Electrolysis

An electrolysis cell has three parts:

• Electrolyte: the ionic compound, MOLTEN or DISSOLVED in water (ions must be free to move).
• Electrodes: two conducting rods (usually carbon or platinum) dipped in the electrolyte.
• DC power supply: a battery or DC source, making one electrode positive (ANODE) and the other negative (CATHODE).

When the supply is switched on:

• Cations (+) move toward the CATHODE (−).
• Anions (−) move toward the ANODE (+).
• At the cathode, cations gain electrons → atoms (REDUCTION).
• At the anode, anions lose electrons → atoms (OXIDATION).

Molten electrolytes contain ONLY the ions from the compound — no water. So the products are simple:

Molten lead bromide (PbBr₂):

• At cathode (−): Pb²⁺ + 2 e⁻ → Pb. Lead metal forms.
• At anode (+): 2 Br⁻ → Br₂ + 2 e⁻. Bromine gas (orange-brown vapour).

Molten sodium chloride (NaCl):

• At cathode: Na⁺ + e⁻ → Na. Sodium metal.
• At anode: 2 Cl⁻ → Cl₂ + 2 e⁻. Chlorine gas.

The metal is always discharged at the cathode and the non-metal at the anode. This is how very reactive metals (K, Na, Ca, Mg, Al) are extracted from their compounds.

Water is slightly ionised: H₂O ⇌ H⁺ + OH⁻. So in an aqueous electrolyte, both the salt's ions AND H⁺/OH⁻ are present at the electrodes.

Rule at the cathode: the LESS REACTIVE species is discharged.

• If the cation is LESS reactive than H⁺ (e.g. Cu²⁺, Ag⁺) → the metal forms.
• If the cation is MORE reactive than H⁺ (e.g. Na⁺, K⁺, Ca²⁺, Mg²⁺) → H₂ gas forms instead.

Rule at the anode: again, less reactive discharged.

• Halide ions (Cl⁻, Br⁻, I⁻) are usually discharged → halogen gas.
• Otherwise OH⁻ (from water) is discharged → O₂ gas.

Examples:

Solution	Cathode	Anode
NaCl (aq)	H₂ (Na too reactive)	Cl₂
CuSO₄ (aq)	Cu (less reactive than H⁺)	O₂ (no halide)
H₂SO₄ (aq)	H₂	O₂
Dilute Na₂SO₄ (aq)	H₂	O₂ (essentially electrolysing water)

CuSO₄ is a great teaching example: blue colour (Cu²⁺) fades as the copper deposits at the cathode, and a coating of bright orange copper builds up.

Electroplating is electrolysis used to put a thin layer of one metal onto an object. The object to be plated is the CATHODE; the plating metal is the ANODE; the electrolyte contains ions of the plating metal.

For silver-plating a spoon:

• Cathode (−): the spoon.
• Anode (+): a piece of pure silver.
• Electrolyte: silver nitrate solution (AgNO₃).

What happens:

• At the cathode (spoon): Ag⁺ + e⁻ → Ag. Silver coats the spoon.
• At the anode (silver): Ag → Ag⁺ + e⁻. Silver dissolves into solution.
• Net effect: silver moves from the anode to the spoon. The AgNO₃ solution stays roughly the same concentration.

Uses include:

• Decoration: jewellery (gold/silver-plated steel).
• Protection: chrome-plating bumpers and tools.
• Improving conductivity: electroplating copper onto electrical contacts.

MYP criterion D extension: why is electroplating preferred over making things from solid silver or gold? Cost — a thin layer needs only a tiny amount of the expensive metal, while the bulk is cheap steel underneath.