1. Physical Properties of Molecules

2. Introduction The physical properties may be thought of as
additive (derived from the sum of the properties of the individual atoms or functional groups within the molecule) e.g. mass
constitutive (dependent on the structural arrangement of the atoms within the molecule) e.g. optical rotation
additive-constitutive: Molar refraction of a compound is the sum of the refraction of atoms and groups making up the compound, however the refractive index will also depend on the arrangement of the atoms within each molecule.

3. Polarization
When liquids are placed in an electromagnetic field, the electron clouds of their molecules become distorted so that a temporary charge separation occurs that is they become polarized. This effect is termed induced polarization.
There may also be alignment of any permanent dipoles within the liquid.
Polarizability is defined as the ease with which an ion or molecule can be polarized by external force.
The polarizability of a medium determines the refractive index, the dielectric constant and the optical rotation.

4. Polarization of nonpolar liquids
Nonpolar molecules in an electrical field acquire a temporary dipole moment (a measure of the polarity of the molecule), the magnitude of which is proportional to the applied field and the molecular polarizability.
Dipole moment characteristic of a dipole unit and equal to the product of one of its charges times the vector distance separating the charges.
On removal of the electric field, the molecules revert to their original state.

5. Polarization of nonpolar liquids
The molar polarization Pi (m3mol-1) is defined by the Clausius-Mossotti equation as
Where:
M is the molar mass (molecular weight) in kgmol-1
 is the dielectric constant
 is the density (in kgm-3).

6. Polarization of nonpolar liquids
Example: The dielectric constant and density of benzene (molecular weight 78.11) are 2.28 and gcm-3 respectively at 20ºC. Calculate the molar polarization.
Answer: Pi (molar polarization) = X 10-4 m3mol-1
Pi represents the induced dipole moment per molecule of a nonpolar substance when the electrical field strength is equal to 1 volt/meter.

7. Polarization of polar liquids
The molecules of polar liquids experience two effects when subjected to an applied electric field.
As with nonpolar liquids, there is an induced polarization which is proportional to the field strength and the polarizability.
A second effect arises because of the alignment of the permanent dipoles in the applied field (orientation polarization).

8. Polarization of polar liquids
The total molar polarization is now the sum of the induced polarization, Pi, and the orientation polarization, Po (P = Pi + Po).
Where k, the Boltzmann constant, is 1.38 X J K-1,  is the dipole moment, T is the temperature, N is Avogadro’s number.

9. Polarization of polar liquids
Orientation of dipoles in an applied electric field (Absolute perfect orientation can never occur due to the thermal energy of the molecules which contributes to agitation against the molecular alignment).

10. Polarization of polar liquids
The orientation of permanent dipoles becomes less effective as the temperature is increased.
The thermal motion of the molecules tends to destroy the alignment of dipoles.
The molar polarization of a polar liquid therefore decreases with increase in temperature, in contrast to molar polarization of nonpolar liquids, which is independent of temperature.

11. Polarization of polar liquids
A is a constant that is equal to
Where k, the Boltzmann constant, is 1.38 X J K-1,  is the dipole moment N is Avogadro’s number.

12. Dielectric constant, 
Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential.
In a capacitor, there are two conducting electrodes which are insulated from one another. The charge on the electrodes is +Q and -Q, and V represents the potential difference between the electrodes. The SI unit of capacitance is the farad; 1 farad = 1 coulomb per volt.

13. Parallel Plate Condenser
Dielectric constant, 
Parallel Plate Condenser

14. Dielectric constant, 
The capacitance of the condenser depends on the type of medium separating the plates as well as on the thickness r.
When vacuum fills the space between the plates, the capacitance is C0.

15. Dielectric constant, 
If water fills the space, then the capacitance increase because the water molecules can orientate themselves so that its negative end lies nearest to the positive condenser plate and its positive end lies nearest the negative plate.
This alignment provides an additional movement of charge because of the increased ease with which electrons can flow between the plates.

16. Dielectric constant, 
The capacitance of the condenser filled with some material, Cx, divided by the reference standard ,C0, is referred to as the dielectric constant, ε.
The dielectric constant of a solvent is a measure of its ability to maintain a charge separation in the solution. x

17. Dielectric constant,   Solvent 78.5 Water 42.5 Glycerin 32.6
Methanol 25
Ethanol
4.34
diethyl ether
3.1
Olive oil
2.28
Benzene
Dielectric constants of some liquids at 25ºC

18. Dielectric constant, 
It is 78.5/42.5 = 1.84 times easier to separate Na+ from Cl- ions in water than in glycerin, i.e. NaCl is more soluble in water than in glycerin.
Polar liquids such as water and methanol have high dielectric constants since alignment of permanent dipoles within these liquids produces an appreciable increase in the capacitance of the condenser.
The polarization of non-polar liquids such as benzene and ether produces a much smaller effect on the capacitance of the condenser and this is reflected by the lower dielectric constants of these liquids.