OCR A A level Chemistry Intermolecular forces notes

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Intermolecular forces

There are 3 types of intermolecular forces:

Induced dipole-dipole (London forces)

Permanent dipole-dipole

Hydrogen bonds

Intermolecular forces determine a molecule's physical properties (ie. melting/boiling point)

London forces

London forces are created through instantaneous and induced dipoles which occur when the distribution of electrons is uneven throughout the molecule just by chance as electrons can be found anywhere within their shell and are constantly moving. This is an instantaneous dipole. The charge on the dipole well then either attract or repel electrons on other molecules around it, causing a domino effect and pulling many molecules closer together. These new dipoles formed as a result are called induced dipoles. London forces occur within all molecules and are the weakest type of intermolecular force. London forces are stronger in molecules with a greater number of electrons.

Permanent dipole-dipole

Similarly to London forces, permanent dipole-dipole interactions use the charges of molecules to attract them, however, permanent dipoles occur when there is a difference in the electronegativities of the bonded atoms. As a result, molecules have permanently charged poles (one δ+ and one δ-).

Hydrogen Bonds

Hydrogen bonds occur only between molecules containing hydrogen and another very electronegative atom such as fluorine, oxygen or chlorine (remembering the acronym FON). They are the strongest type of intermolecular force.

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Simple molecular compounds

Physical properties

Low melting and boiling points (weak intermolecular forces)

Does NOT conduct electricity (No delocalised electrons or free ions)

Solubility

The solubility of simple covalent molecules depends on whether it is polar or non-polar.

Non-polar:

In non-polar solvents: Soluble. Intermolecular forces form between the compound and the solvent and so it dissolves.

In polar solvents: Insoluble. The permanent dipoles of the polar solvent are too strong to break down so the molecule does not dissolve

Polar

The solubility of polar substances is difficult to predict. Similar to ionic compounds, polar molecules can form permanent dipoles or hydrogen bonds with the solvent to break down the lattice and dissolve, however this is not always the case. Solubility can depend on the strength of the dipole, or even the hydrophilic/hydrophobic interactions (attraction or repulsion to water) in biological molecules.

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Hydrogen bonding

Hydrogen bonding occurs between molecules containing a hydrogen atom and a highly electronegative atom (fluorine, oxygen and nitrogen) with a lone pair of electrons. They are the strongest type of intermolecular force and are shown using a dashed line.

Water

Cohesive: Hydrogen bonding is very important for the physical properties of water. For example, hydrogen bonds are responsible for the cohesive forces between water molecules that make them 'stick together'.

Density: Furthermore, the density of ice being lower than the density of water can also be explain using hydrogen bonding. These intermolecular forces are so strong that they hold the water molecules in an open tetrahedral lattice closer together than the molecules are held when frozen.

Melting/boiling point: Water also has an unexpectedly high melting and boiling point. This is because it contains all three intermolecular forces: London forces, permanent dipole-dipole and hydrogen bonding. All three forces must be overcome to turn water into a gas.

Other anomalous properties: High surface tension and viscosity