Properties and Uses of Alkanes Chemistry Tutorial

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:

• Alkanes are relatively unreactive.
  Alkanes do not react with strong acids, bases, oxidising agents (oxidants) or reducing agents (reductants).
• Alkanes combust (react rapidly with oxygen) releasing energy, which makes alkanes useful as fuels.
  (see combustion of hydrocarbons and heat of combustion)
• Alkanes will react with halogens such chlorine gas and bromine water in the presence of ultraviolet light.
  (see halogenation of hydrocarbons)

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

Physical Properties

The table below lists the properties and uses of a number of different alkanes:

Alkanes with flashpoints⁽¹⁾ 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 of Alkanes

• 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 and 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 of Alkanes

• Alkanes are less dense than water (alkanes will float on top of water)
• Density increases with increasing molar mass.

Melting Point and Boiling Point of Alkanes

• Simple alkanes have low melting and boiling points (measured at 1atm or 101.3 kPa pressure).
  ⚛ Methane to butane have boiling points less than 25°C.
  ⚛ Methane to butane are gases at 25°C.
  ⚛ Pentane to decane have melting points less than 25°C.
  ⚛ Pentane to decane are liquids at 25°C.
• Boiling points increase as the molar mass increases as shown in the graph below:

Alkanes are non-polar molecules.
Only weak intermolecular forces (Van der Waal's Forces⁽²⁾, 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 of Alkanes

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.

• The combustion of any alkane produces energy as shown in the graph below:

  Energy released during combustion (kJ mol-1)

  Heat of Combustion of Alkanes (methane to heptane) Molar Mass (g mol-1)

• As the molar mass of a straight-chain alkane increases, the amount of energy released also increases as shown by the graph above.
  The slope of the graph above 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 × 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).

If there is insufficient oxygen available for the alkane to undergo 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.

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 of a substitution reactions is when hexane reacts with bromine in the presence of UV light as shown in the chemical equation below:

Further substitutions are then possible as shown in the chemical equation below:

until all the hydrogen atoms have been replaced by bromine atoms as shown in the chemical equation below: