Electrolytic conduction, equivalent and molar conductivities

Electrolytic Conduction

Electrolytes are substances that conduct electricity due to the drifting of ions. The positive ions are called cations, and the negative ions anions. The common examples of electrolytes are aqueous solutions of inorganic salts, acids and bases. Salts like NaCl, KCl are electrolytes in their molten state. Solutions of organic compounds are poor conductors. Solid state electrolytes (eg. AgI) with mobile ions are also present.

Here, we will see how aqueous solution of common salt conducts electricity. Solid crystalline NaCl is made up of Na⁺ and Cl^- ions bound by a strong force of attraction. The energy required to separate Na⁺and Cl^- ions (i.e., dissociate them) is ~7.9eV per molecule. The thermal energy at room temperature, is only 0.03eV per molecule, and thus cannot dissociate NaCl. However, when NaCl is dissolved in water, the force of attraction is greatly reduced because of the high dielectric constant (= 81) of water. In fact, the force reduces by a factor of 81, and the thermal energy is sufficient to dissociate completely into Na⁺and Cl^- ions. This process is called ionisation.

Electrolyte conductivity is smaller than that of metals by a factor of 10^-5 to 10^-6 at room temperature. This is due to the smaller number density of ions as compared to free electrons, greater viscosity of the medium in which they move and the larger mass of ions.

Specific conductance

It is the reciprocal of specific resistance.

It is the conductance of a solution of 1 cm length having 1 square cm as area of cross section.

fig 5.2

Specific conductance depends upon the concentration of the solution.

Equivalence conductance

It is defined as the conductance of that volume of solution which has one equivalent dissolved in it.

V = Volume of solution which has one equivalent in it.

N normality (Number of equivalents per liter or 1000cc)

Molar Conductivity (^m)

It is defined as the conductance of 1 cm³ of volume of electrolyte which has one mole dissolved in it.

Units: Ohm^-1 cm^-1 or S/cm^-1

l = length of the Conductor

a = Area of cross section of the conductor.