Carbon and Its Compounds — Class 10 Physical Science
"Carbon is unique. No other element can form the VARIETY, the COMPLEXITY, and the SHEER NUMBER of compounds that carbon does. It is the element of LIFE."
1. Why Carbon Is Special
Catenation: Carbon atoms can LINK to form LONG CHAINS and RINGS. Silicon can also catenate — but far less extensively. Tetravalency: Carbon has 4 VALENCE ELECTRONS → forms 4 COVALENT BONDS. C−C bond energy is HIGH (347 kJ/mol) — these bonds are STABLE.
2. Allotropes — Same Element, Different Structures
| Allotrope | Structure | Properties and Uses |
|---|---|---|
| Diamond | Each C bonded to 4 others. 3D rigid network. | HARDEST natural substance. Insulator (no free electrons). Transparent. Used in cutting tools. |
| Graphite | Layers of hexagons. Weak forces between layers. | Layers SLIDE → lubricant. CONDUCTS electricity (one free electron per C atom — delocalised). Pencil 'lead.' Electrodes. |
| Fullerenes (C₆₀) | Soccer-ball shaped molecules | Nanotechnology. Drug delivery. |
3. Hydrocarbons
| Type | General Formula | Bond | Name Ending | Example |
|---|---|---|---|---|
| Alkane | CₙH₂ₙ₊₂ | Single (C−C) | -ane | CH₄ (methane) |
| Alkene | CₙH₂ₙ | Double (C=C) | -ene | C₂H₄ (ethene) |
| Alkyne | CₙH₂ₙ₋₂ | Triple (C≡C) | -yne | C₂H₂ (ethyne) |
Saturated vs Unsaturated
Saturated: ALL single bonds (alkanes). Unsaturated: Contains double or triple bonds (alkenes, alkynes). 'Unsaturated compounds DECOLOURISE bromine water (addition reaction) — this is the TEST for unsaturation.'
4. Functional Groups
| Group | Formula | Name Ending | Example |
|---|---|---|---|
| Alcohol | −OH | -ol | C₂H₅OH (ethanol) |
| Aldehyde | −CHO | -al | HCHO (methanal — formalin) |
| Carboxylic Acid | −COOH | -oic acid | CH₃COOH (ethanoic acid — vinegar) |
| Ketone | >C=O | -one | CH₃COCH₃ (propanone — acetone, nail polish remover) |
Homologous Series
Same general formula. Same functional group. Each member differs from next by −CH₂−. GRADUAL change in physical properties (BP, MP increase with chain length — stronger intermolecular forces).
5. Soaps and Detergents
Soaps: Sodium or potassium salts of long-chain fatty acids (e.g., sodium stearate — C₁₇H₃₅COONa). Made by SAPONIFICATION: fat/oil + NaOH → soap + glycerol.
How Soap Cleans: Soap molecule has HYDROPHILIC head (COO⁻Na⁺ — water-loving, IONIC) and HYDROPHOBIC tail (long hydrocarbon chain — water-hating, NON-POLAR). In water: tails attach to oil/grease. Heads face water. MICELLES form — oil trapped inside, washed away.
Soap vs Detergent: Soaps form SCUM in HARD WATER (Ca²⁺, Mg²⁺ ions react → insoluble precipitate). Detergents are SYNTHETIC — work in hard water (no scum). 'Detergents are usually sulphonated — they don't react with Ca²⁺ or Mg²⁺.'
6. Common Mistakes
- 'All carbon compounds are organic' — CO₂, CO, carbonates (CaCO₃), bicarbonates are INORGANIC — even though they contain carbon.
- 'Alkanes are unreactive' — They undergo SUBSTITUTION reactions (with halogens in UV light) and COMBUSTION.
7. AP SSC Exam Focus
| Topic | Marks |
|---|---|
| Allotropes — diamond vs graphite | 3-4 |
| Hydrocarbons — naming | 3-4 |
| Functional groups | 3-4 |
| Soaps and detergents | 3-4 |
8. Covalent Bonding in Carbon — Detailed
Carbon has 4 valence electrons (2s²2p²). To achieve stable OCTET (8 electrons in valence shell), carbon must either GAIN 4 electrons (→ C⁴⁻ — not favoured because 4 extra electrons would cause high repulsion), LOSE 4 electrons (→ C⁴⁺ — too much energy needed), or SHARE 4 electrons (→ Covalent bonds — the MOST favourable). 'Carbon ALWAYS forms COVALENT bonds. It never forms ionic bonds — the energy cost of gaining or losing 4 electrons is simply too HIGH.'
Types of Covalent Bonds
Single Bond (C−C): 1 shared electron pair. Example: H₃C−CH₃ (Ethane). Bond length ~154 pm. Double Bond (C=C): 2 shared pairs. Example: H₂C=CH₂ (Ethene). Bond length ~134 pm (SHORTER — stronger pull). Triple Bond (C≡C): 3 shared pairs. Example: HC≡CH (Ethyne). Bond length ~120 pm (SHORTEST).
'As the number of shared pairs INCREASES: bond length DECREASES, bond STRENGTH increases, and reactivity CHANGES.'
Properties of Covalent Compounds
- LOW melting and boiling points (weak intermolecular forces — van der Waals forces).
- Do NOT conduct electricity (no free ions or electrons — except graphite).
- Generally INSOLUBLE in water (non-polar). SOLUBLE in organic solvents (benzene, ether, CCl₄).
- Reactions are SLOW (involve breaking covalent bonds — needs activation energy).
9. Nomenclature of Hydrocarbons — IUPAC Rules
- Find the LONGEST carbon chain (parent chain).
- Number the chain from the end NEAREST the functional group/double/triple bond.
- Name SUBSTITUENTS (side chains: methyl, ethyl) with position numbers.
- Write as: [position]-[substituent][parent chain][suffix].
Example: CH₃−CH(CH₃)−CH₂−CH₃ → Longest chain = 4 C atoms (butane). Methyl on C₂ → 2-methylbutane.
Example: CH₃−CH=CH−CH₃ → 4 C chain with double bond → but-2-ene (not 2-butene — IUPAC puts the number BEFORE the suffix).
10. Ethanol (C₂H₅OH) — Properties and Reactions
Physical: Colourless liquid. Characteristic smell. BP = 78°C. MISCIBLE with water (forms hydrogen bonds — the −OH group is polar).
Chemical Reactions:
- Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat (clean blue flame — used as FUEL).
- Oxidation: C₂H₅OH →(alkaline KMnO₄)→ CH₃COOH (ethanoic acid). 'KMnO₄ is an OXIDISING AGENT — it turns ethanol into ethanoic acid (vinegar).'
- Dehydration: C₂H₅OH →(conc. H₂SO₄, 443 K)→ C₂H₄ + H₂O. 'Concentrated sulphuric acid REMOVES water (dehydration) — ethene is formed.'
- Reaction with Sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑. 'The −OH hydrogen in ethanol is slightly acidic — reactive enough to produce H₂ with sodium, but NOT acidic enough to turn blue litmus red.'
Harmful Effects of Alcohol
Excessive consumption → liver damage (cirrhosis). Depresses central nervous system. Impairs judgement and coordination. 'Ethanol is also used as: ANTISEPTIC (70% solution kills bacteria). SOLVENT for medicines (tincture of iodine).'
11. Ethanoic Acid (CH₃COOH) — Properties and Reactions
Physical: Colourless liquid. PUNGENT smell (vinegar = 5-8% ethanoic acid). BP = 118°C. FREEZES at 17°C → 'glacial acetic acid' (ice-like crystals in cold weather).
Chemical Reactions:
- Acidic Nature: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Turns blue litmus RED. Reacts with bases (neutralisation): CH₃COOH + NaOH → CH₃COONa + H₂O.
- Esterification: CH₃COOH + C₂H₅OH ⇌(conc. H₂SO₄)→ CH₃COOC₂H₅ + H₂O. 'Ethyl acetate (ester) — fruity SMELL. Esters are used in FLAVOURINGS and PERFUMES. The reaction is REVERSIBLE.'
- Reaction with Carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑.
Soaps and Detergents — Deeper Dive
Micelle Formation: When soap is added to water, the HYDROPHOBIC tails point INWARD (away from water) and HYDROPHILIC heads point OUTWARD (toward water). These spherical structures are MICELLES (diameter ~10⁻⁷ m). 'Micelles trap DIRT and OIL in the centre. Mechanical agitation (rubbing) helps dislodge dirt from fabric. The micelles remain SUSPENDED in water (emulsification) — and are washed away.'
Hard Water Problem: Ca²⁺ and Mg²⁺ in hard water → react with soap → Ca/Mg stearate (INSOLUBLE precipitate = SCUM). 'Detergents solve this because their calcium and magnesium salts are SOLUBLE — no scum.'
Environmental Impact: Soaps are BIODEGRADABLE (made from natural fats/oils). Some detergents are NON-BIODEGRADABLE (branched hydrocarbon chains resist bacterial breakdown) → cause WATER POLLUTION (foam in rivers). 'Modern detergents have LINEAR (straight) chains — these ARE biodegradable.'
12. Substitution Reaction of Alkanes
Alkanes are generally LESS reactive than alkenes/alkynes. But they undergo SUBSTITUTION with halogens in the presence of SUNLIGHT (UV):
CH₄ + Cl₂ →(UV light)→ CH₃Cl + HCl (chloromethane) CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane) CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (trichloromethane — chloroform) CHCl₃ + Cl₂ → CCl₄ + HCl (tetrachloromethane — carbon tetrachloride)
'This is a SUBSTITUTION reaction — chlorine ATOMS REPLACE hydrogen atoms one by one.'
13. Addition Reaction of Unsaturated Hydrocarbons
Unsaturated compounds ADD atoms across the double/triple bond:
C₂H₄ + H₂ →(Ni catalyst, 300°C)→ C₂H₆ (hydrogenation — used to convert vegetable OILS to SEMI-SOLID fats like VANASPATI ghee) C₂H₄ + Br₂ → C₂H₄Br₂ (bromine water DECOLOURISES — test for unsaturation) C₂H₂ + 2Br₂ → C₂H₂Br₄
14. Self-Test
Q1: Why does carbon form only covalent bonds? A1: Carbon has 4 valence electrons. Gaining 4 electrons (→ C⁴⁻) or losing 4 electrons (→ C⁴⁺) would require TOO MUCH energy. SHARING 4 electrons (covalent bonding) is energetically most favourable.
Q2: Diamond is the hardest natural substance. Graphite is soft enough to write with. Explain. A2: Diamond has each C atom bonded to 4 others in a rigid 3D network — VERY strong. Graphite has layers held by WEAK van der Waals forces — layers can SLIDE over each other.
Q3: Why does soap not work well in hard water? A3: Hard water contains Ca²⁺ and Mg²⁺ ions. These react with soap (sodium stearate) to form INSOLUBLE SCUM (calcium/magnesium stearate) — no lather forms. Detergents work better because their Ca/Mg salts are soluble.
Q4: A compound has the molecular formula C₂H₆O. It reacts with sodium to produce H₂ gas but does NOT turn blue litmus red. Identify the compound. A4: It is ETHANOL (C₂H₅OH). The −OH hydrogen is slightly reactive (produces H₂ with Na) but not acidic enough to affect litmus.
Q5: What is a homologous series? Give one characteristic. A5: A HOMOLOGOUS SERIES is a group of organic compounds with the SAME functional group and SAME general formula. Each member differs from the next by −CH₂−. Characteristic: PHYSICAL properties change GRADUALLY (melting/boiling points increase with chain length).
Q6: Why does ethanoic acid freeze at 17°C? A6: Ethanoic acid has a relatively HIGH melting point due to STRONG intermolecular hydrogen bonding (dimer formation). Below 17°C, the molecules form a crystalline solid — 'glacial acetic acid'.
Q7: Explain how detergents are better than soaps for washing clothes. A7: Detergents work in BOTH hard and soft water (no scum). They have sulphonate groups (R−SO₃⁻Na⁺) that form soluble Ca²⁺/Mg²⁺ salts. However, some non-biodegradable detergents cause water pollution.
