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Properties and Uses of Alkanes

Key Concepts

Physical Properties of Alkanes:

  • Alkanes are colourless.

  • Alkanes are less dense than water (alkanes float on top of water).

  • Alkanes are non-polar molecules so they are more soluble in non-polar solvents than they are in polar solvents. Alkanes are insoluble in water.

  • The melting and boiling points of the shorter chain alkanes is low, but the melting and boilings of alkanes increase as the number of carbon atoms in the carbon chain increases.

Chemical Properties of Alkanes:

Physical Properties of Alkanes

Name Molecular Formula Molecular Mass Melting Point (oC) Boiling Point (oC) State (25oC, 101.3kPa) Density (liquid g cm-3, 20oC) Flashpoint (oC) Enthalpy of Combustion (kJ mol-1) Uses
methane CH4 16 -182 -162 gas     -889 major component of natural gas (fuel)

ethane C2H6 30 -183 -88.6 gas     -1557 component of natural gas (fuel)

propane C3H8 44 -188 -42.1 gas     -2217 component of liquefied petroleum gas (LPG), bottled gas (fuel)

butane C4H10 58 -138 -0.5 gas     -2874 component of liquefied petroleum gas (LPG), cigarette lighters (fuel)

pentane C5H12 72 -130 36.1 liquid 0.626 -49 -3536 component of petrol (fuel)

hexane C6H14 86 -95.3 68.7 liquid 0.659 -22 -4190 component of petrol (fuel)

heptane C7H16 100 -90.6 98.4 liquid   -4 -4847 component of petrol (fuel)

octane C8H18 114 -56.8 126 liquid     -5506 major component of petrol (fuel)

nonane C9H20 128             component of petrol (fuel)

decane C10H22 142 -30 174 liquid 0.730     component of petrol (fuel)

hexadecane C16H34 226 18.5 288 liquid 0.775     component of diesel fuel & heating oil

eicosane C20H42 282 36 343 solid        
 

Alkanes with flashpoints1 below room temperature (the components of petrol for example) should be stored in strong metal containers with narrow mouths and tightly sealed lids to prevent the vapour from escaping and to prevent a naked flame or spark from igniting the vapour/air mixture.

Colour

  • Methane to butane are colourless gases.
        (propane and butane are easily condensed under pressure and are commonly sold as liquids)

  • Alkanes containing 5 carbons up to about 19 are colourless liquids.
        (petrol & kerosene are mixtures of liquid alkanes, dye is added to the fluids for safety reasons)

  • Alkanes with more than about 20 carbon atoms are colourless, waxy solids.
        (paraffin wax is a mixture of solid alkanes)

Density

  • Alkanes are less dense than water (alkanes will float on top of water)

  • Density increases with increasing molar mass.

Melting and Boiling Points

  • Simple alkanes have low melting and boiling points
        (measured at 1atm or 101.3 kPa pressure).
        Methane to butane have boiling points less than 25oC.
        Methane to butane are gases at 25oC.
        Pentane to decane have melting points less than 25oC.
        Pentane to decane are liquids at 25oC.

  • Boiling points increase as the molar mass increases.
AUS-e-TUTE Members should log-in to use the interactive graphs in the Members Only area.

Alkanes are non-polar molecules.
Only weak intermolecular forces (Van der Waal's Forces2, London Forces, Dispersion Forces, Weak Intermolecular Forces) act between the alkane molecules, so little energy is required to break these weak intermolecular forces and separate the molecules so that the compound melts and boils at quite low temperatures.

As the number of carbon atoms in the chain increases, the long carbon chains are increasingly attracted to each other by these weak intermolecular forces, so, as the molar mass of alkanes increases, the melting and boiling points also increase.

Solubility

  • Alkanes are soluble in non-polar solvents.
    Non-polar alkane molecules are attracted to other non-polar molecules by weak intermolecular forces (Van der Waals Forces, Dispersion Forces, London Forces), so non-polar alkane molecules will dissolve in non-polar solvents.

  • Alkanes are insoluble in polar solvents like water.
    The molecules in a polar solvent such as water are strongly attracted to each other as a result of the attraction of partial negative and partial charges within each molecule:
    δ+H-Oδ--Hδ+
    .
    .
    .
    .
    .
    Red dotted lines (...) represent the intermolecular attraction between the partial negative charge on the oxygen atom of one water molecule and the partial positive charge on the hydrogen atom of a different water molecule. This type of intermolecular attraction is known as a hydrogen bond.
    δ+H-Oδ--Hδ+

    When a non-polar alkane is added to a polar solvent:
        (i) the alkane molecules are attracted to each other but are not attracted to the water molecules
        (ii) the water molecules are attracted to each other but are not attracted to the alkane molecules
    so the alkane does not dissolve in the polar solvent.

Chemical Properties

Combustion of Alkanes

In the presence of excess oxygen, O2, alkanes combust to produce carbon dioxide gas and water, as well as energy in the form of heat and light.

General word equation: alkane + oxygencarbon dioxide gas + water+ energy
Example (word equation): butane + oxygencarbon dioxide gas + water+ 2874 kJ mol-1
Example (chemical equation): C4H10(g) + 6½O2(g)4CO2(g) + 5H2O(l)+ 2874 kJ mol-1

  • The combustion of any alkane produces energy.

  • As the molar mass of a straight-chain alkane increases, the amount of energy released also increases as shown by the graph on the right.
    The slope of the graph on the right is approximately 45 kJ g-1, that is, if the molar mass of an alkane increases by 14 g (the molar mass of each additional CH2 in the carbon chain) then the amount of additional energy released by its combustion will be about 45 kJ/g x 14 g = 630 kJ.

  • As the length of the carbon chain increases, the amount of energy released during combustion increases.
    A longer carbon chain contains more C-H bonds and more C-C bonds.
    A longer carbon chain produces more C=O bonds (as in CO2) and more O-H bonds (as in H2O).
    The overall process of breaking C-C and C-H bonds and making C=O and O-H bonds releases energy.
    The more C-C and C-H bonds broken, and the more C=O and O-H bonds formed, the greater the amount of energy released (see bond energy).
AUS-e-TUTE Members should log-in to use the interactive graphs in the Members Only area.

If there is insufficient oxygen available for the alkane to undergoe complete combustion, then the alkane will undergo incomplete combustion.
The products of incomplete combustion include water and carbon monoxide and/or carbon.

Halogenation of Alkanes

Alkanes are not very reactive.

The reaction between an alkane and a halogen such as chlorine or bromine will not occur without energy, in the form of ultraviolet light, being supplied.

reaction conditions reactants products
no ultraviolet light alkane + halogen no reaction
ultraviolet light alkane + halogen UV
halogenated alkanes

Ultraviolet light provides enough energy to break a C-H bond in the alkane molecule and replace the hydrogen atom with a halogen atom.
Reactions in which one atom in an organic molecule is replaced with a different atom are called substitution reactions.
Example:

hexane + bromine UV light
bromohexane + hydrogen bromide
H
|
H
|
H
|
H
|
H
|
H
|
H-C-C-C-C-C-C-H
|
H
|
H
|
H
|
H
|
H
|
H
+ Br-Br UV light
H
|
H
|
H
|
H
|
H
|
H
|
H-C-C-C-C-C-C-Br
|
H
|
H
|
H
|
H
|
H
|
H
+ H-Br

Further substitutions are then possible:
bromohexane + bromine UV light
1,2-dibromohexane + hydrogen bromide
H
|
H
|
H
|
H
|
H
|
H
|
H-C-C-C-C-C-C-Br
|
H
|
H
|
H
|
H
|
H
|
H
+ Br-Br UV light
H
|
H
|
H
|
H
|
H
|
Br
|
H-C-C-C-C-C-C-Br
|
H
|
H
|
H
|
H
|
H
|
H
+ H-Br

until all the hydrogen atoms have been replaced by bromine atoms:
Br
|
Br
|
Br
|
Br
|
Br
|
Br
|
H-C-C-C-C-C-C-Br
|
Br
|
Br
|
Br
|
Br
|
Br
|
Br
+ Br-Br UV light
Br
|
Br
|
Br
|
Br
|
Br
|
Br
|
Br-C-C-C-C-C-C-Br
|
Br
|
Br
|
Br
|
Br
|
Br
|
Br
+ H-Br


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1Flashpoint: the minimum temperature at which the vapour pressure of a liquid is high enough for an explosive mixture to be formed with air. Safety precautions for handling & storing fuels are determined by the flashpoint.

2 Some Chemists refer to all intermolecular forces as Van der Waal's forces, others use the term Van der Waal's forces synonymously with London forces or dispersion forces. It is probably best to avoid using the term Van der Waal's forces at all and use one of the other, unambiguous, terms instead.

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