CBSE Class 10 Science Carbon and its Compounds Notes Set 06

1. Bonding in carbon

Carbon has four electrons in its outer shell (atomic number 6, configuration 2, 4). It cannot easily lose or gain four electrons, so it shares them and forms covalent bonds. A covalent bond is a shared pair of electrons. Examples are methane CH₄, ethene C₂H₄ (a double bond) and ethyne C₂H₂ (a triple bond). Covalent compounds usually have low melting and boiling points and are poor conductors of electricity.

2. Versatile nature of carbon

Carbon forms a huge number of compounds for two reasons. Catenation is the ability of carbon atoms to join with each other in long chains, branched chains and rings. Tetravalency lets each carbon bond with four other atoms such as hydrogen, oxygen, nitrogen, sulphur and halogens. The small size of carbon makes its bonds strong and stable. Allotropes of carbon include diamond (each atom bonded to four others, very hard), graphite (layers of hexagons, soft and conducting) and fullerene (C-60, football shaped).

3. Hydrocarbons and isomerism

Hydrocarbons contain only carbon and hydrogen. Saturated hydrocarbons (alkanes, C_nH_2n+2) have only single bonds. Unsaturated hydrocarbons have double bonds (alkenes, C_nH_2n) or triple bonds (alkynes, C_nH_2n−2). Compounds with the same molecular formula but different structures are isomers. Butane has two isomers while pentane has three.

4. Functional groups and homologous series

A functional group is an atom or group of atoms that decides the chemical properties of a compound. Common ones are halo (−Cl, −Br), alcohol (−OH), aldehyde (−CHO), ketone (C=O inside the chain) and carboxylic acid (−COOH).

A homologous series is a family of compounds with the same functional group and similar chemical properties in which successive members differ by a CH₂ unit (14 u). Physical properties such as boiling point change gradually along the series. Examples: methane, ethane, propane, butane.

5. Nomenclature

1. Count the carbon atoms in the longest chain: 1 meth, 2 eth, 3 prop, 4 but, 5 pent, 6 hex.

2. Add the ending: ane for single bonds, ene for a double bond, yne for a triple bond.

3. For a functional group, replace the final e of the name with its suffix: ol (alcohol), al (aldehyde), one (ketone), oic acid (carboxylic acid). A halogen is a prefix, for example chloropropane.

Examples: CH₃OH is methanol, CH₃CH₂OH is ethanol, CH₃CHO is ethanal, CH₃COCH₃ is propanone, CH₃COOH is ethanoic acid.

6. Chemical reactions of carbon compounds

1. Combustion: carbon compounds burn in oxygen to give CO₂, water, heat and light. CH₄ + 2O₂ → CO₂ + 2H₂O. Saturated hydrocarbons burn with a clean blue flame, while unsaturated ones burn with a yellow sooty flame.

2. Oxidation: alcohols are oxidised to carboxylic acids using alkaline KMnO₄ or acidified K₂Cr₂O₇.

3. Addition: unsaturated hydrocarbons add hydrogen in the presence of nickel or palladium catalyst, for example CH₂=CH₂ + H₂ → CH₃CH₃. This is used to make vegetable ghee from oils.

4. Substitution: saturated hydrocarbons react with chlorine in sunlight, replacing hydrogen: CH₄ + Cl₂ → CH₃Cl + HCl.

7. Ethanol and ethanoic acid

Ethanol is a colourless liquid that is soluble in water and used in medicines, tonics and as a solvent and fuel. It reacts with sodium to give hydrogen: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂. Heating with concentrated sulphuric acid at 443 K removes water and gives ethene (dehydration). Alcohol meant for industrial use is made unfit to drink by adding methanol and a dye, called denatured alcohol.

Ethanoic acid (acetic acid, 5 to 8 percent solution is vinegar) freezes at 290 K to glacial acetic acid. It is a weak acid. It reacts with carbonates and hydrogencarbonates to release CO₂. With ethanol and a few drops of concentrated sulphuric acid it forms a sweet-smelling ester: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. Alkaline hydrolysis of an ester is called saponification and gives soap.

8. Soaps and detergents

Soap is the sodium or potassium salt of a long chain fatty acid. A soap molecule has a water-loving ionic head and a water-hating hydrocarbon tail. In water, soap forms spherical clusters called micelles with tails pointing inward around the grease, which lifts the dirt away. Soaps form an insoluble scum with calcium and magnesium salts in hard water, but detergents (sodium salts of sulphonic acids) do not, so they work well even in hard water.

Worked examples

Example 1. Name the compound CH₃CH₂CH₂CHO.

The chain has 4 carbons, so the base is butane. The group is an aldehyde, so the final e is replaced by al. The name is butanal.

Example 2. How many covalent bonds are present in one molecule of propane, C₃H₈?

There are 2 carbon to carbon bonds and 8 carbon to hydrogen bonds. The total is 10 single covalent bonds, which equals 3n + 1 for n = 3.

Example 3. A compound A, C₂H₆O, gives C₂H₄ (B) on heating with concentrated H₂SO₄, and B gives C₂H₆ (C) on adding hydrogen. Identify A, B, C.

A is ethanol, which loses a molecule of water to give ethene (B). Ethene adds hydrogen over a nickel catalyst to give ethane (C). Concentrated sulphuric acid acts as the dehydrating agent.

Quick Check Questions

Answer these five questions to test your understanding of the chapter.