Molten Lead(II) Bromide (Electrolysis)
The electrolysis of molten lead(II) bromide (PbBr₂) is the classic IGCSE demonstration used to introduce the fundamental principles of electrolysis because it involves a simple binary ionic compound with only one cation and one anion present, making the electrode reactions unambiguous. Lead(II) bromide is an ionic compound consisting of Pb²⁺ cations and Br⁻ anions held together in a giant ionic lattice by strong electrostatic forces of attraction. When heated above its melting point of approximately 373 °C, the ions become mobile and the molten liquid conducts electricity, allowing electrolysis to occur. At the cathode, lead(II) ions are reduced to form grey liquid lead metal, while at the anode, bromide ions are oxidised to form red-brown bromine gas. This investigation is consistently featured in IGCSE practical examinations because it clearly demonstrates the relationship between ionic mobility, electrical conductivity, and the decomposition of a compound by an electric current. The overall process can be summarised by a single decomposition equation — PbBr₂(l) → Pb(l) + Br₂(g) — which makes it the simplest and most conceptually accessible electrolysis to study before progressing to more complex aqueous systems.
Why Must It Be Molten?
The requirement for lead(II) bromide to be in the molten (liquid) state is central to understanding why electrolysis works at all. This requirement stems from the nature of ionic bonding and the conditions necessary for electrical conduction in ionic compounds.
Solid Lead(II) Bromide Does NOT Conduct Electricity
In the solid state, lead(II) bromide exists as a crystalline solid with Pb²⁺ and Br⁻ ions arranged in a regular, repeating three-dimensional pattern — the giant ionic lattice. In this lattice, each Pb²⁺ ion is surrounded by Br⁻ ions and vice versa, held in fixed positions by strong electrostatic attractions. Although the ions are charged particles, they are locked in place and cannot move. Electrical conduction requires the movement of charged particles (either electrons, as in metals and graphite, or mobile ions, as in electrolytes). Since the ions in solid PbBr₂ are immobile, the solid cannot conduct electricity, and no electrolysis can take place.
Molten Lead(II) Bromide DOES Conduct Electricity
When lead(II) bromide is heated to its melting point (approximately 373 °C), the thermal energy supplied overcomes the electrostatic forces holding the ions in the lattice. The regular arrangement collapses, and the ions become free to move independently. In this liquid state, Pb²⁺ cations and Br⁻ anions are mobile charge carriers. When electrodes connected to a DC power supply are inserted into the molten salt, the positively charged Pb²⁺ ions migrate towards the negative electrode (cathode), and the negatively charged Br⁻ ions migrate towards the positive electrode (anode). This movement of ions constitutes an electric current through the liquid. Electrolysis — the chemical decomposition of the compound by the electric current — can now proceed.
Key Principle
Ionic compounds conduct electricity only when molten or dissolved in water, because these are the only conditions under which the ions are free to move. Solid ionic compounds are electrical insulators. This principle is tested frequently in IGCSE multiple-choice and theory papers.
Electrode Reactions
With only two ions present — Pb²⁺ and Br⁻ — the electrode reactions are straightforward and predictable. There is no competition between ions as there is in aqueous electrolysis. Inert electrodes (typically graphite rods) are used so that the electrodes themselves do not participate in the reactions.
At the Cathode (Negative Electrode)
The cathode is connected to the negative terminal of the DC power supply. It attracts positively charged lead(II) ions (Pb²⁺). At the surface of the cathode, each Pb²⁺ ion gains two electrons — it is reduced.
Half-equation at the cathode (reduction):
Pb²⁺(l) + 2e⁻ → Pb(l)
Observations:
- A silvery-grey liquid forms at the cathode.
- This is molten lead metal. Lead has a relatively low melting point (327.5 °C), so it remains liquid at the operating temperature of the molten electrolyte.
- Upon cooling, the lead solidifies into a grey, shiny metallic solid.
- The lead is dense and sinks to the bottom of the crucible.
Identifying the product: The grey metallic globule can be confirmed as lead by its high density, its malleability (it can be flattened when pressed), and its metallic lustre. In a laboratory setting, the lead may be collected and weighed.
At the Anode (Positive Electrode)
The anode is connected to the positive terminal of the DC power supply. It attracts negatively charged bromide ions (Br⁻). At the surface of the anode, each Br⁻ ion loses one electron — it is oxidised. Two bromine atoms then combine to form a bromine molecule.
Half-equation at the anode (oxidation):
2Br⁻(l) → Br₂(g) + 2e⁻
Observations:
- A red-brown gas (bromine vapour) is evolved at the anode.
- The gas has a sharp, choking smell (toxic — this experiment must be performed in a fume cupboard).
- The red-brown colour is characteristic and distinctive.
- Bromine gas is heavier than air and may be seen as a brownish haze around the anode.
Identifying the product: Bromine gas turns damp blue litmus paper red and then bleaches it white. This bleaching effect is a key diagnostic test for bromine (and chlorine) gas. Bromine is toxic and corrosive; direct inhalation must be avoided.
Overall Reaction
Combining the two half-equations (and ensuring electrons cancel) yields the overall decomposition equation:
Cathode: Pb²⁺ + 2e⁻ → Pb(l) Anode: 2Br⁻ → Br₂(g) + 2e⁻
Overall: PbBr₂(l) → Pb(l) + Br₂(g)
This is a decomposition reaction driven by electrical energy. The electrical energy from the DC supply forces the non-spontaneous decomposition of lead(II) bromide into its constituent elements.
Observations Summary Table
| Feature | Detail |
|---|---|
| Electrolyte | Molten (liquid) lead(II) bromide, PbBr₂(l) |
| Appearance before electrolysis | White/colourless crystalline solid when cold; clear colourless liquid when molten |
| Electrodes | Inert electrodes (typically graphite/carbon rods) |
| Temperature required | Above 373 °C (melting point of PbBr₂) |
| Cathode product | Lead metal, Pb(l) — grey, shiny liquid |
| Cathode half-equation | Pb²⁺ + 2e⁻ → Pb(l) — reduction |
| Anode product | Bromine gas, Br₂(g) — red-brown gas with choking smell |
| Anode half-equation | 2Br⁻ → Br₂(g) + 2e⁻ — oxidation |
| Test for cathode product | Grey metallic globule; malleable; high density |
| Test for anode product | Red-brown gas; turns damp litmus red then bleaches white |
| Overall equation | PbBr₂(l) → Pb(l) + Br₂(g) |
| Conduction mechanism | Movement of Pb²⁺ and Br⁻ ions through the liquid |
Key IGCSE Practical Details
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Heating method: In the school laboratory, lead(II) bromide is typically heated in a crucible using a Bunsen burner. A pipe-clay triangle supports the crucible on a tripod. The solid must be heated until it is fully molten before inserting the electrodes; inserting electrodes into a partially solid sample will not produce electrolysis.
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Fume cupboard requirement: Bromine gas is toxic, corrosive, and has a sharp, irritating odour. This practical must always be carried out in a well-ventilated fume cupboard. Students wear eye protection at all times.
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Graphite electrodes: Carbon (graphite) rods are used as inert electrodes. They do not react with the molten salt or the products. Over repeated use, graphite anodes may degrade slightly, but they remain the electrode material of choice for this investigation.
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DC power supply: A low-voltage DC supply (typically 6–12 V) is sufficient. The electrodes are connected via leads with crocodile clips. Alternating current (AC) must NOT be used, as AC would not produce consistent electrode reactions — the products would continuously recombine.
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Confirming ionic mobility: A simple circuit with a bulb in series can demonstrate that solid PbBr₂ does not complete the circuit (bulb remains off) while molten PbBr₂ does (bulb lights up). This is a common IGCSE demonstration question.
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Electrode identification: In IGCSE diagrams, students must label the anode as the electrode connected to the positive terminal and the cathode as the electrode connected to the negative terminal. The cathode attracts cations (Pb²⁺) and the anode attracts anions (Br⁻).
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Post-experiment observations: After the power supply is disconnected, the lead solidifies into a grey solid. The bromine gas disperses or condenses into a dark red-brown liquid if cooled. The remaining electrolyte eventually solidifies as it cools.
Contrast with Aqueous Electrolysis
A common IGCSE extension question asks students to compare the electrolysis of molten lead(II) bromide with the electrolysis of aqueous lead(II) bromide solution. The key differences are:
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Molten PbBr₂: Only Pb²⁺ and Br⁻ ions are present. Electrolysis is simple and predictable — lead at the cathode, bromine at the anode.
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Aqueous PbBr₂: Water introduces H⁺ and OH⁻ ions alongside Pb²⁺ and Br⁻. At the cathode, H⁺ ions may compete with Pb²⁺ for discharge. At the anode, OH⁻ ions may compete with Br⁻ for discharge. The products are therefore not guaranteed to be lead and bromine alone; hydrogen gas and oxygen gas are possible alternatives, depending on concentration and electrode potential.
This contrast underscores the essential principle: in molten electrolysis, only the ions of the compound are present, so only the decomposition of that compound occurs.
Sources
- Cambridge IGCSE Chemistry Coursebook (5th Edition), Richard Harwood and Ian Lodge, Cambridge University Press, 2021, Chapter 5: Electricity and Chemistry, pp. 96–101.
- Cambridge IGCSE Chemistry Syllabus 0620, Topic 4: Electrochemistry, 4.1 Electrolysis.
- Edexcel IGCSE Chemistry Student Book, Jim Clark, Pearson, 2017, Topic 1(i): Electrolysis, Section on molten ionic compounds.
- “Electrolysis of Molten Lead Bromide,” Royal Society of Chemistry (RSC) Classic Chemistry Demonstrations, rsc.org.
- Nuffield Practical Chemistry, “The Electrolysis of Molten Lead Bromide,” Nuffield Foundation, nuffieldfoundation.org.
- GCSE Chemistry (9-1) Revision Guide, CGP Books, 2021.
Common Misconceptions
- Misconception 1: “Solid lead(II) bromide can be electrolysed if a high enough voltage is applied.” This is incorrect. No amount of voltage can produce electrolysis in solid PbBr₂ because the ions are immobile in the solid lattice. Conduction depends on ion mobility, not voltage.
- Misconception 2: “Lead is produced at the anode.” This is incorrect. Pb²⁺ ions are positively charged and are therefore attracted to the negative electrode (the cathode). It is the cathode where reduction to lead metal occurs. The anode is where negatively charged Br⁻ ions are attracted and oxidised.
- Misconception 3: “The bromine produced is a liquid at the anode.” While bromine is a liquid at room temperature, the high temperature of the molten electrolyte (above 373 °C, far exceeding bromine’s boiling point of 58.8 °C) means bromine is produced as a gas (vapour) at the anode.
- Misconception 4: “The electrodes must be made of lead and bromine.” The electrodes in this investigation are inert (graphite or platinum). They do not supply the ions for the reaction. The ions come from the electrolyte. If lead electrodes were used, the anode reaction might change, as reactive metal electrodes can dissolve.
- Misconception 5: “The bulb in the circuit lights up because electrons flow through the molten salt.” This is incorrect. The current through the molten electrolyte is carried by moving ions, not electrons. Electrons flow through the external circuit (wires and power supply). Ion movement through the electrolyte completes the circuit. This distinction between electronic conduction (in metals) and ionic conduction (in electrolytes) is fundamental.