Write short notes on any two. i) Chemistry of rusting theory of iron ii) Extraction of blister copper from copper pyrites iii) Lewis concept of acid and base iv) Laboratory preparation of…

4Fe+3O₂+6H₂OOxygen, moisture, CO₂ (catalyst)4Fe(OH)₃

i) Chemistry of Rusting Theory of Iron

Definition and Composition of Rust

Rust is a reddish-brown, porous, and non-uniform coating formed on the surface of iron when it is exposed to moisture, oxygen, and carbon dioxide for a prolonged period. Chemically, rust is primarily hydrated iron(III) oxide (Fe₂O₃·xH₂O) or iron(III) hydroxide (Fe(OH)₃). The exact composition varies depending on environmental conditions, but the general formula can be represented as Fe₂O₃·nH₂O.

Carbon dioxideCarbon dioxide

Mechanism of Rusting (Electrochemical Theory)

Rusting is an electrochemical process involving oxidation-reduction (redox) reactions at the surface of iron. It occurs in the presence of electrolytes (moisture, acids, or salts) and follows these key steps:

1. Formation of Galvanic Cell

   • Impurities (like carbon, sulfur, or other metals) in iron act as cathodes, while pure iron acts as the anode.
   • A thin film of water (containing dissolved oxygen and salts) acts as an electrolyte, allowing electron flow.

2. Anodic Reaction (Oxidation)

   • At the anode (iron surface), iron loses electrons and forms Fe²⁺ ions: $$\mathrm{Fe}\to \mathrm{Fe}{}^{2+}+2\mathrm{e}{}^{-}$$
   • The Fe²⁺ ions then react with hydroxide ions (from water) to form Fe(OH)₂: $$\mathrm{Fe}{}^{2+}+2\mathrm{OH}{}^{-}\to \mathrm{Fe}(\mathrm{OH}){}_2$$

3. Cathodic Reaction (Reduction)

   • At the cathode (impurity site), oxygen from air is reduced in the presence of water and protons (H⁺ from dissolved CO₂): $$\mathrm{O}{}_2+2\mathrm{H}{}_2\mathrm{O}+4\mathrm{e}{}^{-}\to 4\mathrm{OH}{}^{-}$$
   • Alternatively, if CO₂ is present, it forms carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻, further aiding the reaction.

4. Final Formation of Rust

   • The Fe(OH)₂ produced at the anode reacts with oxygen to form Fe(OH)₃, which then dehydrates to Fe₂O₃·xH₂O (rust): $$4\mathrm{Fe}(\mathrm{OH}){}_2+\mathrm{O}{}_2+2\mathrm{H}{}_2\mathrm{O}\to 4\mathrm{Fe}(\mathrm{OH}){}_3$$ $$2\mathrm{Fe}(\mathrm{OH}){}_3\to \mathrm{Fe}{}_2\mathrm{O}{}_3\cdot x\mathrm{H}{}_2\mathrm{O}+(3\text{-}\mathrm{x})\mathrm{H}{}_2\mathrm{O}$$

Factors Affecting Rusting

• Presence of Water (Moisture): Rusting occurs faster in humid conditions because water acts as an electrolyte.
• Presence of Oxygen: Rusting is an oxidation process; higher oxygen levels accelerate it.
• Presence of Electrolytes (Salts, Acids): Impurities like NaCl (salt) or H₂SO₄ (acid) increase conductivity, speeding up electron transfer.
• pH of the Environment: Acidic conditions (low pH) enhance rusting due to increased H⁺ ion concentration.
• Temperature: Higher temperatures increase the rate of rusting by providing more kinetic energy to reactants.

Prevention of Rusting

To prevent rusting, the following methods are used:

• Barrier Protection: Painting, greasing, or coating iron with substances like zinc (galvanization) or plastic.
• Sacrificial Protection: Using a more reactive metal (e.g., zinc or magnesium) that corrodes instead of iron (e.g., in ships).
• Alloying: Adding chromium (stainless steel) or nickel to form corrosion-resistant alloys.
• Cathodic Protection: Using an external power source to make iron the cathode, preventing oxidation.

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ii) Extraction of Blister Copper from Copper Pyrites

Composition of Copper Pyrites

Copper pyrites (or chalcopyrite) is the primary ore of copper, with the chemical formula CuFeS₂. It contains copper (Cu), iron (Fe), and sulfur (S). The extraction process involves several steps to obtain blister copper (98% pure copper).

Steps in Extraction

1. Concentration of Ore (Froth Floatation Process)

   • Copper pyrites is concentrated by froth floatation to remove impurities like silica, iron pyrites, and gangue.
   • The ore is crushed and mixed with pine oil and water, and air is blown through it.
   • The CuFeS₂ particles attach to air bubbles and float to the surface as froth, while impurities sink.

2. Roasting of Concentrated Ore

   • The concentrated ore is roasted in the presence of excess air to convert it into copper matte (Cu₂S and FeS) and remove sulfur as SO₂: $$2\mathrm{CuFeS}{}_2+\mathrm{O}{}_2\to \mathrm{Cu}{}_2\mathrm{S}+2\mathrm{FeS}+\mathrm{SO}{}_2$$
   • Roasting also oxidizes some iron to FeO (iron(II) oxide), which reacts with silica (SiO₂) to form slag (FeSiO₃): $$2\mathrm{FeO}+\mathrm{SiO}{}_2\to 2\mathrm{FeSiO}{}_3$$

3. Smelting (Formation of Copper Matte)

   • The roasted ore is heated in a reverberatory furnace with silica (SiO₂) to remove iron as slag: $$2\mathrm{FeS}+3\mathrm{O}{}_2\to 2\mathrm{FeO}+2\mathrm{SO}{}_2$$ $$\mathrm{FeO}+\mathrm{SiO}{}_2\to \mathrm{FeSiO}{}_3(\mathrm{slag})$$
   • The remaining Cu₂S (copper(I) sulfide) is called copper matte.

4. Self-Reduction (Conversion to Blister Copper)

   • Copper matte (Cu₂S) is heated in a blast furnace or converter with silica and air:
     • Partial oxidation of Cu₂S produces Cu₂O (copper(I) oxide), which then reacts with remaining Cu₂S to form copper (Cu): $$2\mathrm{Cu}{}_2\mathrm{S}+3\mathrm{O}{}_2\to 2\mathrm{Cu}{}_2\mathrm{O}+2\mathrm{SO}{}_2$$ $$\mathrm{Cu}{}_2\mathrm{S}+2\mathrm{Cu}{}_2\mathrm{O}\to 6\mathrm{Cu}+\mathrm{SO}{}_2$$
   • The molten copper obtained is 98–99% pure and contains impurities like sulfur, oxygen, and iron.
   • This impure copper is called blister copper due to the blistered surface caused by escaping SO₂ gas.

Properties of Blister Copper

• Appearance: Dark brown, porous surface with blisters.
• Purity: ~98–99% copper.
• Impurities: Sulfur (Cu₂S), oxygen (Cu₂O), and traces of iron and noble metals (Ag, Au).

Further Purification (Electrolytic Refining)

Blister copper is further purified by electrolytic refining to obtain 99.9% pure copper:

• Anode: Impure blister copper.
• Cathode: Pure copper sheet.
• Electrolyte: Copper sulfate (CuSO₄) + sulfuric acid (H₂SO₄).
• Result: Pure copper deposits at the cathode, while impurities settle as anode mud (containing Ag, Au, Pt).

Sulphuric acidSulphuric acid

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CuFeS₂+O₂Roasting (heating in air)Cu₂S+FeS+SO₂