Electrolytic Conductance, Electrolysis, Batteries and Corrosion — Part 1: Conductance: visual lesson, ISC Class 12 Chemistry

Lesson

Electrolytic conductance

Part 1 — how solutions of electrolytes carry current, and how we measure and compare it.

In this lesson you will

Compare metallic and electrolytic conductance
Define resistance, specific resistance, conductance, specific conductance and cell constant, and calculate them
Interconvert specific, molar and equivalent conductance and explain their variation with concentration
Apply Kohlrausch’s law to find limiting molar conductance and the degree of dissociation of a weak electrolyte

Concept

Metallic and electrolytic conductance

Metals conduct by movement of electrons; electrolytes (solutions or melts) conduct by movement of ions, with chemical change at the electrodes.

Table comparing metallic and electrolytic conductance
Feature	Metallic conductance	Electrolytic conductance
Charge carriers	Electrons	Ions
Chemical change	None	Decomposition at the electrodes
Transfer of matter	No	Yes, ions move
Effect of rise in temperature	Conductance decreases (vibrating ions scatter electrons)	Conductance increases (less viscosity, more dissociation)
Examples	Copper, silver	Aqueous NaCl, molten KCl

Concept

Resistance, conductance and cell constant

Resistance $R = \rho\dfrac{l}{A}$ , where $\rho$ is the specific resistance (resistivity). Conductance $G = 1/R$ (siemens, S). Specific conductance (conductivity) $\kappa = 1/\rho$ : the conductance of a 1 cm cube of solution. Cell constant $G^* = l/A$ , found by calibrating with a KCl solution of known $\kappa$ .

\kappa = \frac{1}{\rho} = \frac{1}{R}\cdot\frac{l}{A} = \frac{G^*}{R}

Table of conductance quantities with symbols and units
Quantity	Symbol	Unit
Resistance	R	Ω
Specific resistance	ρ	Ω cm
Conductance	G	S = Ω⁻¹
Specific conductance	κ	S cm⁻¹
Cell constant	G* = l/A	cm⁻¹

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