Hydrocarbons and Fuels

Why Hydrocarbons and Fuels Matter for NDA

Every year the NDA General Science paper carries a handful of chemistry questions, and the organic chemistry portion almost always touches hydrocarbons or fuels. These topics are popular with examiners because they connect to everyday life — the gas cylinder at home, the petrol pump, the coal in a power station.

The good news for a busy student: this chapter is more about remembering clean facts than heavy calculation. If you lock down the general formulae, a few key reactions and the properties of common fuels, you can grab these marks quickly. Unlike physics numericals that eat up minutes, most hydrocarbon questions can be answered in a few seconds once the facts are memorised.

This page builds the topic in a logical order: first what hydrocarbons are, then the three main families with their formulae, then how they burn to give energy, and finally the real fuels you see every day. Read it once carefully, then use the 60-second recap at the end for quick revision before the exam.

What Exactly Is a Hydrocarbon?

Carbon has the special ability to form four strong covalent bonds and to link with other carbon atoms in long chains, branched chains and rings. This property is called catenation, and it is why millions of carbon compounds exist.

Hydrocarbons are broadly divided into two big families:

• Saturated — carbon atoms joined only by single bonds (the alkanes).
• Unsaturated — at least one double or triple bond is present (alkenes and alkynes).

A separate group called aromatic hydrocarbons contains the benzene ring, a special stable ring of six carbons.

The word ‘saturated’ literally means the carbon atoms are holding the maximum possible number of hydrogen atoms — there is no room for more. An unsaturated molecule, by contrast, has spare bonding capacity locked up in a double or triple bond, which is why it can take in extra atoms during an addition reaction. This single idea explains most of the chemical behaviour you will be tested on.

Alkanes — The Saturated Hydrocarbons

Alkanes have only single (C−C) bonds and follow the general formula C_nH_2n+2. They form a homologous series — a family where each member differs from the next by a −CH₂ unit and shows a gradual change in properties.

• Methane — CH₄ (n = 1)
• Ethane — C₂H₆
• Propane — C₃H₈
• Butane — C₄H₁₀

Alkanes are also called paraffins because of their low chemical reactivity. Their main reaction is combustion (burning in oxygen), which makes them excellent fuels. They also undergo substitution with halogens in sunlight, where a hydrogen atom is swapped for a chlorine or bromine atom one step at a time.

Physical properties change smoothly down the series, another hallmark of a homologous family. The first four alkanes (methane to butane) are gases at room temperature, the next group up to about C₁₇ are liquids, and the heaviest ones are waxy solids. This is exactly why methane is a gaseous fuel while petrol is a liquid and candle wax is a solid — they are all alkanes of different chain lengths.

Alkenes and Alkynes — The Unsaturated Ones

Alkenes contain at least one carbon–carbon double bond and follow C_nH_2n. The simplest is ethene (ethylene), C₂H₄, used to ripen fruits and to make polythene.

Alkynes contain a carbon–carbon triple bond and follow C_nH_2n−2. The most famous is ethyne (acetylene), C₂H₂, burned in the oxy-acetylene torch for welding and cutting metals.

Because of their reactive multiple bonds, alkenes and alkynes undergo addition reactions — for example, adding hydrogen (hydrogenation) to become saturated, or decolourising bromine water, a classic test for unsaturation. In the laboratory, if a hydrocarbon turns reddish-brown bromine water colourless without sunlight, it must contain a double or triple bond.

These compounds are also industrially priceless. Ethene is the starting material for polythene, the plastic used in bags and bottles, and is the natural gas that ripens fruit such as bananas and mangoes. Ethyne, burnt with oxygen, produces a flame hot enough (around 3000°C) to weld and cut steel, which is why the oxy-acetylene torch is a standard NDA fact.

Aromatic Hydrocarbons and Benzene

Aromatic hydrocarbons are built around the benzene ring, C₆H₆ — a flat ring of six carbon atoms with delocalised electrons that make it unusually stable. Despite having three apparent double bonds, benzene mostly undergoes substitution rather than addition, which is its signature behaviour.

Many useful compounds are aromatic: toluene (in paints and thinners), naphthalene (mothballs) and the building blocks of dyes, drugs and plastics. For NDA, simply remember benzene’s formula, ring structure and its preference for substitution.

Isomerism and Naming Basics

Isomers are compounds with the same molecular formula but different structures. For example, C₄H₁₀ exists as two isomers: straight-chain n-butane and branched isobutane. Isomerism explains why a single formula can describe more than one real substance with different properties.

The modern IUPAC system names hydrocarbons using a root that tells the number of carbons (meth-, eth-, prop-, but-, pent-) and a suffix that tells the bond type:

• -ane → alkane (single bond)
• -ene → alkene (double bond)
• -yne → alkyne (triple bond)

Combustion — How Hydrocarbons Release Energy

When a hydrocarbon burns in plenty of oxygen, it undergoes complete combustion, producing carbon dioxide and water along with a large amount of heat. This is the reaction that powers engines, stoves and turbines.

If oxygen is limited, incomplete combustion occurs, giving a sooty yellow flame and producing carbon monoxide (CO) and carbon (soot). CO is a poisonous gas because it binds tightly to haemoglobin in blood — a frequent fact in exams.

Fuels and Calorific Value

A fuel is a substance that burns to release usable heat energy. Its quality is measured by calorific value — the amount of heat produced by completely burning 1 kilogram of the fuel, usually expressed in kilojoules per kilogram (kJ/kg).

A good fuel should have:

• High calorific value
• Moderate ignition temperature (not too low — that is dangerous)
• Low ash and pollution
• Easy availability and storage, and low cost

Fuels are classified by state as solid (coal, wood, charcoal), liquid (petrol, diesel, kerosene) and gaseous (LPG, CNG, biogas). Gaseous fuels generally burn cleanest.

Common Fuels You Must Know

NDA loves direct questions on the composition and uses of everyday fuels. Lock these in:

• LPG (Liquefied Petroleum Gas) — mainly butane and propane; the cooking gas in cylinders. An odorant (ethyl mercaptan) is added so leaks can be smelled.
• CNG (Compressed Natural Gas) — mainly methane; a cleaner fuel for buses, autos and cars.
• Petrol and diesel — liquid fractions of petroleum (crude oil) obtained by fractional distillation.
• Coal — a solid fossil fuel; its types in increasing carbon content are peat → lignite → bituminous → anthracite.
• Biogas (gobar gas) — produced by anaerobic decomposition of organic waste; mainly methane, a renewable fuel.

Petroleum, Natural Gas and Coal

Petroleum (crude oil) is a dark, thick mixture of hydrocarbons formed over millions of years from buried marine organisms — hence it is a fossil fuel. It is separated into useful products by fractional distillation in a refinery, where different fractions boil off and condense at different heights of the tower.

From lightest to heaviest, common fractions include: petroleum gas, petrol, kerosene/aviation fuel, diesel, lubricating oil and finally bitumen used for roads. Natural gas, found above petroleum deposits, is largely methane and is a clean, important fuel.

Worked Example

So butane is C₄H₁₀, butene is C₄H₈ and butyne is C₄H₆. The alkyne, with its triple bond, undergoes addition reactions most readily.

Common Mistakes to Avoid

• Confusing the general formulae — alkene is C_nH_2n, not C_nH_2n+2.
• Calling LPG ‘natural gas’. LPG is mainly butane; CNG/natural gas is mainly methane.
• Forgetting that incomplete combustion produces toxic carbon monoxide, not just less heat.
• Writing CO₂ as a product of incomplete combustion — the dangerous product is CO plus soot.
• Assuming aromatic compounds add reagents like alkenes; benzene prefers substitution.

Previous-Year Style Question

Practising such direct factual MCQs builds speed. Always pause on options that swap LPG and CNG — that single swap is the favourite trap of the General Science paper.

Quick Revision