Hydrocarbons and Fuels
~9 min read
- Hydrocarbons: Compounds of carbon and hydrogen only. Three families: Alkanes (C-C single), Alkenes (C=C double), Alkynes (C≡C triple).
- Fossil fuels: Petroleum (crude oil → fractions), Natural gas (mostly methane), Coal.
- Alternative fuels: LPG (liquefied petroleum gas — butane/propane), CNG (compressed natural gas, mostly methane), Biogas (methane from biomass).
Hydrocarbons are the building blocks of life and the world's fuels. NDA tests families, common fuels, and combustion.
Three Families of Hydrocarbons
| Family | General formula | Bond type | Example |
|---|---|---|---|
| Alkanes (paraffins) | CₙH₂ₙ₊₂ | Only C-C single bonds (saturated) | Methane CH₄, Ethane C₂H₆, Propane C₃H₈ |
| Alkenes (olefins) | CₙH₂ₙ | At least one C=C double bond | Ethene (ethylene) C₂H₄, Propene C₃H₆ |
| Alkynes | CₙH₂ₙ₋₂ | At least one C≡C triple bond | Ethyne (acetylene) C₂H₂ |
Saturated = only single bonds. Unsaturated = at least one double/triple bond.
Petroleum and Its Fractions
Petroleum (crude oil) is a complex mixture of hydrocarbons formed from ancient marine organisms over millions of years. Refined by fractional distillation into:
| Fraction | BP range | Use |
|---|---|---|
| LPG (liquefied petroleum gas) | <20°C | Cooking gas — propane + butane |
| Petrol (Gasoline) | 40-200°C | Vehicle fuel |
| Naphtha | 70-200°C | Petrochemical feedstock |
| Kerosene | 150-275°C | Jet fuel, lamps, heaters |
| Diesel | 200-350°C | Diesel engines, trucks, trains |
| Lubricating oil | 300-400°C | Lubrication |
| Bitumen / Asphalt | >400°C residue | Roads, roofing |
Common Domestic Fuels
| Fuel | Main component | Use |
|---|---|---|
| LPG | Propane (C₃H₈) + Butane (C₄H₁₀) | Domestic cooking. Stored as pressurised liquid in cylinders |
| CNG (Compressed Natural Gas) | Methane (CH₄) | Vehicles (cars, buses). Cleaner than petrol/diesel |
| Natural gas (piped) | Methane (CH₄) | Domestic heating, cooking, electricity generation |
| Biogas / Gobar gas | Methane (~50-60%) + CO₂ | Rural cooking and lighting. From animal/plant waste fermentation |
| Producer gas | CO + N₂ | Industrial fuel |
| Water gas | CO + H₂ | Industrial fuel |
Combustion
- Complete combustion: Hydrocarbon + plenty of O₂ → CO₂ + H₂O + lots of heat.
- Example: CH₄ + 2O₂ → CO₂ + 2H₂O.
- Incomplete combustion: Insufficient O₂ → CO (carbon monoxide, toxic) + C (soot) + H₂O.
- Calorific value: Heat produced per kg/litre of fuel.
- Hydrogen: 150 kJ/g (highest).
- Natural gas: ~55 kJ/g.
- LPG: ~46 kJ/g.
- Petrol: ~46 kJ/g.
- Coal: ~25-35 kJ/g.
- Octane number: Petrol quality. Higher = better resistance to knocking. Premium petrol ~91-95.
- Cetane number: Diesel quality.
NDA PYQ Examples
Q: Methane is the main component of:
(a) Petrol (b) Diesel (c) Natural gas (d) Kerosene
Answer: (c) Natural gas — mostly CH₄.
Q: LPG mainly contains:
(a) Methane (b) Propane and butane (c) Hydrogen (d) Carbon monoxide
Answer: (b) Propane and butane.
Q: Acetylene is:
(a) Alkane (b) Alkene (c) Alkyne (d) Aromatic
Answer: (c) Alkyne — C₂H₂ with triple bond.
Q: Biogas mainly contains:
(a) Hydrogen (b) Methane (c) Butane (d) Octane
Answer: (b) Methane — ~50-60%.
Q: Incomplete combustion of hydrocarbons produces toxic:
(a) CO₂ (b) CO (c) H₂O (d) O₂
Answer: (b) CO — carbon monoxide.
Drill Hydrocarbons and Fuels for NDA
NDA-pattern items on Hydrocarbons and Fuels with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is CNG considered cleaner than petrol or diesel?
CNG burns more completely → less CO, less unburnt hydrocarbons, less soot/particulate matter. No lead. CO₂ emissions are lower per unit energy. Used by Delhi's public buses since the 1990s under court order.
What is the difference between LPG and CNG?
LPG = Liquefied Petroleum Gas — propane and butane, stored as liquid under moderate pressure. Heavier than air. Used in domestic cylinders. CNG = Compressed Natural Gas — mostly methane, compressed to high pressure. Lighter than air. Used in vehicles.
How is petroleum formed?
From marine organisms (algae, plankton) that died and settled to ocean floors millions of years ago. Buried under sediments, subjected to heat and pressure over millions of years → transformed into hydrocarbons (oil and natural gas). Hence 'fossil fuel'.
Why does a candle flame have a yellow tip and a blue base?
Yellow zone — incomplete combustion, soot (carbon particles) glowing. Blue zone — sufficient oxygen for complete combustion. A blue flame is hotter and cleaner than yellow. Bunsen burner shows this principle clearly.
What is the calorific value?
Energy released per unit mass (or volume) of fuel on complete combustion. Hydrogen has the highest calorific value among common fuels (150 kJ/g). It's why hydrogen is being explored as a future fuel — but storage and production challenges remain.