Conductance

Electrolytic Conductance: The substances which allow the passage of electric current are called conductors. The best conductors are metals such as copper, silver, tin, etc. On the other hand, the substances which do not allow the passage of electric current through them are called non-conductors or insulators. e.g. rubber, wood, wax, etc.

Types of conductors: The conductors are classified as Metallic conductors and Electrolytic conductors
i) Metallic conductors: These are metallic substances which allow the electricity to pass through them without undergoing any chemical change. The conductivity of metals is due to the flow of electrons the metal atoms. e.g. copper, silver, etc.

ii) Electrolytic conductors: These are substances which allow the electricity to pass through them in their molten states or in the form of their aqueous solutions and undergo a chemical change. The flow of electric current through an electrolytic solution is due to the movement of ions produced by the electrolytes in their aqueous solution. e.g. acids, bases, salts, etc.

Types of electrolytes: The electrolytes are two types namely, Weak electrolytes & Strong electrolytes

1. Strong electrolytes: The electrolytes which are almost completely dissociated into ions in solution are called strong electrolytes. e.g. NaCl, KCI, HCI, NaOH, NH₄NO₃, etc.,
2. Weak electrolytes: The electrolytes which do not dissociated completely into ions in solution are called weak electrolytes. Thus, in case of weak electrolytes, equilibrium is established between the unionized electrolytes and the ions formed in solution. The extent of ionization of a weak electrolyte is expressed in terms of degree of ionization or degree of dissociation. e.g. CH₃COOH, H₂CO₃, HCN, ZnCl₂, NH₄OH, HgCl₂, H₃BO₃, etc.

Factors Affecting Electrolytic conduction: In aqueous solution or molten state, the electrolytes split up into oppositely charged ions. These ions are free to move in solutions. This free movement is responsible for the conduction of electricity. Some of the factors on which electrical conductivity of an electrolyte depends are-

1) Interionic Attractions: These depend upon the interactions between the ions of the solute molecules. If the solute-solute interactions are large, the extent of dissociation will be less. These interactions are also responsible for the classification of electrolytes as strong and weak electrolytes.

2) Solvation of Ions: These depend upon the interactions between the ions of the solute and the molecules of solvent. If the solute-solvent interactions (solvation) are strong, the ions of the solute will be highly solvated and their electrical conductivity will be low.

3) Viscosity of the solvent: The viscosity of the solvent depends upon the solvent-solvent interactions. Larger the solvent-solvent interactions, larger will be the viscosity of the solvent, therefore conductivity decrease.

4) Temperature: All above factors decreases with increase in temperature. The average K.E. of the ions of electrolytes increases with increase in temperature. Consequently, the conductance of electrolytic solutions increases with rise in temperature. On the other hand, conductivity of a metal decreases with increase in temperature.

Faraday’s Law First of electrolysis: According to Faraday’s first law of electrolysis, “the amount of substances deposited over the cathode as a result of electrolysis is directly proportional to the quantity of electricity passed through the electrolytes”.

If m is the mass of substances deposited over the cathode and Q is the quantity of electricity in coulomb passed through the electrolytes. Then,

m α Q

Or, m = Z Q

Or, m = Z.c.t.

Where,

Z = Proportionality constant, called E.C.E. , Q= Quantity of electricity = c x t, c = Current strength in amperes, t = Time in second.

In the above expression, if C = 1 ampere and t = 1 second, then,  m = Z

Therefore, electrochemical equivalent (ECE) of a substance is defined as the mass of substance deposited over the cathode when one ampere of current is passed through the electrolyte for one second.

Faraday’s Law Second of electrolysis: According to Faraday’s second law of electrolysis, “when the same quantity of electricity passed through different electrolytes connected in series, then mass of different substances deposited over the electrodes are directly proportional to their chemical equivalent or equivalent mass”. If M₁and M₂ be the mass of two substances of equivalent mass E₁ and E₂ respectively set free at different electrodes by the passes same quantity of electricity Q coulomb, Then,

M_1/ M₂ =    E₁/ E₂

Molar conductivity (Ʌ_m): Molar conductivity is the conducting power of all the ions produced by dissolving one gram mole of an electrolyte in solution. It is related to specific conductance k as follows:

Ʌ_m = χ x V

Variation of Molar conductance with concentration: The variation of molar conductance with concentration can be explained on the basis of conducting ability of ions for weak and strong electrolytes.

1) Conductance behaviour of weak electrolytes:-The variation of Ʌ with dilution can be explained on the basis of number of ions furnished by an electrolyte in solution depends upon the degree of dissociation with dilution. ‘With the increase in dilution, the degree of dissociation increases and as a result molar conductance increases. The limiting value of molar conductance (Ʌ_m^α) corresponds to degree of dissociation equal to 1 i.e. the whole of the electrolyte dissociates’.

Thus, the degree of dissociation can be calculated at any concentration as:

α = Ʌ_m^c / Ʌ_m^α

Where iis the degree of dissociation, Ʌ_m^c is the molar conductance at concentration C and Ʌ_m^αis the molar conductance at infinite dilution.

2) Conductance behaviour of strong electrolytes:-For strong electrolytes, there is no increase in the number of ions with dilution because strong electrolytes are completely ionizes in solution at all However, in concentrated solutions of strong electrolytes there are strong forces of attraction between the ions of opposite charges called inter-ionic forces. Due to these inter-ionic forces the conducting ability of the ions is less in the concentrated solution. With dilution, ions become far apart from one another and inter-ionic forces decrease. As a result, molar conductance increases with dilution. When the concentration of the solution becomes very low, the inter-ionic attractions become negligible and the molar conductance approaches the limiting value called molar conductance at infinite dilution.

3) Temperature: The conductance of an electrolyte depends upon the temperature. With increase in temperature, the conductance of an electrolyte increases.