Half-equations and charge
How the charge moves
- Electrons flow through the wires from the power supply.
- Inside the electrolyte, ions move: positive ions to the cathode, negative ions to the anode.
- At the cathode: positive ions gain electrons → reduction.
- At the anode: negative ions lose electrons → oxidation.
Practice
At the cathode, positive ions:
At the negative cathode, positive ions gain electrons — that is reduction.
Practice
Losing electrons is:
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). Anodes oxidise.
Half-equations
- A half-equation shows what happens at one electrode. For molten lead(II) bromide:
$$\text{Pb}^{2+} + 2e^{-} \rightarrow \text{Pb} \quad (\text{cathode, reduction})$$
$$2\text{Br}^{-} \rightarrow \text{Br}_2 + 2e^{-} \quad (\text{anode, oxidation})$$
Practice
The half-equation Pb2+ + 2e- → Pb shows reduction at the cathode.
The lead ion gains 2 electrons (reduction), which happens at the cathode.
You've got it
Key idea
- electrons travel in the wires; ions move in the electrolyte
- cathode: gain electrons = reduction; anode: lose electrons = oxidation
- half-equations: $\text{Pb}^{2+} + 2e^{-} \rightarrow \text{Pb}$ and $2\text{Br}^{-} \rightarrow \text{Br}_2 + 2e^{-}$