Dispersion Forces

What are Dispersion forces?

Dispersion forces are the weakest intermolecular attractive forces. The existence of dispersion forces accounts for the fact that low molecular weight, non-polar substances, such as hydrogen (H₂), Neon (Ne), and methane (CH₄) can be liquified.

To visualize the origin of dispersion forces, it is necessary to think in terms of the instantaneous distribution of electron density rather than the average distribution.

For example, consider neon which is a gas at room temperature.

• It can be liquefied when cooled to -246^oC.
• From the heat of vaporization, it can be calculated that the neon-neon attractive interaction in the liquid state is approximately 0.3kJ.

Table of Contents

• Characteristic and Effect of Dispersion Force
• Dispersion Force and Polarity
• Consequences of Dispersion Forces
• Dispersion Force Examples

Characteristic and Effect of Dispersion Force

On the basis of polarity, molecules can be classified as polar and nonpolar molecules. The polar molecules have partial charges on both poles of the molecules whereas non-polar molecules have no charges on both sides of the molecules. For example; hydrochloric acid or HCl is a polar molecule. In this molecule, both hydrogen and chlorine are bonded with a covalent bond.

Since covalent bonds are formed by equal sharing of electrons, therefore, there must be no charge on both sides of the molecule but because of the high electronegativity of chlorine, the bonding electrons shifted towards the chlorine atom. This partial shifting creates a partial negative charge on the chlorine atom and a partial positive charge on hydrogen. Due to these partial charges on both poles of molecules, they attract each other and have weak interaction forces between molecules.

On the contrary, in non-polar molecules like chlorine molecules, the bonding electrons are placed exactly at the center of the bond. Therefore there is no partial charge over the molecule. Then how these molecules will attract each other, Or there will be no interaction between molecules. The force of attraction between nonpolar molecules is known as dispersion forces.

The distribution of electron density is symmetrical in neon and there is no dipole moment in neon. However, at any instant, there is a non-zero probability that its electron density will be polarized more toward one part of the atom than toward another.

This temporary polarization creates a temporary dipole moment, which in turn induces a temporary dipole moment on adjacent atoms.

1. The distribution of electron density averaged over time in neon is symmetrical and there is no net polarity.
2. Temporary polarization of one neon atom induces temporary polarization in adjacent atoms. Electrostatic attraction between temporary dipoles is called dispersion forces.

Dispersion forces are inversely proportional to the sixth power of the distance between interacting atoms or molecules. For them to be important the interacting atoms or molecules must be in virtual contact with one another.

Dispersion Force and Polarity

• Dispersion forces are always present whether the molecules have permanent dipoles, or not.
• The intermolecular force between permanent molecular dipoles is the result of the polarity and the dispersion forces.
• For example, HCl is significantly more polar than HI, yet the boiling point of HCl is much lower than that of HI.
• The ‘I’ atom is much bigger than the Cl atom. It has more electrons and the distance over which the electrons can spread is greater, giving greater dispersion forces in HI than in HCl.
• The dispersion force is usually of more significance than the polarity of the molecules. However, in two molecules with the same number of electrons and similar size, the polarity becomes significant.
• Ethane and fluoro-ethane both have 18 electrons, but fluoro-ethane is significantly more polar, and so has a higher boiling point.

Consequences of Dispersion Forces

The principal aspect of dispersion force is the determination of the order of magnitude of the attractive force. The main feature of dispersion force (London dispersion force) is

• Dispersion forces are long-range and can be effective from large distances (>10nm) down to interatomic distances.
• Dispersion forces may be repulsive or attractive. The expression of the dispersion force does not follow a simple power law.
• Dispersion interaction between two bodies is affected by the presence of other bodies nearby. (non-addictive interaction)

Dispersion Force Examples

Dispersion forces act over a smaller surface area. Hence the attraction between molecules decreases and the boiling point decreases. For any group of alkane constitutional isomers, the least branched isomer generally has the highest boiling point and the most branched isomer generally has the lowest boiling point.

For example, the difference in boiling point due to the dispersion forces for hexane and 2,2-dimethyl butane are shown below.

The boiling point of some five isomeric alkanes within the molecular formula C₆H₁₄ are listed below.

As branching increases, the alkane molecule becomes more compact and it’s surface area decreases. As surface area decreases, London dispersion forces act over a smaller surface area. Hence the attraction between molecules decreases and the boiling point decreases. Thus for any group of alkane constitutional isomers, the least branched isomer generally has the highest boiling point and the most branched isomer generally has the lowest boiling point.