GCSE Chemistry revision

Electrolysis of molten ionic compounds

Electrolysis

AQA 4.4.3.2 Foundation & Higher

Your specification

AQA student objectives

Learning pathway

ALL 🎯

MOST 🎯🎯

SOME 🎯🎯🎯

Revision summary

Read on screen, then print for Cornell-style active revision.

Electrolysis literally means 'splitting up with electricity' and uses an electric current to separate the elements within an ionic compound.
It can be used to decompose insoluble ionic compounds, such as lead bromide, into their pure elemental forms.

The electrolyte is a liquid or solution containing an ionic compound whose ions are free to move.
Soluble ionic compounds, such as copper sulfate, can simply be dissolved in water to form an aqueous electrolyte.
Insoluble ionic compounds, such as lead bromide, must be melted to form a molten electrolyte, ensuring all ions are free to move.
Molten lead bromide contains free-moving positive lead ions (Pb²⁺) and negative bromide ions (Br⁻).

2. The Electrolyte AQA

The electrolyte is a liquid or solution containing an ionic compound whose ions are free to move.
Soluble ionic compounds, such as copper sulfate, can simply be dissolved in water to form an aqueous electrolyte.
Insoluble ionic compounds, such as lead bromide, must be melted to form a molten electrolyte, ensuring all ions are free to move.
Molten lead bromide contains free-moving positive lead ions (Pb²⁺) and negative bromide ions (Br⁻).

The electrolyte is placed in a beaker, and two solid conducting electrodes — generally made of metal or carbon — are inserted into it.
The positive electrode is called the anode, and the negative electrode is called the cathode.
The electrodes are connected by wire to a power supply, such as a battery, which drives the flow of electrons around the circuit.

3. The Equipment AQA

The electrolyte is placed in a beaker, and two solid conducting electrodes — generally made of metal or carbon — are inserted into it.
The positive electrode is called the anode, and the negative electrode is called the cathode.
The electrodes are connected by wire to a power supply, such as a battery, which drives the flow of electrons around the circuit.

Negative ions (anions) are attracted to the positive anode, while positive ions (cations) are attracted to the negative cathode.
At the electrodes, ions are discharged, meaning they change from charged ions into neutral atoms.
In molten lead bromide, bromide ions are discharged at the anode and pair up to form bromine gas (Br₂), which escapes.
Lead ions are discharged at the cathode to form pure molten lead, which sinks to the bottom of the beaker.

4. What Happens at the Electrodes? AQA

Negative ions (anions) are attracted to the positive anode, while positive ions (cations) are attracted to the negative cathode.
At the electrodes, ions are discharged, meaning they change from charged ions into neutral atoms.
In molten lead bromide, bromide ions are discharged at the anode and pair up to form bromine gas (Br₂), which escapes.
Lead ions are discharged at the cathode to form pure molten lead, which sinks to the bottom of the beaker.

Oxidation occurs at the anode, where bromide ions each lose one electron to form bromine atoms, shown by the half equation: 2Br⁻ → Br₂ + 2e⁻.
Reduction occurs at the cathode, where each lead ion gains two electrons to form a neutral lead atom, shown by the half equation: Pb²⁺ + 2e⁻ → Pb.
A helpful way to remember this is OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

5. Oxidation and Reduction at the Electrodes AQA

Oxidation occurs at the anode, where bromide ions each lose one electron to form bromine atoms, shown by the half equation: 2Br⁻ → Br₂ + 2e⁻.
Reduction occurs at the cathode, where each lead ion gains two electrons to form a neutral lead atom, shown by the half equation: Pb²⁺ + 2e⁻ → Pb.
A helpful way to remember this is OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Electrons from the negative bromide ions are transferred to the anode, travel along the wire powered by the battery, and are then given to the positive lead ions at the cathode.
Overall, electrolysis converts the ions in an ionic compound back into their pure elemental forms by transferring electrons from negative ions to positive ions.

6. The Overall Process AQA

Electrons from the negative bromide ions are transferred to the anode, travel along the wire powered by the battery, and are then given to the positive lead ions at the cathode.
Overall, electrolysis converts the ions in an ionic compound back into their pure elemental forms by transferring electrons from negative ions to positive ions.

Electrolysis of molten ionic compounds is a key method for extracting reactive metals that cannot be obtained by reduction with carbon.
The same principles apply to the electrolysis of aqueous solutions, where the presence of water introduces additional ions that affect what is produced at each electrode.
Electrolysis of metal oxides can also be used to extract pure metals, extending the technique beyond simple binary compounds like lead bromide.

7. Uses and Further Applications AQA

Electrolysis of molten ionic compounds is a key method for extracting reactive metals that cannot be obtained by reduction with carbon.
The same principles apply to the electrolysis of aqueous solutions, where the presence of water introduces additional ions that affect what is produced at each electrode.
Electrolysis of metal oxides can also be used to extract pure metals, extending the technique beyond simple binary compounds like lead bromide.