Oxidation and reduction
Each electrode has its own half reaction, and each kind of half reaction has a name.
Oxidation
A species (atom, molecule or ion) losing electrons.
At the zinc: Zn (s) → Zn2+ (aq) + 2e
The electrode where oxidation happens is the anode.
Reduction
A species gaining electrons.
At the copper: 2H+ (aq) + 2e → H2 (g)
The electrode where reduction happens is the cathode.
So the zinc is the anode of the cell, and equation (1) is the oxidation half reaction at the anode. The copper is the cathode, and equation (2) is the reduction half reaction.
Which end is negative
Zinc — the negative terminal
Zinc atoms dissolve and leave their electrons on the strip, so it is negatively charged compared with the copper.
Copper — the positive terminal
Electrons flow to it and are taken away, so it is positively charged compared with the zinc.
| Electrode | Name | Terminal | Half reaction |
|---|---|---|---|
| Zinc | Anode | Negative | Oxidation |
| Copper | Cathode | Positive | Reduction |
A way to keep them straight
Anode and oxidation both start with a vowel; cathode and reduction both start with a consonant. Oxidation is loss of electrons, reduction is gain.
Why both always happen together
Electrons cannot pile up or vanish. Every electron that zinc gives up has to be taken by something else, so an oxidation never happens without a reduction happening at the same time — together they are called a redox reaction. You met oxidation before as a substance gaining oxygen, as when magnesium burns. The electron definition covers that case too: magnesium atoms lose electrons to become Mg2+ in magnesium oxide. It also covers reactions with no oxygen at all, like zinc in acid.
Oxidation at the anode, reduction at the cathode
In the zinc-copper cell the zinc is oxidised at the anode, the negative terminal; hydrogen ions are reduced at the copper cathode, the positive terminal.