Intermolecular Forces

Intermolecular Force

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Introduction to Intermolecular Force

The forces of attraction existing among the molecules of a substance (gaseous, liquid or solid) are called Intermolecular Forces. These forces are responsible for the structural features and physical form of the substance. On the other hand, intramolecular forces that is, forces which exist within the same molecule or a polyatomic ion, affect the chemical properties of the substance. 

Greater the intermolecular forces, higher is the boiling point. Conversely, by comparing the boiling points of different substances, strengths of their intermolecular forces can be compared. This is because the boiling point, the heat absorbed is used to cut off the intermolecular forces to convert the liquid into vapor. Similarly, greater the strength of intermolecular forces, higher is the melting point. 

The intermolecular forces arise due to any one of the following interactions: 

  • Dipole-dipole 
  • Ion-dipole 
  • Ion-induced dipole 
  • Dipole-induced dipole 
  • Dispersion forces
  • Hydrogen bonding

Dipole-dipole, dipole-induced dipole and dispersion forces are collectively called as Van Der Waals Forces in honor of the Dutch scientist, van der Waals who studied about these forces. 
Note that ion-dipole and ion-induced dipole forces are not van der Waals forces. Further, hydrogen bonding is only a special type of dipole-dipole attraction shown only by limited number of elements.

Types of Intermolecular Forces

Dipole-Dipole Interactions

These forces of attraction occur among the polar molecules. The reason for the origin of these forces is quite obvious. Polar molecules have permanent dipoles. The positive pole of one molecule is thus attracted by the negative pole of the other molecule. A simple example is that of HCl in which chlorine being more electronegative acquires a slight negative charge whereas the hydrogen end becomes slightly positively charged. The dipole-dipole interactions then take place among the HCl molecules

Dipole-Dipole attractions among HCl Molecule Fig. No. 1 Dipole-Dipole attractions among HCl Molecule

The magnitude of dipole-dipole forces in different polar molecules can be predicted on the basis of polarity of the molecules, which in turn depends upon the electro negativities of the atoms present in the molecule and the geometry of the molecule (in case of polyatomic molecules, containing more than two atoms in a molecule) The poIarities of the molecules are usually expressed in terms of dipole moments of the molecules. For Example, dipole moments of PH3 and H2S are 0.55 D and 1.10 D respectively that is, dipole moment of H2S is double than that of PH3. Thus, though both have nearly same molecular mass, the melting and boiling point of H2S are higher than those of PH3. (Melting Points PH3 = -134° C, H2S = -86° C and Boiling Points PH3= -88° C, H2S = -61° C).

The existence of these forces was studied by Keesom in 1912. Hence, these forces are also called Keesom Forces and the effect is called Orientation Effect

Note: Dipole-dipole interaction energy between stationary polar molecules (as present in the solids) is proportional to 1/r3 and that between the rotating molecules is proportional to l/r6, where r is the distance between the polar molecules.

Ion-Dipole Interactions

This is the attraction between an ion (cation or anion) and a polar molecule. For Example, when NaCl is dissolved in water, the polar water molecules are attracted towards Na+ ion as well as towards Cl- ion (a process called Hydration of Ions) as shown in figure below. The strength of this interaction depends upon the charge and size of the ion and the magnitude of dipole moment and size of the polar molecule. Due to greater charge density on the cation, this interaction is usually stronger with the cation than with the anion having the same charge cm bigger size. Further, CCl4 being non-polar, cannot interact with Na+ and Cl- cations. Hence, NaCl insoluble in CCl4.

Ion-dipole attraction forces are stronger than dipole-dipole interactions because the charge of any ion is much greater than the charge of a dipole moment.

Ion-dipole attractions between Na+ and H2O molecules and Cl- ion and H2O moleculesFig. No. 2 Ion-dipole attractions between Na+ and H2O molecules and Cl- ion and H2O molecules

Ion-Induced dipole Interactions

A non-polar molecule may be polarized by the presence of an ion near it that is, it becomes an induced dipole. The interactions between them are called Ion-Induced Dipole Interactions. The strength of these interactions depends upon the charge on the ion and the case with which the non-polar molecule gets polarized. A cation polarizes the molecule by attraction of the electron cloud whereas an anion does it by repulsion. For Example
In the presence of nitrate ion (NO3-), iodine molecule (I2), which is non-polar, gets polarized as ( I(δ+) – I(δ-)) as shown in figure below.

Ion-induced dipole attractions between Fe2+ and O2 MoleculeFig. No. 3 Ion-induced dipole attractions between Fe2+ and O2 Molecule

Dipole-Induced dipole Interactions

A non-polar molecule may be polarized by the presence of a polar molecule (dipole) near it, thereby making it an induced dipole. The interactions between them are then called Dipole-Induced Dipole Interactions. Their strength will depend upon the strength of the dipole and the ease of polarisability of the non-polar molecule. 

For Example, Noble gases get polarized in the presence of polar molecules as shown in figure below.

Dipole-induced dipole attractions           Fig. No. 4 Dipole-induced dipole attractions

The existence of these forces was studied by Debye in 1920 and this effect is called Induction Effect.

London forces or Dispersion forces

For studying intermolecular forces that attract non-polar molecules like H2, O2, N2, etc. or monoatomic gases like He, Ne, Ar etc. to each other in the liquid and the solid state a new force came into theory, this force was called London force. The origin of these forces was proposed by Fritz London in 1930. Hence, they are termed as London forces. These forces are thought to arise from the motion of the electrons. It is believed that at any instant of time, the electron cloud of the molecule may be distorted so that an instantaneous dipole or momentary dipole (i.e. a dipole for a short while) is produced in which one part of the molecule is slightly more negative than the rest.

lt is interesting to note that after a momentary dipole is formed, at the next instant, the orientation of the momentary dipole will be different because the electrons have moved. Over a period of time (a very short period of time), electrons move rapidly, the effects of these momentary dipoles cancel so that a non-polar molecule has no permanent dipole moment.

The momentary dipoles induce dipoles in the neighboring molecules. These are then attracted to each other exactly in the same way as the permanent dipoles. The forces of attraction between the induced momentary dipoles are called London Dispersion Forces. The origin of these forces in case of helium as shown in figure below.

London Forces in Helium AtomsFig. No. 5 London Forces in Helium Atoms

Note: London dispersion forces operate only over very short distances. So the molecules must be really very close in order for the forces to have any effect. The energy of interaction varies as 1/r6. Thus, if the distance between the particles is doubled, the energy decreases by a factor of 26 = 64. 

It may be pointed out that since all molecules contain electrons, London forces also exist in the polar molecules. However, in case of non-polar molecular substances, London forces are the only intermolecular forces that exist. 

Magnitude of the London forces

London forces are the weakest intermolecular forces.Their magnitude depends upon the following two factors: 

Size/Complexity of the molecules

Larger or more complex are the molecules, greater is the magnitude of London forces. This is obviously due to the fact that the large electron clouds are easily distorted or polarized.
Since the larger molecular size amounts to larger molecular mass, it is often suggested that the magnitude of London forces increases with increasing molecular mass. 
For Example, for the hydrides of Group 14, as we move down the group, the size of the molecules increases. Hence, the magnitude of London forces increases and so does their boiling points, For Example

CH4     <          SiH4    <          GeH4

(112 K)           (161 K)          (183 K) 

Similarly, for noble gases, as we move down the group, the size of the atoms increases that is, size of the electron cloud increases which gets more and more easily distorted. As a result, London forces increase and so do their boiling points.

Geometry of the molecules

The shape of the molecules has a significant effect on the magnitude of London forces. For Example, n-pentane and neo-pentane have the same molecular formula, C5H12, yet the boiling point of n-pentane is about 27° higher than that of neo-pentane. The difference can be attributed to the different shapes of the two molecules; the n-pentane being a zig-zag chain whereas neo-pentane is nearly spherical.

(a) Less contact between neo-pentane molecules (b) Greater contact between n-pentane molecules
Fig. No. 6 (a) Less contact between neo-pentane molecules (b) Greater contact between n-pentane molecules

The overall attraction between molecules is greater in the case of n-pentane because there are more sites of interaction; the molecules are able to come in contact with the entire length of the molecule. (Fig. no. 6b)

In case of neo-pentane, molecules there is less contact and hence less force of attraction. (Fig. no. 6a)

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