intermolecular forces

INTERMOLECULAR FORCES

LONDON FORCES aka van der Waals forces cause all atoms and molecules to be attracted to each other electron charge clouds are always moving, at a specific moment electrons in one atom are more likely to be to one side than the other. This creates a temporary dipole. this dipole can cause another temporary dipole in the opposite direction on neighbouring atom(s), this results in them being attracted to each other. (instantaneous dipole-induced dipole interactions). electrons are constantly moving, so dipoles are being created and destroyed all the time larger molecules have larger electron clouds, meaning stronger London forces. molecules w/ greater S.A. have stronger London forces, as more of their electron cloud is exposed. HYDROCARBONS:  LONGER CHAINS HAVE MORE MOLECULAR S.A., MORE ELECTRONS INTERACT SO STRONGER LONDON FORCES. BRANCHED-CHAIN ALKANES HAVE SMALLER MOLECULAR S.A. AND CANNOT AS CLOSELY TOGETHER, SO LONDON FORCES ARE LOWER.THUS LONG-CHAIN AND STRAIGHT-CHAIN ALKANES HAVE HIGHER BOILING POINTS.

POLARITY permanent dipole-dipole interactions charges on polar molecules cause weak electrostatic forces of attraction between molecules permanent dipole-dipole interactions > instantaneous dipole-induces dipole interactions think of electrostatically charged rod and jet of water; will attract the liquid molecules TOWARDS the rod. a bond within a mole may be polar but if the molecule is symmetrical , overall the molecule would not be polar.

HYDROGEN BONDING strongest intermolecular forces only form when hydrogen is bonded to fluorine, nitrogen or oxygen, this is because these elements are very electronegative, so they draw the bonding electrons away from the hydrogen atom, bond is so polarised and hydrogen has such a high charge density that hydrogen atoms form weak bonds with lone pairs of electrons on the F, N, O atoms of other molecules. likely molecules to have H bonding are usually organic, containing -OH or -NH groups water and ammonia both have hydrogen bonding. H bonds increase melting and boiling points. ice is less dense than liquid water due to H bonds too.

New Page