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4.3Organic Chemistry

Alkanes — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 18 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
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Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

Alkanes: Structure, Properties and Reactions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the structure, trends in physical properties and the combustion and substitution reactions of alkanes.Alkanes are hydrocarbons with the general formula CnH2n+2.

  • Complete combustion
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Volatility

Section 1 of 8Structure of alkanes

Alkanes are hydrocarbons with the general formula CnH2n+2.

Structure of alkanes
Saturated
All the carbon–carbon bonds are single covalent bonds, so alkanes are saturated: each carbon holds the maximum possible number of hydrogen atoms.
Each carbon atom forms four single bonds arranged tetrahedrally, with bond angles of about 109.5°.
The first members are methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12, hexane C6H14.
From butane onward, structural isomers are possible, such as 2-methylpropane.
AlkaneFormulaBoiling point (°C)State at 25 °C
MethaneCH4−162Gas
EthaneC2H6−89Gas
PropaneC3H8−42Gas
ButaneC4H10−1Gas
PentaneC5H1236Liquid
DecaneC10H22174Liquid

Section 2 of 8Physical properties

As the chain length increases, the intermolecular forces between molecules become stronger, so the boiling point, melting point and viscosity increase and volatility and flammability decrease.

Physical properties
Alkanes are non-polar,
  • so they do not dissolve in water but dissolve in other non-polar solvents,
    • and they do not conduct electricity because they have no free ions or electrons.
Branched isomers have lower boiling points than straight-chain isomers of the same formula since they pack less closely.

Section 3 of 8Chemical reactions

Alkanes are relatively unreactive because of the strong C–C and C–H bonds, but they do two important reactions.

Complete combustion
Complete combustion in excess oxygen gives carbon dioxide and water and releases large amounts of energy: C3H8 + 5O2 → 3CO2 + 4H2O.
Incomplete combustion
Incomplete combustion in limited oxygen produces carbon monoxide and/or carbon (soot) and water: 2CH4 + 3O2 → 2CO + 4H2O.
  • Carbon monoxide is toxic, so gas appliances must be serviced and rooms ventilated.
Substitution
Substitution with halogens in the presence of UV light: CH4 + Cl2 → CH3Cl + HCl.
  • One hydrogen atom is replaced by a chlorine atom per step;
    • further substitution can occur to give CH2Cl2, CHCl3 and CCl4.

Tests

  • alkanes do not decolourise bromine water in the dark, which distinguishes them from alkenes.
  • Under UV light bromine does react by substitution, slowly, and the red colour fades with fumes of hydrogen bromide.

Section 4 of 8Uses and the environment

Methane in natural gas is used for heating and generating electricity; propane and butane are sold as bottled gas; petrol, kerosene and diesel are alkane mixtures. Burning alkanes releases carbon dioxide (a greenhouse gas); methane itself is also a potent greenhouse gas leaking from landfill, cattle and gas pipes. Reduce emissions through efficient engines, catalytic converters and alternative fuels.

Section 5 of 8Worked calculations with alkanes

Volume calculation

  • 20 cm3 of methane burns completely in excess oxygen.
  • CH4 + 2O2 → CO2 + 2H2O.
  • At the same temperature and pressure the volumes of gases are in the same ratio as the moles, so 40 cm3 of oxygen is used and 20 cm3 of carbon dioxide forms.

Mass calculation8.0 g of methane (Mr 16) is 0.50 mol, which produces 0.50 mol of CO2, a mass of 0.50 × 44 = 22 g.

Worked calculations with alkanes
This shows that burning a fuel produces more mass of carbon dioxide than the mass of fuel, because oxygen from the air is added to the carbon.
Formula practice: an alkane with 12 carbon atoms has 2 × 12 + 2 = 26 hydrogens, so dodecane is C12H26.
The percentage of carbon by mass increases along the series from 75% in methane to about 85% in long-chain alkanes,
  • which explains why larger alkanes burn with a more smoky flame, and why diesel produces more soot than natural gas.

Practical noteto test the products of burning a hydrocarbon, pass the gases through a cooled U-tube to collect the liquid water (test with anhydrous copper(II) sulfate, which turns blue, or cobalt chloride paper, which turns pink) and then through limewater, which turns milky showing carbon dioxide.

Section 6 of 8Worked examples

  1. Write the balanced equation for the complete combustion of butane. [2]2 marks

  2. Name the type of reaction between methane and chlorine in UV light, and the products. [2]2 marks

  3. Explain why decane has a higher boiling point than propane. [2]2 marks

  4. Why is incomplete combustion of alkanes dangerous in a closed room? [2]2 marks

Section 7 of 8Key terms

TermMeaning
AlkaneSaturated hydrocarbon CnH2n+2
SaturatedOnly single bonds between carbon atoms
Complete combustionBurning in excess oxygen to CO2 and H2O
SubstitutionOne atom replaced by another
VolatilityTendency to evaporate
IsomerSame formula, different structure

Section 8 of 8Exam technique

1
Saying alkanes react with bromine water.
2
Forgetting that UV light is needed for halogen substitution.
3
Thinking that alkanes are polar and dissolve in water.
4
Mixing up the general formulae of alkanes and alkenes.

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