Conductance B.Sc. 4th Semester Notes

Conductance: B.Sc. Chemistry 4th Semester

Conductance (π)

Any substance that allows an electric current to pass through is called conductor and the capacity to conduct electricity is called the conductivity or conductance of a conductor. Conductor offers to resistance to flow the electric current.
So, the conductance is resiprocal to resistance (R)
π = 1/R

Unit of π = 1/ohm = ohm⁻¹ = mhos
SI Unit is Siemen (S)

Specific or Electrolytic Conductance (κ)

The resiprocal of specific resistance (ρ) is called specific conductance.
We know that-
R ∝ l/a
where, l is length of the wire and a is cross sectional area of the wire
or, R = ρ. l/a
or, 1/ρ = 1/R . l/a
or, κ = π . l/a

Unit of κ = ohm⁻¹ . cm/cm²
ohm⁻¹ . cm⁻¹
Its S.I. unit is Sm⁻¹
when, l = 1 and a = 1
then, κ = π

So, specific conductance is the conductance of a conductor of unit length and unit cross sectional area. For electrolytic solutions, the conductance of one cc of the solution is called its specific conductance.

Equivalent Conductance (Λ)

It is denoted by capital lambda(Λ) and is conductance of V cc of the solution containing one gm-equivalent of an electrolyte. So, it is the product of Specific or Electrolytic Conductance (κ) and the volume of the solution (V) in cc containing one-gm equivalent of the electrolyte.
or, Λ = κ . V

If the concentration of a solution is C g-equivalent/liter, then-
C g-equivalent is present in 1000cc of the solution.
so, 1 g-equivalent is present in 1000cc/C of the solution.
or, Volume in cc containing one gm-equivalent = 1000/C
so, Λ = 1000.κ/C

Unit of Λ = κ/C = ohm⁻¹ . cm⁻¹/eq-cm⁻³
or, ohm⁻¹ . cm² eq⁻¹

Molecular Conductance (μ)

It is denoted by capital mu(μ) and is conductance of V cc of the solution containing one mole of an electrolyte. So, it is the product of Specific or Electrolytic Conductance (κ) and the volume of the solution (V) in cc containing one mole of the electrolyte.
or, μ = κ . V

If the concentration of a solution is C mole/liter, then-
C mole is present in 1000cc of the solution.
so, 1 mole is present in 1000cc/C of the solution.
or, Volume in cc containing one mole = 1000/C
so, μ = 1000.κ/C

When concentration approaches zero, the molar conductivity is known as limiting molar conductivity and is represented by E^o_m.

Unit of μ = κ/C = ohm⁻¹ . cm⁻²mol⁻¹
or, Sm² . cm²mol⁻¹

Relationship between molar conductivity and Concentration

For strong electrolyte, molar conductivity increases with dilution and can be represented by the equation-
μ = E^o_m − AC^1/2
where A is a constant depending upon the type of the electrolyte, the nature of the solvent and the temperature.
The equation is called Debye Huckel-Onsager equation and is found to hold good at low concentrations.
If we plot μ against c^1/2, we obtain a straight line with intercept equal to E^o_m and slope equal to '–A'.

Effect of dilution on conductivity

The specific conductance depends on the number of ions present per cc of the solution. Though degree of dissociation increases on dilution but the number of ions per cc decreases. So, the specific conductance decreases on dilution.
The equibvalent conductance is the product of specific conductance and volume of the solution containing one gm-equivalent of the electrolyte.
Λ = κ . V
As the decreasing κ value is more than compensated by the increasing V value, hence, Λ increases. on dilution.

Kohlrausch's Law

The equivalent conductivity of an electrolyte at infinite dilution (Λ_o) is the sum of the ionic conductivities of their cations and anions.
Λ_o = λ₊ + λ₋
where- λ₊ and λ₋ are cationic and anionic conductivities at infinite dilution respectively.
Applications:
Useful in
calculating equivalent conductivity at infinite dilution.
Calculation of degree of dissociation of an electrolyte.
Calculation of solubility of sparingly soluble salt,
Calculation of ionic product of water.

Conductometric Titrations

The determination of the equivalence point of titration by measurement of conductance is called as conductometric titration. These titrations are based on the principle that the conductance of a solution depends upon number of ions present and their velocity. In such titrations, either the number of ions in the solution changes or one of the ion is replaced by another ion having different velocity. Hence, the conductance of a solution changes before and after the end point.

Titration of a Strong Acid with Strong Base

Consider, the titration between HCl and NaOH.
HCl + NaOH → NaCl + H₂O

Titration of Weak Acid with Strong Base

Consider the titration between CH₃COOH with NaOH.
CH₃COOH + NaOH → CH₃COONa + H₂O

Advantages of Conductometric Titrations

Color doesn't interfere with conductance measurements, unlike some other titration methods. It can be used for weak acids or bases. The equipment is relatively straightforward.

Conductance B.Sc. 4th Semester MCQs