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van der Waals’ Forces

It has been pointed out earlier in the beginning of this unit that the attractive forces existing between polar as well non-polar molecules are collectively called van der Waals’ forces. van der Waals’ forces can be classified into following categories:

(i) Dipole-Dipole forces

(ii) Dipole-induced dipole forces

(iii) Dispersion forces.

Let us proceed to discuss the nature of these attractive forces.

 

Dipole-Dipole Forces

Dipole-dipole forces are attractive forces operating between the molecules possessing permanent dipole. Ends of the dipole possess “partial charges” and these charges are shown by Greek letter delta (8). Partial charges are always less than the unit charge (1.6 x I0-19 C) because of electron sharing effect. The polar molecules interact with neighbouring molecules. This interaction is weak as compared to ion-ion interactions because only partial charges are involved. The interaction energy decreases with the increase of distance between the dipoles. It is also proportional to ( 1/ r6 ) where ‘r’ is the distance between interacting polar molecules. Now, greater the dipole moment of polar molecule, greater is the magnitude of dipole-dipole forces. This effect was studied · by Keesom (1912) and is also referred to as orientation effect. Fig. 8.8 shows electron cloud distribution. and attractive interactions between H-Cl dipoles.

Fig. 8.8. Attractive interactions between H-CI dipoles.

 The magnitude dipole-dipole forces in different polar molecules can be predicted on the basis of the polarity. In general, for the molecules of similar molecular masses, the magnitude of dipole-dipole forces is higher in case of more polar molecules. For example, molecular masses of PH3 and S is same but H2S is more polar (µ= 1.10D) than PH3 (µ= 0.55D). Hence boiling point of H2S (187K) is higher than that of PH3 (Tb = 137 k).

The magnitude dipole-dipole forces in different polar molecules can be predicted on the basis of the polarity. In general, for the molecules of similar molecular masses, the magnitude of dipole-dipole forces is higher in case of more polar molecules. For example, molecular masses of PH3 and S is same but H2S is more polar (µ= 1.10D) than PH3 (µ= 0.55D). Hence boiling point of H2S (187K) is higher than that of PH3 (Tb = 137 k).

 

Dipole-Induced Dipole Forces

This type of attractive forces operate between polar molecules having µ> 0 and the non-polar molecules having µ= 0. Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming or polarizing its electron cloud. The attractive forces between permanent dipole and induced dipole are called dipole-induced dipole forces. In this case also interaction energy is proportional to ( 1/ r6) where r is the distance between two molecules. Induced dipole moment depends upon the dipole moment of the pannanent dipole and the polarizability of the electrically neutral molecule. As the size of molecule/atom increase, the influence of permanent electrical dipole on it also increases, resulting in the increase in magnitude of these forces. The existence of these forces was studied by Debye (1920) and this effect was termed as induction effect.

Fig. 8.9. Dipole-induced dipole forces between permanent dipole’ and induced dipole.

 

In this case also there is a cummulative effect of dispersion forces and dipole-induced dipole interactions.

 

Dispersion Forces or London Forces

These are the interparticle forces among the monoatornic or non-polar molecules such as N2, He, H2 , CO2 , etc. Atoms and other non-polar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distributed. However, an instantaneous dipole may develop in such atoms and molecules. In order to understand this let us assume two atoms of neon ‘A’ and ‘B’ in the close vicinity of each other (Fig. 8.10 (a)). Rapid movement of electrons in A may cause momentary accumulation of electron density on one side, thus, causing the charge distribution to become unsymmetrical. In other word, the charge cloud is more concentrated on one side than the other. (Figs. 8.10 (b) and 8.10 (c)). This will result in the development of temporary instantaneous dipole on the atom A for a very short time. This instantaneous or transient dipole distorts the electron density of the atom ‘B’, which is close to it. In other words, a dipole is induced in the atom ‘B’ also.

Fig. 8.10. Dispersion forces or London forces between atoms.

 

The temporary dipoles of atom ‘A’ and ‘B’ attract each other. Similar temporary dipoles are induced in non-polar molecules also. Magnitude of such a force of attraction was first calculated by the German physicist Fritz London. For this reason, force of attraction between two temporary dipoles is known as London force. Another name for this force is dispersion force. These forces are always attractive and interaction energy is proportional to ( 1/ r6 ) where r is the distance between two interacting particles. These forces are important only at short distances ( -500 pm) and their magnitude depends on the ability of the particles to undergo polarization. Dispersion forces are present among all the particles.

 

• molecular size (Molar mass)

•. number of electrons and

• ·surface area of molecule

 

Strength of van der Waals’ Forces

Strength and magnitude of van der Waals’ forces depend on the following factors:

(i) Molecular Size (Molar mass). Larger the molecular size or its complexity greater is the probability of polarisation or distortion and higher is the magnitude of van der Waals’ forces

(ii) Number of electrons per molecule. Larger the number of electrons per molecule more are the chances of polarisation and higher is magnitude of van der Waals’ forces

(iii) Surface area and geometry of molecule. Larger the surface area of the molecule more are the chances of intermolecular interactions

 

Let us now compare the boiling points of halogens. All halogens are non-polar molecules. But molecular size and number of electrons per molecule increases from F2 to I2.

Consequently, magnitude of van der Waals’ forces and boiling points increase in the same order.

In the similar way the boiling points of noble gases increase from He to Xe due to increase in the molecular mass and number of electrons per molecule.