Revision:Van der Waals Forces

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Van der Waal's forces are the weakest intermolecular force, are a temporary induced dipole-dipole and last for only a fraction of a second. They are produced when a symmetrical atoms electron configuration is disturbed and losses its symmetrical electron configuration. This causes the atom to become polarised. The polar force of the atom can convert another atom nearby to become polarised as well. With both atoms having a charge they attract each other like magnets. Van der Waals force is increased with the size of the atom and number of electrons.

Summary

• Act between all atoms and molecules.

• Van der Waals forces are caused by the formation of temporary dipoles due to constant movement of electrons.

• The size of the force depends on the size of the molecule. In a large molecule, there are more electrons therefore the Van der Waals forces between large molecules are stronger than between smaller molecules.

Permanent dipole forces

• Occur between molecules with permanent dipoles.

• Permanent dipoles are formed when there is a large difference in electronegativity between two atoms bonded together in a covalent bond. This causes the shared pair of electrons to be shared unequally. They are pulled towards the more electronegative atom.

• e.g. H---F has a permanent dipole. The shared pair of electrons are shared unequally and are pulled towards the electronegative F atom.

Examples

One example of Van der waals forces is the boiling points of Ethane and Pentane. Ethane is a gas at room temperature and has a boiling point of 185K while Pentane is a liquid at room temperature and has a boiling point of 309K.

Pentane is a larger molecule than Ethane and so has more Van der Waal forces; this is the cause of the higher boiling point and states of matter.

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