Electrolysis is the passage of an electric current through an electrolyte causing it to decompose.
- Electrode – The point at which current enters and leaves the electrolyte
- Anode- positive electrode.
- Cathode – negative electrode
- Cations (+) move towards the cathode. Please note that cations are positively-charged ions and are attached to the cathode (negative electrode).
- Anions (-) move towards the anode. Please note that anions are negatively-charged ions and are attached to the anode (positive electrode).
- Electrodes may be active or inert.
During electrolysis, ions lose their charge (discharge).
At the anode: Xn- -ne → X (Oxidation (loss of electrons))
e.g. 2Cl- – 2e → Cl2
At the cathode: Mn+ + ne → M (Reduction (gain of electrons))
e.g. Cu2+ + 2e → Cu
When ions are discharged they become neutral atoms.
In the electrolysis of MgO, show what happens at the anode and the cathode.
- Metal deposit at the cathode
- Hydrogen evolve at the cathode
- Metals break away from the anode
- Non – metals discharged at the anode
How do we know which ion is discharged?
We use the electrochemical/ reactivity series
Example: Dilute sulphuric acid(H2SO4)
From water H+ OH-
From acid H+ So42-
At cathode: 2H+ + 2e → H2
At anode: 4OH- – 4e → 2H2O + O2
Electrolysis of dilute sodium hydroxide (NaOH)
Ions present – from water: H+ OH-
– from NaOH: Na+ OH-
Cathode: 2H+ + → H2
Anode: 4OH- – 4e → 2H2O + O2
e.g. electrolysis of copper sulphate using copper electrodes
At cathode: Cu2+ + 2e → Cu [Cu2+ leaves the solution and is deposited at the cathode.]
At anode: Cu – 2e → Cu2+ [ copper anode breaks down to Cu2+ and enters the solution.
This is used in copper refining and electroplating