Saturated and Unsaturated Carbon Compounds

Hydrocarbons — The Simplest Carbon Compounds

'Hydrocarbon' = a compound made of only hydrogen and carbon atoms.

These are the simplest carbon compounds.

Examples of Hydrocarbons

• $CH_4$ (methane — natural gas)
• $C_2H_6$ (ethane)
• $C_3H_8$ (propane — LPG)
• $C_4H_{10}$ (butane — LPG)
• $C_8H_{18}$ (octane — petrol)
• $C_{16}H_{34}$ (cetane — diesel)
• …long polymers like polyethylene

Two Major Types

1. Saturated Hydrocarbons: Only single bonds between carbons. 'Saturated' = full of hydrogens. Cannot accept more H atoms.

Examples: methane, ethane, propane. Series: Alkanes.

2. Unsaturated Hydrocarbons: Have double or triple bonds between carbons. 'Unsaturated' = can accept more H atoms. Less H than the saturated version with same C.

Sub-types:

• Alkenes — have double bonds.
• Alkynes — have triple bonds.

Why is This Distinction Important?

Saturated and unsaturated hydrocarbons have very different properties:

• Saturated: less reactive (single bonds — strong, hard to break).
• Unsaturated: more reactive (double/triple bonds — can be 'opened' to add atoms).

This affects:

• How they burn.
• How they react with other chemicals.
• Their uses (fuel, raw material for plastics, etc.).

Saturated vs Unsaturated — Quick Comparison

Property	Saturated	Unsaturated
Bonds	All single (C-C)	Some double (C=C) or triple (C≡C)
Series	Alkane	Alkene, Alkyne
Reactivity	Less	More
Test	No reaction with bromine water	Decolourises bromine water

A Common Example

Cooking gas (LPG) is mostly butane ($C_4H_{10}$) — saturated. Burns cleanly with blue flame.

Industrial plastic raw material is ethene ($C_2H_4$) — unsaturated. Reacts to form polyethene (plastic).

Alkanes — Saturated Hydrocarbons

'Alkane' family — only single bonds between C atoms.

General Formula

$$C_nH_{2n+2}$$

Where n = number of carbon atoms.

Calculating H Atoms

If 1 C: H = 2(1)+2 = 4 → CH₄ If 2 C: H = 2(2)+2 = 6 → C₂H₆ If 3 C: H = 2(3)+2 = 8 → C₃H₈ If 10 C: H = 2(10)+2 = 22 → C₁₀H₂₂

First Few Alkanes

n	Formula	Name	Common Use
1	CH₄	Methane	Natural gas, biogas
2	C₂H₆	Ethane	In LPG mix
3	C₃H₈	Propane	LPG
4	C₄H₁₀	Butane	LPG
5	C₅H₁₂	Pentane	Petrol component
6	C₆H₁₄	Hexane	Petrol component
7	C₇H₁₆	Heptane	Petrol component
8	C₈H₁₈	Octane	Petrol
9	C₉H₂₀	Nonane	—
10	C₁₀H₂₂	Decane	Diesel component

Naming Pattern

First 4: meth-, eth-, prop-, but- (historical names) 5+: pent-, hex-, hept-, oct-, non-, dec- (Greek numbers)

Suffix for alkane: -ane.

Structure of Alkanes

Methane ($CH_4$)

        H
        |
    H — C — H
        |
        H

1 C with 4 H. Tetrahedral.

Ethane ($C_2H_6$)

    H   H
    |   |
H — C — C — H
    |   |
    H   H

2 C joined by single bond. 6 H atoms total (3 on each C).

Propane ($C_3H_8$)

    H   H   H
    |   |   |
H — C — C — C — H
    |   |   |
    H   H   H

3 C in a row. 8 H atoms.

Properties of Alkanes

1. State at room temperature:

• C₁ to C₄: gases.
• C₅ to C₁₇: liquids.
• C₁₈ and above: solids.

2. Combustibility:

• All burn with clean blue flame in plenty of O₂.
• Used as fuels (natural gas, LPG, petrol, diesel).
• Reaction: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$

3. Stability:

• Single C-C bonds — strong and stable.
• Don't react easily.
• Called 'paraffins' (Latin: 'parum affinis' = little affinity).

4. Insolubility in water:

• Non-polar molecules.
• Soluble in non-polar solvents (kerosene, benzene).

Alkenes — Hydrocarbons with Double Bonds

'Alkene' family — have at least one C=C double bond.

General Formula

$$C_nH_{2n}$$

Where n = number of carbon atoms (n ≥ 2).

Note: 2 fewer H atoms than alkane with same n.

Calculating H Atoms

If 2 C: H = 2(2) = 4 → C₂H₄ If 3 C: H = 2(3) = 6 → C₃H₆ If 4 C: H = 2(4) = 8 → C₄H₈

First Few Alkenes

n	Formula	Name
2	C₂H₄	Ethene (Ethylene)
3	C₃H₆	Propene (Propylene)
4	C₄H₈	Butene
5	C₅H₁₀	Pentene

Suffix for alkene: -ene.

Structure of Alkenes

Ethene ($C_2H_4$) — The Simplest

    H       H
     \     /
      C = C
     /     \
    H       H

1 double bond between 2 C. 4 H atoms (2 per C).

Each C has only 2 H (compared to 3 in ethane) — because double bond uses 2 of the 4 bonds.

Propene ($C_3H_6$)

    H       H   H
     \     /   |
      C = C — C — H
     /         |
    H          H

1 double bond between C₁ and C₂. 1 single bond between C₂ and C₃. 6 H atoms.

Properties of Alkenes

1. Reactivity:

• More reactive than alkanes.
• The double bond can break to add atoms.
• Addition reactions are characteristic.

2. State at room temperature:

• C₂ to C₄: gases.
• C₅ to C₁₅: liquids.
• Higher: solids.

3. Combustibility:

• Burn — but with slightly yellow flame (more soot due to higher C content per H).

4. Bromine Water Test:

• Alkenes decolourise brown bromine water (orange → colourless).
• This is a standard test for unsaturation (presence of double bond).

Reaction: $$C_2H_4 + Br_2 \rightarrow C_2H_4Br_2$$ (1,2-dibromoethane — colourless)

Importance — Why Alkenes Matter

Alkenes are raw materials for:

• Plastics (polyethene from ethene).
• Synthetic fibres (polyester from ethylene glycol).
• Antifreeze.
• Drugs and chemicals.

Ethene alone — produces 130+ million tonnes worldwide annually!

A Common Use

Ethene is also a plant hormone! It causes fruits to ripen. Bananas, mangoes, tomatoes — release ethene as they ripen. That's why one ripe fruit makes others ripen faster — ethene gas spreads.

Alkynes — Hydrocarbons with Triple Bonds

'Alkyne' family — have at least one C≡C triple bond.

General Formula

$$C_nH_{2n-2}$$

Where n = number of carbon atoms (n ≥ 2).

Note: 4 fewer H atoms than alkane with same n.

Calculating H Atoms

If 2 C: H = 2(2)-2 = 2 → C₂H₂ If 3 C: H = 2(3)-2 = 4 → C₃H₄ If 4 C: H = 2(4)-2 = 6 → C₄H₆

First Few Alkynes

n	Formula	Name
2	C₂H₂	Ethyne (Acetylene)
3	C₃H₄	Propyne
4	C₄H₆	Butyne

Suffix for alkyne: -yne.

Structure of Alkynes

Ethyne ($C_2H_2$) — Acetylene

    H — C ≡ C — H

1 triple bond between 2 C atoms. Only 2 H atoms (1 per C). Linear molecule.

Each C has only 1 H (compared to 3 in ethane, 2 in ethene) — because triple bond uses 3 of the 4 bonds.

Propyne ($C_3H_4$)

H — C ≡ C — C — H
            |
            H
            |
            H

Triple bond + 1 single bond.

Properties of Alkynes

1. Highest Reactivity:

• Most reactive among hydrocarbons.
• The triple bond is rich in electrons.
• Many addition reactions possible.

2. State:

• C₂ to C₄: gases.
• Higher: liquids/solids.

3. Combustibility:

• Burn with smoky flame (high C content per H).
• High heat output → used in welding.

4. Bromine Water Test:

• Alkynes decolourise bromine water — same as alkenes.
• Test for unsaturation.

Importance of Ethyne (Acetylene)

$C_2H_2$ is industrially very important:

1. Oxyacetylene welding:

• Burning C₂H₂ + O₂ → temperature ~3000°C.
• Used to weld and cut metals.

2. Manufacture of:

• Vinyl chloride (for PVC plastic).
• Acrylonitrile (for synthetic fibres).
• Acetic acid (vinegar component).

3. Source for synthetic rubber.

Comparison — Alkanes, Alkenes, Alkynes

Feature	Alkane	Alkene	Alkyne
Bonds	All single	At least 1 double	At least 1 triple
Formula	$C_nH_{2n+2}$	$C_nH_{2n}$	$C_nH_{2n-2}$
Suffix	-ane	-ene	-yne
2-C example	Ethane (C₂H₆)	Ethene (C₂H₄)	Ethyne (C₂H₂)
Reactivity	Low	High	Highest
H per C ratio	High	Medium	Low
Bromine water	No reaction	Decolourises	Decolourises
Flame	Clean blue	Slight yellow	Smoky

Tests and Identification

How to Tell Saturated from Unsaturated?

Three quick tests:

1. Bromine Water Test (Most Famous)

Bromine water is reddish-brown ($Br_2$ dissolved in water).

Add to the hydrocarbon:

Saturated (alkane):

• No reaction.
• Bromine water remains brown.

Unsaturated (alkene/alkyne):

• Reacts immediately.
• Bromine water becomes colourless.

Why? Br₂ adds across the double/triple bond.

Examples: $$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$ (1,2-dibromoethane, colourless)

2. Combustion Test (Flame Colour)

Burn the hydrocarbon in air:

Type	Flame
Alkane	Clean blue, hardly any soot
Alkene	Slightly yellow with some soot
Alkyne	Yellow/sooty, much smoke

Why? As C:H ratio increases, less oxygen is needed per carbon. Incomplete combustion → soot.

Saturated has more H per C → cleaner burn. Unsaturated has less H per C → sootier burn.

3. KMnO₄ Test (Optional, Advanced)

Dilute potassium permanganate solution (purple) is added.

Saturated: No change. Solution remains purple. Unsaturated: Decolourises (purple → colourless).

Reason: KMnO₄ oxidises the double/triple bond.

Naming Conventions Quick Reference

For straight-chain hydrocarbons:

C atoms	Prefix	Alkane	Alkene	Alkyne
1	meth-	methane	(none)	(none)
2	eth-	ethane	ethene	ethyne
3	prop-	propane	propene	propyne
4	but-	butane	butene	butyne
5	pent-	pentane	pentene	pentyne
6	hex-	hexane	hexene	hexyne

Suffixes:

• -ane = alkane (saturated)
• -ene = alkene (1 double bond)
• -yne = alkyne (1 triple bond)

Why So Many Hydrocarbons?

Carbon's catenation + tetravalency:

• Different chain lengths.
• Different bond types.
• Different arrangements.

Result: thousands of hydrocarbons known. And we can keep making more!

A Quick Quiz

Identify each:

• $CH_4$ → alkane (1 C, all single bonds)
• $C_3H_6$ → alkene (matches $C_nH_{2n}$)
• $C_2H_2$ → alkyne (matches $C_nH_{2n-2}$)
• $C_5H_{12}$ → alkane (matches $C_nH_{2n+2}$)

🧠 Memory Capsule

A quick glance just before the board exam.

1. Hydrocarbons — Definition

Compounds of only C and H.

2. Two Main Types

Saturated (only single bonds) vs Unsaturated (double/triple bonds).

3. Three Series

Series	Bond Type	Formula
Alkane	Single	$C_nH_{2n+2}$
Alkene	Double	$C_nH_{2n}$
Alkyne	Triple	$C_nH_{2n-2}$

4. First Members

Alkanes:

• CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane)

Alkenes:

• C₂H₄ (ethene), C₃H₆ (propene), C₄H₈ (butene)

Alkynes:

• C₂H₂ (ethyne), C₃H₄ (propyne), C₄H₆ (butyne)

5. Suffixes

• -ane = alkane
• -ene = alkene
• -yne = alkyne

6. Tests for Unsaturation

• Bromine water test: decolourises (brown → colourless).
• KMnO₄ test: decolourises (purple → colourless).
• Saturated: no change in colour.

7. Combustion

• Alkane: clean blue flame.
• Alkene: slightly yellow.
• Alkyne: smoky/yellow.

8. Reactivity Order

Alkyne > Alkene > Alkane

9. Important Hydrocarbons

• Methane: natural gas, biogas, marsh gas.
• Butane, propane: LPG (cooking gas).
• Octane: petrol.
• Ethene: plastic raw material; plant hormone.
• Ethyne (acetylene): welding.

10. Board's 'Golden' Questions

1. Saturated vs Unsaturated — difference.
2. Test to differentiate (bromine water).
3. Why does ethene decolourise bromine water?
4. General formulas of three series.
5. Calculate H in $C_5H_{?}$ for alkane / alkene / alkyne.

Final Formula: Saturated = full of H = unreactive; Unsaturated = less H = reactive.

Solved Examples

Example 1: NCERT — Saturated and Unsaturated

What are saturated and unsaturated hydrocarbons? Give examples.

Solution:

Hydrocarbons — Recap

Compounds containing only C and H atoms.

Saturated Hydrocarbons

Definition: Hydrocarbons in which all bonds between C atoms are single bonds.

'Saturated' = filled with maximum H atoms. Cannot accept any more H.

General Formula: $C_nH_{2n+2}$

Series Name: Alkanes

Examples:

• Methane ($CH_4$)
• Ethane ($C_2H_6$)
• Propane ($C_3H_8$)
• Butane ($C_4H_{10}$)
• Octane ($C_8H_{18}$)

Properties:

• Less reactive (single bonds — strong, stable).
• Don't decolourise bromine water.
• Burn with clean blue flame.

Unsaturated Hydrocarbons

Definition: Hydrocarbons that have double or triple bonds between C atoms.

'Unsaturated' = can accept more H atoms.

Two types:

A. Alkenes (1 double bond)

• General Formula: $C_nH_{2n}$
• Examples: Ethene ($C_2H_4$), Propene ($C_3H_6$)

B. Alkynes (1 triple bond)

• General Formula: $C_nH_{2n-2}$
• Examples: Ethyne ($C_2H_2$), Propyne ($C_3H_4$)

Properties:

• More reactive (multiple bonds can break).
• Decolourise bromine water (test for unsaturation).
• Burn with sootier (yellow) flame.

Comparison Table

Property	Saturated	Unsaturated
Bonds	All single	Double or triple
Formula	$C_nH_{2n+2}$	$C_nH_{2n}$ or $C_nH_{2n-2}$
Reactivity	Less	More
Bromine water	No reaction	Decolourises
Flame	Clean blue	Yellow, sooty

[NCERT — every year]

Example 2: NCERT — Bromine Water Test

How can you distinguish between saturated and unsaturated hydrocarbons using bromine water?

Solution:

Bromine Water Test

'Bromine water' = $Br_2$ dissolved in water. Colour: reddish-brown (or orange).

Procedure

Add bromine water to the hydrocarbon (in a test tube):

Observation 1: Saturated Hydrocarbon (e.g., methane, ethane)

Bromine water remains brown/orange.

No reaction at room temperature. Single C-C bonds — strong, no reaction with Br₂.

If exposed to UV light, slow substitution may happen — but at room temperature in dark, no change.

Observation 2: Unsaturated Hydrocarbon (e.g., ethene, ethyne)

Bromine water becomes colourless.

Br₂ adds across the double/triple bond.

Reaction with ethene: $$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$ (1,2-dibromoethane, colourless)

The Br₂ is consumed → no more colour.

Reaction with ethyne: $$HC≡CH + 2Br_2 \rightarrow CHBr_2-CHBr_2$$ (1,1,2,2-tetrabromoethane, colourless)

Triple bond can absorb 2 Br₂ molecules.

Why This Test Works?

Br₂ has a brown colour due to its electronic structure. When Br₂ adds to a multiple bond, it breaks into 2 Br atoms, each forming a single bond. New molecule (with Br atoms attached) is colourless.

Important Notes

1. Both alkenes and alkynes decolourise bromine water. 2. Test doesn't distinguish between alkene and alkyne — it only confirms unsaturation. 3. To find the type: observe how much Br₂ is consumed.

• Alkene: 1 Br₂ per double bond.
• Alkyne: 2 Br₂ per triple bond.

A Practical Demonstration

Take 3 test tubes:

• A: Bromine water + methane → brown remains.
• B: Bromine water + ethene → colourless quickly.
• C: Bromine water + ethyne → colourless quickly.

Result:

• A: saturated (alkane).
• B, C: unsaturated.

Practical Significance

Used in industry to:

• Determine if a hydrocarbon is saturated or not.
• Quality test for organic compounds.
• Identification of new compounds.

[NCERT — important, every year]

Example 3: NCERT — Calculate Formulas

Calculate the molecular formula for: (a) Alkane with 5 C atoms (b) Alkene with 4 C atoms (c) Alkyne with 3 C atoms (d) Alkane with 8 C atoms

Solution:

Use General Formulas

• Alkane: $C_nH_{2n+2}$
• Alkene: $C_nH_{2n}$
• Alkyne: $C_nH_{2n-2}$

(a) Alkane with 5 C atoms

n = 5 H = 2(5) + 2 = 12

Formula: $C_5H_{12}$ (Pentane)

(b) Alkene with 4 C atoms

n = 4 H = 2(4) = 8

Formula: $C_4H_8$ (Butene)

(c) Alkyne with 3 C atoms

n = 3 H = 2(3) - 2 = 4

Formula: $C_3H_4$ (Propyne)

(d) Alkane with 8 C atoms

n = 8 H = 2(8) + 2 = 18

Formula: $C_8H_{18}$ (Octane)

Summary Table

Compound	n	Formula	Name
(a) Alkane	5	$C_5H_{12}$	Pentane
(b) Alkene	4	$C_4H_8$	Butene
(c) Alkyne	3	$C_3H_4$	Propyne
(d) Alkane	8	$C_8H_{18}$	Octane

Verification — Tetravalency Check

Each C has 4 bonds:

Pentane (C₅H₁₂):

• 5 C in chain.
• C-C single bonds: 4.
• C-H bonds per C: 3, 2, 2, 2, 3 (total 12 H).
• Each C has 4 bonds ✓

Butene (C₄H₈):

• 4 C in chain, 1 double bond.
• C=C: counts as 2 bonds.
• 8 H atoms attached.
• Each C has 4 bonds ✓

Propyne (C₃H₄):

• 3 C, 1 triple bond.
• C≡C: counts as 3 bonds.
• 4 H atoms.
• Each C has 4 bonds ✓

Octane (C₈H₁₈):

• 8 C in chain.
• 18 H atoms.
• Each C has 4 bonds ✓

Practical Application

These calculations:

• Help identify compound type.
• Useful in chemistry and pharmacology.
• Foundation for organic chemistry.

[Board: 3-mark numerical]

Example 4: NCERT — Combustion of Hydrocarbons

Write the balanced equation for the combustion of methane and ethyne.

Solution:

Combustion of Methane ($CH_4$)

Reactants: Methane + Oxygen Products: Carbon dioxide + Water + Heat

Unbalanced: $$CH_4 + O_2 \rightarrow CO_2 + H_2O$$

Balancing: Carbon: 1 = 1 ✓ Hydrogen: 4 = 2 — need 2 H₂O.* Oxygen: 4 = 4 — need 2 O₂.*

Balanced: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$$

This is exothermic — releases heat. This is why methane is the main component of LPG and natural gas (used as fuel).

Combustion of Ethyne ($C_2H_2$)

Unbalanced: $$C_2H_2 + O_2 \rightarrow CO_2 + H_2O$$

Balancing: Carbon: 2 = 2 — need 2 CO₂.* Hydrogen: 2 = 2 — need 1 H₂O.* Oxygen: 2(2) + 1 = 5 — need 5/2 O₂.*

Balanced (multiply by 2 to remove fraction): $$2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O + \text{heat}$$

Or simplified per molecule: $$C_2H_2 + \frac{5}{2}O_2 \rightarrow 2CO_2 + H_2O$$

Heat of Combustion

Methane: 891 kJ/mol Ethyne: 1300 kJ/mol

Ethyne releases more heat per mole — because of triple bond breaking. Hence ethyne (acetylene) used in welding — high temperature ~3000°C.

Comparison — Why Different Flames?

Methane (CH₄): C:H ratio = 1:4 → enough O₂ available → clean blue flame. Ethyne (C₂H₂): C:H ratio = 1:1 → needs more O₂ → if insufficient, soot forms (yellow/sooty flame).

That's why:

• Pure O₂ + C₂H₂ → very hot, clean welding flame.
• Air + C₂H₂ → smoky, sooty.

Practical Significance

Methane:

• Natural gas (homes, factories).
• LPG component.
• Biogas (from animal waste).

Ethyne:

• Welding (oxyacetylene torch).
• Cutting metals.
• Industrial chemicals.

[NCERT — important]

Example 5: NCERT — Drawing Structures

Draw the structures of: (a) Ethane (b) Ethene (c) Ethyne

Count the number of C-H and C-C bonds in each.

Solution:

(a) Ethane ($C_2H_6$)

    H   H
    |   |
H — C — C — H
    |   |
    H   H

Bonds:

• C-C single bonds: 1
• C-H bonds: 6
• Total bonds: 7

All single bonds (saturated). Each C has 4 bonds: 1 to other C + 3 to H.

(b) Ethene ($C_2H_4$)

    H       H
     \     /
      C = C
     /     \
    H       H

Bonds:

• C=C double bond: 1 (2 shared pairs)
• C-H bonds: 4
• Total bonds: 5 (in terms of bond count) or 6 (counting double as 2)

1 double bond (unsaturated alkene). Each C has 4 bonds: 2 in double bond + 2 to H.

(c) Ethyne ($C_2H_2$)

    H — C ≡ C — H

Bonds:

• C≡C triple bond: 1 (3 shared pairs)
• C-H bonds: 2
• Total bonds: 3 (in terms of bond count) or 5 (counting triple as 3)

1 triple bond (unsaturated alkyne). Each C has 4 bonds: 3 in triple bond + 1 to H.

Comparison Table

Compound	C-C bonds	C=C bonds	C≡C bonds	C-H bonds
Ethane	1 (single)	0	0	6
Ethene	0	1	0	4
Ethyne	0	0	1	2

Total Shared Electrons

Ethane:

• 1 C-C single = 2 e⁻
• 6 C-H = 12 e⁻
• Total: 14 e⁻

Ethene:

• 1 C=C double = 4 e⁻
• 4 C-H = 8 e⁻
• Total: 12 e⁻

Ethyne:

• 1 C≡C triple = 6 e⁻
• 2 C-H = 4 e⁻
• Total: 10 e⁻

Tetravalency Verification

Each C must have exactly 4 bonds:

Ethane: Each C: 1 (to other C) + 3 (to H) = 4 ✓ Ethene: Each C: 2 (in C=C) + 2 (to H) = 4 ✓ Ethyne: Each C: 3 (in C≡C) + 1 (to H) = 4 ✓

Pattern

As we go from ethane → ethene → ethyne:

• Multiplicity of C-C bond increases (single → double → triple).
• H atoms decrease (6 → 4 → 2).
• Reactivity increases.
• Bond length decreases.
• Bond strength increases.

[NCERT — fundamental]

Example 6: NCERT — Identifying Type of Hydrocarbon

Identify the type (alkane/alkene/alkyne) of: (a) $C_4H_{10}$ (b) $C_5H_{10}$ (c) $C_2H_2$ (d) $C_3H_8$ (e) $C_6H_{12}$ (f) $C_4H_6$

Solution:

Use General Formulas

• Alkane: $C_nH_{2n+2}$
• Alkene: $C_nH_{2n}$
• Alkyne: $C_nH_{2n-2}$

(a) $C_4H_{10}$

n = 4

Check alkane: 2(4)+2 = 10 ✓

Type: Alkane (Butane)

(b) $C_5H_{10}$

n = 5

Check alkane: 2(5)+2 = 12 ≠ 10 ✗ Check alkene: 2(5) = 10 ✓

Type: Alkene (Pentene)

(c) $C_2H_2$

n = 2

Check alkane: 2(2)+2 = 6 ≠ 2 ✗ Check alkene: 2(2) = 4 ≠ 2 ✗ Check alkyne: 2(2)-2 = 2 ✓

Type: Alkyne (Ethyne)

(d) $C_3H_8$

n = 3

Check alkane: 2(3)+2 = 8 ✓

Type: Alkane (Propane)

(e) $C_6H_{12}$

n = 6

Check alkane: 2(6)+2 = 14 ≠ 12 ✗ Check alkene: 2(6) = 12 ✓

Type: Alkene (Hexene)

Note: Could also be cyclohexane (a ring with 6 C and single bonds), which has same formula! We'll learn rings in Section 4.

(f) $C_4H_6$

n = 4

Check alkane: 2(4)+2 = 10 ≠ 6 ✗ Check alkene: 2(4) = 8 ≠ 6 ✗ Check alkyne: 2(4)-2 = 6 ✓

Type: Alkyne (Butyne)

Summary Table

Formula	n	Verifies	Type	Name
$C_4H_{10}$	4	Alkane (10)	Alkane	Butane
$C_5H_{10}$	5	Alkene (10)	Alkene	Pentene
$C_2H_2$	2	Alkyne (2)	Alkyne	Ethyne
$C_3H_8$	3	Alkane (8)	Alkane	Propane
$C_6H_{12}$	6	Alkene (12)	Alkene	Hexene
$C_4H_6$	4	Alkyne (6)	Alkyne	Butyne

Quick Trick

To identify type from formula $C_nH_x$:

• If $x = 2n+2$: alkane.
• If $x = 2n$: alkene.
• If $x = 2n-2$: alkyne.

Each formula has a 'gap' of 2 H atoms between consecutive types.

[Board: 3-mark]

Example 7: NCERT — Why Alkanes are Less Reactive

Why are alkanes generally less reactive than alkenes and alkynes?

Solution:

The Fundamental Reason

Alkanes have only single bonds (C-C and C-H).

All electrons are used in stable single bonds. No 'extra' electrons available for reactions.

Comparison with Alkenes and Alkynes

Alkenes:

• Have a C=C double bond.
• 2 pairs of electrons between the same 2 C atoms.
• 1 pair (called σ bond) is strong.
• The other pair (π bond) is weaker and exposed.
• π electrons can attack reactants → reactions.

Alkynes:

• Have a C≡C triple bond.
• 3 pairs between same 2 C atoms.
• 1 σ bond + 2 π bonds.
• 2 π bonds → very reactive.
• Even more reactive than alkenes.

The π electrons are like 'sticking out' — easy targets for reactants.

Key Idea — Reactivity Comes from Multiple Bonds

Multiple bonds are 'sites of reactivity'. They can break (the π part), and atoms can add across. This is called an addition reaction.

Reactions Showing Difference

Alkane + Bromine

No reaction at room temperature in dark. Slow substitution if exposed to UV light: $$CH_4 + Br_2 \xrightarrow{\text{UV}} CH_3Br + HBr$$

Alkene + Bromine (Addition — fast!)

$$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$

Br₂ adds across the double bond. Decolourises bromine water at room temperature.

Alkyne + Bromine

$$HC≡CH + 2Br_2 \rightarrow CHBr_2-CHBr_2$$

2 Br₂ molecules add across the triple bond. Decolourises bromine water at room temperature.

Why Single Bonds Are Stable

Single bond:

• Strong σ overlap.
• Symmetric.
• All electron density is in the bond.
• Difficult to break.

Double bond:

• σ + π.
• π is above and below the line of bond.
• Weaker → easier to break.

Triple bond:

• σ + 2π.
• Even more electron density exposed.

Real-World Implications

Why alkanes are good fuels?

• Stable when stored.
• Burn cleanly in air.
• Provide good energy.
• LPG, petrol, diesel — all alkanes.

Why alkenes/alkynes are good for industry?

• Reactive nature → can be used to make polymers, plastics.
• React with H₂, Cl₂, Br₂, water.
• Form chains by addition (polymerisation).

Hence:

• Alkanes: good fuels (stable).
• Alkenes/Alkynes: good raw materials (reactive).

Summary

Single bonds = stable + less reactive (alkanes). Multiple bonds = reactive + good for chemical reactions (alkenes, alkynes).

This is why hydrocarbons have such varied applications in our daily life.

[NCERT — important]

Example 8: Numerical — Combustion of Methane

How much CO₂ and water is produced when 16 g of methane is completely burnt? (C=12, H=1, O=16)

Solution:

Reaction

$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$

Molecular masses

• $CH_4$ = 12 + 4(1) = 16 g/mol
• $CO_2$ = 12 + 2(16) = 44 g/mol
• $H_2O$ = 2(1) + 16 = 18 g/mol

Moles of CH₄

16 g ÷ 16 g/mol = 1 mol

Moles of CO₂ produced

From equation: 1 mol CH₄ → 1 mol CO₂ So 1 mol CH₄ → 1 mol CO₂ = 44 g

Moles of H₂O produced

From equation: 1 mol CH₄ → 2 mol H₂O So 1 mol CH₄ → 2 mol H₂O = 36 g

Answers

CO₂ produced: 44 g H₂O produced: 36 g

Verification (Mass Conservation)

Reactants total mass:

• CH₄: 16 g
• O₂ used: 2 × 32 = 64 g
• Total = 80 g

Products total mass:

• CO₂: 44 g
• H₂O: 36 g
• Total = 80 g ✓

Mass balanced — physics says no atoms are lost in chemical reactions.

Heat Released

1 mol of CH₄ on combustion releases ~891 kJ of heat.

1 mol = 16 g. So 16 g CH₄ releases ~891 kJ heat.

That's enough heat to:

• Cook food for hundreds of meals.
• Heat a small room for hours.
• Power a small generator.

Volume of CO₂ at NTP (Bonus)

1 mol CO₂ at NTP = 22.4 L. So 1 mol CH₄ → 22.4 L CO₂ at NTP.

Volume of CO₂ at NTP

1 mol H₂O is liquid at NTP, takes negligible space.

Real-World Meaning

When you burn 16 g (~1 cubic foot) of natural gas:

• You get 44 g of CO₂ (greenhouse gas).
• You get 36 g of water (steam).
• You get 891 kJ of heat (useful energy).

This is why methane (natural gas) is one of the cleanest fossil fuels:

• Burns cleanly to CO₂ + H₂O only.
• No SO₂, NOₓ produced (compared to coal).
• Less CO₂ per unit of energy than coal/oil.

But CO₂ is still a greenhouse gas — contributes to climate change.

[Board: 3-mark numerical]

Example 9: NCERT — Why Hydrocarbons Are Important Fuels?

Why are hydrocarbons used as fuels? Give examples.

Solution:

Why Hydrocarbons as Fuels?

1. Easy to burn:

• React with O₂ in air.
• Combustion is exothermic — releases heat.

2. High energy output:

• Per gram, hydrocarbons release a lot of heat.
• Methane: 55 kJ/g.
• Octane (petrol): 47 kJ/g.

3. Easy to transport and store:

• Liquid (petrol, diesel) — pumped through pipes.
• Gas (natural gas) — pressurised in cylinders.
• Solid (coke) — convenient.

4. Burn relatively cleanly:

• Products: CO₂ + H₂O (mostly).
• Less harmful emissions than coal.

5. Naturally available:

• Found in petroleum, natural gas, coal.
• Easy extraction.

Examples of Hydrocarbon Fuels

1. Natural Gas

Mostly methane (CH₄) + ethane (C₂H₆) + propane (C₃H₈) + butane (C₄H₁₀). Used in: homes (LPG), factories, power plants, vehicles (CNG).

Reaction: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$

2. LPG (Liquefied Petroleum Gas)

Mainly butane (C₄H₁₀) + propane (C₃H₈). Used in: cooking gas cylinders.

Reaction: $2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O$

3. Petrol

Mostly C₅H₁₂ to C₁₀H₂₂. Used in: cars, motorbikes, generators.

4. Diesel

Mostly C₁₀H₂₂ to C₁₆H₃₄. Used in: trucks, buses, generators, agricultural machinery.

5. Kerosene

Mostly C₁₀H₂₂ to C₁₅H₃₂. Used in: lamps, stoves (in rural areas), aviation fuel.

6. Coal/Coke

Mostly carbon, with hydrocarbons. Used in: power plants, industries.

7. CNG (Compressed Natural Gas)

Mostly methane. Used in: vehicles (cleaner than petrol/diesel).

Comparison of Fuels

Fuel	Main hydrocarbon	Energy (kJ/g)	Use
Natural gas/CNG	CH₄	55	Homes, vehicles
LPG	C₄H₁₀	50	Cooking
Petrol	C₈H₁₈ (avg)	47	Cars
Diesel	C₁₂H₂₆ (avg)	45	Trucks
Kerosene	C₁₂H₂₆ (avg)	43	Lamps
Coal	C + others	25-35	Power plants

Choosing the Right Fuel

Different fuels for different needs:

• Cooking: LPG (clean, controllable).
• Vehicles: petrol/diesel/CNG (compact, energy-dense).
• Power plants: coal/natural gas (cheap, large volumes).
• Aviation: jet fuel (high energy, special properties).

Advantages and Disadvantages

Advantages:

• High energy.
• Convenient.
• Established infrastructure.

Disadvantages:

• Non-renewable (oil/gas reserves limited).
• Burning produces CO₂ → greenhouse effect.
• Can produce pollutants (SO₂, NOₓ, CO if incomplete burning).

The Future

Hydrocarbons are still major fuels. But shifting to:

• Electric vehicles.
• Solar, wind energy.
• Hydrogen fuel.

Hydrocarbons may give way to cleaner alternatives in the next decades.

[NCERT — important]

Example 10: An Interesting — Octane Number

What is octane number? Why does premium petrol have higher octane?

Solution:

Petrol's Problem — 'Knocking'

In a car engine, petrol+air mixture is compressed in the cylinder, then ignited by a spark.

If the fuel ignites before the spark (due to high pressure), it causes:

• Knocking sound.
• Loss of efficiency.
• Damage to engine.

This is called 'knocking'.

What Causes Knocking?

Branched alkanes resist knocking. Straight-chain alkanes knock more easily.

So we want fuels that don't knock.

Octane Number — A Rating

Octane number rates a fuel's resistance to knocking.

Reference: $iso$-octane (2,2,4-trimethylpentane, a branched C₈H₁₈) — rated 100. n-Heptane (straight chain C₇H₁₆) — rated 0.

A petrol with octane rating 87 = behaves like a mix of 87% iso-octane + 13% n-heptane.

Octane Numbers in Real Petrol

Type	Octane Number
Regular petrol	87
Mid-grade	89
Premium petrol	91-93
Aviation fuel	100+

Higher Octane = Better?

Higher octane:

• Resists knocking more.
• Allows higher compression ratio.
• More efficient engines.
• Used in high-performance cars.

Regular cars don't need premium — engine designed for 87 will run fine on 87. Higher octane in regular car = wasteful (no benefit).

How Is High Octane Made?

Petroleum refineries do 'cracking' and 'reforming':

Cracking:

• Long alkanes broken into smaller ones.
• Some smaller ones have multiple isomers.
• Branched isomers preferred.

Reforming:

• Straight-chain alkanes converted to branched alkanes.
• Or to aromatic compounds (benzene, toluene).
• These have high octane.

Octane Boosters

Some chemicals added to increase octane:

• Tetraethyl lead (banned now — toxic).
• MTBE (methyl tert-butyl ether) — controversial.
• Ethanol (used in some countries).
• Aromatic compounds (benzene, toluene, xylenes).

A Lesson in Chemistry

Same chemical formula (C₈H₁₈ has many isomers):

• Straight chain: poor fuel.
• Branched chain: excellent fuel.

This is one reason why structural isomerism matters in industry.

(More on isomers in Section 4!)

Cetane Number — For Diesel

Diesel uses cetane number (similar concept). Cetane (n-hexadecane, C₁₆H₃₄) — rated 100. Higher cetane = better diesel fuel.

Typical diesel: 40-50 cetane number.

[Board + General Knowledge]

Example 11: Identifying from Properties

A compound burns with a clean blue flame, doesn't decolourise bromine water, and has formula $C_3H_x$. Find x and name the compound.

Solution:

Analysis Step by Step

Clue 1: Clean blue flame

Indicates alkane (saturated hydrocarbon). High C:H ratio of alkane → enough O₂ → clean burn.

Clue 2: Doesn't decolourise bromine water

Confirms saturated. Alkanes don't react with bromine water.

If it had decolourised → alkene or alkyne.

Conclusion

The compound is alkane.

Formula Verification

General alkane formula: $C_nH_{2n+2}$

Given: $C_3H_x$ (3 C atoms)

For alkane with 3 C: x = 2(3)+2 = 8

Answer

x = 8, so the compound is $C_3H_8$ (Propane).

Verification

Propane:

• Formula: C₃H₈ ✓ (matches alkane formula).
• Property: clean blue flame ✓ (alkane, saturated).
• No reaction with bromine water ✓ (no double/triple bonds).

Structure of Propane

    H   H   H
    |   |   |
H — C — C — C — H
    |   |   |
    H   H   H

3 C atoms in chain. 8 H atoms total. All single bonds — saturated.

Practical Use

Propane:

• Major component of LPG (cooking gas).
• Used in heaters, generators.
• Liquid in cylinders, gas at room temperature when released.
• Clean burning fuel.

Mixed with butane (C₄H₁₀) in standard LPG cylinders for Indian homes.

Alternative Compounds — Why Not Others?

If $C_3H_6$:

• Matches alkene formula (2n=6 ✓).
• Would decolourise bromine water (contradicts clue 2).
• ✗ Not this.

If $C_3H_4$:

• Matches alkyne formula (2n-2=4 ✓).
• Would decolourise bromine water (contradicts).
• Burns with sooty flame (contradicts clue 1).
• ✗ Not this.

Only $C_3H_8$ fits all clues — propane.

A Detective's Approach

This is how organic chemists identify unknown compounds:

1. Check physical properties (state, smell, colour).
2. Test with various reagents (bromine water, KMnO₄, etc.).
3. Burn and observe flame.
4. Determine molecular formula.
5. Deduce structure.

[Board: 3-mark practical]

Example 12: NCERT — Importance of Ethene

Why is ethene ($C_2H_4$) important in industry and biology?

Solution:

Industrial Importance

Ethene is the most produced organic chemical in the world. Annually, ~140 million tonnes of ethene are produced.

1. Polyethene (Polythene) — Plastic

Polymerisation: $$nCH_2=CH_2 \rightarrow (-CH_2-CH_2-)_n$$

Many ethene molecules join via the double bond → long polymer chain.

Uses of polyethene:

• Plastic bags.
• Bottles.
• Containers.
• Pipes.
• Insulation.
• Toys.

World's most-produced plastic.

2. Ethylene Glycol — Antifreeze

Ethene → ethylene oxide → ethylene glycol

Uses:

• Antifreeze in car radiators (lowers freezing point).
• Polyester production (fabrics).
• Coolant in some applications.

3. Ethanol

Ethene + water → ethanol $$CH_2=CH_2 + H_2O \xrightarrow{\text{acid catalyst}} CH_3CH_2OH$$

Industrial ethanol used in:

• Solvent.
• Fuel additive.
• Disinfectant.
• Pharmaceutical manufacturing.

4. Vinyl Chloride → PVC

Ethene + chlorine → vinyl chloride → PVC plastic.

Uses of PVC:

• Pipes (water, drainage).
• Wire insulation.
• Flooring.

5. Other Products

Ethene → many other chemicals:

• Acetic acid.
• Detergents.
• Synthetic fibres.
• Pharmaceuticals.

Biological Importance

Plant Hormone — Ripening

Ethene is a natural plant hormone. Released by ripening fruits. Causes:

• Softening of fruits.
• Conversion of starch to sugar.
• Colour change (green → ripe colour).
• Aroma development.

This is why:

• One ripe fruit ripens others nearby (ethene gas spreads).
• 'Don't put ripe bananas with green ones' — they'll ripen too fast.
• Apples emit ethene → use to ripen tomatoes.

Commercial Use in Agriculture

Farmers use ethene-releasing chemicals:

• To ripen fruits artificially (during transport).
• To trigger flower opening.
• To synchronise harvesting.

Calcium carbide ($CaC_2$) + water → acetylene (similar to ethene) — used illegally to ripen fruits in some markets.

This is harmful to health — should be avoided.

A Curious Observation

If you place a ripe banana with green tomatoes in a paper bag:

• Banana releases ethene.
• Ethene is trapped in the bag.
• Tomatoes ripen faster.

Try this at home — it works!

Industrial Production Methods

Ethene is produced from:

• Cracking of petroleum (most common).
• Cracking of ethane.
• Dehydration of ethanol.

Production: ~140 million tonnes annually worldwide. Major producers: USA, China, Saudi Arabia, India.

Summary — Why Ethene Matters

Industrial:

• Plastic raw material.
• Antifreeze.
• Ethanol production.
• Many chemicals.

Biological:

• Plant hormone.
• Fruit ripening.
• Agriculture.

Ethene — a simple 2-C alkene, but one of the most important molecules in modern industry and agriculture.

[Board + General Knowledge]

Example 13: NCERT — Importance of Ethyne (Acetylene)

What are the uses of ethyne ($C_2H_2$)?

Solution:

Ethyne — Acetylene

$C_2H_2$ — simplest alkyne. Has triple bond between 2 C atoms. Most reactive among simple hydrocarbons.

Use 1: Oxyacetylene Welding/Cutting

The most famous use!

When ethyne burns in O₂: $$2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O + \text{heat}$$

Temperature reached: ~3000°C — among hottest flames.

Used to:

• Weld metals (joining iron pieces, steel).
• Cut metals (large steel plates).
• Forge in blacksmithing.

A handheld torch:

• One cylinder of acetylene.
• One cylinder of O₂.
• Mixed at the nozzle.
• Lit → blue flame.

Used in industries: shipbuilding, construction, automotive.

Use 2: Manufacture of Vinyl Chloride → PVC

Ethyne + HCl → vinyl chloride $$HC≡CH + HCl \rightarrow CH_2=CHCl$$

Vinyl chloride is polymerised to make PVC plastic.

PVC uses:

• Pipes (water, drainage, electrical).
• Wire insulation.
• Floor tiles.
• Credit cards.
• Inflatable products.

Use 3: Manufacture of Acrylonitrile

Ethyne + HCN → acrylonitrile $$HC≡CH + HCN \rightarrow CH_2=CHCN$$

Acrylonitrile is polymerised to make:

• Synthetic fibres (acrylic).
• Plastics (ABS).
• Used in clothing, carpets.

Use 4: Manufacture of Acetic Acid

Ethyne + water → acetaldehyde → acetic acid $$HC≡CH + H_2O \rightarrow CH_3CHO \rightarrow CH_3COOH$$

Acetic acid uses:

• Vinegar (food).
• Solvent.
• Drug manufacturing.

Use 5: Source of Synthetic Rubber

Ethyne + 2H₂ + chlorine → chloroprene Chloroprene polymerises to → neoprene rubber.

Used in: tyres, gaskets, mats.

Use 6: Lighting (Historical)

Calcium carbide ($CaC_2$) + water → ethyne: $$CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$$

Old miners' lamps and street lamps used this 'carbide light' in the 19th-20th century. Burnt acetylene gives bright yellow-white light.

Use 7: Ripening Fruits (Illegal in Some Areas)

Calcium carbide + moisture → acetylene gas (similar to ethene). Used to artificially ripen fruits. Banned in many countries due to harmful side effects.

Production of Ethyne

Two main methods:

1. From Calcium Carbide (Lab): $$CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2\uparrow$$

2. From Methane (Industry): Cracking methane at very high temperature (~1500°C): $$2CH_4 \xrightarrow{\Delta} C_2H_2 + 3H_2$$

Properties Summary

• Colourless gas.
• Slight garlic-like smell (in pure form, odourless).
• Highly reactive.
• Burns with smoky flame in air.
• Burns with hot, clean flame in pure O₂.
• Slightly soluble in water.

A Note on Safety

Acetylene is highly explosive when compressed. Cannot be stored as compressed gas. In cylinders, it's dissolved in acetone (sealed in porous material) for safety.

Summary

Ethyne ($C_2H_2$):

• Welding/cutting metals (oxyacetylene).
• PVC plastic.
• Synthetic fibres.
• Acetic acid.
• Synthetic rubber.
• (Historical) lighting.

A small molecule with huge industrial impact.

[NCERT — important]

Example 14: A Concluding Question

(a) Define saturated and unsaturated hydrocarbons. (b) Give the general formulas of three series. (c) How does bromine water test work? (d) Why is methane a good fuel?

Solution:

(a) Saturated and Unsaturated

Saturated: Hydrocarbons in which all bonds between C atoms are single bonds. Cannot accept more H. Less reactive. Series: Alkanes. Example: methane (CH₄).

Unsaturated: Hydrocarbons that have double or triple bonds between C atoms. Can accept more H. More reactive. Series: Alkenes (1 double), Alkynes (1 triple). Examples: ethene (C₂H₄), ethyne (C₂H₂).

(b) General Formulas

Series	General Formula	Bond Type
Alkane	$C_nH_{2n+2}$	All single
Alkene	$C_nH_{2n}$	At least 1 double
Alkyne	$C_nH_{2n-2}$	At least 1 triple

Note: 2 fewer H atoms each time we add a double or triple bond.

(c) Bromine Water Test

Procedure: Add bromine water (orange-brown $Br_2$ in water) to the hydrocarbon.

Saturated (alkane):

• No reaction at room temperature.
• Bromine water remains orange-brown.

Unsaturated (alkene/alkyne):

• Bromine water becomes colourless.
• Br₂ adds across the double/triple bond.

Reaction (alkene example): $$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$

Conclusion: Bromine water decolourised → unsaturated hydrocarbon. Bromine water unchanged → saturated hydrocarbon.

This is the standard test for unsaturation.

(d) Why Methane is a Good Fuel

Methane (CH₄) has many advantages as a fuel:

1. High energy:

• 891 kJ released per mole burnt.
• 55 kJ per gram (one of the highest among hydrocarbons).

2. Clean combustion:

• Burns to CO₂ + H₂O only.
• No SO₂, NOₓ, particulate matter (unlike coal/oil).
• Less greenhouse gas per energy unit.

3. Easy to handle:

• Gas — easy to pipe.
• Easy to ignite.
• Burns with clean blue flame.

4. Naturally available:

• Found in natural gas reserves.
• Produced from decomposing organic matter (biogas, marsh gas).
• Plentiful supply.

5. Versatile use:

• Cooking gas (in homes via LPG).
• Power plants (large scale).
• Vehicles (CNG).
• Industrial heating.
• Hydrogen production (industrial).

Reaction: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + 891 \text{ kJ}$$

Compared to other fuels:

• Coal: 25-35 kJ/g.
• Oil: 40-45 kJ/g.
• Methane: 55 kJ/g (best!).

This is why natural gas is replacing coal in many power plants — cleaner and more efficient.

Final Insight

Hydrocarbons — simple compounds, but powerful fuels and raw materials for modern life.

From the gas in your kitchen to the plastic of your phone case — hydrocarbons are everywhere.

[Board: 5-mark mixed]

Example 15: An Application — Hydrocarbons in India

Discuss the importance of hydrocarbons in the Indian economy and daily life.

Solution:

Major Hydrocarbon Industry in India

1. Petroleum (Crude Oil) Industry

Major companies:

• ONGC (Oil and Natural Gas Corporation).
• Reliance Industries Ltd.
• Indian Oil Corporation (IOC).
• Bharat Petroleum (BPCL).
• Hindustan Petroleum (HPCL).

Major refineries:

• Jamnagar (Reliance) — world's largest refinery.
• Mathura (IOC).
• Visakhapatnam (HPCL).

India produces about 30% of its oil needs. Imports rest from Saudi Arabia, Iraq, UAE, etc.

2. Petroleum Products Used Daily

From crude oil, refineries make:

• LPG (kitchen cooking).
• Petrol (cars, motorbikes).
• Diesel (trucks, buses, generators).
• Kerosene (lamps, jet fuel).
• Naphtha (raw material for plastics).
• Bitumen (roads).

3. Natural Gas

Major fields in India:

• Krishna-Godavari Basin (KG basin).
• Mumbai High.
• Tripura.

Used for:

• PNG (Piped Natural Gas) for homes.
• CNG (Compressed Natural Gas) for vehicles.
• Power plants.
• Fertiliser industry (urea production).

4. Petrochemical Industry

Hydrocarbons → many products:

• Plastics (PE, PVC, PP).
• Synthetic fibres (polyester, nylon).
• Rubber.
• Detergents.
• Pesticides.
• Drugs.

Major centres:

• Jamnagar (Reliance).
• Vadodara (IOC).
• Hazira (Reliance).

Daily Life Examples

Cooking: LPG cylinders in 95%+ Indian homes (PMUY scheme). Transport: petrol/diesel for vehicles; CNG in cities like Delhi, Mumbai. Plastics: plastic bags, bottles, electronics, clothing. Roads: bitumen (asphalt) — covers most Indian roads. Lighting (rural): kerosene lamps in remote areas. Fertilisers: urea (made from natural gas) — used in agriculture.

Energy Consumption

India's energy mix (~2024):

• Coal: 50%
• Oil: 30%
• Natural gas: 6%
• Renewables: 14%

Major dependency on hydrocarbons.

Indian Achievements

1. World's largest refinery in Jamnagar. 2. Pradhan Mantri Ujjwala Yojana (PMUY) — providing free LPG cylinders to poor households. 3. Successful CNG conversion of Delhi public transport. 4. Polyester capital — large textile industry uses synthetic fibres from hydrocarbons.

Challenges

1. Import dependency — oil price fluctuations affect economy. 2. Pollution — vehicle emissions a major issue in cities. 3. Climate change — CO₂ emissions from hydrocarbons. 4. Plastic waste — non-biodegradable.

Future Direction

India is shifting towards:

• Electric vehicles (Tata, Mahindra, Maruti).
• Biofuels (ethanol blending in petrol).
• CNG infrastructure.
• Solar/wind energy.
• Hydrogen fuel research.

A Lesson

Hydrocarbons:

• Underpin modern economy.
• Enabled India's growth.
• Now require sustainable management.

Future India: balance of hydrocarbons + renewables.

[Board + General Knowledge]

Example 16: Numerical — Combustion of Ethyne

If 26 g of ethyne ($C_2H_2$) is completely burnt, calculate: (a) Mass of $O_2$ needed. (b) Mass of $CO_2$ produced. (c) Mass of $H_2O$ produced. (C=12, H=1, O=16)

Solution:

Reaction

$$2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O$$

Molar Masses

• $C_2H_2$ = 2(12) + 2(1) = 26 g/mol
• $O_2$ = 2(16) = 32 g/mol
• $CO_2$ = 12 + 2(16) = 44 g/mol
• $H_2O$ = 2(1) + 16 = 18 g/mol

Moles of $C_2H_2$

26 g ÷ 26 g/mol = 1 mol

From Reaction Stoichiometry

From: 2 $C_2H_2$ → 5 O₂ → 4 CO₂ → 2 H₂O

For 1 mol $C_2H_2$:

• O₂ used: $\frac{5}{2}$ mol = 2.5 mol
• CO₂ produced: $\frac{4}{2}$ mol = 2 mol
• H₂O produced: $\frac{2}{2}$ mol = 1 mol

(a) Mass of $O_2$

2.5 mol × 32 g/mol = 80 g

(b) Mass of $CO_2$

2 mol × 44 g/mol = 88 g

(c) Mass of $H_2O$

1 mol × 18 g/mol = 18 g

Final Answers

O₂ needed: 80 g CO₂ produced: 88 g H₂O produced: 18 g

Verification (Mass Conservation)

Reactants:

• 26 g (C₂H₂) + 80 g (O₂) = 106 g

Products:

• 88 g (CO₂) + 18 g (H₂O) = 106 g

Mass balanced ✓

Heat Released

1 mol C₂H₂ releases ~1300 kJ. So 26 g C₂H₂ → 1300 kJ heat.

Compare to methane (16 g → 891 kJ):

• Per gram: ethyne 50 kJ/g, methane 55 kJ/g.
• Per mole: ethyne 1300 kJ/mol, methane 891 kJ/mol.

Ethyne releases more heat per molecule (because of triple bond breaking).

Why Ethyne in Welding?

Combustion temp: ~3300°C in pure O₂! Hottest practical flame. Used to weld steel pipes, cut metals.

Comparison Table

Hydrocarbon	M (g/mol)	Heat (kJ/mol)	Heat (kJ/g)
CH₄ (methane)	16	891	55.7
C₂H₆ (ethane)	30	1560	52
C₂H₄ (ethene)	28	1410	50.4
C₂H₂ (ethyne)	26	1300	50

Methane has highest heat per gram — best home fuel. Ethyne has highest flame temperature — best for welding.

Practical Significance

This kind of calculation:

• Used in industry to plan O₂ supply.
• Used to size storage tanks.
• Used to calculate CO₂ emissions.
• Used in safety planning.

Chemistry — backbone of modern industry.

[Board: 3-5 mark numerical]