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Unit 7: Studies of Heavy Metals

Chemistry - Class 12

This chapter covers the occurrence, extraction, chemical properties and industrial uses of copper, zinc, mercury, iron and silver. It also details the preparation and applications of their important compounds such as blue vitriol, red and black oxides, white vitriol, calomel, corrosive sublimate, steel manufacturing processes and silver halides.

No MCQ questions available for this chapter.

Unit 7: Studies of Heavy Metals

7.1 Copper (Cu)

Occurrence

Copper is primarily found in sulfide minerals. The most important ores are copper pyrite (CuFeS2), also known as chalcopyrite, and copper glance (Cu2S), also called chalcocite. These minerals occur in hydrothermal veins and are associated with other metal sulfides.

Extraction from Copper Pyrite

  1. Crushing and Grinding: The ore is crushed to fine particles to increase surface area.
  2. Concentration (Froth Flotation): Powdered ore is mixed with water, pine oil and collectors; air bubbles carry the hydrophobic sulfide particles to the froth, which is skimmed off.
  3. Roasting: The concentrated sulfide is heated in excess air to convert it to copper(II) oxide and sulfur dioxide: 2 CuFeS2 + 4 O2 → 2 CuO + 2 FeO + 3 SO2
  4. Smelting: The roasted product is mixed with silica flux and heated in a blast furnace. Iron oxide forms slag (FeSiO3) while copper oxide is reduced to copper(I) sulfide (copper matte): CuO + FeS → Cu2S + FeO (simplified)
  5. Bessemerization: Air is blown through the molten matte in a converter; sulfur is oxidized to SO2 and iron to slag, leaving blister copper (~98% Cu): 2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2 Cu2O + Cu2S → 6 Cu + SO2
  6. Electrolytic Refining: Blister copper is cast into anodes and placed in an electrolyte of acidified copper sulfate. Pure copper deposits on cathodes (Cu^{2+} + 2e^- → Cu) while impurities settle as anode mud.

Properties

  • Reaction with Air: Copper slowly oxidizes in moist air to form a green patina of basic copper carbonate; when heated strongly it forms black copper(II) oxide: 2 Cu + O2 → 2 CuO
  • Reaction with Acids:
    • Dilute sulfuric acid: no reaction (copper is below hydrogen in the reactivity series).
    • Dilute nitric acid: forms copper(II) nitrate and nitrogen oxides: 3 Cu + 8 HNO3 (dil) → 3 Cu(NO3)2 + 2 NO + 4 H2O
  • Reaction with Aqueous Ammonia: Copper(II) ions form a deep‑blue tetraammine complex: [Cu(NH3)4]^{2+} (deep blue)
  • Displacement Reactions: Copper can be displaced by more reactive metals such as zinc or iron: CuSO4 + Zn → ZnSO4 + Cu

Uses

  • Electrical wiring and cables due to high conductivity.
  • Plumbing pipes and fittings (corrosion resistant).
  • Alloys: brass (Cu‑Zn) and bronze (Cu‑Sn).
  • Coinage, heat exchangers, and roofing.

Blue Vitriol (Copper(II) Sulfate Pentahydrate – CuSO4·5H2O)

Preparation: Treat copper oxide or copper carbonate with dilute sulfuric acid: CuO + H2SO4 → CuSO4 + H2O Then crystallize the solution to obtain the pentahydrate.

Properties: Bright blue crystalline solid; soluble in water giving a blue solution due to [Cu(H2O)6]^{2+}; loses water on heating to form anhydrous white CuSO4.

Uses:

  • Fungicide (Bordeaux mixture).
  • Electrolyte in electroplating and electrorefining of copper.
  • Reagent in laboratory tests for water (anhydrous CuSO4 turns blue).

Red Oxide (Copper(I) Oxide – Cu2O) and Black Oxide (Copper(II) Oxide – CuO)

  • Red Oxide (Cu2O): Prepared by reducing CuO with glucose or by heating copper metal in limited oxygen. Used as a pigment, antifouling paint, and in rectifier diodes.
  • Black Oxide (CuO): Obtained by heating copper in air or by heating copper nitrate. Used as a catalyst, in ceramics, and as a pigment.

7.2 Zinc (Zn)

Occurrence

Zinc occurs mainly as sulfide and carbonate ores. The principal ore is zinc blende (ZnS, sphalerite). Another important ore is calamine (ZnCO3, smithsonite).

Extraction from Zinc Blende

  1. Roasting: Zinc sulfide is roasted in excess air to give zinc oxide and sulfur dioxide: 2 ZnS + 3 O2 → 2 ZnO + 2 SO2
  2. Reduction with Carbon: The ZnO is mixed with coke and heated in a retort (or vertical kiln) where carbon reduces the oxide: ZnO + C → Zn + CO Zinc vapor is distilled off and condensed.
  3. Electrolytic Refining (optional): For high‑purity zinc, the impure metal is dissolved in sulfuric acid and electrolyzed; pure Zn deposits on the cathode (Zn^{2+} + 2e^- → Zn).

Properties

  • Reaction with Air: Forms a protective layer of zinc oxide: 2 Zn + O2 → 2 ZnO
  • Reaction with Acids: Reacts readily with dilute acids to produce hydrogen: Zn + 2 HCl → ZnCl2 + H2
  • Reaction with Alkali: Amphoteric nature; reacts with concentrated NaOH to give sodium zincate: Zn + 2 NaOH + 2 H2O → Na2[Zn(OH)4] + H2
  • Displacement Reactions: Zinc displaces copper from its salts: Zn + CuSO4 → ZnSO4 + Cu

Uses

  • Galvanizing iron and steel to prevent rust.
  • Alloying with copper to make brass.
  • Dry‑cell batteries (as the anode).
  • Die‑casting, roofing, and pharmaceuticals (zinc oxide ointment).

White Vitriol (Zinc Sulfate Heptahydrate – ZnSO4·7H2O)

Preparation: React zinc oxide or zinc metal with dilute sulfuric acid: ZnO + H2SO4 → ZnSO4 + H2O Crystallize the solution to obtain the heptahydrate.

Properties: Colourless crystalline solid; soluble in water giving a neutral solution; loses water on heating to form anhydrous ZnSO4.

Uses:

  • Mordant in dyeing and printing.
  • Supplement in agriculture (zinc fertilizer).
  • Electrolyte in zinc‑based batteries.
  • Precursor for lithopone pigment.

7.3 Mercury (Hg)

Occurrence

Mercury is found almost exclusively as the sulfide mineral cinnabar (HgS).

Extraction

The ore is roasted in a stream of air; mercury sulfide decomposes to metallic mercury vapor and sulfur dioxide: HgS + O2 → Hg + SO2 The mercury vapour is condensed in cooled receivers to give liquid mercury.

Properties

  • Only metal that is liquid at room temperature (‑38.8 °C melting point, 356.7 °C boiling point).
  • Forms amalgams with many metals (e.g., Na‑Hg, Au‑Hg).
  • Reaction with nitric acid (concentrated): Hg + 2 HNO3 → Hg(NO3)2 + NO2 + H2O Dilute nitric acid gives mercurous nitrate: 2 Hg + 2 HNO3 → Hg2(NO3)2 + H2
  • High surface tension and uniform volumetric expansion make it suitable for thermometers and barometers.

Calomel (Mercury(I) Chloride – Hg2Cl2)

Preparation: Reaction of mercury metal with hydrochloric acid in the presence of oxygen, or by disproportionation of mercuric chloride: 2 Hg + 2 HCl → Hg2Cl2 + H2

Properties: White crystalline solid; poorly soluble in water; turns black on exposure to light due to formation of mercury.

Uses:

  • Antiseptic and fungicidal preparations (historically).
  • Reference electrode in electrochemistry (calomel electrode).

Corrosive Sublimate (Mercury(II) Chloride – HgCl2)

Preparation: Direct combination of mercury and chlorine gas, or by reacting mercury with hydrochloric acid and oxidizing agent: Hg + Cl2 → HgCl2

Properties: White crystalline solid; highly toxic; sublimes readily (hence the name). Soluble in ethanol and ether.

Uses:

  • Strong antiseptic and disinfectant.
  • Preservative for biological specimens.
  • Poison (historically used as a murder weapon).

7.4 Iron (Fe)

Occurrence

Iron is abundant in the Earth's crust, mainly as oxide and carbonate ores. Important ores include:

  • Hematite (Fe2O3) – reddish brown
  • Magnetite (Fe3O4) – black, magnetic
  • Siderite (FeCO3) – pale brown

Extraction – Blast Furnace

In a blast furnace, iron ore, coke (carbon) and limestone (flux) are

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