Electric Polarization

Electric Polarization of Molecules

Electric polarization or polarisation arises when a non-polar substance placed between two parallel plates with an applied electric field. The electric field tends to attract the negatively charged electron particles towards the positive plate and positive charge nucleus towards a negative plate. Therefore, under this condition, there will be observed an electrical distortion on the molecule to form an electrical dipole. Such type of distortion in the molecules is called the electric polarization. But the polarization disappears as soon as the field is withdrawn and the molecule comes back to its original state.

Electric polarization of molecules

Induced Polarization in Chemistry

Let induced polarization = Pi and induced dipole moment = µi. The induced dipole moment or simply the induced moment directly proportional to the strength of the electric field applied(F). Therefore, μi ∝ F. But when force is very low, otherwise, hyperpolarization may occur among the polar molecules. Hence, μi = αi F, where αi = proportionality constant. This constant is called induced polarizability constant of the molecule. Therefore, the definition of induced polarization in chemistry, the amount of induced moment in the molecule when the unit electric field strength applied.

Unit and Dimension of Polarizability

From the definition, we can prove that the polarization has the dimension of the volume. Unit of dipole moment = esu × cm and force unit = esu cm-2, obtained from Coulomb’s law. Therefore the unit of polarization constant, αi = r3 where r = radius of the molecule assuming it to have a spherical shape.

The polarizability of the atom increases with the increasing atomic size, atomic number, and ionization energy. Hence the atom behaves like a dipole and this dipole moment induced by the applied electric field.

Clausius mossotti Electric Polarization Equation

Mossotti derived a relation between the polarizability and the dielectric constant of the non-polar medium between two plates from electromagnetic theory.

    \[ \boxed{P_{i}=\left ( \frac{D-1}{D+2} \right )\times \frac{M}{\rho }=\frac{4}{3}\pi \, N_{0}\, \alpha _{i}} \]

where N0 = Avogadro number,
M = molar mass,
ρ = density of the medium,
and αi = polarizability constant.

Induced polarizability constant and given when the distortion produced in the 1 mole of the substance by a unit electric field. Therefore, the polarization constant value for a given molecule and independent of temperature for learning chemistry or physics.

Dielectric constant (D) = C/C0, where C = capacitance of the condenser containing the substance and C0 in the vacuum. Hence the dielectric constant is a dimensionless quantity having the value unity for vacuum. Other substances value of dielectric constant greater than unity. Therefore, the induced dipole moment of the crystalline solid, liquid, and gas substance calculated by measuring dielectric constant, density (ρ), and molar mass (M) of the substance.

Debye Equation for Electric Polarization

For the polar molecules like methyl chloride, water, hydrogen fluoride, etc, the molar polarization not constant. It decreases with increasing temperature. Therefore, the Clausius Mossotti equation fails very badly for the polarity of the bond. The reason for the failure of the equation put forward by P Debye for molar polarization.

According to him, when an electric field applied between two parallel plates containing polar gas molecules, two effects will occur, induced polarization and orientation polarization. Here induced polarization tends to increase the induced moment among the molecules which discuss broadly before.

Definition of Orientation Polarization

When an electric field produces on the polar molecules, the dipolar molecules tend to orient in the direction of the field. This is called orientation polarization. It expresses as

    \[ P_{o}=\frac{4}{3}\, \pi \, N_{0}\, \alpha _{o} \]

where α0 = orientation polarisability.

Debye calculated the value of α0 = μ2/3KT. Hence the total polarization (Pt) = Pi + P0. But the orientation of chemical elements has two tendencies. Polar molecules tend to orient in the direction of the applied field (applied). Thermal energy or specific heat tends to destroy the alignment of the molecules.

Temperature Dependence of Electric Polarization

A polar chemical bond fixed and unable to orient in the fixed direction. Therefore, the orientation polarization equal to zero. Hence for the condensed system where strong intermolecular forces prevent the free rotation of the molecules. But at very high temperatures tending to infinity.

1/T = 0 and Pt = 0
∴ Pt = Pi

Experimental determination for molar polarization of may substances like carbon dioxide (greenhouse gas), hydrocarbon like methane, ethane, propane, nitrogen, hydrogen, etc remains constant and did not vary with temperature. But in many molecules like hydrochloric acid, methyl chloride, nitrobenzene, benzyl alcohol, etc the molar electric polarization dependent on temperature.