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7

Electrochemistry

Introduction

This chapter commences by describing cells and redox chemistry. Faraday’s laws of
electrolysis describe the way that charge and current passage necessarily consume
and produce redox materials. The properties of each component within a cell are
described in terms of potential, current and composition.
Next, the nature of half-cells is explained, together with the necessary thermody-
namic backgrounds of the theory of activity and the Nernst equation.
In the final sections, we introduce several key electrochemical applications, such
as the pH electrode (a type of concentration cell), nerve cells (which rely on junction
potentials) and batteries.

7.1 Introduction to cells: terminology
and background

Why does putting aluminium foil in the mouth cause
pain?

Introduction to electrochemistry

Most people have at some time experienced a severe pain in their teeth after acci-
dentally eating a piece of sweet wrapper. Those teeth that hurt are usually nowhere
near the scrap of wrapper. The only people who escape this nasty sensation are those
without metal fillings in their teeth.
The type of sweet wrapper referred to here is generally made of aluminium metal,
even if we call it ‘silver paper’. Such aluminium dissolves readily in acidic, conductive
electrolytes; and the pH of saliva is about 6.5–7.2.
The dissolution of aluminium is an oxidative process, so it gener-
Two redox states of
ates several electrons. The resultant aluminium ions stay in solution
the same material are
next to the metal from which they came. We generate a redox cou- called a redox couple.
ple, which we define as ‘two redox states of the same material’.