Lead also represented chemically as Pb, was first mined in the first century A.D. although it is not a very reactive metal, it occurs combined in nature. The principal ore of PB is galena, PbS, which is found in many parts of the world. Others include cerussite, PbCO3, and anglesite, PbSO4.
In Nigeria, small quantities of lead are found in Ogoja, Abakaliki and along the Benue River in Gongola State.
Extraction of Lead
Pb is extracted mainly from galena, PbS. The ore first roasted in air to yield lead(II) oxide. The PbO is then heated in a blast furnace together with the same coke, limestone and scrap iron.
2PbS(s) + 3O2(g)
The limestone forms a slag with the impurities and this can be tapped off separately, while the lead(II) oxide is reduced to lead by the carbon. Any unchanged PbS which might still be present is also reduced to lead by the iron.
Physical Properties Of Lead
Appearance: Greyish-white solid with lustre
Relative density: 11.3
Malleability: Very mallable metal; soft enough to be cut with a knife
Ductility: Not ductile enough to be drawn into wires
Tensile Strength: Fairly strong
Melting point: 327C
Conductivity: Good conductor of heat and electricity
Chemical Properties of Lead
Reaction with air: The metal is attacked by most air to form a thin white coating of PbO, lead(II) hydroxide and lead(II) trioxocarbonate(IV) on the outside. On heating strongly in air, Pb combines slowly with oxygen PbO.
However, continued heating in air to a temperature of about 450C results in the formation of red dilead(II) oxide.
Reaction with acids: It does not dissolve in dilute hydrochloric or tetraoxosulphate(VI) acids due to the formation of an insoluble outer layer of PbCl2 chloride or tetraoxosulphate(VI) which protects the metal form further attack. However, lead is attacked rapidly by hot concentrated hydrochloric acid and slowly by hot concentrated tetraoxosulphate(VI) acid to liberate hydrogen and sulphur(IV) oxide respectively.
Pb is readily soluble in trioxonitrate(V) acid, reacting to form nitrogen(II) oxide with the dilute acid and nitrogen(IV) oxide with the concentrated acid.
Reaction with alkalis: Pb, being fairly far down the activity series, has no action with alkalis.
Tests for Lead(II) ions
These tests can be used to identify the presence of Pb in a compound
Aqueous ammonia: Add a few drops of aqueous ammonia to the unknown salt solution. The formation of the white precipitate, which does not dissolve in an excess of aqueous ammonia, indicates the presence of Pb2+
NOTE: A similar reaction is also given by aluminum salts. Use ammonia and KI as valid tests for Pb2+.
Potassium tetraoxochromate (VI) or iodide: Add some potassium tetraoxochromate(VI) or iodide solution to the unknown salt solution. The formation of a yellow precipitate in either case indicates the presence of lead(II) ions.
Concentrated hydrochloric acid: Add a few drops of concentrated hydrochloric acid to the unknown salt solution. The formation of a white precipitate, which dissolves when warmed but reappears on cooling, confirms the presence of Pb2+ .
6 Important Uses of Lead
- Lead is used for roofing, for making water and gas pipes, and for sheathing cables because of its resistance to corrosion, its flexibility, low melting point and cheapness. However, if it is used to make water pipes, there is a danger of lead poisoning because water containing dissolved oxygen attacks lead slowly to form lead(II) hydroxide which is slightly soluble. This poisonous effect can be avoided if the water is slightly hard, because the dissolved tetraoxosulphates(VI) and hydrogentrioxocarbonates(VI) will soon react with the lead to form a protective layer of insoluble salts.
- It is used for making bullets, Pb shots and weights because of its high density.
- This metal is also used for making lead accumulators.
- Pb is used in making alloys such as type metal, pewter and plumber’s solder.
- It is used for making lead compounds, e.g. the oxides and lead(VI) tetraethyl.
- Pb is used as a shield against X-rays and radioactive radiation because it is the cheapest of the very dense metals and has a high absorbing power.