When liqids change to gases, energy is required to bring this about. This energy is required to overcome forces between the molecules, i.e. to separate them. In the series of alkanes, the larger the molecule, the higher the boiling point. Straight-chained alkanes have a higher boiling point than branched chains. The intermolecular forces between the molecules arise due to the presence of dipoles in the molecules.
A dipole arises when charge is not shared equally between two atoms. This is shewn in hydrogen chloride below:
When a molecule has a dipole it is said to be polarised. There are several ways a molecule can become polarised.
All intermolecular forces arise from the attractive forces between dipoles. There are three types:
How do we know if there is a dipole present in a molecule? Bond polarity depends on any difference in electronegativity between the atoms in the bond. A table of electronegativity values for different elements was drawn up by Pauling.
Elements in the top right of the Periodic Table are the most electronegative.
Note that carbon and hydrogen have similar electronegativities, hence a C - H bond is taken as non-polar.
These will usually occur in molecules which contain the elements, F, O, N and Cl. Examples of this are aldehydes, ketones, esters, chlorinated hydrocarbons, PVC.
Hydrogen bonding is a particularly strong intermolecular force that involves three features:
Hydrogen bonds are much stronger than other types of intermolecular forces. They are however much weaker than covalent bonds.
Water is a liquid due to the hydrogen bonds between the molecules.
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