What is it?

Electrolysis is the breakdown of an ionic compound, when molten or in aqueous solution, by passing an electric current through it.

The compound being broken down is the electrolyte. The current enters and leaves through two electrodes, usually inert graphite (carbon) or platinum:

• the anode is the positive electrode
• the cathode is the negative electrode

The electrolyte must be molten or dissolved because the ions must be free to move. In a solid, the ions are held in a lattice, so it does not conduct.

PANIC — Positive Anode, Negative Is Cathode

What happens at the electrodes?

Opposite charges attract, so the ions move:

• cations (positive, such as Pb²⁺) move to the cathode and gain electrons — this is reduction
• anions (negative, such as Br⁻) move to the anode and lose electrons — this is oxidation

For a molten compound of two elements, the metal forms at the cathode and the non-metal at the anode.

Electrons released at the anode travel round the wire to the cathode. Inside the electrolyte the charge is carried by ions, not electrons.

Pb²⁺ + 2e⁻ → Pb  (cathode)
2Br⁻ → Br₂ + 2e⁻  (anode)

Why are solutions different?

An aqueous solution also contains a few H⁺ and OH⁻ ions from water, so two cations compete at the cathode and two anions at the anode.

Only one of each is discharged — turned into an element. The other ion stays in the solution.

The ions left behind can change the solution. Electrolysing concentrated sodium chloride leaves sodium hydroxide.

2H⁺ + 2e⁻ → H₂  (cathode)
4OH⁻ → O₂ + 2H₂O + 4e⁻  (anode)

Which ion is discharged?

At the cathode

• metal less reactive than hydrogen (copper, silver) → the metal forms
• metal more reactive than hydrogen (sodium, potassium, magnesium) → hydrogen forms

At the anode

• concentrated halide (Cl⁻, Br⁻, I⁻) → the halogen forms
• dilute halide, or no halide (sulfate, nitrate) → oxygen forms

Copper electrodes Supp.

With copper electrodes in copper(II) sulfate solution, copper moves from the anode to the cathode.

• anode: copper atoms lose electrons and dissolve, so it loses mass
• cathode: Cu²⁺ ions gain electrons and copper builds up, so it gains mass
• the blue colour stays the same, because Cu²⁺ is replaced as fast as it is used

To purify copper, make the impure copper the anode and pure copper the cathode. The impurities fall to the bottom as a sludge.

Cu → Cu²⁺ + 2e⁻  (anode)
Cu²⁺ + 2e⁻ → Cu  (cathode)

Electroplating

Electroplating coats an object with a thin layer of metal, using the same idea:

• the object to be plated is the cathode
• the plating metal is the anode
• the electrolyte contains ions of the plating metal

Metals are electroplated to improve their appearance and to resist corrosion.

Compare inert carbon electrodes: oxygen forms at the anode instead, and the blue colour fades as Cu²⁺ is used up.