Class 10 Science Notes Chapter 4 — Carbon and its Compounds#

Covalent bonding#

• Carbon has atomic number 6 (2, 4): 4 valence electrons. It can neither lose 4 nor gain 4 electrons easily, so it shares electrons.
• Covalent bond — a bond formed by sharing a pair of electrons. A single bond shares 1 pair, a double bond 2 pairs, a triple bond 3 pairs.
• Examples: H₂ (H–H, single), O₂ (O=O, double), N₂ (N≡N, triple), CH₄ (four C–H single bonds).
• Covalent compounds have low melting and boiling points (weak forces between molecules) and are poor conductors of electricity (no ions).

Allotropes of carbon#

Why carbon forms so many compounds#

1. Catenation — carbon bonds with other carbon atoms to form long chains, branches and rings.
2. Tetravalency — with a valency of 4, it bonds with other carbon atoms and with H, O, N, S, Cl and others.

The C–C bond is strong, so these compounds are stable.

Hydrocarbons#

• Structural isomers — compounds with the same molecular formula but different structures. Butane (C₄H₁₀) has two: a straight chain and a branched chain.
• Cyclic compounds: cyclohexane C₆H₁₂ (saturated ring); benzene C₆H₆ (ring with alternate double bonds).

Functional groups#

Homologous series#

A homologous series is a series of compounds with the same functional group, in which each member differs from the next by –CH₂– (14 u).

• Similar chemical properties (decided by the functional group).
• Physical properties change gradually — melting point, boiling point and solubility change as molecular mass increases.

Naming: count the carbons for the stem (meth-, eth-, prop-, but-, pent-), add the functional group’s suffix, and drop the final “e” of the stem if the suffix begins with a vowel (propane → propanol).

Chemical properties#

Combustion: carbon compounds burn in oxygen to give CO₂, water, heat and light.

• Unsaturated compounds, or saturated ones burnt in limited air, give a sooty flame. A blackened cooking vessel means the air holes are blocked.
• Coal and petroleum burning give oxides of sulphur and nitrogen, which pollute.

Oxidation: alkaline KMnO₄ or acidified K₂Cr₂O₇ (oxidising agents) add oxygen.

Addition (hydrogenation): unsaturated compounds add hydrogen in the presence of a nickel or palladium catalyst. Used to turn vegetable oils into fats (vanaspati ghee). Animal fats are saturated and less healthy; oils with unsaturated fatty acids are better for cooking.

Substitution: saturated hydrocarbons react with chlorine in sunlight, replacing H atoms one by one.

Ethanol (C₂H₅OH)#

• Liquid at room temperature; soluble in water. Used in drinks, medicines (tincture iodine, cough syrups) and as a solvent.
• Absolute alcohol (pure ethanol) can be lethal. Methanol even in small amounts can cause blindness and death.
• Denatured alcohol — industrial ethanol made unfit to drink by adding methanol, coloured blue with dye.

Reactions:

(sodium ethoxide; hydrogen is given off)

(conc. H₂SO₄ acts as a dehydrating agent)

Ethanoic acid (CH₃COOH)#

• Also called acetic acid. A 5–8% solution is vinegar, used to preserve pickles.
• Pure ethanoic acid freezes at 290 K, so it is called glacial acetic acid.
• It is a weak acid, unlike HCl, which ionises completely.

Reactions:

• Esterification: acid + alcohol → ester (sweet-smelling), in the presence of an acid catalyst. $$\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{acid}} \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$$ Esters are used in perfumes and flavourings.
• Saponification: ester + NaOH → alcohol + sodium salt of the acid. This is how soap is made.
• With base: $\text{NaOH} + \text{CH}_3\text{COOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$
• With carbonates and hydrogencarbonates: gives salt, water and CO₂. $$2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$$

Soaps and detergents#

• Soap — sodium or potassium salt of a long-chain carboxylic acid.
• A soap molecule has an ionic (hydrophilic) end that dissolves in water and a long carbon chain (hydrophobic) end that dissolves in oil.
• In water, the molecules form micelles: clusters with the carbon tails pointing inward around the oily dirt, and the ionic heads outward. The dirt is trapped in the centre and washed away.
• Hard water contains calcium and magnesium salts. Soap reacts with them to form an insoluble scum, so it lathers poorly.
• Detergents — sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions. They do not form scum with hard water, so they work in hard water. Used in shampoos and cleaning products.

Common mistakes#

• Saying carbon forms ionic bonds. It forms covalent bonds by sharing.
• Mixing up alkene ($\text{C}_n\text{H}_{2n}$) and alkane ($\text{C}_n\text{H}_{2n+2}$) formulas.
• Saying members of a homologous series have the same physical properties. Only the chemical properties are similar.
• Calling hydrogenation a substitution reaction. It is an addition reaction.