Session Objectives

• Conductance of electrolytic solution

• Specific conductance, Molar conductance

• Kohlrausch's law

Electrolytes

Substances whose aqueous solution does not conduct electricity are called non electrolytes.

Examples are solutions of cane sugar, glucose, urea etc.

Substances whose solution in water conducts electric current. Conduction takes place by the movement of ions.

Examples are salts, acids and bases.

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Types of Electrolytes

Strong electrolyte are highly ionized in the solution.

Examples are HCl, H2SO4, NaOH, KOH etc

Weak electrolytes are only feebly ionized in the solution.

Examples are H2CO3, CH3COOH, NH4OH etc

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Difference between electronic & electrolytic conductors

(3) Conduction increases with increase in temperature

(3) Conduction decreases with increase in temperature

(2) Flow of electricity is due to the movement of ions

(2) Conduction is due to the flow of electron

(1)Flow of electricity takes place by the decomposition of the substance.

(1) Flow of electricity take place without the decomposition of substance.

Electrolytic conductorsElectronic conductors

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Resistance refers to the opposition to the flow of current.

For a conductor of uniform cross section(a)and length(l); Resistance R,

a

lWhere is called resistivity orspecific resistance. Its SI units is Ohm metre (Ωm)

Resistance

R l

R A

R = ρ x l/A

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Conductance

The reciprocal of the resistance is called conductance. It is denoted by G.

G=1/R

Conductors allows electric current to pass through them. Examples are metals, aqueous solution of acids, bases and salts etc.

Insulators do not allow the electric current to pass through them.

Examples are pure water, urea, sugar etc.

Unit of conductance is ohm-1 or mho or Siemen(S)

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Specific conductance 1

Specific Conductivity

Unit of specific conductance is ohm–1cm–1

SI Unit of specific conductance is Sm–1 where S is Siemen

l/A is known as cell constant

Conductance of unit volume of cell is specific conductance.

K = l

AR

K = x Condactance lA

But ρ = x R Al

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Metallic and Electrolytic Conductance

• Electrical conductance through metals is called metallic or electronic conductance and is due to the movement of electrons. The electronic conductance depends on

• (i) the nature and structure of the metal

• (ii) the number of valence electrons per atom

• (iii) temperature (it decreases with increase of temperature).

• The conductance of electricity by ions present in the solutions is called electrolytic or ionic conductance. The conductivity of electrolytic (ionic) solutions depends on: 

• (i) the nature of the electrolyte added • (ii) size of the ions produced and their 

solvation • (iii) the nature of the solvent and its 

viscosity • (iv) concentration of the electrolyte • (v) temperature (it increases with the 

increase of temperature). 

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Measurement of the Conductivity of Ionic Solutions

R = ρ lA = ρ

lкA

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Measurement of Resistance

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Molar Conductivity

The conductivity of solutions of different electrolytes in the same solvent and at a given temperature differs due to

● charge and size of the ions in which they dissociate,● the concentration of ions or ease with which the ions

move under a potential gradient. ● It, therefore, becomes necessary to define a physically

more meaningful quantity called molar conductivity denoted by the symbol Λm (Greek, lambda).

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Molar Conductivity

• Molar Conductivity , m is related to the conductivity Λof the solution by the equation:

Λm =

Κc

units of Λ m are in Sm 2mol –1 and Scm 2 mol –1.

It is the conductance of a solution containing 1 mole of the electrolyte in V cc of solution. it is represented as Λ.

Molar conductance

Where V = volume solution in cc

ΛMolar conductance

k = Specific conductance

M = molarity of the solution.

=k x 1000/M

Λ = k x V

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Exercises

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Effect of Dilution on Conductivity

Specific conductivity decreases on dilution.

molar conductance increases with dilution and reaches a maximum value.

The conductance of all electrolytes increases with temperature.

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Illustrative Example

The resistance of 0.01N NaCl solution at 250C is 200 ohm. Cell constant of conductivity cell is unity. Calculate the equivalent conductance and molar conductance of the solution.

Solution:

Conductance of the cell=1/resistance =1/200 =0.005 S.

Specific conductance=conductance x cell constant =0.005 x 1 =0.005 S cm-1

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Solution Cont.

Molar Conductivity = Equivalent conductivity x n-factor = 500 x 1 = 500 ohm-1mol-1cm2

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Kohlrausch’s Law

The law states that “limiting molar conductivity of an electrolyte

can be represented as the sum of the individual contributions

of the anion and cation of the electrolyte..”

In general, if an electrolyte on dissociation gives ν

+ cations and ν

– anions then its limiting molar conductivity is

given by: Λ Λ °°mm = ν = ν

++ λ λ 0 0++ + ν + ν

–– λ λ 00––

Here, λ0

+ and λ0

− are the limiting molar conductivities of the cation and

anion respectively.

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Strong Electrolyte And Weak Electrolyte

For strong electrolytes, Λm increases slowly with dilution and can

be represented by the equation: Λm = Λ

m° – A c 1⁄2

a straight line with intercept equal to Λm

° and slope equal to ‘–A’.

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Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations and hence for such electrolytes, the change in Λ

m

with dilution is due to increase in the degree of dissociation

Λm increases steeply on dilution, especially near lower concentrations.

Therefore, Λm

° cannot be obtained by extrapolation of Λm to zero

concentration.

At infinite dilution (i.e., concentration c → zero) electrolyte dissociates completely (α =1),but at such low concentration the conductivity of the solution is so low that it cannot be measured accurately.

Weak electrolytes

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Application of Kohlrausch’s law

(2). For obtaining the equivalent conductivities of weak electrolytes at infinite dilution.

(1). It is used for determination of degree of dissociation of a weak electrolyte.

Where,

v

�

0

�

vrepresents equivalent conductivity at infinite dilution.

represents equivalent conductivity at dilution v.

0

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Exercise

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