Chemistry of Heavy Metals

Chemistry of Heavier Metals: Extraction, Properties & Compounds

A concise, exam-oriented guide to the occurrence, extraction, and chemistry of heavier metals—Iron (Fe), Copper (Cu), Silver (Ag), Gold (Au), Zinc (Zn), Mercury (Hg), Tin (Sn), and Lead (Pb). Ideal for board prep and JEE-level revision.

Iron (Fe)

Occurrence & Important Ores

Common ores include magnetite (Fe3O4), haematite (Fe2O3), limonite (Fe2O3·3H2O), siderite (FeCO3), and sulphides such as pyrites (FeS2).

Extraction (Blast Furnace Overview)

Iron is extracted mainly from oxide ores via concentration, roasting, and smelting in a blast furnace with coke (reducing agent) and limestone (flux). Distinct temperature zones enable reduction of iron oxides and formation of a protective slag (CaSiO3). The molten product is pig iron, further processed to wrought iron or steel by controlled removal of impurities and carbon adjustment.

Forms of Iron & Heat Treatment

Cast (pig) iron: highest C; wrought iron: lowest C; steel: medium C (including mild, hard, and alloy steels). Heat-treatments such as annealing, quenching, and tempering tune hardness and elasticity.

Key Compounds (Exam Hits)

FeO (basic), Fe2O3 (amphoteric), Fe3O4 (mixed oxide, magnetic); FeSO4·7H2O (reducing agent; forms nitroso complex with NO); FeCl3 (oxidizing, forms Prussian blue with ferricyanide); Mohr’s salt: FeSO4·(NH4)2SO4·6H2O—primary standard for titrations.

Copper (Cu)

Occurrence & Extraction

From sulphide ores (chalcopyrite CuFeS2, chalcocite Cu2S), oxide (cuprite Cu2O), and carbonates (malachite, azurite). Steps: froth flotation → roasting → smelting with silica (slag formation) → Bessemerization (auto-reduction to blister copper) → refining (poling & electrolytic) to 99.99% Cu.

Properties & Compounds

Red-brown, highly conductive; forms green patina (basic copper carbonate) in moist air. Important salts: CuSO4·5H2O (blue vitriol; forms deep blue [Cu(NH3)4]2+), Cu2O (glass red), CuO (black), reactions with KI give Cu2I2 with iodine liberation.

Silver (Ag)

Cyanide Process & Refining

Argentite and related ores dissolve in NaCN (in air) to form NaAg(CN)2; Ag is precipitated with Zn and refined electrolytically (Ag anode → AgNO3 bath). Notable compounds: AgNO3 (lunar caustic; mirror silvering, reagent), AgBr (light-sensitive; photography).

Gold (Au)

Extraction & Purity

Processed similarly by cyanidation with aeration; Au is precipitated with zinc and refined (electrolytic). Purity is expressed in carats (24-carat = pure gold). Gold(III) chloride forms tetrachloroaurate in HCl; Au2S is an insoluble sulphide.

Zinc (Zn)

Ores, Roasting & Reduction

From zinc blende (ZnS), zincite (ZnO), calamine (ZnCO3). Concentrated ore is roasted to ZnO; reduced with coke at high tem

IRON
Symbol: Fe
Atomic number: 26
Electronic configuration: [Ar]3d⁶, 4s²
It is a reactive metal and does not occur in free state.

Occurrence
1. Oxide ores:
(i) Magnetite (Fe₃O₄)—It is richest ore of iron.
(ii) Haematite (Fe₂O₃)
(iii) Limonite (Fe₂O₃·3H₂O)
2. Carbonate ore: Siderite (FeCO₃)
3. Sulphide ores:
(i) Iron pyrites (FeS₂)
(ii) Chalcopyrites (CuFeS₂)

Extraction - The extraction of iron is usually made from its oxide ores, viz., magnetite, haematite and limonite ores involves the following steps:

(i) Concentration of the ore: Crushing and washing with water and then electromagnetic dressing.

(ii) Calcination and Roasting: Heating in excess of air.

• (a) Moisture and carbon dioxide are removed.

Fe₂O₃·3H₂O → Fe₂O₃ + 3H₂O

FeCO₃ → FeO + CO₂

• (b) Impurities (such as P, S, C, As, Sb etc.) are removed as their volatile oxides.

S + O₂ → SO₂

• (c) Ferrous oxide is oxidized to ferric oxide.

2FeO + ½ O₂ → Fe₂O₃

Ferric oxide does not form slag at lower temperature so early formation of fusible slag is checked.

• (d) The entire mass becomes porous to facilitate reduction.

(iii) Smelting: The calcined ore mixed with limestone, CaCO₃ (a flux) and coke (a reductant) in the ratio 8:1 and charged into the blast furnace from the top. Different temperature zones are formed. The temperature decreases on moving upwards in the blast furnace.

• (a) Combustion zone: It is the lowest zone with a temperature range of 1500–1600°C.

Reactions:

1. C + ½ O₂ → CO; ΔH = −ve
2. CO + O₂ → CO₂; ΔH = −ve
3. CO₂ + C —(1500°C)—> 2CO; ΔH = −ve

Reduction zone: It is the upper most zone with a temperature range of 250°C–750°C.

3Fe₂O₃ + CO 300–400°C → 2Fe₃O₄ + CO₂

Fe₃O₄ + CO 500–600°C → 3FeO + CO₂

3FeO + CO 700°C → Fe (Spongy iron) + CO₂

(c) Slag formation zone: It is the central zone with a temperature range of 800–1100°C.

CaCO₃——>1000°C CaO + CO₂

CaO + SiO₂(Sandy impurities)——> CaSiO₃(Slag)

(d) Zone of fusion: It is just above the zone of combustion and with a temperature range of 1200–1500°C. The spongy iron from reduction zone melts in this zone and is collected at the bottom, while the slag (CaSiO₃) being lighter floats over the molten metal and this prevents the oxidation of Fe by blast of hot air. The molten iron is then allowed to run into boat shaped moulds, called pigs, hence the name pig iron.

Types of Iron

It is available in three commercial varieties.

1. Cast iron or pig iron: Most impure form of iron with highest carbon content (2.5–4%).

2. Wrought iron: Purest form of iron with lowest carbon content (0.12-0.25%).

3. Steel: Most important variety of commercial iron with medium carbon content (0.2-1.5%).

Steel is also available in different varities.

(a) Mild steel: Low carbon content of 0.2-0.5%.

(b) Hard steel: High carbon content of 0.5-1.5%.

(c) Alloy steel: It contains small amount of Ni, Cr, Co, W, Mn, V etc. Stainless steel is an alloy of Fe, Cr and Ni and tool steel is an alloy of Fe, W, V etc.

Steel is generally manufactured from cast iron by

(i) Bessemer process in a large pear-shaped furnace called Bessemer converter.

(ii) L.D process

(iii) Open hearth process, spiegeleisen (alloy of Fe, Mn and C) is added during the manufacture of steel.

Heat treatment of steel:

(i) To develop special properties like hardness, elasticity etc.

(ii) To remove undesirable properties like internal stresses, strains etc.

The various methods used for heat treatment are:

(a) Annealing: Heating steel to redness followed by slow cooling.

(b) Quenching: Heating steel to redness followed by sudden cooling by plunging red hot steel into water or oil.

(c) Tempering: Heating the quenched steel to a temperature much below redness
(473-623 K) followed by slow cooling.

(d) Case hardening: Producing thin coating of hardened steel on the surface of wrought iron or mild steel by heating it in contact with charcoal followed by quenching in oil.

(e) Nitriding: Heating steel at about 700°C in an atmosphere of ammonia resulting in a hard coating of iron nitride on the surface.

Compounds of Iron
1. Oxides

(a) Ferrous oxide, FeO: Obtained when ferrous oxalate is heated to about 430 K in the absence of air.
(COO)₂Fe → FeO + CO + CO₂
It is a black powder, basic in nature and reacts with dilute acids to give ferrous salts.
FeO + H₂SO₄ → FeSO₄ + H₂O
It is used in glass industry to impart green colour to glass.

(b) Ferric oxide, Fe₂O₃: Obtained by heating carbonate, nitrate or oxalate in the presence of air. It is a reddish brown powder, not affected by air or water and is insoluble in water. It is amphoteric in nature.
Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O
Fe₂O₃ + Na₂CO₃ → 2NaFeO₃ + CO₂
Fe₂O₃ + NaOH → 2NaFeO₂ + H₂O
It is reduced to Fe on heating with C or CO.
Fe₂O₃ + 3C → 2Fe + 3CO
Fe₂O₃ + 3CO → 2Fe + 3CO₂
It is used as red pigment and also as a polishing powder by jewellers.

(c) Ferrosoferric oxide, Fe₃O₄ (FeO.Fe₂O₃): More stable than FeO and Fe₂O₃, magnetic in nature and dissolves in acid giving a mixture of iron (II) and iron (III) salts.
Fe₂O₃ + 4H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4H₂O

Ferrous sulphate, FeSO₄·7H₂O (Green vitriol)

It can be obtained by treating scrap iron with dilute sulphuric acid.

Fe + H₂SO₄ → FeSO₄ + H₂

Commercially, it is obtained by exposing big heaps of moist iron pyrites to air when slow oxidation takes place.

2FeS₂ + 2H₂O + 7O₂ → 2FeSO₄ + 2H₂SO₄

1. On standing, there is a loss of water of crystallization and also it turns brown on exposure to air due to oxidation to basic ferric sulphate.

   4FeSO₄ + 2H₂O + O₂ → 4Fe(OH)SO₄

2. Strong heating of ferrous sulphate gives ferric oxide with the evolution of SO₂ and SO₃.

   2FeSO₄ — Δ → Fe₂O₃ + SO₂ + SO₃

3. It can reduce MnO₄⁻ to Mn²⁺ and Cr₂O₇²⁻ to Cr³⁺ in acidic medium. It also reduces nitric acid to nitric oxide, Hg²⁺ to Hg and Sn⁴⁺ to Sn²⁺.
4. It absorbs nitric oxide forming dark brown addition compound, nitroso ferrous sulphate, which is used in the ring test for nitrate ions.

   FeSO₄ + NO → FeSO₄·NO

5. It forms double salts of the composition R₂SO₄·FeSO₄·6H₂O (where R = an alkali metal or NH₄⁺ radical).
6. It reacts with potassium cyanide (excess) forming potassium ferrocyanide.

   FeSO₄ + 6KCN → K₄[Fe(CN)₆] + K₂SO₄

3. Ferric chloride, FeCl₃

Anhydrous ferric chloride is prepared by passing dry chlorine gas over heated iron filings while hydrated ferric chloride (FeCl₃·5H₂O) is prepared by the action of HCl on Fe(OH)₃ or Fe₂(CO₃)₃ or Fe₂O₃.

1. Anhydrous FeCl₃ is dark red deliquescent solid and gives acidic solution in water.
   FeCl₃ + 3H₂O → Fe(OH)₃ + 3HCl
2. It is a good oxidizing agent and exists as a dimer in vapour state, Fe₂Cl₆.

3. It forms prussian blue with potassium ferricyanide.
   4FeCl₃ + 3K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃ + 12KCl
   Ferri–ferrocyanide (Prussian blue)

4. Mohr’s salt, FeSO₄(NH₄)₂SO₄·6H₂O

It is a green coloured double salt used as a primary standard in volumetric analysis.

COPPER

Symbol: Cu

Atomic number: 29

Electronic configuration: [Ar]3d¹⁰, 4s¹

Occurrence

1. Sulphide ores:

(i) Chalcopyrite (CuFeS₂)

(ii) Chalcocite or copper glance (Cu₂S)

2. Oxide ore: Cuprite Cu₂O - Red colour

3. Carbonate ores:

(i) Malachite CuCO₃.Cu(OH)₂- Green colour

(ii) Azurite [(2CuCO₃)Cu(OH)₂] - Blue colour

Extraction

Copper is mostly (about 75%) is extracted from its sulphide ore, copper pyrite, which contains varying amounts of copper and iron sulphides.
Ores which contain copper more than 3% are extracted by dry process or smelting process.

(i) Concentration of ore: By froth floatation process.

(ii) Roasting: Heating strongly in current of air on the hearth of the reverberatory furnace to remove volatile impurities.
Main reaction:

2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂

Side reaction:

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ 2FeS + 3O₂ → 2FeO + 2SO₂

(iii) Smelting: The roasted ore is mixed with coke and silica and smelted into a small blast furnace in the presence of excess of air.
(a) The cuprous oxide reacts with ferrous sulphide.

FeS + Cu₂O → FeO + Cu₂S

(b) Ferrous oxide combines with silica and forms slag.

FeO + SiO₂ → FeSiO₃ (Slag)

The ‘matte’ mostly cuprous sulphide with a little iron sulphide left at the bottom after the removal of slag is taken out.

Bessemerization:

Molten matte is introduced into a Bessemer converter along with silica and a blast of hot air is blown through the molten mass. The following changes take place:

• (a) Remaining ferrous sulphide gets oxidized.
  2FeS + 3O₂ → 2FeO + 2SO₂
• (b) Ferrous oxide combines with silica to form slag which is drained out.
  FeO + SiO₂ → FeSiO₃
• (c) A part of cuprous sulphide is oxidized which combines with remaining cuprous sulphide to form metal.
  2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
  Cu₂S + 2Cu₂O → 6Cu + SO₂

This is an example of auto-reduction. The molten copper is poured off and allowed to cool so that the dissolved sulphur-dioxide comes out and large blisters are formed on the surface. Hence, the name ‘blister copper’ with 98% copper and 2% impurities.

Refining:

• (a) By poling: Impure metal is first oxidized whereby impurities are either volatilize or combine with silica forming slag. The small amount of copper is oxidized to cuprous oxide and then reduced by introducing poles of green wood and stirring them vigorously. It gives copper of 99.5% purity.
• (b) By electrolytic refining: The electrolytic bath contains an acidified solution of copper sulphate, impure copper acts as anode while cathode is of pure strip. It gives copper of about 99.99% purity. The impurities settle down below the anode as anode mud.

Properties of Copper

• It has reddish brown colour.
• It is highly malleable and ductile.
• It has high electrical and thermal conductivity.
• In presence of CO₂ and moisture, Cu is covered with a green layer of CuCO₃.Cu(OH)₂.
  2Cu + H₂O + CO₂ + O₂ → CuCO₃.Cu(OH)₂
• It undergoes displacement reactions with less reactive metals, e.g. with Ag. It can displace Ag from AgNO₃.

Compounds of Copper
1. Oxides

• (a) Cuprous oxide, Cu₂O: It is a reddish brown powder insoluble in water but soluble in ammonia solution, where it forms diamminine Cu (I) ion.
  Cu⁺ + 2NH₃ → [Cu(NH₃)₂]⁺
  It is used to impart red colour to glass.
• (b) Cupric oxide, CuO: It is dark black, hygroscopic powder which can be reduced to Cu by hydrogen, CO, etc. It is prepared by heating copper nitrate.
  2Cu(NO₃)₂ —Δ→ 2CuO + 4NO₂ + O₂
  It is used to impart light blue colour to glass.

2. Copper sulphate, CuSO₄.5H₂O (blue vitriol)
It may be prepared by reacting either CuO or CuCO₃ with dilute H₂SO₄. On an industrial scale, it is obtained by blowing air through a hot mixture of Cu and dilute H₂SO₄.
2Cu + 2H₂SO₄ + O₂ (air) → CuSO₄ + 2H₂O

• (i) The salt loses water of crystallization on heating.
  CuSO₄.5H₂O —(100°C)→ CuSO₄.H₂O —(250°C)→ CuSO₄
  CuSO₄ — strong heat → CuO + SO₃

(ii) With ammonia, the cupric hydroxide is first precipitated which dissolves in more of ammonia giving tetraammine copper (II) sulphate complex.
 CuSO₄ + 4NH₄OH ⟶ [Cu(NH₃)₄]SO₄ + 4H₂O

(iii) With KI, it gives white ppt. of Cu₂I₂.
 4KI + 2CuSO₄ ⟶ 2K₂SO₄ + Cu₂I₂ + I₂
 It does not react with KCl, KBr or KF.

(iv) With K₄[Fe(CN)₆], CuSO₄ gives reddish brown precipitate [or chocolate ppt.]
 2CuSO₄ + K₄[Fe(CN)₆] ⟶ Cu₂[Fe(CN)₆] + 2K₂SO₄
      Reddish brown ppt.

Uses:

• Electroplating and electrorefining of copper.
• In agriculture as a fungicide in Bordeaux mixture [11 parts lime + 16 parts CuSO₄ in 1000 parts of water].
• In dyeing as a mordant.

Cupric sulphide, CuS.
It is prepared as follows:
Cu(NO₃)₂ + H₂S ⟶ CuS + 2HNO₃
      Black ppt.

4. Cupric chloride, CuCl₂
It is a dark brown deliquescent solid, readily soluble in water. Its dilute solution is blue in colour due to complex cation, [Cu(H₂O)₄]²⁺, and yellow colour due to complex [CuCl₄]²⁻. The colour of solution however changes to green (due to presence of both complex ions) on addition of conc. HCl.

5. Cuprous chloride, Cu₂Cl₂
It is a white solid, insoluble in water but soluble in excess of hydrochloric acid.
Cu₂Cl₂ + 4HCl → 2H₂CuCl₃
Cu₂Cl₂ + 6HCl → 2H₃CuCl₄

SILVER

Symbol: Ag

Atomic number: 47

Electronic configuration: [Kr]4d¹⁰, 5s¹

Occurrence

Native state: Associated with copper, gold and platinum metals.

Combined state:

1. Sulphide ores:

(i) Argentite or silver glance (Ag₂S)

(ii) Ruby silver (3Ag₂S.Sb₂S₃)

(iii) Silver copper glance [(CuAg)₂S]

2. Halide ore: Horn silver (AgCl)

Extraction

The process used for extraction is cyanide process. It is also known as Mac-Arthur and Forest process.

1. Silver compounds dissolve in sodium cyanide solution forming a complex salt, NaAg(CN)₂ , in presence of air.

2. Silver is precipitated from this complex salt by the addition of zinc. The process involves the following steps:

(i) Concentration: By froth floatation process.

(ii) Cyanidation: The powdered ore is treated with dilute solution (0.4 to 0.6%) of sodium cyanide in presence of a current of air to form sodium argento cyanide.

Compounds of Silver

1. Silver nitrate, AgNO₃ (Lunar caustic)
It is a colourless crystalline compound, soluble in water. It decomposes on exposure to light and thus stored in brown bottles.
2AgNO₃ → 2AgNO₂ + O₂

It can be prepared by heating silver with dilute nitric acid.
3Ag + 4HNO₃(dilute, heat) → 3AgNO₃ + NO + 2H₂O

It is used
(i) for silvering of mirrors.
(ii) as a laboratory reagent.
(iii) in the preparation of inks and hair dyes.

2. Silver bromide, AgBr
It is obtained by adding sodium bromide to silver nitrate solution.
AgNO₃ + NaBr → AgBr + NaNO₃

The air blown in shifts the equilibrium in forward direction by converting Na₂S to Na₂SO₄.

(iii) Recovery of silver: On putting finely divided zinc, zinc enters the complex ion while Ag precipitates as amorphous mass.
2NaAg(CN)₂ + Zn → Na₂Zn(CN)₄ + 2Ag

(iv) Refining: Silver is refined by electrolysis of silver nitrate solution containing 10% nitric acid using pure silver as cathode and impure silver as anode. Copper, if present as impurity dissolves in the electrolyte and gold, if present is deposited as anode mud.

Compounds of Silver

1. Silver nitrate, AgNO₃ (Lunar caustic)
   It is a colourless crystalline compound, soluble in water. It decomposes on exposure to light and thus stored in brown bottles.
   2AgNO₃ —Δ→ 2AgNO₂ + O₂
   It can be prepared by heating silver with dilute nitric acid.
   3Ag + 4HNO₃ (dilute) —heat→ 3AgNO₃ + NO + 2H₂O
   
   It is used
   1. for silvering of mirrors.
   2. as a laboratory reagent.
   3. in the preparation of inks and hair dyes.
2. Silver bromide, AgBr
   It is obtained by adding sodium bromide to silver nitrate solution.
   AgNO₃ + NaBr → AgBr + NaNO₃
   It is a pale yellow solid, insoluble in water and concentrated acids and is partially soluble in strong solution of ammonium hydroxide due to complex formation.
   AgBr + 2NH₄OH → Ag(NH₃)₂Br + 2H₂O
   
   It is used in photographic solutions as it is most sensitive to light.
   2AgBr —light→ 2Ag + Br₂

GOLD
Symbol: Au
Atomic number: 79
Electronic configuration: [Xe]4f¹⁴, 5d¹⁰, 6s¹

Occurrence
Some important ores are,
(i) Bismuthaurite (BiAu₂)
(ii) Syvanite (AgAuTe₂)
(iii) Calverite (AuTe₂)

Extraction
The extraction of gold is carried out by using Mac–Arthur and Forest process.
The gold bearing quartz is mined by blasting. The rock is crushed to very fine powder in stamp mills and a pulp of powdered ore and water is made alkaline with slaked lime. The slurry is treated with a dilute solution of sodium cyanide (0.03 to 0.08%) and the solution is agitated by passing air through it.

4Au + 8Na(CN) + 2H₂O + O₂ → 4[NaAu(CN)₂] + 4NaOH

The gold recovered from the solution by precipitation with Zn dust in deaerated cyanide solution.
2[NaAu(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Au

Gold obtained in this way contains some silver and other impurities.

Refining: The crude gold is made anode and the cathode is pure gold and gold chloride in hydrochloric acid is used as an electrolyte.

Fineness of Gold
Gold is a precious, noble and soft metal. Its bright surface remains untarnished and is therefore used for making ornaments.

To make gold harder, small quantities of other metals, especially copper are added to it. The purity of gold is expressed in carats, pure gold being known as 24 carats.

∴ % of gold in 22 carat gold sample = (22/24) × 100 = 91.66%

Compounds of Gold

1. Gold Halide, AuCl₃
   It is a reddish solid, soluble in water. It reacts with HCl to give H[Au(Cl)₄] which is used in toning process in photography.
   HCl + AuCl₃ ⟶ H[Au(Cl)₄]
2. Gold sulphide, Au₂S
   It is a dark brown solid, insoluble in water. It is prepared as follows:
   2K[Au(CN)₂] + H₂S ⟶ Au₂S + 2KCN + 2HCN

ZINC
Symbol: Zn
Atomic number: 30
Electronic configuration: [Ar]3d¹⁰, 4s²

Occurrence
Important ores:
(i) Zincite (ZnO)
(ii) Franklinite (ZnO.Fe₂O₃)

(iii) Zinc blende (ZnS)

(iv) Calamine (ZnCO₃)

Extraction:
The extraction of zinc from zinc blende (ZnS) follows the below mentioned steps:

1. Concentration: By froth floatation process.

2. Roasting:

   The concentrated ore is roasted at 1200 K in excess of air.

   2ZnS + 3O₂ → 2ZnO + 2SO₂

   Some ZnSO₄ is also formed but at high temperature the sulphate decomposes to give ZnO.

   ZnS + 2O₂ → ZnSO₄

   2ZnSO₄ → 2ZnO + 2SO₂ + O₂

3. Reduction of ZnO:

   The oxide is mixed with crushed coke and heated to about 1670 K in fire clay retorts (Belgian process), zinc being volatile distils over and is received in an earthenware pot where it condenses. The crude metal obtained is called zinc spelter.

4. Refining: It is refined by distillation and by electrolytic method.

Compounds of Zinc

1. 1. Zinc oxide, ZnO (Philosopher’s wool)
      Zinc oxide is formed when zinc carbonate, nitrate or hydroxide is strongly heated. It is also obtained by burning zinc in air. It is a white powder and amphoteric in nature.
      ZnO + 2HCl → ZnCl₂ + H₂O
      ZnO + 2NaOH → Na₂ZnO₂ + H₂O

1. Zinc sulphide, ZnSO₄.7H₂O (White vitriol)
   It can be prepared by the action of dilute H₂SO₄ on zinc or its oxide and carbonate. It is used to prepare lithopone, a white pigment. On heating it loses its molecules of water as:
   ZnSO₄.7H₂O _375K → ZnSO₄.H₂O _725K → ZnSO_41075K → ZnO + SO₂ + O₂

MERCURY
Symbol: Hg
Atomic number: 80
Electronic configuration: [Xe]4f¹⁴, 5d¹⁰, 6s²
Occurrence
Important ore: Cinnabar (HgS)

Extraction
(i) Concentration of the ore: By froth floatation process.
(ii) Roasting: The concentrated ore is roasted in the presence of excess of air at 770 to 780 K to form HgO, which further decomposes to mercury vapours and oxygen.
2HgS + 3SO₂ → 2HgO + 2SO₂
2HgO → Hg + O₂

(iii) Refining: It is refined by filtering impure mercury through thick canvass. It is then dropped into 5% nitric acid solution where metallic impurities like Fe, Cu, Zn etc., get converted into their respective nitrates and go into solution and mercury free from impurities is received in the receiver. It is further refined by distillation under reduced pressure. More volatile mercury is distilled first.

Compounds of Mercury
1. Mercurous chloride, Hg₂Cl₂ (Calomel)
It is a white solid insoluble in water. It turns black when treated with ammonium hydroxide forming a mixture of mercury and mercuric amino chloride.
Hg₂Cl₂ + 2NH₄OH → Hg(NH₂)Cl + Hg + NH₄Cl + 2H₂O
Black
It is used as a purgative in medicine.

2. Mercuric chloride, HgCl₂ (Corrosive sublimate)

It is a colourless solid, sparingly soluble in water. Aqueous ammonia gives a white precipitate of mercuric amino chloride with HgCl₂ solution.
HgCl₂ + NH₄OH → Hg_(NH₂)Cl + NH₄Cl + 2H₂O

3. Mercuric iodide, HgI₂

1. It is a yellow solid below 400 K but changes to red solid above 400 K.
   HgI₂ (Red) ⇄ HgI₂ (Yellow) (at 400 K)
2. It dissolves in excess of KI forming K₂HgI₄.
   HgI₂ + 2KI → K₂HgI₄
   Alkaline solution of K₂HgI₄ is called Nessler’s reagent.

TIN
Symbol: Sn
Atomic number: 50
Electronic configuration: [Kr4d¹⁰, 5s², 5p²]

Occurrence
Tin exhibits enantiotropy among its three allotropic forms.
Grey tin → White tin → Rhombic tin

It liberates H₂ with dilute HCl and H₂SO₄; with hot concentrated H₂SO₄ it liberates SO₂.
Sn + 2SO₄²⁻ + H⁺ ⟶ Sn⁴⁺ + 2SO₂ + 4H₂O
Tin also dissolves in hot alkaline forming stannates and liberating hydrogen.

Compounds of Tin
1. Stannous oxide, SnO
It is a dark grey or black powder. It is an amphoteric oxide and dissolves in both acids and alkalis.

SnO + 2HCl ⟶ SnCl₂ + H₂O
SnO + 2NaOH ⟶ Na₂SnO₂ + H₂O

Stannite ion acts as a powerful reducing agent. For example, MnO₄⁻ is reduced to MnO₂ and Bi³⁺ ions to Bi.

2. Stannic oxide, SnO₂
It is a white powder and does not react with common acids except sulphuric acid. Tin dioxide is also soluble in alkalis.

SnO₂ + 2KOH ⟶ K₂SnO₃ + H₂O

3. Stannous chloride, SnCl₂.2H₂O

It is soluble in water. However, on standing, the precipitate of Sn(OH)Cl is obtained. Stannous chloride in concentrated HCl is a powerful reducing agent. With ammonia, SnCl₂ forms a number of double salts (SnCl_­2.NH₃, SnCl₂.2NH₃ and 3SnCl₂.2NH₃)

4. Stannic chloride, SnCl₄

It is a colourless fuming liquid. With limited water, it form a series of hydrated salts (SnCl₄.3H₂O, SnCl₄.5H₂O, SnCl₄.6H₂O and SnCl₄.8H₂O). With excess water it is hydrolysed.

LEAD
Symbol: Pb
Atomic number: 82
Electronic configuration: [Xe]4f¹⁴, 5d¹⁰, 6s², 6p²

Occurrence
It is a soft, bluish-grey and highly malleable metal. Dry air has no action on lead but in moist air a protective coating of basic carbonate is formed and protects it from further oxidation. When heated in air, it forms litharge which at very high temperature is converted to red lead.

6Pb + 3O₂ →(Litharge, T>725K, O₂) 6PbO →(Red lead) 2Pb₃O₄

Compounds of Lead
1. Oxides:
Lead forms five oxides, namely, monoxide (PbO), dioxide (PbO₂), mixed oxide (Pb₃O₄, red lead), suboxide (Pb₂O), and sesquioxide (Pb₂O₃).

(i) Lead monoxide is amphoteric in nature.
PbO + 2HNO₃ → Pb(NO₃)₂ + H₂O
PbO + 2NaOH → Na₂PbO₂ + H₂O

(ii) Lead dioxide is a chocolate brown powder and is a powerful oxidizing agent. It dissolves in concentrated HCl as well as in concentrated NaOH.

(iii) Red lead is used as a protective paint for iron and steel and as an oxidizing agent in laboratory.

(iv) Lead suboxide is obtained when lead oxalate is heated in absence of air.
2PbC₂O₄ → Pb₂O + 3CO₂ + CO

(v) Lead sesquioxide (Pb₂O₃) is obtained when lead monoxide is heated in air up to 775 K.
4PbO + O₂ → 2Pb₂O₃

Solved Example

1. The substance that sublimes on heating is

(A) MgCl₂

(B) AgCl

(C) HgCl₂

(D) NaCl

Sol. (C).

2. The purple colour of [Ti(H₂O)₆]³⁺ ion is due to

(A) unpaired d-electron

(B) transfer of an electron

(C) intermolecular vibrations

(D) presence of water molecules

Sol. (A).

3. When calomel reacts with ammonium hydroxide we get

(A) HgNH₂Cl

(B) NH₂–Hg–Hg–Cl

(C) Hg₂O

(D) HgO

Sol. (A).

4. Transition elements are coloured

(A) due to small size

(B) due to metallic nature

(C) due to unpaired d-electrons

(D) all of the above

Sol. (C).

5. A white powder soluble in NH₄OH but insoluble in water is

(A) BaSO₄

(B) CuSO₄

(C) PbSO_­4

(D) AgCl

Sol. (D).

6. Which of the following transition metal cation has maximum unpaired electrons?

(A) Mn²⁺

(B) Fe²⁺

(C) Ni³⁺

(D) Cu⁺

Sol. (A).

7. Which of the following ore is not an ore of iron?

(A) Haematite

(B) Magnetite

(C) Cassiterite

(D) Limonite

Sol. (C).

8. The colour of FeSO₄(NH₄)₂SO₄.6H₂O is

(A) red

(B) white

(C) blue

(D) green

Sol. (D).

9. Iron is manufactured from the ore

(A) cryolite

(B) bauxite

(C) haematite

(D) chalcopyrite

Sol. (C)