At the anode
Rules 5, 6 and 7 deal with the positive electrode.
Rule 5
Anions (negative ions) in the solution travel to the positive electrode and give up electrons — they are oxidised. For example, Cl- ions in the solution lose electrons and become Cl2 molecules.
2Cl- (aq) → Cl2 (g) + 2e
When there are several negative ions in the solution, deciding which is oxidised first depends on several factors. They go beyond this syllabus and are not discussed here; the examples in this chapter tell you which one it is.
Rule 6
Because oxidation happens at the positive electrode, the positive electrode is the anode.
Rule 7
If a metal other than platinum is used as the anode, then instead of the negative ions being oxidised, the metal atoms themselves give up electrons and are oxidised. For example, with a silver rod as the anode, the oxidation at the positive electrode is:
Ag (s) → Ag+ (aq) + e
Why a metal anode dissolves
The anode is where electrons are pulled out of the cell by the battery. Something there has to give them up. With carbon or platinum, which do not easily lose electrons, it is the negative ions that give them. A metal like silver, copper or zinc loses electrons more easily than those ions do, so the anode itself gives them up and goes into solution as metal ions. So a metal anode slowly wears away — just as the zinc did in 12.1 — and that is what electroplating and copper refining rely on.
Oxidation at the positive anode
In electrolysis the positive electrode is the anode, where anions are oxidised — unless the anode is a metal other than platinum, which is then oxidised itself.