Electrolysis (Chemistry)

What is electrolysis?

Breaking down a molten or dissolved ionic compound using electricity

What is an electrolyte?

Molten/dissolved ionic compound that conducts electricity — ions are free to move

Why can't solid ionic compounds conduct electricity?

Ions are locked in a fixed lattice — cannot move freely

What are electrodes?

Rods (usually graphite/metal) that carry current into/out of the electrolyte

What is the cathode?

Negative electrode — positive ions (metal-cations) move here

What happens at the cathode (-) in molten electrolysis?

Positive metal ions gain electrons → reduced → pure metal forms

What happens at the anode (+) in molten electrolysis?

Negative non-metal ions lose electrons → oxidised → non-metal element forms

Half-equation for lead forming at cathode (PbBr₂)

Pb²⁺ + 2e⁻ → Pb

Half-equation for bromine forming at anode (PbBr₂)

2Br⁻ → Br₂ + 2e⁻

What extra ions are present in aqueous solutions?

Hydrogen ions (H⁺) and hydroxide ions (OH⁻) from water

Rule for what's produced at the cathode in aqueous electrolysis

•Metal more reactive than H → H₂ gas produced

•Metal less reactive than H → pure metal produced

Rule for what's produced at the anode in aqueous electrolysis

•Halide ions present → halogen gas forms

•No halide ions → oxygen gas forms (from OH⁻)

Why is aluminium extracted by electrolysis?

Aluminium is more reactive than carbon — can't be reduced by carbon alone

What is aluminium oxide mixed with? Why?

Mixed with cryolite → lowers melting point → saves energy & money

Where does aluminium form?

At the negative cathode

Where does oxygen form?

At the positive anode

Why must the graphite anodes be replaced regularly?

Oxygen reacts with hot graphite → forms carbon dioxide → anode wears away

Why can ionic compounds only be electrolysed when molten or dissolved?

Ions must be free to move and carry charge — solid ions can't move

What gas is always produced at the cathode if the metal is more reactive than hydrogen?

Hydrogen gas (H₂)

The metals less reactive than hydrogen

Copper, silver, gold, platinum

Metals extracted from reduction by carbon (below carbon on reactivity)

Copper, hydrogen, lead, tin, iron, zinc (carbon more reactive than them all)

Molten electrolysis aluminium half equation at anode

2O²⁻ → O₂ + 4e⁻

Molten electrolysis aluminium half equation at cathode

Al³⁺ + 3e⁻ → Al

Metals extracted by electrolysis

Aluminium, magnesium, calcium, sodium, potassium

What forms at each electrode for NaCl(aq)?

Cathode = Hydrogen gas

Anode = Chlorine gas

What forms at each electrode for CuSO₄(aq)?

Cathode = Copper metal

Anode = Oxygen gas