Separating oxidation and reduction
Electrochemical cells generate electrical energy from spontaneous redox reactions. Oxidation and reduction are separated so that electron transfer between the two processes can occur through an external circuit.
Each redox system forms a half-cell. At the boundary between an electrode and the species with which it is in contact, a reversible redox equilibrium is established.
For a zinc half-cell, the equilibrium is:
Zinc atoms can undergo oxidation to form , leaving electrons at the electrode. The reverse process is reduction: ions accept electrons at the electrode and form zinc atoms.
The balance between these opposing processes determines the potential established at the electrode–solution boundary.
Two coupled redox systems
Different half-cells establish different potentials. Connecting two suitable half-cells therefore creates a potential difference between their electrodes.
Oxidation
In the zinc–copper cell, zinc supplies electrons:
Reduction
Copper(II) ions accept those electrons:
Oxidation occurs at the negative electrode and reduction at the positive electrode. Electrons pass through the external circuit from the negative electrode towards the positive electrode.
When electrode reactions are written for electrode-potential work, the convention is to write the reversible half-equation in the reduction direction, with electrons on the left.