An electrochemical cell converts chemical energy into electrical energy using a redox reaction.

Since, metals can be oxidised or reduced depending upon their chemical environment, then such an arrangement as shown below may be set-up.

The Daniell cell, specifically, uses Zn_(s)/Zn²⁺_(aq) and Cu_(s)/Cu²⁺_(aq) reactions. Note that the two rods in the diagram are called electrodes.

The zinc rod is the negative electrode and the copper rod the positive electrode.

Each metal in contact with a solution of its ions is called a half-cell. Half-cells are often represented by half-cell equations, which show the electrode processes:

At the negative electrode: Oxidation

Zn_(s) → Zn²⁺_(aq) + 2e^-

At the positive electrode: Reduction:

Cu²⁺_(aq) + 2e^- → Cu_(s)

So the overall reaction equation is:

Remember this is a REDOX reaction.

Note in the diagram:

1. The electrons flow in a clockwise direction, from the zinc rod to the copper.
2. The salt bridge, completes the circuit by allowing the passage of ions from the copper sulphate solution to the zinc sulphate solution. This salt bridge is usually a strip of filter paper soaked in saturated potassium sulphate.

Electrochemical cells can be made, as long as you pair up two half-cells of different potential so that an electrical current can be produced.

How do we find the potential of any half-cell individually? The answer is that we use a standard hydrogen electrode (s.h.e) and give it a potential of zero.

Therefore, when it is connected to another half-cell, the electromagnetic field (e.m.f) between the s.h.e and the second half-cell is equal to the potential of the second half-cell.

The diagram below outlines the main characteristics and conditions required to establish the s.h.e:

The reaction that takes place is:

2H⁺_(aq) + 2e^- → 2H_{2 (g)}