Carbon and Its Compounds Exercise Solutions | KalviGavani

Carbon has 4 valence electrons and each oxygen has 6 valence electrons. Carbon shares 2 electrons with each oxygen atom, forming two double bonds:

Structural formula: O=C=O (carbon forms a double bond with each oxygen). Each oxygen atom now has 8 electrons in its outer shell (4 from its own lone pairs + 4 shared with carbon). Carbon also achieves 8 electrons (4 shared with each oxygen). This satisfies the octet rule for all three atoms.

Sulphur (atomic number 16) has 6 valence electrons. In the S₈ molecule, each sulphur atom forms 2 single covalent bonds with two adjacent sulphur atoms and retains 2 lone pairs. The 8 sulphur atoms arrange in a crown-shaped ring:

Each S–S bond is formed by sharing one pair of electrons. Every sulphur atom completes its octet: 4 electrons in 2 shared pairs (2 bonds) + 4 electrons in 2 lone pairs = 8 electrons.

Pentane has the molecular formula C₅H₁₂. It has 3 structural isomers:

1. n-Pentane (pentane): Straight chain CH₃–CH₂–CH₂–CH₂–CH₃ (Boiling point: 36 °C)

2. Isopentane (2-methylbutane): One branch

(Boiling point: 28 °C)

3. Neopentane (2,2-dimethylpropane): Two branches

(Boiling point: 9.5 °C)

All three have the same molecular formula (C₅H₁₂) but different structures, hence different physical properties. As branching increases, boiling point decreases because surface area decreases, weakening van der Waals forces.

1. Catenation: Carbon atoms have the unique ability to form bonds with other carbon atoms, giving rise to long chains, branched chains, and rings. The C–C bond energy (~348 kJ/mol) is nearly as strong as the C–H bond, making these chains stable. No other element shows catenation to this extent — silicon chains, for example, are far less stable.

2. Tetravalency: Having 4 valence electrons, a carbon atom can form bonds with 4 other atoms simultaneously. This allows carbon to bond with a wide variety of elements (H, O, N, S, Cl, Br, P) and also with other carbon atoms, creating an enormous diversity of structures — chains, branches, rings, and combinations of all three.

Cyclopentane is a cyclic alkane with 5 carbon atoms. Since it is cyclic, two C–H bonds are "lost" (the ends of the chain join), so the formula is C₅H₁₀ (general formula for cycloalkanes: CₙH₂ₙ).

Electron dot structure: Each carbon shares one electron pair with each of its 2 neighbouring carbon atoms and one electron pair with each of 2 hydrogen atoms. Every carbon has 4 single bonds.

Each carbon atom satisfies the octet through 2 C–C bonds and 2 C–H bonds.

(a) CH₃–CH₂–Br

Step-by-step IUPAC naming:

• Longest carbon chain: 2 carbons → root = "eth"
• All single bonds → suffix = "ane"
• Functional group: –Br → prefix = "bromo"
• Position of Br: carbon 1 (or 2, both same for 2-carbon chain)
• Name: Bromoethane

(b) CH₃–CHO

• Longest carbon chain containing –CHO: 2 carbons → root = "eth"
• Functional group: –CHO → suffix = "al" (aldehyde always at terminal carbon, position 1 by default)
• Name: Ethanal

(c) CH₃–CO–CH₃

• Longest carbon chain containing –CO–: 3 carbons → root = "propan"
• Functional group: –CO– (ketone) → suffix = "one"
• Position of carbonyl: carbon 2 (lowest possible number from either end)
• Name: Propanone (common name: acetone)

This indicates incomplete combustion of the fuel. When oxygen supply is insufficient, the carbon in the fuel does not burn completely to form CO₂. Instead, unburnt carbon particles (soot) deposit as a black layer on the vessel bottom. Carbon monoxide (CO) is also produced, which is a poisonous gas. A yellow, luminous flame (rather than a clean blue flame) is a sign of incomplete combustion. To fix this, increase the air supply to the burner so that complete combustion occurs: fuel + O₂ → CO₂ + H₂O + heat.

Chloromethane (CH₃Cl) is formed entirely by covalent bonds through electron sharing:

• Carbon (4 valence electrons) needs 4 more electrons to complete its octet.
• Three hydrogen atoms (1 valence electron each) each share 1 electron with carbon → 3 C–H single covalent bonds.
• Chlorine (7 valence electrons) shares 1 electron with carbon → 1 C–Cl single covalent bond.

Electron dot structure:

Each shared pair constitutes one covalent bond. Since electrons are shared (not transferred), no ions are formed. Covalent compounds like CH₃Cl have low melting and boiling points, are poor conductors of electricity, and exist as discrete molecules with weak intermolecular forces.

(i) Ethanoic acid (CH₃COOH):

Electron dot representation:

• C–H: 3 single covalent bonds (3 shared pairs)
• C–C: 1 single covalent bond (1 shared pair)
• C=O: 1 double bond (2 shared pairs)
• C–O: 1 single bond (1 shared pair)
• O–H: 1 single bond (1 shared pair)

Each atom achieves an octet (duplet for H).

(ii) H₂S:

Sulphur (6 valence electrons) shares 1 electron with each hydrogen, forming 2 S–H single bonds. Sulphur retains 2 lone pairs. H₂S has a V-shaped (bent) structure, similar to water.

(iii) Propanone (CH₃COCH₃):

Electron dot: The central carbon forms a C=O double bond with oxygen and single bonds with two –CH₃ groups. All atoms satisfy the octet rule.

(iv) F₂:

Each fluorine has 7 valence electrons. They share 1 pair of electrons, forming a single F–F bond. Each F now has 8 electrons (1 shared pair + 3 lone pairs).

A homologous series is a series of carbon compounds in which:

• All members have the same functional group and hence similar chemical properties.
• Each successive member differs from the previous one by a –CH₂– unit (14 u difference in molecular mass).
• All members share a general formula (e.g., CₙH₂ₙ₊₂ for alkanes, CₙH₂ₙ₊₁OH for alcohols).
• There is a gradual change in physical properties (boiling point, density, etc.) as molecular mass increases.

Example — Homologous series of alkanes:

Each successive alkane has one more –CH₂– unit than the previous. All alkanes undergo similar combustion and substitution reactions, but boiling points increase with chain length.

The litmus test and sodium carbonate test are the simplest ways to distinguish them: ethanoic acid turns blue litmus red and produces CO₂ with Na₂CO₃; ethanol does neither.

A soap molecule (e.g., sodium stearate, C₁₇H₃₅COONa) has two distinct parts:

1. Hydrophilic head (–COO⁻Na⁺): Ionic, water-loving, dissolves readily in water. 2. Hydrophobic tail (long hydrocarbon chain C₁₇H₃₅–): Non-ionic, water-repelling, cannot dissolve in water.

When soap is added to water, the hydrophilic heads dissolve in water while the hydrophobic tails cannot. To minimise contact with water, the tails cluster together, pointing inward, while the heads face outward toward water. This arrangement forms a micelle — a spherical cluster typically containing 50–100 soap molecules. Micelles form above a certain soap concentration called the critical micelle concentration (CMC).

In ethanol: Micelles will not form. Ethanol is an organic solvent that can interact with both the hydrophilic and hydrophobic parts of the soap molecule (it has both polar –OH and non-polar alkyl parts). Since there is no strong repulsion driving the tails to cluster together, soap dissolves uniformly in ethanol without forming micelles.

Carbon and its compounds are widely used as fuels because:

1. High calorific value: They release large amounts of energy per unit mass on complete combustion. Example: CH₄ + 2O₂ → CO₂ + 2H₂O + 890 kJ/mol. 2. Complete combustion products are relatively clean: CO₂ and H₂O are the main products, which are less polluting than residues from other fuels. 3. Easy availability: Coal, petroleum, natural gas are abundant natural resources. 4. Convenient storage and transport: Can be stored as solids (coal), liquids (petrol, diesel), or gases (LPG, CNG). 5. Controllable combustion: Flame temperature and rate can be adjusted by controlling air supply. 6. Ignition temperature is suitable: Not too low (unsafe) or too high (impractical).

The high energy released during combustion makes carbon-based fuels the most practical choice for cooking, heating, transportation, and industrial applications.

Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts (chlorides, sulphates, bicarbonates). Soap is a sodium salt of a fatty acid (e.g., sodium stearate, C₁₇H₃₅COONa). When soap is added to hard water, the sodium ions are replaced by calcium or magnesium ions:

The calcium/magnesium salts of fatty acids are insoluble in water and appear as a sticky, greyish-white precipitate called scum. Problems caused by scum:

• Wastes soap (a portion of soap is consumed in removing Ca²⁺/Mg²⁺ rather than cleaning).
• Does not produce lather, making washing difficult.
• Leaves deposits on clothes, skin, and utensils.

Detergents do not form scum because their calcium and magnesium salts are soluble in water, which is why detergents are more effective in hard water.

Soap solution is alkaline (basic) in nature because it is a sodium/potassium salt of a fatty acid, and the aqueous solution has a pH > 7 due to slight hydrolysis.

• Red litmus paper turns blue — confirms the basic nature of soap.
• Blue litmus paper remains blue — no change.

This confirms that soap is basic. Note: if the soap solution is very dilute, the colour change may be faint.

Hydrogenation is the chemical process of adding hydrogen (H₂) across a C=C double bond of an unsaturated hydrocarbon, converting it into a saturated hydrocarbon. The reaction takes place in the presence of a nickel or palladium catalyst at about 150–200 °C:

R–CH=CH–R' + H₂ →(Ni catalyst, 200°C) R–CH₂–CH₂–R'

The double bond breaks and hydrogen atoms attach to each carbon of the former double bond, forming single bonds.

Industrial application — manufacture of vegetable ghee (vanaspati): Vegetable oils (groundnut oil, sunflower oil, palm oil) are unsaturated — they contain C=C double bonds and are liquids at room temperature. Hydrogenation converts them into saturated fats (solid/semi-solid at room temperature) called vegetable ghee or dalda. This extends shelf life and changes texture. The nickel catalyst is filtered out before the product is packaged.

Only unsaturated hydrocarbons (those with C=C or C≡C bonds) undergo addition reactions. Saturated hydrocarbons (alkanes with only single bonds) do not.

C₃H₆ (propene) and C₂H₂ (ethyne) undergo addition reactions because they contain multiple bonds. For example, propene adds H₂, Br₂, or HBr across its double bond. Ethyne can add H₂ (once to give ethene, twice to give ethane) or Br₂ across its triple bond.

Bromine water test (or Baeyer's test):

• Add a few drops of orange-brown bromine water to each sample and shake.
• Cooking oil (unsaturated): Bromine water is decolourised (turns colourless). The C=C double bonds in the unsaturated fat undergo addition reaction with bromine, consuming it.
• Butter (mostly saturated): Little or no decolourisation of bromine water. Saturated fats lack C=C double bonds, so bromine is not consumed.

Alternative — Baeyer's test with alkaline KMnO₄:

• Add dilute alkaline KMnO₄ (purple) to each.
• Cooking oil: purple colour disappears (KMnO₄ oxidises the double bond, gets reduced).
• Butter: purple colour persists (no double bonds to oxidise).

Both tests exploit the presence or absence of unsaturation (C=C bonds) in the fat.

The cleaning action of soap involves several steps:

Step 1 — Dissolution and micelle formation: When soap is dissolved in water, the molecules arrange into clusters called micelles. The hydrophobic (water-repelling) hydrocarbon tails point inward, and the hydrophilic (water-attracting) ionic heads (–COO⁻Na⁺) point outward into the water.

Step 2 — Attachment to dirt/oil: When a dirty cloth is soaked in soapy water, the hydrophobic tails of soap molecules attach themselves to the oily dirt particles on the cloth surface. The hydrophilic heads remain embedded in the surrounding water.

Step 3 — Emulsification: Multiple soap molecules surround each oil/dirt particle, with their tails embedded in the oil and heads facing water. This forms a micelle around the dirt particle, effectively emulsifying (breaking into tiny droplets) the oil.

Step 4 — Removal by agitation: When the cloth is agitated (rubbed or machine-washed), the micelles enclosing the dirt are dislodged from the cloth surface. Because the hydrophilic heads face the water, the entire micelle (with trapped dirt) is pulled away into the water and rinsed off.

Step 5 — Rinsing: The dirty micelles remain suspended in the water and are carried away when the cloth is rinsed with clean water. The cloth is left clean.

This mechanism works because soap acts as a bridge between the water (polar) and oil (non-polar) — a process called emulsification. Without soap, water and oil do not mix, and the oily dirt cannot be removed.