fe(OH)₃, Al(OH)₃

Physical Properties of Bases 

1. Taste: Bases are usually bitter. 
2. Nature: Bases are slippery to feel/soapy in nature. 
3. Action of an Indicator. 

When the red litmus paper dissolved in the solution of base, it turns into blue 

4. Action of Electricity: Bases also conduct electricity when dissolved in water.  

This is because, when bases dissolved in the water, they dissociate into their respective ions (hydroxyl ions) which is helpful to conduct the electricity. 

 

Chemical Properties of Bases

  

Methods Of Preparation of Base

1. By the Action of Oxygen on Metals 

When the metals are heated with oxygen or air, they give their respective basic metal oxides. 

The general chemical reaction can be written as: 

Metal + Oxygen    →    Metallic oxide [Basic Oxide] 

For example:

• when the metals like sodium, potassium, calcium burn in the air, they give their corresponding oxides. 

 

2. By the Action of Water / Steam on Reactive Metals 

Some reactive metals react with water or stream to give basic hydroxide with the evolution of hydrogen gas. 

Metal     +     Water/Steam     →   Basic hydroxide/oxide+ H₂(g) 

Metals like sodium and potassium react with cold water to form hydroxides of sodium and potassium, respectively, with liberation hydrogen gas. This reaction is very violent. 

Magnesium reacts with steam to give magnesium oxide with the evolution of hydrogen gas. 

When superheated steam is passed over red-hot iron, ferric oxide is formed, and hydrogen gas is evolved. 

By Heating Carbonate s(Decomposition) of Some Metals: 

When the carbonates of some metal are heated, they decompose into their respective oxides with the evolution of carbon dioxide gas. 

When calcium carbonate is heated, calcium oxide and carbon dioxide are formed. 

 

Uses of Bases

1. Sodium Hydroxide (NaOH) 

• Used in the manufacture of soaps, detergents. 
• Used in the manufacture of textiles, papers. 

2. Magnesium Hydroxide (Mg(OH)₂) 

• Used as antacid to relief from acidity. 

3. Calcium Hydroxide (Ca(OH)₂) 

• Used to neutralize acidity of soil. 
• Used as dry mixes in decorating, painting. 

 

3. Salts

Salts

The ionic compounds formed when acid and base react with each other are called Salts. That is, a salt is a compound consisting of positive metallic ion and a negative ion.

e.g. NaCl; Sodium chloride

Acid + Base → Salt + Water 

1. Salts are neutral in nature. 
2. The salts are formed because of neutralization reaction as both acid and base neutralize each other’s effect. 

Examples:

• Sodium chloride (common salt) CaCl_2,
• Barium sulphate (BaSO₄)
• Sodium nitrate (NaNO₃) 

Sodium chloride or common salt is a product of the neutralisation reaction between the hydrochloric acid (acid) and sodium hydroxide (base). 

 

Neutralization:  

The reaction between a base and an acid to produce salt and water is called Neutralisation. 

For example:

• When sodium hydroxide (NaOH), a base reacts with sulphuric acid (H₂SO₄), it forms salt, i.e., sodium sulphate and water. 

In all neutralization reactions, the product is always water due to the combination of hydrogen ions present in the acid and hydroxyl ions present in the base.  

The formation of water in an acid-base occurs as follows: 

Types Of Salts

Five main types of salts exist which are:

1. Normal salt: Normal salt is formed when all the replaceable hydrogen ions in an acid have been completely replaced by metallic ions of a base. E.g. NaCl, CaCl₂, MgSO₄, Na₂SO₄.

A normal salt contains no replaceable hydrogen, H⁺ or hydroxyl, OH^-, ion. Aqueous solution of normal salt is neutral to litmus and it has a pH of 7.

2. Acid salt: An acid salt is formed when the replaceable hydrogen ions in the acid are partially replaced by a metal. An acid salt contains replaceable hydrogen ion E.g NaHCO₃; sodium hydrogen trioxocarbonate(iv). NaHSO₄; Sodium hydrogen tetraoxosulphate (vi) Ca(HCO₃)₂;Calcium hydrogen trioxocarbonate(iv).

A solution of an acidic salt turns blue litmus paper red, and it can react with excess alkali to form

a normal salt and water. Eg.

3. Basic salt: A basic salt is formed when a base reacts with a limited quantity of an acid. That is, the oxonium ion, H₃O⁺, is insufficient for the complete neutralization of the base. A basic salt therefore contains replaceable hydroxyl ions, OH^-.

 

4. Double salt: Double salt is a salt which ionizes to yield three different types of ions in solution.

Two of than are positively charged (metallic ions) while the other is negatively charged. Eg

(NH₄)SO₄FeSO₄.6H₂O Ammonium iron(ii) tetraoxosulphate(vi) hexahydrate

KAl(SO₄)₂ .12H₂O Ammonium potassium tetroxosulphate (vi) dodecahydrate

5. Complex salt: This is a salt that contains complex ions. A complex ion is the ion consisting of a

charged group of atoms. E.g.