CBSE Class 10 Science Carbon And Its Compounds Notes Set 05

These CBSE Class 10 Science notes for Chapter 4, Carbon and Its Compounds, cover covalent bonding, the allotropes of carbon, the versatile nature of carbon, hydrocarbons and isomers, functional groups, homologous series and nomenclature, the chemical properties of carbon compounds, ethanol and ethanoic acid, and soaps and detergents.

Important Notes

Bonding in Carbon: The Covalent Bond

1. Carbon is a non-metal of atomic number 6 with electronic configuration 2, 4. The earth's crust has only 0.02% carbon, in the form of minerals, and the atmosphere has 0.03% carbon dioxide.
2. Carbon has four valence electrons. To reach a noble gas configuration it would have to gain four electrons (forming C⁴⁻, which a nucleus of six protons cannot hold) or lose four electrons (forming C⁴⁺, which needs a very large amount of energy).
3. Carbon therefore shares its valence electrons with other atoms of carbon or of other elements. A bond formed by sharing a pair of electrons between two atoms is a covalent bond. The number of electrons an atom shares shows its covalency.
4. A single bond is a shared pair (–), a double bond is two shared pairs (=) and a triple bond is three shared pairs (≡). Examples: H₂ (single), O₂ (double), N₂ (triple), CH₄, H₂O, NH₃, CO₂ and C₂H₄.

Electron dot structures (dots for electrons of one atom, crosses for the other):

Methane, CH₄ (four single bonds)

Ethene, C₂H₄ (one double bond between the carbon atoms)

Ethyne, C₂H₂ (one triple bond between the carbon atoms)

Carbon dioxide, CO₂ (two double bonds)

Properties of Covalent Compounds

1. Forces between the molecules are weak, so covalent compounds generally have low melting and boiling points.
2. No ions are formed, so they are generally poor conductors of electricity.
3. They are generally insoluble in water but soluble in organic solvents.

Allotropes of Carbon

Allotropy is the property by which an element exists in more than one form having different physical properties but similar chemical properties. Carbon exists as diamond, graphite and fullerene (all of them burn in oxygen to give carbon dioxide).

Buckminsterfullerene is named after the architect Buckminster Fuller, whose geodesic dome resembles its structure.

Versatile Nature of Carbon

1. Catenation: carbon atoms can link to each other by covalent bonds to form long chains, branched chains and rings. It is maximum in carbon because the small size of carbon makes the C–C bond very strong. Silicon forms chains of only seven or eight atoms, and these compounds are very reactive.
2. Tetravalency: carbon has a valency of four, so it can bond with four other atoms of carbon or of elements such as hydrogen, oxygen, nitrogen, sulphur and chlorine.
3. Carbon atoms may be linked by single, double or triple bonds. Compounds with only single bonds between carbon atoms are saturated; compounds with at least one double or triple bond are unsaturated.
4. Because carbon is small, the bonds it forms with most other elements are strong, which makes carbon compounds very stable.

Hydrocarbons

Compounds of carbon and hydrogen only are hydrocarbons.

1. Carbon compounds may have straight chains, branched chains or rings. Cyclohexane (C₆H₁₂) is a saturated ring compound and benzene (C₆H₆) is an unsaturated ring compound.
2. Structural isomers have the same molecular formula but different structures. Butane (C₄H₁₀) has two isomers: CH₃–CH₂–CH₂–CH₃ and CH₃–CH(CH₃)–CH₃ (2-methylpropane). Pentane has three isomers and hexane has five. Methane, ethane and propane have no isomers because they cannot be branched.

Functional Groups

An atom or a group of atoms that makes a carbon compound reactive and decides its chemical properties is called a functional group. It replaces a hydrogen atom of the carbon chain (for example, ethanol is ethane with one hydrogen replaced by –OH).

Homologous Series

A homologous series is a series of carbon compounds with the same functional group and similar chemical properties, in which any two successive members differ by a –CH₂– unit.

1. All members have the same general formula, for example C_nH_2n+2 for alkanes.
2. Successive members differ by CH₂, that is, by 14 u in molecular mass.
3. Chemical properties are similar, since they depend on the functional group.
4. Physical properties, such as melting and boiling points, show a gradation with increasing molecular mass.

Nomenclature of Carbon Compounds

1. Count the carbon atoms of the longest chain and write the word root: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).
2. Add the suffix for the bond type: -ane (single bond), -ene (double bond), -yne (triple bond). If the chain has a functional group, replace the final -e with its suffix: -ol, -al, -one, -oic acid.
3. Halogens are written as prefixes, for example chloro- before the name.
4. Examples: CH₃CH₂CH₂OH propanol; CH₃CH₂CHO propanal; CH₃COCH₂CH₃ butanone; CH₃CH₂COOH propanoic acid; CH₃CH₂Br bromoethane.

Chemical Properties of Carbon Compounds

1. Combustion: carbon compounds burn in oxygen to give carbon dioxide and water with heat and light. CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light. Saturated hydrocarbons burn with a clean blue flame in plenty of air; unsaturated hydrocarbons burn with a yellow sooty flame. Blackened cooking vessels show that the air holes are blocked and the fuel is burning incompletely.
2. Oxidation: alcohols are oxidised to carboxylic acids by alkaline KMnO₄ or acidified K₂Cr₂O₇, which add oxygen. CH₃CH₂OH → CH₃COOH.
3. Addition reaction: unsaturated hydrocarbons add hydrogen in the presence of a nickel or palladium catalyst to give saturated hydrocarbons (hydrogenation). CH₂=CH₂ + H₂ → CH₃–CH₃. This is used to make vegetable ghee from vegetable oils. Unsaturated compounds also decolourise bromine water.
4. Substitution reaction: in sunlight, chlorine replaces the hydrogen atoms of a saturated hydrocarbon one at a time. CH₄ + Cl₂ → CH₃Cl + HCl.

Ethanol

1. Ethanol (C₂H₅OH) is a colourless liquid, miscible with water in all proportions, with a boiling point of 351 K. It is the active ingredient of all alcoholic drinks and is a good solvent, used in medicines such as tincture of iodine and cough syrups.
2. Reaction with sodium: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂ (sodium ethoxide and hydrogen gas).
3. Dehydration: heating with excess conc. H₂SO₄ at 443 K gives ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O. Conc. H₂SO₄ acts as a dehydrating agent.
4. Effect on living beings: large amounts of ethanol depress the nervous system, causing lack of coordination, mental confusion and drowsiness, and long-term use damages the liver. Methanol is oxidised in the liver to methanal and even a small quantity can cause blindness or death.
5. To stop misuse of industrial ethanol it is made unfit for drinking by adding poisonous substances such as methanol and a blue dye. This is called denatured alcohol.

Ethanoic Acid

1. Ethanoic acid (CH₃COOH), commonly called acetic acid, is a carboxylic acid. A 5–8% solution in water is vinegar, used as a preservative in pickles. Pure ethanoic acid freezes at 290 K and is called glacial acetic acid.
2. It is a weak acid because it ionises only partly in water, unlike HCl.
3. Esterification: an acid and an alcohol react in the presence of conc. H₂SO₄ to form a sweet-smelling ester. CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. Esters are used in perfumes and as flavouring agents.
4. Saponification: an ester reacts with an alkali to give an alcohol and the sodium salt of the acid. CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa. This reaction is used to make soap.
5. With a base: CH₃COOH + NaOH → CH₃COONa + H₂O.
6. With carbonates and hydrogencarbonates: brisk effervescence of CO₂. 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The gas turns lime water milky.

Soaps and Detergents

1. Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). Detergents are generally sodium salts of sulphonic acids (or ammonium salts) of long-chain compounds.
2. A soap molecule has two ends: a water-loving ionic head (hydrophilic) and a water-hating hydrocarbon tail (hydrophobic) that dissolves in oil.
3. In water, soap molecules gather into micelles with the tails inside and the ionic heads outside. Oily dirt collects in the centre; the micelles stay suspended in water and are rinsed away. Agitation helps the dirt leave the cloth.

1. Hard water contains calcium and magnesium salts. Soap reacts with Ca²⁺ and Mg²⁺ ions to form an insoluble curdy precipitate called scum, so soap does not lather and is wasted.
2. Detergents do not form scum with hard water, so they clean well in both soft and hard water.
3. Many detergents are non-biodegradable and cause water pollution.

Quick Revision

1. Carbon forms covalent bonds by sharing electrons; catenation and tetravalency give millions of compounds.
2. Alkanes C_nH_2n+2, alkenes C_nH_2n, alkynes C_nH_2n−2; neighbours in a series differ by CH₂ (14 u).
3. Four reactions: combustion, oxidation, addition (hydrogenation) and substitution.
4. Ethanol: reacts with sodium, dehydrates to ethene, oxidises to ethanoic acid. Ethanoic acid: forms esters, reacts with bases and carbonates.
5. Soap forms micelles and scum in hard water; detergents do not form scum.