Home › Learning Hub › IGCSE Chemistry › 11a Fuels, alkanes and alkenes
Topic 11 · 11.1–11.5

Organic formulae, fuels, alkanes and alkenes

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Draw and interpret displayed formulae; use general formulae; define functional group, homologous series, saturated and unsaturated.
  • Write structural formulae, define structural isomers, and describe the characteristics of a homologous series EXTENDED.
  • Name the fossil fuels; describe the fractional distillation of petroleum, how properties change up the column, and the uses of the fractions.
  • Describe alkanes as saturated and generally unreactive; the substitution reaction with chlorine EXTENDED.
  • Describe alkenes, cracking and the reasons for it, and the bromine water test; addition reactions with bromine, hydrogen and steam EXTENDED.

📚The chemistry

Formulae and families

A displayed formula shows every atom and every bond. A homologous series is a family of compounds with similar chemical properties because they have the same functional group — the atom or group of atoms that determines how the compounds react. General formulae:

alkanes \( \mathrm{C_{n}H_{2n+2}} \) (names end -ane)
alkenes \( \mathrm{C_{n}H_{2n}} \) (-ene)
alcohols \( \mathrm{C_{n}H_{2n+1}OH} \) (-ol)
carboxylic acids \( \mathrm{C_{n}H_{2n+1}COOH} \) (-oic acid)

The first part of the name counts the carbons: meth- 1, eth- 2, prop- 3, but- 4. A saturated compound has only single carbon–carbon bonds; an unsaturated compound has at least one C=C double bond.

Displayed formulae. Methane, CH4: one carbon with four hydrogens. Ethane, C2H6: two carbons joined by a single bond, each with three hydrogens. Ethene, C2H4: two carbons joined by a double bond, each with two hydrogens. Propene, C3H6: a chain of three carbons with a double bond between the first two. Methane and ethane are alkanes; ethene and propene are alkenes.
Every carbon has four bonds and every hydrogen one. Count them on your own drawings — it catches most mistakes.

EXTENDED A structural formula describes the arrangement unambiguously without drawing every bond: CH2=CH2, CH3CH2OH. Members of a homologous series have the same functional group and general formula, differ by a –CH2– unit, show a trend in physical properties (boiling point rises with chain length) and have similar chemical properties. Structural isomers have the same molecular formula but different structural formulae:

Two pairs of structural isomers. C4H10: butane, a straight chain of four carbons, and methylpropane, a chain of three carbons with a CH3 branch on the middle carbon. C4H8: but-1-ene, with the double bond between carbons 1 and 2, and but-2-ene, with the double bond between carbons 2 and 3.
The syllabus examples. The number in but-1-ene and but-2-ene says where the double bond starts. EXTENDED

Fuels and fractional distillation

The fossil fuels are coal, natural gas (mainly methane) and petroleum. Petroleum (crude oil) is a mixture of hydrocarbons — compounds of hydrogen and carbon only. It is separated into useful fractions by fractional distillation: the oil is vaporised, and the vapours rise up a column that is hotter at the bottom than at the top, so each fraction condenses at a different height.

A fractionating column. Crude oil is heated in a furnace and the vapour enters at the bottom. Fractions leave from top to bottom: refinery gas for heating and cooking; gasoline or petrol for cars; naphtha as a chemical feedstock; kerosene or paraffin for jet fuel; diesel oil or gas oil for diesel engines; fuel oil for ships and home heating; lubricating oil for lubricants, waxes and polishes; bitumen for making roads. Towards the top: shorter chains, lower boiling points, more volatile, less viscous.
From bottom to top: chain length decreases, volatility increases, boiling point decreases and viscosity decreases.

Alkanes

Alkanes have only single covalent bonds: they are saturated hydrocarbons. They are generally unreactive, except that they burn (combustion) and react with chlorine by substitution.

EXTENDED In a substitution reaction one atom or group of atoms is replaced by another. The reaction of an alkane with chlorine is photochemical: ultraviolet light provides the activation energy.

\[ \mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl} \]

Alkenes and cracking

Alkenes contain a C=C double bond: they are unsaturated hydrocarbons. They are made by cracking larger alkane molecules, using a high temperature and a catalyst; cracking also produces hydrogen.

\[ \mathrm{C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4} \]
Laboratory cracking. A boiling tube held horizontally contains mineral wool soaked in liquid paraffin at the closed end and pieces of porous pot in the middle, heated strongly by a Bunsen burner. A delivery tube leads to an upturned test tube in a trough of water, where the gaseous alkenes are collected.
In the laboratory the porous pot is the catalyst. Remove the delivery tube from the water before you stop heating, or water is sucked back into the hot tube.

Why crack? Fractional distillation gives more of the long-chain fractions than are needed, and less of the short-chain ones such as petrol. Cracking converts the surplus into shorter, more useful alkanes, plus alkenes (to make polymers and ethanol) and hydrogen.

Test for unsaturation: shake the hydrocarbon with aqueous bromine (bromine water).

Two test tubes after shaking with bromine water. With an alkane the liquid stays orange: saturated. With an alkene the liquid turns from orange to colourless: unsaturated.
Alkene: orange to colourless. Alkane: stays orange. Say “colourless”, never “clear”.

Addition reactions EXTENDED

In an addition reaction two molecules join to form only one product. The double bond opens and the atoms add across it:

Four reactions drawn with displayed formulae. Substitution: methane plus chlorine, in UV light, gives chloromethane plus hydrogen chloride. Addition of bromine: ethene plus bromine at room temperature gives 1,2-dibromoethane. Addition of hydrogen: ethene plus hydrogen with a nickel catalyst gives ethane. Addition of steam: ethene plus steam with an acid catalyst gives ethanol.
Substitution gives two products; each addition gives one. EXTENDED

✏️Worked example

Dodecane, C12H26, is cracked to give octane and ethene only. (a) Write the balanced equation. [2] (b) Give two reasons why an oil company cracks dodecane. [2] (c) Describe a test to show that ethene is unsaturated and octane is not. [2] (d) EXTENDED Ethene reacts with hydrogen. Name the catalyst and the product, and state the type of reaction. [3]

(a) \( \mathrm{C_{12}H_{26} \rightarrow C_8H_{18} + 2C_2H_4} \): 12 C and 26 H on each side.

(b) Shorter alkanes like octane are in greater demand (petrol) than long ones; ethene is needed to make polymers and ethanol.

(c) Shake each with bromine water: ethene turns it from orange to colourless; octane leaves it orange.

(d) Nickel; ethane; addition.

Check it. In cracking, carbon and hydrogen atoms must balance, and the alkane product must fit CnH2n+2: C8H18 ✓, the alkene CnH2n: C2H4 ✓.
“Cracking splits a compound into elements.” It breaks a long hydrocarbon into shorter hydrocarbons (and sometimes hydrogen) — it is thermal decomposition, not splitting into carbon and hydrogen.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which formula is an alkene? A: C3H8. B: C4H8. C: C5H12. D: C2H6.
B. Alkenes are CnH2n: 4 carbons, 8 hydrogens. The others fit CnH2n+2.
2. (Theory.) Give the molecular formula of the alkane with six carbon atoms, and state whether it is saturated or unsaturated. [2]
C6H14 (\( 2 \times 6 + 2 = 14 \)); saturated (only single C–C bonds).
3. (Theory.) Name the fraction used as jet fuel, and state how the viscosity of the fractions changes towards the bottom of the column. [2]
Kerosene (paraffin). Viscosity increases towards the bottom.
4. (Theory.) Complete the cracking equation: \( \mathrm{C_{14}H_{30} \rightarrow C_{10}H_{22} + \ ?\ C_2H_4} \). [1]
2: \( \mathrm{C_{14}H_{30} \rightarrow C_{10}H_{22} + 2C_2H_4} \). Carbon: \( 10 + 4 = 14 \); hydrogen: \( 22 + 8 = 30 \).
5. (Theory.) Hexane and hex-1-ene are both colourless liquids. Describe a chemical test to tell them apart, with the result for each. [3]
Add bromine water and shake. Hex-1-ene decolourises it (orange to colourless); hexane does not (it stays orange).
6. (Theory.) EXTENDED Ethane reacts with chlorine. State the condition needed, and draw the displayed formula of the organic product of monosubstitution. [3]
Ultraviolet light. Chloroethane, CH3CH2Cl: two carbons joined by a single bond, five H atoms and one Cl atom, every carbon with four bonds.
7. (Theory.) EXTENDED Propene reacts with bromine. Name the product and give its structural formula. What type of reaction is this? [3]
1,2-dibromopropane, CH2BrCHBrCH3. Addition.
8. (Theory.) EXTENDED Define structural isomers, and write the structural formula of an isomer of but-1-ene. [3]
Compounds with the same molecular formula but different structural formulae. But-2-ene, CH3CH=CHCH3.
9. (Theory.) EXTENDED Give three characteristics of a homologous series. [3]
Any three: same functional group; same general formula; neighbouring members differ by –CH2–; a trend in physical properties; similar chemical properties.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • MolView — build and rotate 3D models of alkanes, alkenes and their isomers
  • Royal Society of Chemistry — cracking hydrocarbons, a class practical