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MAKING THE MODERN WORLD
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module:Polymers and intermolecular bonds

page:Intermolecular forces: Part 2

Permanent dipole–permanent dipole attractions

Chloroform (CHCl3) was one of the first anaesthetics and has been widely used as a solvent in the chemical industry. Like bromine, it is a liquid but with a higher boiling point. This indicates that the intermolecular forces between CHCl3 molecules are stronger than between Br2 molecules.


CHCl3 molecule

So what holds neighbouring molecules of CHCl3 together? As with any molecule there will be instantaneous dipole-induced dipole attractions between them. But in addition there is another type of force.

Carbon and chlorine both contribute one electron to the covalent bond that holds them together. But chlorine is a more electronegative atom than carbon. This means that it pulls the electrons in the bond towards itself. It ends up with a slight excess of negative charge and the carbon ends up with a slight deficiency of positive charge.

In other words each of the three carbon-chlorine bonds are polarised. We say that the molecule is polar. It has a positive end and a negative end. It possesses a permanent dipole.


CHCl3 molecule

What happens if we add another molecule of CHCl3? The molecules orientate themselves so the positive end of one attracts the negative end of another. These are permanent dipole-permanent dipole attractions.

In general these types of attractions will always be stronger than instantaneous dipole-induced dipole attractions (for similarly sized molecules).

To decide whether a molecule possesses a permanent dipole and therefore the ability to attract other molecules by permanent dipole-permanent dipole attractions, look at the atoms in the molecules and think about their electronegativity values.

If there are bonds between atoms with very different values then these bonds will be polar. Provided the symmetry of the molecule doesn’t result in a number of polar bonds cancelling each other out, the molecule will be polar overall. Remember that carbon and hydrogen have similar electronegativities.

A C-H bond is considered to be non-polar. Oxygen, nitrogen, and the halogens are all more electronegative than carbon or hydrogen.

Use the illustration below to explore your understanding of these bonds so far by printing it out and using the illustration to note the correct locations of polar bonds.


Permanent dipole-permanent dipole attractions

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Answers
    Ethanol
Oxygen is more electronegative than hydrogen and carbon. The molecule is polar.
    Water (H2O)
Oxygen is more electronegative than hydrogen. It is polar.
    Carbon Dioxide (CO2)
Oxygen is more electronegative than carbon. There are two polar bonds. The centre of negative charge is in the same place as the centre of positive charge so the molecule is non-polar.
    Butane (C4H10)
There are only C-C bonds and C-H bonds in this molecule. Carbon and hydrogen have similar electronegativity values. There are no polar bonds and the molecule is non-polar.
    Dichloromethane (CH2Br2)
Bromine is more electronegative than carbon. There are two polar bonds in the molecule. It is polar.
    cis-1,2-difluoroethene
Fluorine is more electronegative than carbon. There are two polar bonds in the molecule. The molecule is polar.
    trans-1,2-difluoroethane
Fluorine is more electronegative than carbon. There are two polar bonds in the molecule. However, the centre of negative charge is in the same place as the centre of positive charge and so the molecule is non-polar.

 

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