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Electric Polarization

Electric Polarization of Molecules

Electric polarization occurs 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 or clouds towards the positive plate and positive charge nucleus towards a negative plate. Therefore, in the presence of an electric field or current, there will be observed an electrical distortion or polarized the molecule to form an electrical dipole. In learning chemistry or physics, such type of distortion process in the molecules is called the electric polarization but the polarization disappears as soon as the polarized field is withdrawn and the polarizing molecule comes back to its original state.

Induced Electric Polarization in Chemistry

Electric polarization definition and example of polarized molecules in chemistry

Let induced electric 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 electric force is very low, otherwise, hyperpolarization may occur among the polarized molecules. Hence, μi = αi F, where αi = proportionality constant. This constant is called the induced polarizability constant of the polarizing molecule. Therefore, the induced electric polarization in chemistry meaning the amount of induced moment in the polarized molecule when the unit electric field of current strength applied.

Unit and Dimension of Polarizability

From the definition and polarizing formula, we can prove that the electric polarization constant 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 electric polarization constant (αi)= r3, where r = radius of the polarize molecule assuming it to have a spherical shape.

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

Clausius mossotti Equation

Mossotti derived a relation between the polarizability of substances 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 polarizing substance by a unit electric field. Therefore, the electric polarization constant formula 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 polarized 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 the dielectric constant, density (ρ), and molar mass (M) of the polarized substance.

Debye Equation for Electric Polarization

For the examples of polarized molecules like methyl chloride, water, hydrogen fluoride, etc, the molar electric 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 linear parallel plates containing polarized gas molecules, two effects will occur, induced and orientation polarization. Here induced polarization tends to increase the induced moment among the molecules which discuss broadly before.

Definition of Orientation Electric Polarization

When an electric field produces on the polarized 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. Polarized 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 polarize 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 polarized molecules. But at very high temperatures tending to infinity.

1/T = 0 and Pt = 0
∴ Pt = Pi

For example, experimental determination for molar electric 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 examples of polarized molecules like hydrochloric acid, methyl chloride, nitrobenzene, benzyl alcohol, etc the molar electric polarization-dependent on temperature.