Which ion goes where
To predict what happens in any electrolysis, the textbook sets out seven conventions. The first two say which electrode is which and where each ion goes.
Rule 1 — naming the electrodes
The electrode connected to the positive terminal of the outside supply (the battery) is the positive electrode; the one connected to the negative terminal is the negative electrode.
Rule 2 — where ions go
The positive ions in the solution or liquid are attracted to the negative electrode, and the negative ions to the positive electrode.
| Ion | Charge | Moves to |
|---|---|---|
| Na+, H+, Cu2+ | positive (cation) | the negative electrode |
| Cl-, OH-, SO42- | negative (anion) | the positive electrode |
Why opposites meet
The battery keeps one electrode short of electrons (positive) and crowds the other with electrons (negative). Opposite charges attract, so every positive ion in the liquid drifts towards the negative electrode, and every negative ion towards the positive one. This drift of ions through the liquid is the current inside the electrolyte — in the wires the current is carried by electrons, but in the liquid it is carried by moving ions. That is why a liquid needs free ions to conduct, as activity 12.2.1 showed.
Positive ion, negative electrode
A common slip is to send positive ions to the positive electrode. Like charges repel: cations go to the negative electrode, anions to the positive.
Cations to the negative, anions to the positive
The electrode on the battery's positive terminal is positive. Positive ions move to the negative electrode, negative ions to the positive electrode.