Chapter 4: Carbon and its Compounds

Solution

Carbon has 4 valence electrons and needs 4 more to complete its octet. It achieves this by forming 4 covalent bonds with other atoms. This property is called tetravalency.

Solution

Each nitrogen atom has 5 valence electrons and needs 3 more. Two nitrogen atoms share 3 pairs of electrons (6 electrons total) to form a triple bond: N≡N. This makes N₂ a very stable and unreactive molecule.

Solution

Catenation is the unique ability of carbon atoms to form bonds with other carbon atoms, resulting in straight chains, branched chains, and rings. This is possible because the C—C bond is very strong (bond energy ≈ 346 kJ/mol) due to the small size of carbon atoms.

Solution

Alkenes are unsaturated hydrocarbons with one carbon-carbon double bond (C=C). Their general formula is C_nH_2n. For example: ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈).

Alkanes: C_nH_2n+2; Alkynes: C_nH_2n−2

Solution

CH₃OH has 1 carbon atom (prefix: meth-) and an —OH (hydroxyl) functional group (suffix: -ol). Therefore, the IUPAC name is methanol. Its common name is methyl alcohol or wood spirit.

Solution

Saturated hydrocarbons (alkanes) have a lower percentage of carbon compared to unsaturated hydrocarbons. They undergo complete combustion in sufficient air, producing CO₂ and H₂O with a clean blue flame. Unsaturated hydrocarbons burn with a sooty yellow flame due to incomplete combustion.

Solution

C₂H₅OH →^{conc. H₂SO₄, 443 K} CH₂=CH₂ + H₂O

Concentrated H₂SO₄ acts as a dehydrating agent, removing water from ethanol to form ethene (an alkene). This is a dehydration reaction.

Solution

Pure (100%) ethanoic acid has a freezing point of 16.6°C. Below this temperature, it solidifies into ice-like crystals. Because of this resemblance to glacial ice, pure acetic acid is called glacial acetic acid.

Solution

A micelle is a spherical structure formed by soap molecules in water. The hydrophobic (water-repelling) tails point inward, away from water, while the hydrophilic (water-attracting) heads point outward into the water. Oil and grease get trapped inside the micelle and are washed away.

Solution

Using the general formulae:

• Alkane C₃: C₃H₂₍₃₎₊₂ = C₃H₈ — doesn't match
• Alkene C₃: C₃H₂₍₃₎ = C₃H₆ — doesn't match
• Alkyne C₃: C₃H₂₍₃₎₋₂ = C₃H₄ — matches!

C₃H₄ is propyne, an alkyne with a C≡C triple bond.

Solution

Covalent compounds are formed by sharing of electrons between atoms. They exist as molecules held together by weak intermolecular forces (van der Waals forces). Since these forces are weak, covalent compounds have low melting and boiling points, not high. They are generally poor conductors of electricity (no free ions) and are usually soluble in organic solvents like alcohol and acetone rather than water.

Option A ('high melting and boiling points') is incorrect for covalent compounds — this is a characteristic of ionic compounds.

Solution

CH₃COOH has 2 carbon atoms (prefix: eth-) and a —COOH (carboxyl) functional group (suffix: -oic acid). Therefore, the IUPAC name is ethanoic acid. Its common name is acetic acid (found in vinegar at 5-8% concentration).

Solution

Vegetable oils contain unsaturated fatty acids (with C=C double bonds). When hydrogen gas is added in the presence of a nickel (Ni) catalyst, the double bonds are converted to single bonds, making the fat saturated and solid. This process is called hydrogenation.

The product (vanaspati ghee/margarine) is a solid or semi-solid fat.

Solution

CH₄ + Cl₂ →^sunlight CH₃Cl + HCl

One hydrogen atom of methane is replaced (substituted) by a chlorine atom. This is a substitution reaction, characteristic of saturated hydrocarbons. Unsaturated hydrocarbons undergo addition reactions instead.

Solution

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑

The reaction produces brisk effervescence due to CO₂ gas. This is a standard test to confirm the presence of a carboxylic acid — it reacts with carbonates/bicarbonates to release CO₂ (tested with lime water turning milky).

Solution

Hard water contains dissolved Ca²⁺ and Mg²⁺ ions. These react with soap (sodium stearate) to form insoluble calcium or magnesium stearate, which is the white, sticky scum:

2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl

This scum wastes soap and reduces its cleaning action. Detergents do not form scum with hard water, which is why they are preferred in areas with hard water.

Solution

Esterification is the chemical reaction between an alcohol and a carboxylic acid in the presence of an acid catalyst (usually concentrated H₂SO₄) to form an ester and water.

Balanced equation:

CH₃COOH + C₂H₅OH →^{conc. H₂SO₄} CH₃COOC₂H₅ + H₂O

(Ethanoic acid + Ethanol → Ethyl ethanoate + Water)

Properties of the ester (ethyl ethanoate):

• Has a sweet, fruity smell
• Insoluble in water but soluble in organic solvents
• Used in perfumes, flavouring agents (artificial fruit flavours), and nail polish remover

Reverse reaction (Saponification): When an ester is heated with NaOH, it gives back the alcohol and sodium salt of the acid. This is used in soap making.

Solution

Homologous series: A family of organic compounds having the same general formula, same functional group, and similar chemical properties, where successive members differ by a —CH₂— group.

Characteristics:

1. All members have the same general formula (e.g., C_nH_2n+2 for alkanes).
2. Successive members differ by —CH₂— (mass difference of 14 u).
3. All members share similar chemical properties due to the same functional group.
4. Physical properties (melting point, boiling point, density) change gradually with increasing molecular mass.

First three alkanes:

1. Methane — CH₄ (M.W. = 16)
2. Ethane — C₂H₆ (M.W. = 30)
3. Propane — C₃H₈ (M.W. = 44)

Solution

(a) CH₃CH₂OH

2 carbons (eth-) + hydroxyl group (—OH → -ol) = Ethanol

(b) CH₃COCH₃

3 carbons (prop-) + ketone group (>C=O → -one) = Propanone (common name: acetone)

(c) HCOOH

1 carbon (meth-) + carboxyl group (—COOH → -oic acid) = Methanoic acid (common name: formic acid — found in ant stings)

(d) CH₃CH₂CH₃

3 carbons (prop-) + all single bonds (alkane → -ane) = Propane (used as LPG fuel)

Solution

Alkaline KMnO₄ (potassium permanganate) or acidified K₂Cr₂O₇ (potassium dichromate) can be used as oxidising agents to convert ethanol to ethanoic acid:

CH₃CH₂OH →^{alkaline KMnO₄ / K₂Cr₂O₇} CH₃COOH

The colour of KMnO₄ (purple/pink) is decolorised during the reaction.

Solution

In a homologous series, each successive member differs by —CH₂—.

Current alkane: C₅H₁₂

Next higher homologue: C₅₊₁H₁₂₊₂ = C₆H₁₄ (hexane)

Verification using alkane formula: C_nH_2n+2 for n = 6: C₆H₂₍₆₎₊₂ = C₆H₁₄ ✓

Solution

Cleansing action of soap:

A soap molecule has two parts:

• Hydrophobic tail (long hydrocarbon chain) — repels water, attracted to oil/grease.
• Hydrophilic head (—COO⁻Na⁺) — attracted to water.

Mechanism:

1. When soap is dissolved in water and applied to a dirty surface, the hydrophobic tails of soap molecules penetrate and attach to the oil/grease.
2. The hydrophilic heads remain in the water phase.
3. Many soap molecules surround each oil droplet, forming a spherical cluster called a micelle, with tails pointing inward (towards oil) and heads pointing outward (into water).
4. The micelle is now emulsified (suspended) in water and can be rinsed away, removing the dirt.

Why detergents work better in hard water:

Hard water contains Ca²⁺ and Mg²⁺ ions. With soap, these ions form insoluble scum (e.g., calcium stearate), reducing cleaning ability.

Detergents (sodium salts of sulphonic acids) form soluble calcium and magnesium salts — no scum. Hence, detergents clean effectively in both hard and soft water.

Solution

Danger of methanol (CH₃OH):

When methanol enters the body, it is oxidised by liver enzymes to:

1. Formaldehyde (HCHO) — highly toxic
2. Formic acid (HCOOH) — damages the optic nerve

Even 10 mL of methanol can cause permanent blindness, and 30 mL can cause death.

Ethanol (C₂H₅OH) is metabolised to acetaldehyde and then acetic acid, which are relatively less toxic and can be processed by the body in moderate amounts. However, excessive ethanol consumption damages the liver and brain.

Denatured alcohol: Ethanol that is made unfit for drinking by adding toxic substances like methanol, pyridine, or copper sulphate. The purpose is to prevent the misuse of industrial ethanol as an alcoholic beverage. It is used as a solvent, fuel, and in industry.

Solution

Why covalent compounds are poor conductors:

Covalent compounds are formed by sharing of electrons between atoms. They exist as molecules (not ions). Since there are no free ions or free electrons to carry electric charge, covalent compounds are poor conductors of electricity.

Comparison:

Solution

Identification:

• X = Ethanol (C₂H₅OH) — molecular formula C₂H₆O, reacts with Na to give H₂
• Y = Ethene (CH₂=CH₂) — molecular formula C₂H₄, decolourises bromine water (unsaturated)

Reaction 1 (X with sodium):

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑

Type: Displacement reaction (Na displaces H from the —OH group)

Reaction 2 (X to Y — dehydration):

C₂H₅OH →^{conc. H₂SO₄, 443 K} CH₂=CH₂ + H₂O

Type: Dehydration reaction (elimination of water). H₂SO₄ acts as a dehydrating agent.

Why Y decolourises bromine water: Ethene has a C=C double bond. It undergoes an addition reaction with bromine:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)

The orange colour of bromine water disappears, confirming unsaturation.