Characteristic and Effect of Dispersion Force
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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 instantaneous distribution of electron density rather than average distribution.
For example, consider neon which is a gas at room temperature.
It can be liquefied when cooled to -246o
From the heat of vaporization, it can be calculated that the neon-neon attractive interaction in the liquid state is approximately 0.3kJ.
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 temporary dipole moments in adjacent atoms.
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 forces are always present whether the molecules are 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 the higher boiling point.
The principle aspect of dispersion force is the determination of the order of magnitude of the attractive force. The main features of dispersion force (London force) is
- Dispersion force are long range and can be effective from large distance (>10nm) down to inter atomic distances.
- Dispersion forces may be repulsive or attractive. 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.(nonaddictive interaction)
Dispersion forces act over a smaller surface area. Hence the attraction between molecules decreases and 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.
The boiling point of some five isomeric alkanes within the molecular formula C6
are listed below.
As branching increases the alkane molecule become more compact and its surface area decreases. As surface area decreases, London dispersion forces act over a smaller surface area. Hence the attraction between molecules decreases and 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.
For example, the difference in boiling point due to dispersion force for hexane and 2,2-dimeyl butane are shown below.