To properly discuss dipoles and the effects of solvation, one must understand the concept of dielectric constant. Placing a molecule in an electric field is one way to induce a dipole. In a capacitor (condenser), there are two parallel conducting plates separated by a medium across a distance (r). Electricity will flow from the battery through the plates until the potential difference (V) of the plates equals that of the battery supplying the initial potential difference. Charge on the electrodes is +q and -q, and V represents the potential difference between the electrodes.
Parallel Plate Condenser Dielectric constant:
The SI unit of capacitance is the farad; 1 farad = 1 coulomb per volt. and Capacitance is a measure of the quantity of the electric charge stored on the plates (q, in coulombs) for a given electric potential difference (V, in volts). C = q/ V.
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.
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.
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.
Table 1 – Dielectric constants of some liquids at 25ºC
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.
Multiple choice questions (MCQs)
1.The forbidden energy gap of dielectrics is __________
a)Less than 1.2 eV
b)Greater than or equal to 1.2 eV
c)Less than 3 eV
d)Greater than or equal to 3 eV
2.Dipoles are created when dielectric is placed in __________
3.CO2is a polar dielectric.
4.The torque induced on a dipole when placed in an electric field E is given by _________
5.The dipole is most stable when the angle between the dipole and the field is __________
6.What is the value of 1 Debye in cm?
a)33 X 10-28cm
b)33 X 10-30cm
c)33 X 10-32cm
d)33 X 10-34cm
7.Polarization is defined as the dipole moment __________
a)per unit length
b)per unit area
c)per unit volume
d)per unit time
8.In a water drop of radius 1 mm all the molecular dipole points are in the same direction. If the dipole moment of a water molecule is 6 X 10-30m, the polarization in the water drop is ___________
a)6.4 X 10-13m-2
b)7.4 X 10-13m-2
c)8.4 X 10-13m-2
d)9.4 X 10-13m-2
9.What is the direction of electric field?
a)Right to Left
b)Up to Down
c)Down to Up
d)Left to Right
10.H2O is a polar dielectric.
11.In the formula, P = χeE, what is χecalled
12.What is the new capacitance of the plates, when a slab of Dielectric Constant K and thickness one-fourth of the separation of plate is inserted between the plates?
a)4KCo/K + 2
b)4KCo/1 + 2K
c)4KCo/K + 3
d)4KCo/1 + 3K
13.The expression for electric susceptibility is ___________
c)εoE (K + 1)
d)εoE (K – 1)
14.The relative dielectric constant of polystyrene is 2.5. What is the polarization produced when 0.5 mm thick sheet of polystyrene is subjected to 220 V?
a)2.78 X 10-6C/m
b)3.91 X 10-6C/m
c)4.12 X 10-6C/m
d)5.84 X 10-6C/m
15.Which of the following is not correct for dipole moment?
a)lone pair of electrons present on the central atom can give rise to the dipole moment
b)the dipole moment is a vector quantity
c)CO2molecule has no dipole moment since C-O bonds are nonpolar
d)the difference in electronegativities of combining atoms can lead to the dipole moment
- d)Greater than or equal to 3 eV
- b)Electric field
- b)3.33 X 10-30 cm
- c)per unit volume
- c)8.4 X 10-13 m-2
- d)Left to Right
- c)Electric susceptibility
- d)4KCo/1 + 3K
- c)εoE (K + 1)
- d)5.84 X 10-6 C/m
- c) CO2molecule has no dipole moment since C-O bonds are nonpolar
1. Martins Physical Pharmacy, 6th edition 2011, page no. 332-355.