Short Answer Questions — Chapter 11: Carbon and its Compounds
Carbon forms over 10 million compounds — more than any other element. Two unique properties explain this:
1. Catenation: Carbon can form strong covalent bonds with other carbon atoms to create:
- Long chains: CH₃–CH₂–CH₂–CH₃ (butane)
- Branched chains: Isobutane
- Rings: Cyclohexane, benzene (C₆H₆)
- Chains with double/triple bonds: Ethene, ethyne
No other element (except silicon, to a small extent) shows catenation as strongly as carbon. C–C bond is strong (347 kJ/mol) and stable.
2. Tetravalency: Carbon has 4 valence electrons → forms 4 covalent bonds. It can bond with:
- Hydrogen (C–H)
- Oxygen (C–O, C=O)
- Nitrogen (C–N)
- Halogens (C–Cl, C–Br)
- Other carbons (C–C, C=C, C≡C)
The combination of catenation and tetravalency creates enormous structural diversity.
A functional group is an atom or group of atoms that determines the chemical properties of an organic compound. Compounds with the same functional group have similar chemical behaviour.
| Functional Group | Name | Example |
|---|---|---|
| –OH | Hydroxyl | Methanol (CH₃OH), Ethanol (C₂H₅OH) — alcohols |
| –COOH | Carboxyl | Acetic acid (CH₃COOH) — carboxylic acids |
| –CHO | Aldehyde | Formaldehyde (HCHO), Acetaldehyde (CH₃CHO) |
| –CO– | Ketone (carbonyl) | Acetone (CH₃COCH₃) |
| –NH₂ | Amino | Methylamine (CH₃NH₂) — amines |
| –Cl, –Br | Halo | Chloroethane (C₂H₅Cl) — halides |
| –COOR | Ester | Ethyl acetate (CH₃COOC₂H₅) |
The carbon chain (without functional group) is the backbone; the functional group gives reactivity.
Homologous series: A series of organic compounds in which each member differs from the next by a –CH₂– unit (14 in molecular mass), has the same functional group, and can be represented by a general formula.
Properties of a homologous series: 1. Same general formula (e.g., CₙH₂ₙ₊₂ for alkanes) 2. Successive members differ by –CH₂– 3. Same functional group → similar chemical properties 4. Physical properties change gradually (boiling point, melting point, density increase with molecular mass)
Alkane Homologous Series:
| Name | Formula | State at room temp |
|---|---|---|
| Methane | CH₄ | Gas |
| Ethane | C₂H₆ | Gas |
| Propane | C₃H₈ | Gas |
| Butane | C₄H₁₀ | Gas |
| Pentane | C₅H₁₂ | Liquid |
| Hexane | C₆H₁₄ | Liquid |
| Heptane | C₇H₁₆ | Liquid |
| Octane | C₈H₁₈ | Liquid |
Saponification is the hydrolysis of fats or oils (esters) with a strong alkali (NaOH or KOH) to produce soap (fatty acid salt) and glycerol (glycerin).
Reaction:
Fat (triglyceride) + 3NaOH → 3 Soap (sodium fatty acid salt) + Glycerol
Example: Glyceryl tristearate + 3NaOH → 3 Sodium stearate (soap) + Glycerol
- Fats from animals (lard, tallow) or plants (coconut oil, palm oil) are used
- Sodium soaps = hard bar soaps; Potassium soaps = soft/liquid soaps
How soap cleans (micelle formation):
Soap molecule has two parts: 1. Hydrophilic head (–COO⁻Na⁺): Water-loving, ionic end 2. Hydrophobic tail (long hydrocarbon chain): Water-repelling, oil-loving end
When soap is added to oily water: 1. Hydrophobic tails of soap molecules insert into the oil/grease 2. Hydrophilic heads remain in water 3. Soap molecules surround the oil droplet forming a micelle (spherical cluster) 4. The micelle is surrounded by water on the outside → can be rinsed away 5. The oily dirt is thus emulsified and removed
Carbon exists in different physical forms (allotropes) with different structures and properties:
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