Intermolecular Forces

'The volatility of a liquid decreases as the number and strength of the intermolecular forces that need to be broken to vaporise it increase'

There are three important types of intermolecular force;

• van der Waals forces
• Permanent dipole - permanent dipole forces
• Hydrogen Bonds

van der Waals forces

'van der Waals forces are attractive forces between molecules that arise when random electron movements in one molecule cause an instantaneous dipole which in turn causes an induced dipole in the other molecule'

• According to orbital theory, electrons do not have fixed positions within atoms and molecules. Instead, electrons move in a somewhat random way within their orbitals.
• As a result, even when on time-average the electrons are evenly distributed in a molecule, it is possible that at any one moment the electrons will happen to be more on one side or at one end than the other

• The molecule will then have an instantaneous dipole 
• If another molecule is close to this first one then the electrons in this molecule will be influenced by the nearby instantaneous dipole, causing an induced dipole in this second molecule
• As a result, the two molecules are attracted to each other. The time average of these instantaneous dipole - induced dipole attraction is an attractive intermolecular force called the van der Waals force

The effectiveness of van der Waals forces in attracting molecules together depends on two main factors;

• The number of electrons in the molecule - the more electrons the stronger the van der Waals forces
• The surface area of the molecule - a larger surface area means more contacts with other molecules can be made, so more van der Waals forces have to be broken to seperate the molecule (e.g. on seperation)

The boiling point increases progressively as both the number of electrons per molecule and the accessible surface area increase

• All molecules have van der Waals intermolecular forces. However, when molecules contain atoms of different electronegativity, the electrons will not be shared equally, even on a time average and in this case the molecules may have a dipole moment all the time
• This is called a permanent dipole
• When a simple molecular substance has a permanent dipole, it can form additional intermolecular forces called permanent dipole - permanent dipole forces
• Because the dipole is there all the time, permanent dipole - permanent dipole forces are generally stronger than van der Waals forces which depend on transient dipoles
• Therefore, polar substances are likely to have higher boiling points than non-polar substances with molecules of the same size

Diatomic molecules will always have a permanent dipole unless that are made of two of the same type of atom. The bigger the electronegativity difference, the bigger the dipole moment

Polyatomic molecules: Molecules with more than two atoms can also have a dipole moment, if at least some of the bonds are polar but now there is an additional issue;

Effect of symmetry on dipole moments

• In highly symmetrical molecules, the individual dipoles may cancel out, so that even though there are polar bonds, the molecule as a whole is not polar
• Shapes where symmetry causes the dipoles to cancel are;

• Linear
• Trigonal Planar
• Tetrahedral
• Octahedral

All of the above molecules are non-polar, even though they contain polar bonds. If the symmetry is destroyed by introducing different atoms, polar molecules may result

Pyramidal and non-linear molecules are less symmetrical, so the dipoles do not cancel

Hydrogen bonds are a special case of permanent dipole - permanent dipole forces which occur between;

• A hydrogen atom
• A small, very electronegative atom (oxygen, nitrogen or fluorine). A lone pair of electrons on this atom points directly towards the hydrogen

These hydrogen bonds are much stronger than other intermolecular forces

'A hydrogen bond is the intermolecular attraction between hydrogen covalently bonded to N, O or F on one molecule and the lone pair on N, O or F in another molecule'

Water

Important example of a molecule whose properties are profoundly affected by hydrogen bonding

• In liquid water, the molecules more around in clusters containing two or more molecules held together by hydrogen bonding, with these hydrogen bonds being continually broken and re-formed. To boil water, the molecules must be given enough energy to overcome these hydrogen bonds between the molecules

• In ice, the water molecules are held together by hydrogen bonds in a rigid three dimensional network

Water has anomalous properties caused by hydrogen bonding. These anomolous properties are;

1. Relatively high melting and boiling points (compared with other molecules of a similar size)

This is because hydrogen bonds are stronger than intermolecular forces, so more energy is needed to break them

2. High Surface Tension (means that more force is needed to puncture the surface that have molecules of a similar size)

This is because a lot of energy is needed to break the hydrogen bonds that hold together the network of water molecules in the surface layer

3. Water is less denes in the solid state than in the liquid state

This is becuause ice has an open lattice structure; the hydrogen bonds keep all of the water molecules fixed in position. So on average the spaces between the molecules are slightly larger than in liquid water