CBSE Class 10 Science Carbon And Its Compounds Notes Set 04

Covalent Bond: The chemical bond formed by the sharing of electrons between two atoms is called covalent bond.

• Single covalent bond: A covalent bond formed by sharing of one pair of electrons between two atoms is known as single covalent bond. For example, two hydrogen atoms share their electrons to form a molecule of hydrogen, \( \text{H}_2 \).
  Single bond between two Hydrogen atoms
• Double covalent bond: The covalent bond formed by sharing of two pairs of electrons between two atoms is known as double covalent bond. For example, the two electrons contributed by each oxygen atom give rise to two shared pairs of electrons. This is said to constitute a double bond between the two atoms.
  The electron dot structure of \( \text{O}_2 \) and its double bond.
  Double bond between two oxygen atoms
• Triple covalent bond: The covalent bond formed by the sharing of three pairs of electrons between two atoms is known as triple covalent bond. In the case of a diatomic molecule of nitrogen, each nitrogen atom in a molecule of nitrogen contributes three electrons giving rise to three shared pairs of electrons. This is said to constitute a triple bond between the two atoms.
  The electron dot structure of \( \text{N}_2 \) and its triple bond.
  Triple bond between two nitrogen atoms

Covalent compounds exist as solids, liquids and gases. They are generally soluble in non-polar solvents like ether, benzene etc. and generally insoluble in polar solvents like water.

Molecules of covalent compounds are held together by relatively weaker forces as compared to ionic compounds. Therefore, covalent compounds have relatively lower melting and boiling points.

Covalent compounds are poor conductors of electricity because they contain neither the ions nor free electrons necessary for conduction.

VERSATILE NATURE OF CARBON

The nature of the covalent bond enables carbon to form a large number of compounds. Two factors noticed in the case of carbon are –

• Catenation: Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. The self-linking property of carbon atoms through covalent bonds to form long chains of carbon, branched chains of carbon or even carbon atoms arranged in rings. In addition, carbon atoms may be linked by single, double or triple bonds.
  Compounds of carbon, which are linked by only single bonds between the carbon atoms, are called saturated compounds.
  Compounds of carbon having double or triple bonds between their carbon atoms are called unsaturated compounds.
• Tetravalency: Carbon has a valency of four, it is capable of bonding with four other atoms of carbon or atoms of some other mono-valent element. Compounds of carbon are formed with oxygen, hydrogen, nitrogen, sulphur, chlorine and many other elements giving rise to compounds with specific properties which depend on the elements other than carbon present in the molecule.

Homologous Series

It is a family of organic compounds having the same functional group in which the formula of successive members differs by \( -\text{CH}_2 \) group. For example,

• For alkanes \( \text{CH}_4 \), \( \text{C}_2\text{H}_6 \), \( \text{C}_3\text{H}_8 \), \( \text{C}_4\text{H}_{10} \) etc.
• For alkenes \( \text{C}_2\text{H}_4 \), \( \text{C}_3\text{H}_6 \), \( \text{C}_4\text{H}_8 \) and \( \text{C}_5\text{H}_{10} \) etc.
• For alkynes \( \text{C}_2\text{H}_2 \), \( \text{C}_3\text{H}_4 \), \( \text{C}_4\text{H}_6 \) and \( \text{C}_5\text{H}_8 \) etc.

For example, the chemical properties of \( \text{CH}_3\text{OH} \), \( \text{C}_2\text{H}_5\text{OH} \), \( \text{C}_3\text{H}_7\text{OH} \) and \( \text{C}_4\text{H}_9\text{OH} \) are all very similar. Hence, such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.

The melting and boiling points increase with increasing molecular mass.

Nomenclature of Carbon Compounds

Some functional groups in carbon compounds

Saturated and Unsaturated Carbon Compounds

The carbon compounds which contain only carbon and hydrogen are called hydrocarbons. Among these, the saturated hydrocarbons are called alkanes. Methane, Ethane, Propane etc.

Methane has a formula \( \text{CH}_4 \). Hydrogen has a valency of 1. Carbon is tetravalent because it has four valence electrons. In order to achieve noble gas configuration, carbon shares these electrons with four atoms of hydrogen as shown in Fig. is given below:

Electron dot structure for methane

Structure of ethane formed between carbon and hydrogen with a formula of \( \text{C}_2\text{H}_6 \).

The structure of ethane is arrived in the following steps –

• (a) Carbon atoms linked together with a single bond
• (b) Each carbon atom bonded to three hydrogen atoms
• (c) Electron dot structure of ethane

Electron dot structure for ethane

The unsaturated hydrocarbons which contain one or more double bonds are called alkenes. Ethene, Propene etc.

Those containing one or more triple bonds are called alkynes. Ethyne, Propyne etc.

The electron dot structure for Ethene.

Another compound of hydrogen and carbon has the formula \( \text{C}_2\text{H}_2 \) and is called Ethyne.

\( \text{H} - \text{C} \equiv \text{C} - \text{H} \)

Chains, Branches and Rings

The carbon compounds methane, ethane and propane, containing respectively 1, 2 and 3 carbon atoms. Such 'chains' of carbon atoms can contain many more carbon atoms.

Formulae and structures of saturated compounds of carbon and hydrogen (Alkanes)

Formulae and structures of unsaturated compounds of carbon and hydrogen (Alkenes)

Formulae and structures of unsaturated compounds of carbon and hydrogen (Alkynes)

CHEMICAL PROPERTIES OF CARBON COMPOUNDS

Combustion

Carbon, in all its allotropic forms, burns in oxygen to give carbon dioxide along with the release of heat and light.

• (i) \( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 + \text{heat and light} \)
• (ii) \( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat and light} \)
• (iii) \( \text{CH}_3\text{CH}_2\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} + \text{heat and light} \)

Saturated hydrocarbons will generally give a clean flame while unsaturated hydrocarbons will give a yellow flame with lots of black smoke. Limiting the supply of air results in incomplete combustion of even saturated hydrocarbons giving a sooty flame.

Oxidation

Carbon compounds can be easily oxidised on combustion. In addition to this complete oxidation, in which ethyl alcohol is converted to ethanoic acid upon heating in the presence of alkaline potassium permanganate or acidified potassium dichromate (oxidising agents).

\( \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{alkaline } \text{KMnO}_4 + \text{Heat}} \text{CH}_3\text{COOH} \)

Addition reaction

Unsaturated hydrocarbons add hydrogen in the presence of catalyst such as palladium or nickel to give saturated hydrocarbons.

\( \text{R}-\text{C(R)}=\text{C(R)}-\text{R} + \text{H}_2 \xrightarrow{\text{Nickel catalyst}} \text{R}-\text{CH(R)}-\text{CH(R)}-\text{R} \)

Substitution reaction

Saturated hydrocarbons are fairly unreactive and are inert in the presence of most reagents. However, in the presence of sunlight, chlorine is added to methane in very fast reaction. Chlorine can replace the hydrogen atoms one by one. It is called a substitution reaction.

\( \text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{in the presence of sunlight}} \text{CH}_3\text{Cl} + \text{HCl} \)

SOME IMPORTANT CARBON COMPOUNDS - ETHANOL AND ETHANOIC ACID

Properties of ethanol

Ethanol is a liquid at room temperature. Ethanol is commonly called alcohol and is the active ingredient of all alcoholic drinks. Ethanol is also soluble in water in all proportions.

Reactions of Ethanol

• (i) Reaction with sodium: When ethyl alcohol reacts with sodium leading to the evolution of hydrogen and the other product is sodium ethoxide.
  \( 2\text{CH}_3\text{CH}_2\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2 \)
  (Sodium ethoxide)
• (ii) Reaction to give unsaturated hydrocarbon: Heating ethanol at 443 K with excess concentrated Sulphuric acid results in the dehydration of ethanol to give Ethene
  \( \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Hot Conc. } \text{H}_2\text{SO}_4} \text{CH}_2 = \text{CH}_2 + \text{H}_2\text{O} \)
  The concentrated Sulphuric acid can be regarded as a dehydrating agent who removes water from ethanol.

Uses

It is a good solvent; it is also used in medicines such as tincture iodine, cough syrups, and many tonics. Consumption of small quantities of dilute ethanol causes drunkenness. However, intake of even a small quantity of pure ethanol (called absolute alcohol) can be lethal. Also, long-term consumption of alcohol leads to many health problems.

Properties of ethanoic acid

Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic acids. Carboxylic acids are obviously characterized by their acidic nature. Carboxylic acids are weak acids. The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter in cold climates. This gave rise to its name glacial acetic acid.

Reactions of ethanoic acid

• (i) Esterification reaction: Esters are most commonly formed by reaction of an acid and an alcohol. Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester
  \( \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Acid}} \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \)
  (Ethanoic acid) (Ethanol) (Ester)
  On treating with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap. Soaps are sodium or potassium salts of long chain carboxylic acid.
  \( \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{CH}_3\text{CH}_2\text{OH} \)
• (ii) Reaction with a base: Like mineral acids, ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water:
  \( \text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} \)
• (iii) Reaction with carbonates and hydrogencarbonates: Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to a salt, carbon dioxide and water. The salt produced is commonly called sodium acetate.
  \( 2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2 \)
  \( \text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2 \)

Uses

Generally, esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents. 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles.

SOAPS AND DETERGENTS

• Soaps: They form scum when reacted to hard water. Soaps are derived from natural substances such as vegetable oils and animal fats.
• Detergents: They do not form scum. Detergents are generally a derivative of a synthetic compound.

Preparation of soap

On heating with sodium hydroxide, vegetable oil or animal fat forms a sodium salt of fatty acid and glycerol. This process is known as saponification.

\( \text{Vegetable oil/Animal fat} + \text{NaOH} \xrightarrow{\text{Saponification}} \text{Glycerol} + \text{Sodium salt of fatty acid (Soap)} \)

Cleansing action of soaps

A soap molecule is made up of two chemically distinct parts that interact with water in different ways. It has one polar end with a short head carboxylate group (\( -\text{COONa} \)) and one non-polar end with a long tail made of the hydrocarbon chain.

Hydrophilic and hydrophobic end

The polar end is hydrophilic (water-loving) in nature, and it is drawn to water. The non-polar end is hydrophobic (hates water) in nature, and it is attracted to dirt or oil on the cloth but not to water. As a result, the hydrophobic part of the soap molecule traps the dirt while the hydrophilic part makes the entire molecule water-soluble.

When soap or detergent is dissolved in water, the molecules form clusters known as 'micelles'.

Formation of micelles

Their long hydrocarbon chains bind to the oil and dirt. As a result, the dirt is surrounded by the non-polar end of the soap molecules. The micelles are water-soluble because of the charged carboxylate end of the soap molecules. As a result, the soap washes away the dirt.

Detergents are generally sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions, etc. Both have long hydrocarbon chain. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, they remain effective in hard water. Detergents are usually used to make shampoos and products for cleaning clothes.