Electrochemistry — Part 2: Conductance, electrolysis, batteries and corrosion: visual lesson, CBSE Class 12 Chemistry

Lesson

Ions on the move

Part 2 — how solutions conduct, how electricity makes new substances, how batteries store energy and why iron rusts.

In this lesson you will

Define conductivity and molar conductivity and explain how they vary with concentration
Apply Kohlrausch’s law to find limiting molar conductivities and the degree of dissociation of weak electrolytes
Apply Faraday’s laws and predict the products of electrolysis
Describe primary and secondary batteries and the hydrogen–oxygen fuel cell, and explain corrosion and its prevention

Concept

Resistance, conductance and conductivity

Resistance $R = \rho \dfrac{l}{A}$ . Its reciprocal is conductance $G$ (unit siemens, S). The reciprocal of resistivity is conductivity $\kappa$ (kappa): the conductance of a solution between electrodes $1\ \text{cm}^2$ in area and $1\ \text{cm}$ apart.

G = \dfrac{1}{R}, \qquad \kappa = \dfrac{1}{\rho} = \dfrac{1}{R}\cdot\dfrac{l}{A}

Table of resistance, resistivity, conductance, conductivity and cell constant with their symbols and units
Quantity	Symbol	Unit
Resistance	$R$	$\Omega$
Resistivity	$\rho$	$\Omega\ \text{cm}$
Conductance	$G$	$\text{S}$ ( $= \Omega^{-1}$ )
Conductivity	$\kappa$	$\text{S cm}^{-1}$
Cell constant	$G^* = \dfrac{l}{A}$	$\text{cm}^{-1}$

Concept

Measuring conductivity: the cell constant

A conductivity cell has two platinised platinum electrodes a fixed distance apart. Its cell constant $G^* = \dfrac{l}{A}$ is found once using a potassium chloride solution of known $\kappa$ . Alternating current is used, so that the solution is not electrolysed.

\kappa = \dfrac{G^*}{R}

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