Lead

Lead (Pb) is a dense, low-reactivity metal used at IGCSE mainly to teach the electrolysis of a simple molten ionic compound and to discuss environmental toxicity.

Where it appears in 0620

Lead is a small but reliable source of exam marks in two areas:

• Electrolysis: molten lead(II) bromide is the textbook example of splitting a binary ionic compound into its elements. It is the go-to “predict the products” and “why molten?” question.
• Reactivity series and extraction: lead sits just above hydrogen and below carbon, so it is extracted by carbon reduction, not electrolysis.
• Preparation of salts: lead salts are mostly insoluble, making lead(II) iodide a classic precipitation reaction.

Position in the periodic table

• Symbol: Pb (from Latin plumbum)
• Atomic number: 82
• Group 14 (IV), Period 6
• Electron configuration: 2, 8, 18, 32, 18, 4
• Common ion: Pb2+

Physical properties

Property	Detail
Appearance	Grey, dull, soft
Melting point	327 C
Density	11.3 g/cm3 (very dense)
Conductivity	Moderate conductor
Malleability	Very malleable

Reactivity

Lead is low in the reactivity series, just above hydrogen. It:

• Reacts only very slowly, if at all, with dilute acids (an unreactive layer of insoluble salt can form)
• Does not react with water
• Is extracted by carbon reduction because it is below carbon: 2PbO(s) + C(s) -> 2Pb(l) + CO2(g)

Lead(II) oxide is amphoteric — like aluminium and zinc oxides, it reacts with both acids and alkalis.

Electrolysis of molten lead(II) bromide

The classic introductory electrolysis experiment.

Setup: PbBr2 is heated until molten. Carbon (graphite) electrodes are dipped in the melt and connected to a d.c. supply.

Why it must be molten: in solid PbBr2 the Pb2+ and Br- ions are locked in a lattice and cannot move. Melting frees them so they carry the current.

At the cathode (reduction): Pb2+ + 2e- -> Pb Observation: a silvery bead of molten lead collects at the cathode.

At the anode (oxidation): 2Br- -> Br2 + 2e- Observation: brown/orange fumes of bromine gas are given off.

Overall: PbBr2(l) -> Pb(l) + Br2(g)

This shows how electrolysis breaks an ionic compound down into its elements.

Lead compounds

Compound	Formula	Colour	Notes
Lead(II) oxide	PbO	Yellow	Formed when lead is heated in air; amphoteric
Lead(II) bromide	PbBr2	White	Classic electrolysis compound
Lead(II) iodide	PbI2	Bright yellow	Classic precipitation reaction
Lead(II) nitrate	Pb(NO3)2	White, soluble	The one common soluble lead salt
Lead(II) chloride	PbCl2	White	Slightly soluble

Lead(II) iodide precipitation

A visually striking precipitation reaction:

Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s) + 2KNO3(aq)

Observation: a bright yellow precipitate of PbI2 appears when the two colourless solutions are mixed.

Environmental concerns

Lead is toxic and its uses have been cut back sharply:

Former use	Why discontinued
Petrol additive (tetraethyllead)	Lead pollution in exhaust; neurotoxic
Water pipes	Lead dissolves slowly, contaminating water
Paints	Toxic if ingested, especially by children

Lead accumulates in the body (bioaccumulation), causing neurological damage, learning difficulties in children, and kidney problems.

Key facts at a glance

Fact	Detail
Symbol	Pb
Proton number	82
Group / Period	14 (IV) / 6
Ion formed	Pb2+
Extraction	Carbon reduction (below carbon)
Signature experiment	Electrolysis of molten PbBr2

Common exam mistakes

• Saying solid PbBr2 conducts. It does not — the ions are fixed. It conducts only when molten, once the ions are free to move.
• Wrong anode half-equation. It is 2Br- -> Br2 + 2e-, not “Br- -> Br2 + e-”. Bromine is diatomic, so two bromide ions are needed.
• Muddling the electrode observations. The cathode gives silvery molten lead; the anode gives brown bromine vapour — not the other way round.
• Extracting lead by electrolysis. Lead is below carbon, so it is extracted by carbon reduction. Electrolysis is only needed for metals above carbon, such as aluminium.

Exam-style questions

Molten lead(II) bromide is electrolysed using carbon electrodes. Write the half-equation at each electrode and state what is observed. Explain why the compound must be molten. (4 marks)

Mark scheme

• cathode: Pb2+ + 2e- -> Pb; a silvery/grey bead of molten lead forms [1]
• anode: 2Br- -> Br2 + 2e-; brown/orange bromine fumes are given off [1]
• in the solid the ions are held in a fixed lattice and cannot move [1]
• when molten the ions are free to move and carry the current [1]

Examiner note: half-equations must show the electrons and be balanced — a word answer alone caps the marks.

A student mixes colourless lead(II) nitrate solution with colourless potassium iodide solution. Describe what is seen and write a balanced equation with state symbols. (3 marks)

Mark scheme

• a bright yellow precipitate (solid) forms [1]
• Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s) + 2KNO3(aq) [1 for correct products, 1 for balancing and state symbols]

Examiner note: most lead salts are insoluble — expecting a clear mixture instead of a precipitate is the usual error.