Chapter 4: Carbon and its Compounds

Answer: Carbon (atomic number 6) has 4 valence electrons. Oxygen (atomic number 8) has 6 valence electrons. In CO₂, carbon shares two electrons with each oxygen atom to complete its octet. Electron dot structure: :O::C::O: (each oxygen has two lone pairs and two shared pairs).

Answer: Each sulphur atom has 6 valence electrons. In the S₈ ring, each sulphur atom forms two single covalent bonds with its two neighbouring sulphur atoms. Thus, each sulphur atom completes its octet (two bonds + four lone electrons). The structure is a closed ring of eight sulphur atoms.

Answer: Pentane (C₅H₁₂) has three structural isomers:
(1) n-pentane (straight chain)
(2) isopentane (2-methylbutane)
(3) neopentane (2,2-dimethylpropane).

Answer: (i) Catenation – the ability of carbon atoms to form long chains, branched chains, and rings by bonding with other carbon atoms. (ii) Tetravalency – carbon has four valence electrons, allowing it to form four covalent bonds with other atoms (H, O, N, halogens, etc.).

Answer: Formula: C₅H₁₀. Cyclopentane is a ring of five carbon atoms, each bonded to two hydrogen atoms (except each carbon already shares two bonds with neighbours). Electron dot structure shows each carbon forming single bonds with two adjacent carbons and two hydrogens.

Answer: (i) Ethanoic acid: CH₃-COOH
(ii) Bromopentane: C₅H₁₁Br (many isomers possible depending on Br position)
(iii) Butanone: CH₃-CO-CH₂-CH₃
(iv) Hexanal: CH₃-CH₂-CH₂-CH₂-CH₂-CHO
Yes, bromopentane has structural isomers (1-bromopentane, 2-bromopentane, 3-bromopentane, etc.).

Answer:
(i) Bromoethane (or ethyl bromide)
(ii) Methanal (formaldehyde)
(iii) Propan-1-ol (or n-propyl alcohol)
(iv) Butanoic acid (butyric acid)

Answer: Only (c) C₄H₁₀ and (d) C₅H₁₂ show structural isomerism. C₂H₄ (ethene) and C₃H₈ (propane) have no structural isomers.

Answer: In this conversion, ethanol (CH₃CH₂OH) loses hydrogen atoms (or gains oxygen) to form ethanoic acid (CH₃COOH). The loss of hydrogen or gain of oxygen is oxidation. Therefore, it is an oxidation reaction.

Answer: Air contains less oxygen (about 21%). When ethyne burns in air, it produces a sooty flame due to incomplete combustion, which does not generate enough heat for welding. Pure oxygen gives a high-temperature oxy-acetylene flame (approx. 3000°C) used for welding.

Answer: Add a small amount of sodium hydrogencarbonate (baking soda) to both. Carboxylic acid will produce brisk effervescence of CO₂ gas, while alcohol will not react. Alternatively, use litmus paper: carboxylic acid turns blue litmus red, alcohol has no effect.

Answer: (b) 7 covalent bonds – one C–C bond and six C–H bonds.

Answer: (c) ketone – butanone (CH₃COCH₂CH₃) has a carbonyl group (C=O) bonded to two alkyl groups.

Answer: (b) the fuel is not burning completely – incomplete combustion produces unburnt carbon particles which blacken the vessel.

Answer: In CH₃Cl, carbon (4 valence electrons) forms single covalent bonds with three hydrogen atoms and one chlorine atom by sharing electrons. All atoms achieve stable octet/duplet. The bonds are formed by mutual sharing of electron pairs, with no transfer of electrons, hence covalent.

Answer:
(a) Ethanoic acid: CH₃-COOH (carbonyl and OH group).
(b) H₂S: H–S–H, sulphur shares one electron with each H.
(c) Propanone: CH₃–CO–CH₃, central carbon double-bonded to O.
(d) F₂: F–F, each fluorine shares one electron.

Answer: A homologous series is a group of organic compounds having the same functional group and similar chemical properties, where each successive member differs by a –CH₂– unit. Example: Alkanes (methane CH₄, ethane C₂H₆, propane C₃H₈, etc.).

Answer: Physical: Ethanol is a liquid with a pleasant smell, miscible with water; ethanoic acid has a pungent vinegar-like smell, also miscible but freezes at 17°C (glacial).
Chemical: Ethanoic acid reacts with NaHCO₃ to produce CO₂ effervescence; ethanol does not. Ethanoic acid turns blue litmus red; ethanol has no effect.

Answer: Soap molecules have a hydrophobic (hydrocarbon) tail and a hydrophilic (ionic) head. In water, the hydrophobic tails cluster together to avoid water, forming micelles. In ethanol, which is a polar organic solvent, both parts of soap may be soluble, so micelles do not form.

Answer: Carbon compounds (hydrocarbons) undergo combustion releasing large amounts of heat energy. They are easily available, can be stored and transported safely, and have high calorific values. This makes them suitable as fuels.

Answer: Hard water contains calcium and magnesium ions. Soap (sodium or potassium salts of fatty acids) reacts with these ions to form insoluble precipitates called scum (calcium or magnesium salts of fatty acids). This scum reduces the cleansing action of soap and wastes soap.

Answer: Soap solution is basic (pH around 9-10). It will turn red litmus paper blue and will not change blue litmus paper.

Answer: Hydrogenation is the process of adding hydrogen to unsaturated hydrocarbons (alkenes/alkynes) in the presence of a catalyst (e.g., Ni, Pd) to convert them into saturated hydrocarbons (alkanes). Industrial application: converting vegetable oils (unsaturated) into vanaspati ghee (saturated fat).

Answer: Unsaturated hydrocarbons undergo addition reactions. C₃H₆ (propene) and C₂H₂ (ethyne) are unsaturated (contain double or triple bonds). C₂H₆, C₃H₈, and CH₄ are saturated alkanes and do not undergo addition under normal conditions.

Answer: Bromine water test or alkaline KMnO₄ (Baeyer's test). When an unsaturated compound (with double bond) is added to bromine water, the reddish-brown colour of bromine disappears. Similarly, purple colour of KMnO₄ decolorises, indicating unsaturation.

Answer: (i) Physical: Colourless liquid, pungent smell, sour taste, miscible with water, freezes at 17°C (glacial acetic acid).
(ii) Chemical: Acidic – turns blue litmus red, reacts with bases, carbonates, and active metals to give salts. Reacts with ethanol in presence of conc. H₂SO₄ to form ester (esterification).

Answer: Soaps are sodium or potassium salts of long-chain fatty acids. Cleansing action: Soap molecules form micelles with hydrophobic tails pointing inward (trapping oily dirt) and hydrophilic heads outward (dissolving in water). The micelles are washed away with water, removing the dirt.

Answer: Catenation is the ability of an element to form long chains or rings by bonding with its own atoms. Carbon and silicon show catenation, but carbon is most prominent.

Answer: (i) Benzene: C₆H₆ (six-carbon ring with alternating double bonds). (ii) Cyclohexane: C₆H₁₂ (six-carbon saturated ring).

Answer:
(a) CH₄ + 2O₂ → CO₂ + 2H₂O
(b) C₂H₄ + H₂ → C₂H₆ (with Ni catalyst)
(c) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄)