Intermolecular forces are the forces of attraction and repulsion between interacting particles (atoms and molecules). Attractive intermolecular forces are known as van der Waals forces
Dispersion Forces or London Forces
In atoms and nonpolar molecules electronic charge cloud is symmetrical but when the charge cloud becomes more on one side than the other. Dipole develops for few moments. The attraction force between developed dipole is known as dispersion force or London forces. These forces are always attractive.
Interaction energy, E ∝ 1/ r6
r is the distance between two interacting particles.
These forces are important only at short distances (~500 pm)
Dipole – Dipole Forces
Dipole-dipole forces act between the molecules possessing permanent dipole. Ends of the dipoles possess “partial charges”. This interaction is stronger than the London forces but is weaker than ion-ion interaction because only partial charges are involved..
Interaction energy for stationary polar molecules, E ∝ 1/ r3
Interaction energy for rotating polar molecules, E ∝ 1/ r6
Where r is the distance between two interacting particles.
polar molecules can interact by London forces also so cumulative effect of forces in polar molecules increase.
Dipole–Induced Dipole Forces
Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming its electronic cloud. Attractive forces operate between the polar molecules having permanent dipole and induced dipole.
Interaction energy, E ∝1/r 6
Where r is the distance between two molecules.
In this case also cumulative effect of dispersion forces and dipole-induced dipole interactions exists.
Strong type of dipole-dipole interaction is hydrogen bonding. This is found in the molecules in which highly polar N–H, O–H or H–F bonds are present. Energy of hydrogen bond varies between 10 to 100 kJ mol–1. hydrogen bonds are powerful force in determining the structure and properties of many compounds,
Properties of gases
Gases are highly compressible.
Gases exert pressure equally in all directions.
Gases have much lower density than the solids and liquids.
The volume and the shape of gases are not fixed. These assume volume and shape of the container.
Gases mix evenly and completely in all proportions without any mechanical aid.
Boyle’s Law (Pressure – Volume Relationship)
According to Boy’s law, at constant temperature, the pressure of a fixed amount of gas varies inversely with its volume.
Mathematically, P ∝1/ V
At constant T and n
P = k(1/ V) … (1)
k is proportionality constant
If a fixed amount of gas at constant temperature T occupying volume V1 at pressure p1 undergoes expansion, so that volume becomes V2and pressure becomes p2,
p1V1= p2V2= constant
Curve at different constant temperature is known as an isotherm
Density ‘d’ is related to the mass ‘m’ and the volume ‘V’ by the relation
d = m / V
V = m/d
Plug in equation (1), we get
P = k(d/m)
So that at a constant temperature, pressure is directly proportional to the density of a fixed mass of the gas