Q1. Write the names of the following

a. Alloy formed by sodium and mercury.

Answer: Sodium amalgam

Written as: Na(Hg)

Properties and Uses:

  • An alloy of sodium (highly reactive) with mercury (liquid metal)
  • Less reactive than pure sodium, safer to handle
  • Used as a reducing agent in organic chemistry
  • Historically used in the Castner-Kellner process for NaOH production
  • Also used in sodium vapor lamps

Note: Amalgams are alloys containing mercury. Other examples: dental amalgam (Hg with Ag, Sn, Cu), zinc amalgam.

b. Molecular formula of the common ore of aluminium.

Answer: Al₂O₃·2H₂O (Bauxite)

Bauxite Details:

  • Primary ore of aluminium
  • Hydrated aluminium oxide with variable water content
  • General formula: Al₂O₃·xH₂O where x = 1-3
  • Common impurities: Fe₂O₃ (red bauxite), SiO₂, TiO₂
  • Major producers: Australia, China, Guinea, Brazil

Other aluminium ores:

  • Cryolite: Na₃AlF₆ (used in Hall-Héroult process)
  • Corundum: Al₂O₃ (gem quality: ruby, sapphire)

c. An oxide that reacts with both acids and bases to form salt and water.

Answer: Amphoteric oxide

Example: Al₂O₃ (Aluminium oxide)

Reactions of Al₂O₃:

With acid (acts as base):

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

With base (acts as acid):

Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O

(Sodium aluminate)

Other amphoteric oxides:

  • ZnO (Zinc oxide): ZnO + 2HCl → ZnCl₂ + H₂O
  • PbO (Lead(II) oxide)
  • SnO (Tin(II) oxide)
  • Cr₂O₃ (Chromium(III) oxide)

Importance in metallurgy: Amphoteric nature of Al₂O₃ is exploited in Bayer's process for alumina purification.

d. The apparatus used for crushing an ore.

Answer: Ball mill

[Diagram of ball mill showing rotating cylinder with steel balls]

Fig: Ball mill - cylindrical device containing grinding media (steel balls)

How it works:

  1. Cylindrical shell rotates on horizontal axis
  2. Contains grinding media (steel balls, ceramic balls, or flint pebbles)
  3. As cylinder rotates, balls cascade and impact the ore
  4. Produces fine powder through impact and attrition

Other crushing/grinding equipment:

  • Jaw crusher: Primary crushing (large rocks to smaller pieces)
  • Gyratory crusher: Similar to jaw but conical
  • Rod mill: Uses steel rods instead of balls
  • Stamp mill: Historical method using heavy stamps

Purpose: Increase surface area for better chemical reaction during extraction.

e. A non-metal that conducts electricity.

Answer: Graphite

Chemical formula: C (Allotrope of carbon)

[Diagram showing graphite layered structure with delocalized electrons]

Fig: Graphite structure - hexagonal layers with free electrons between layers

Why graphite conducts electricity:

  1. Layered structure with hexagonal rings of carbon atoms
  2. Each carbon forms 3 sigma bonds (sp² hybridization)
  3. Fourth electron of each carbon is delocalized between layers
  4. These delocalized electrons can move freely, allowing conduction
  5. Conductivity is anisotropic: Good along layers, poor perpendicular

Applications:

  • Electrodes in electrolysis (inert, conductive)
  • Pencil "lead" (graphite-clay mixture)
  • Lubricant (slippery layers)
  • Moderator in nuclear reactors

Other conducting non-metals (under specific conditions):

  • Silicon (semiconductor, not at room temp without doping)
  • Iodine (when pressurized)
  • Some organic polymers (doped polyacetylene, etc.)

f. The chemical reagent capable of dissolving noble metals.

Answer: Aqua regia

Composition: 1:3 mixture of concentrated nitric acid and hydrochloric acid

HNO₃ : HCl = 1 : 3 (by volume)

Chemical reactions in aqua regia:

Initial reactions:

HNO₃ + 3HCl → NOCl + 2H₂O + 2[Cl]

(Nitrosyl chloride and chlorine)

Dissolving gold:

Au + 3HNO₃ + 4HCl → HAuCl₄ + 3NO₂ + 3H₂O

Or: Au + NOCl + HCl + H₂O → HAuCl₄ + NO

(Chloroauric acid - soluble complex)

Metals dissolved by aqua regia:

  • Gold (Au) - "Royal water" name from ability to dissolve "royal" metal
  • Platinum (Pt) - Forms chloroplatinic acid H₂PtCl₆
  • Palladium (Pd)
  • Some other platinum group metals

Why it works: Chlorine atoms and NOCl oxidize metals to form soluble chloride complexes.

Historical note: Discovered by alchemists; name means "royal water" in Latin.

Q4. Explain the following terms

a. Metallurgy

Definition: Metallurgy is the science and technology of extracting metals from their ores, refining them, and preparing them for use.

Three main branches:

  1. Extractive Metallurgy: Extraction of metals from ores
    • Pyrometallurgy (using heat)
    • Hydrometallurgy (using aqueous solutions)
    • Electrometallurgy (using electricity)
  2. Physical Metallurgy: Study of structure and properties of metals
  3. Mechanical Metallurgy: Metal forming and fabrication processes

Stages of extractive metallurgy:

  1. Ore concentration (removal of gangue)
  2. Extraction of crude metal (reduction)
  3. Refining (purification)

Importance: Forms basis of modern industry - construction, transportation, electronics, etc.

b. Ores

Definition: Ores are naturally occurring mineral deposits from which metals can be extracted economically and conveniently.

Key characteristics:

  • Contain metal in sufficient quantity to make extraction profitable
  • Located in accessible regions (mining feasible)
  • Can be processed with available technology
  • Contain gangue (unwanted materials) that must be removed

Types of ores based on mineral composition:

Mineral Type Example Metal
Oxide ores Haematite (Fe₂O₃) Iron
Sulphide ores Galena (PbS) Lead
Carbonate ores Calamine (ZnCO₃) Zinc
Halide ores Rock salt (NaCl) Sodium
Native ores Gold nuggets Gold

Note: Not all minerals containing metals are ores - only those economically viable to process.

c. Minerals

Definition: Minerals are naturally occurring, inorganic, crystalline solids with definite chemical composition and ordered atomic arrangement.

Characteristics:

  • Formed by geological processes
  • Have specific physical properties (hardness, cleavage, luster, color)
  • Definite chemical formula (e.g., Quartz: SiO₂)
  • Ordered crystal structure
  • Inorganic origin (with few exceptions like amber)

Difference from ores:

Aspect Mineral Ore
Definition Naturally occurring inorganic compound Mineral deposit economically viable for metal extraction
Metal content May or may not contain metal Must contain sufficient metal
Purpose Scientific/geological interest Commercial extraction
Examples Quartz, Feldspar, Mica Bauxite, Haematite, Galena

Important mineral groups:

  • Silicates (most abundant: quartz, feldspar)
  • Carbonates (calcite, dolomite)
  • Oxides (haematite, magnetite)
  • Sulphides (pyrite, galena)
  • Native elements (gold, silver, copper, diamond)

d. Gangue

Definition: Gangue (pronounced "gang") refers to the commercially worthless material that surrounds, or is closely mixed with, the wanted mineral in an ore deposit.

[Diagram showing ore with valuable mineral and gangue matrix]

Fig: Ore deposit showing valuable mineral particles embedded in gangue matrix

Common gangue materials:

  • Silica (SiO₂): Most common gangue (quartz, sand)
  • Clay minerals: Aluminium silicates
  • Limestone (CaCO₃): In some iron ores
  • Feldspar, mica: In various ores
  • Rock fragments: Various silicate rocks

Problems caused by gangue:

  1. Increases bulk (transportation costs)
  2. Dilutes metal content
  3. May interfere with extraction processes
  4. May form slag that needs separate processing
  5. Can consume reagents unnecessarily

Removal methods (ore concentration/beneficiation):

  • Gravity separation (difference in density)
  • Magnetic separation (for magnetic minerals)
  • Froth flotation (for sulphide ores)
  • Leaching (chemical dissolution)
  • Hand picking (for large gangue pieces)

Note: Sometimes gangue minerals can be commercially valuable themselves (e.g., silica for glass, limestone for cement).

Q5. Give scientific reasons

a. Lemon or tamarind is used to clean copper vessels that have turned green.

Scientific Explanation:

Chemical Process:

Step 1: Green coating formation (patina)

2Cu + H₂O + CO₂ + O₂ → CuCO₃·Cu(OH)₂

Basic copper carbonate (malachite) - green coating

Or alternatively:

Cu + H₂O + CO₂ + O₂ → Cu₂(OH)₂CO₃

Same compound, different representation

Step 2: Cleaning action

Lemon/tamarind contain citric acid (C₆H₈O₇):

Cu₂(OH)₂CO₃ + 2C₆H₈O₇ → 2Cu(C₆H₇O₇) + CO₂ + 3H₂O

Copper citrate (soluble complex) + carbon dioxide + water

Detailed mechanism:

  1. Formation of patina: Copper reacts with atmospheric oxygen, carbon dioxide, and moisture to form basic copper carbonate (verdigris).
  2. Acidic nature of lemon/tamarind: Both contain organic acids:
    • Lemon: Citric acid (5-8%), ascorbic acid (vitamin C)
    • Tamarind: Tartaric acid, citric acid, malic acid
  3. Chemical reaction: Acid reacts with basic copper carbonate to form soluble copper salts and complexes.
  4. Physical removal: Rubbing helps mechanically remove the loosened coating.
  5. Restoration: Clean copper surface is exposed, restoring shine.

Why these specific agents?

  • Safe: Food-grade acids, non-toxic
  • Effective: Strong enough to dissolve coating but not damage copper
  • Available: Common household items
  • Abrasive action: Natural fibers in tamarind help scrub

Alternative cleaning agents: Vinegar (acetic acid), buttermilk (lactic acid), tomato sauce, commercial copper cleaners (often contain phosphoric or oxalic acid).

b. Ionic compounds generally have high melting points.

Scientific Explanation:

[Diagram showing ionic lattice with strong electrostatic forces]

Fig: Ionic crystal lattice showing alternating positive and negative ions

Key factors:

  1. Strong electrostatic forces:
    • Ionic compounds consist of positive cations and negative anions
    • These are held together by strong Coulombic (electrostatic) forces
    • Force ∝ (q₁ × q₂)/r² (product of charges divided by square of distance)
  2. Lattice energy:
    • Energy released when gaseous ions form solid crystal
    • Higher lattice energy = stronger bonds = higher melting point
    • Lattice energy increases with:
      • Higher charge on ions (NaCl vs MgO)
      • Smaller ionic radii (LiF vs CsI)
  3. Three-dimensional network:
    • Each ion is surrounded by several oppositely charged ions
    • Creates extensive network of strong bonds
    • Requires breaking many bonds simultaneously to melt

Examples and comparison:

Compound Ions Melting Point (°C) Reason
NaCl Na⁺, Cl⁻ 801 Moderate charges, moderate sizes
MgO Mg²⁺, O²⁻ 2852 Higher charges (2+ and 2-), strong attraction
LiF Li⁺, F⁻ 845 Small ions, close approach
CsI Cs⁺, I⁻ 621 Large ions, weaker attraction

Contrast with covalent compounds:

  • Covalent molecular: Weak intermolecular forces (e.g., CO₂ m.p. -78°C)
  • Covalent network: Strong covalent bonds throughout (e.g., diamond m.p. >3550°C)
  • Metallic: Metal cations in electron sea (variable m.p.)

Importance in metallurgy: High melting points affect extraction methods (need high temperatures for reduction).

c. Sodium is always stored in kerosene.

Scientific Explanation:

Reactions of sodium with air components:

With oxygen:

4Na + O₂ → 2Na₂O (sodium oxide)
2Na + O₂ → Na₂O₂ (sodium peroxide, with excess O₂)

With moisture/water:

2Na + 2H₂O → 2NaOH + H₂ + Heat

Vigorous, exothermic reaction, hydrogen gas produced (flammable!)

With carbon dioxide:

4Na + CO₂ → 2Na₂O + C

Why kerosene specifically?

  1. Non-reactive: Kerosene doesn't react with sodium
    • Paraffinic hydrocarbon mixture
    • No functional groups that react with alkali metals
  2. Density: Kerosene density (~0.8 g/mL) < Sodium density (0.97 g/mL)
    • Sodium sinks in kerosene (unlike in water where it floats and reacts)
    • Complete immersion prevents air contact
  3. Low volatility: Doesn't evaporate quickly like ether
  4. Availability and cost: Cheap, easily available
  5. Safety: Higher flash point than gasoline (less flammable)

Alternative storage methods:

  • Mineral oil: Similar properties to kerosene
  • Argon atmosphere: In laboratories, under inert gas
  • Vacuum sealing: For long-term storage
  • Historical: Naphtha was used before kerosene

Other alkali metals storage:

  • Lithium: Less reactive, can be stored in paraffin oil
  • Potassium: More reactive than sodium, also stored in kerosene
  • Rubidium & Cesium: Extremely reactive, stored in sealed glass ampules

Safety precautions:

  • Never use water to extinguish sodium fires (use dry sand, class D fire extinguishers)
  • Handle with forceps, never bare hands (reacts with skin moisture)
  • Cut under kerosene/mineral oil to prevent oxidation

d. Pine oil is used in the froth flotation process.

Scientific Explanation:

[Diagram showing froth flotation process with pine oil creating froth]

Fig: Froth flotation cell showing ore particles attaching to air bubbles via pine oil

Froth Flotation Process Overview:

  1. Crushed ore mixed with water to form slurry
  2. Additives: Collectors, frothers, modifiers, depressants
  3. Air blown through mixture creating bubbles
  4. Hydrophobic ore particles attach to bubbles
  5. Bubbles rise to form froth at surface
  6. Froth skimmed off, concentrated ore recovered

Role of Pine Oil (Frother):

Function How Pine Oil Achieves It
Stabilizes bubbles Reduces surface tension of water, allowing stable bubble formation
Creates persistent froth Forms elastic film around bubbles preventing coalescence
Selective action Doesn't make gangue particles hydrophobic (unlike collectors)
Optimal bubble size Creates bubbles of appropriate size for ore attachment
Hydrophobic nature Organic compounds in pine oil repel water

Chemical composition of pine oil:

  • Main component: α-Terpineol (C₁₀H₁₈O)
  • Other terpenes: Borneol, fenchyl alcohol
  • Natural product from pine tree distillation
  • Amphipathic molecules (hydrophobic hydrocarbon tail, polar -OH head)

Mechanism at molecular level:

  1. Pine oil molecules adsorb at air-water interface
  2. Hydrophobic tails point toward air, hydrophilic heads toward water
  3. Reduces surface tension from ~72 mN/m (pure water) to ~40 mN/m
  4. Creates stable foam lamellae between bubbles
  5. Ore particles (rendered hydrophobic by collectors like xanthates) attach to bubble surfaces

Alternative frothers:

  • Cresylic acid (mixture of cresols)
  • Polyglycol ethers (synthetic)
  • MIBC (Methyl isobutyl carbinol)
  • DF-250 (commercial frother)

Ores processed by froth flotation: Mainly sulphide ores (Cu, Pb, Zn, Ni), also potash, phosphates, coal.

e. Anodes are replaced periodically during the electrolysis of alumina.

Scientific Explanation:

[Diagram of Hall-Héroult cell showing carbon anode consumption]

Fig: Electrolytic cell for aluminium production showing anode reaction and consumption

Hall-Héroult Process for Aluminium Extraction:

  • Electrolyte: Molten cryolite (Na₃AlF₆) + Alumina (Al₂O₃)
  • Temperature: ~950°C
  • Cathode: Carbon lining of cell
  • Anode: Carbon blocks (graphite)

Anode reactions:

Primary reaction (desired):

2O²⁻ (from Al₂O₃) → O₂ + 4e⁻

But oxygen reacts with carbon anode:

C + O₂ → CO₂

Or at higher temperatures: 2C + O₂ → 2CO

Overall anode reaction:

C + 2O²⁻ → CO₂ + 4e⁻

Carbon is consumed in the process!

Why anodes need replacement:

  1. Consumption by reaction:
    • Theoretical: 0.33 kg C per kg Al produced
    • Actual: 0.4-0.5 kg C per kg Al (some side reactions)
    • Anodes literally burn away producing CO₂/CO
  2. Mechanical wear:
    • Thermal stress from high temperature
    • Physical erosion from bubbling gases
    • Electrochemical erosion
  3. Impurity accumulation:
    • Ash from anode materials accumulates
    • Electrolyte salts may deposit
    • Reduces electrical conductivity
  4. Shape deterioration:
    • Uneven consumption leads to irregular shape
    • Affects current distribution
    • Causes inefficient operation

Anode replacement cycle:

  • Prebaked anodes: Replaced every 20-28 days typically
  • Söderberg anodes: Continuously added as paste, baked in place
  • Replacement is major operational task in aluminium smelters

Environmental concern: Anode consumption produces greenhouse gases:

  • CO₂: ~1.5 kg per kg Al (from anode + electricity generation)
  • Perfluorocarbons (PFCs) during "anode effects"

Research alternatives: Inert anodes (ceramic/metal alloys) that don't consume, but not yet commercial.

Q6. When a copper coin is immersed in silver nitrate solution, it begins to shine after some time. Why does this occur? Write the chemical equation.

Displacement Reaction:

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

Ionic form:

Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s)

Detailed Explanation:

[Diagram showing displacement reaction on copper coin]

Fig: Copper coin in AgNO₃ solution with silver deposition

Step-by-step process:

  1. Electrochemical series consideration:
    • Copper is above silver in reactivity series
    • Cu → Cu²⁺ + 2e⁻ (E° = +0.34V oxidation potential)
    • Ag⁺ + e⁻ → Ag (E° = +0.80V reduction potential)
    • Overall E°cell = 0.34 + 0.80 = 1.14V (spontaneous)
  2. At the copper surface:
    • Copper atoms lose electrons: Cu → Cu²⁺ + 2e⁻
    • Copper ions go into solution
    • Solution turns blue (characteristic Cu²⁺ color)
  3. Silver deposition:
    • Silver ions in solution gain electrons: Ag⁺ + e⁻ → Ag
    • Silver atoms deposit on copper surface
    • Forms thin, shiny silver layer
  4. Visual changes:
    • Coin develops silvery shine (silver coating)
    • Solution color changes from colorless to blue (Cu²⁺ formation)
    • Possible formation of silver crystals (dendrites) if concentrated

Observations in detail:

  • Initial: Clean copper coin in colorless AgNO₃ solution
  • After few minutes: Coin develops silvery patches
  • After longer time: Complete silver coating, solution turns blue
  • Possible secondary reaction: If chloride ions present, white AgCl precipitate may form

Practical applications of this principle:

  • Electroplating: Silver plating of cheaper metals
  • Displacement plating: Without external current (electroless plating)
  • Recovery of precious metals: From solution using more reactive metals
  • Teaching tool: Demonstrates reactivity series

Related displacement reactions:

Fe + CuSO₄ → FeSO₄ + Cu (iron in copper sulfate)
Zn + CuSO₄ → ZnSO₄ + Cu (zinc in copper sulfate)
Mg + 2AgNO₃ → Mg(NO₃)₂ + 2Ag (magnesium would react even faster)

Q7. The electronic configuration of metal A is 2,8,1 and metal B is 2,8,8,2.

(a) Identification of metals:

  • Metal A: Sodium (Na) - Atomic number 11
  • Metal B: Calcium (Ca) - Atomic number 20

(a) Which metal is more reactive?

Answer: Sodium (A) is more reactive than Calcium (B)

Reason:

  1. Ionization energy:
    • Sodium: First IE = 496 kJ/mol (loses 1 electron easily)
    • Calcium: First IE = 590 kJ/mol, Second IE = 1145 kJ/mol
    • Calcium needs to lose 2 electrons for compound formation
  2. Electron configuration:
    • Na: [Ne]3s¹ - single valence electron far from nucleus
    • Ca: [Ar]4s² - two valence electrons, but higher nuclear charge
  3. Atomic radius:
    • Na atomic radius: 186 pm
    • Ca atomic radius: 197 pm (slightly larger but other factors dominate)
  4. Standard electrode potential:
    • Na⁺/Na: E° = -2.71 V (strong reducing agent)
    • Ca²⁺/Ca: E° = -2.87 V (even stronger theoretically, but kinetics slower)
    • In practice, sodium reacts more vigorously with water

(b) Write their reactions with dilute hydrochloric acid.

Sodium (A) with dilute HCl:

2Na(s) + 2HCl(aq) → 2NaCl(aq) + H₂(g)

Observations:

  • Vigorous reaction (may be explosive with concentrated acid!)
  • Fizzing/bubbling (hydrogen gas evolution)
  • Heat generation (exothermic)
  • Solution becomes basic if excess sodium (forms NaOH with water)
  • Safety: Reaction with acids is extremely dangerous for sodium!

Calcium (B) with dilute HCl:

Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)

Observations:

  • Moderately vigorous reaction
  • Steady bubbling of hydrogen
  • Calcium dissolves
  • Less heat generated than with sodium
  • Solution remains acidic unless excess calcium

Comparison of reactivity:

Aspect Sodium (Na) Calcium (Ca)
Reaction vigor Very vigorous (dangerous) Moderately vigorous
Heat produced High (may ignite H₂) Moderate
Gas evolution rate Rapid fizzing Steady bubbling
Safety concern Extreme (explosive potential) Moderate (handle with care)
Practical demonstration Not recommended for labs Can be demonstrated with precautions

General metal-acid reaction pattern:

Metal + Acid → Salt + Hydrogen gas

Condition: Metal must be above hydrogen in reactivity series.

Exceptions: Nitric acid (HNO₃) doesn't produce H₂ (oxidizing acid), metals like Cu, Ag, Au don't react with dilute HCl.

Q8. Write balanced chemical equations for the following reactions

a. Aluminium reacts with air.

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

Details:

  • Forms protective oxide layer (5-10 nm thick)
  • Prevents further oxidation (passivation)
  • Reaction is highly exothermic (used in thermite)
  • Fine aluminium powder can be explosive in air

With moist air: Also forms hydroxide

2Al + 6H₂O + 3O₂ → 2Al(OH)₃

b. Iron filings are added to copper sulphate solution.

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Ionic form:

Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)

Observations:

  • Blue solution (Cu²⁺) turns pale green (Fe²⁺)
  • Brown copper deposits on iron
  • Iron dissolves gradually
  • Demonstrates iron is more reactive than copper

Applications: Copper recovery, cementation process

c. Reaction between ferric oxide and aluminium.

Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + Heat

Commonly written as:

2Al + Fe₂O₃ → Al₂O₃ + 2Fe

Details:

  • Thermite reaction - highly exothermic
  • Temperature reaches ~2500°C
  • Molten iron produced
  • Used for welding railway tracks, incendiary devices
  • Aluminium reduces iron oxide (aluminium more reactive)

Initiation: Requires high temperature to start (Mg ribbon fuse)

d. Electrolysis of alumina.

Overall reaction:

2Al₂O₃(l) → 4Al(l) + 3O₂(g)

Electrode reactions (Hall-Héroult process):

Cathode (reduction):

Al³⁺ + 3e⁻ → Al(l)

Anode (oxidation):

2O²⁻ → O₂(g) + 4e⁻

But oxygen reacts with carbon anode:

C + O₂ → CO₂

Conditions: Molten cryolite (Na₃AlF₆), ~950°C, carbon electrodes

e. Zinc oxide reacts with dilute hydrochloric acid.

ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)

Details:

  • Zinc oxide is amphoteric (reacts with both acids and bases)
  • White solid dissolves in acid
  • Forms zinc chloride solution
  • Reaction is exothermic

With base (showing amphoteric nature):

ZnO + 2NaOH → Na₂ZnO₂ + H₂O

(Sodium zincate)

Uses of ZnO: Sunscreen, pigment, rubber additive, medicinal ointments

Q9. Complete the following statements related to extraction of aluminium

a. Impurities present in bauxite.

Answer: The main impurities in bauxite are:

  1. Silica (SiO₂): As quartz or clay minerals
  2. Ferric oxide (Fe₂O₃): Gives red color to bauxite
  3. Titanium oxide (TiO₂): As rutile or ilmenite
  4. Other minor impurities: Calcium oxide, magnesium oxide, organic matter

Problems caused by impurities:

  • Silica: Forms sodium aluminium silicate (waste)
  • Iron oxide: Consumes NaOH, forms red mud
  • Titanium oxide: Inert, accumulates in red mud

Red mud: Waste product (1-1.5 tons per ton of Al), environmental concern, contains Fe₂O₃, SiO₂, TiO₂, Na₂O.

b. Use of leaching in ore concentration.

Answer: Leaching is used in Bayer's process to selectively dissolve aluminium oxide from bauxite while leaving impurities behind.

Bayer's Process Steps:

  1. Digestion: Bauxite + concentrated NaOH at 150-200°C under pressure
    Al₂O₃·xH₂O + 2NaOH → 2NaAlO₂ + (x+1)H₂O

    (Sodium aluminate formed, impurities don't dissolve)

  2. Filtration: Separate sodium aluminate solution from red mud (impurities)
  3. Precipitation: Dilute, cool, seed with Al(OH)₃ crystals
    NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH
  4. Calcination: Heat Al(OH)₃ to get pure Al₂O₃
    2Al(OH)₃ → Al₂O₃ + 3H₂O

Advantages of leaching: Selective dissolution, high purity product, works for low-grade ores.

c. Chemical reaction converting bauxite to alumina by Hall's process.

Note: Actually Hall's process refers to the electrolytic reduction of alumina to aluminium. Bayer's process converts bauxite to alumina.

Hall-Héroult Process (Electrolysis):

2Al₂O₃(l) → 4Al(l) + 3O₂(g) (overall)

Bayer's Process (Purification):

Al₂O₃·xH₂O + 2NaOH → 2NaAlO₂ + (x+1)H₂O (digestion)
NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH (precipitation)
2Al(OH)₃ → Al₂O₃ + 3H₂O (calcination)

d. Heating aluminium ore with concentrated sodium hydroxide.

Answer: Aluminium oxide (from bauxite) reacts with concentrated NaOH to form soluble sodium aluminate.

Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂(aq) + H₂O(l)

(For hydrated bauxite: Al₂O₃·xH₂O + 2NaOH → 2NaAlO₂ + (x+1)H₂O)

Why this reaction occurs:

  • Al₂O₃ is amphoteric (reacts with both acids and bases)
  • With base: Acts as acidic oxide
  • Forms aluminate ion AlO₂⁻ (tetrahedral coordination)
  • Impurities like Fe₂O₃ don't react (basic oxide)

Conditions: Concentrated NaOH (~50%), 150-200°C, pressure.

Reverse reaction (precipitation): When solution diluted and cooled:

NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH

Q10. Classify the metals Cu, Zn, Ca, Mg, Fe, Na, and Li into the following groups

a. Highly reactive metals: Na, Li, Ca

Characteristics:

  • React vigorously with cold water
  • Stored under oil to prevent air reaction
  • Extracted by electrolysis of molten compounds
  • Strong reducing agents
  • Form stable oxides/hydroxides

Order of reactivity: Li > Ca > Na (though Li less vigorous than Na due to kinetics)

b. Moderately reactive metals: Mg, Zn, Fe

Characteristics:

  • React with steam or hot water (not cold water)
  • React with dilute acids to produce hydrogen
  • Extracted by reduction of oxides with carbon/CO
  • Form protective oxide layers (except iron rusts)
  • Used in construction, alloys

Order of reactivity: Mg > Zn > Fe

Note: Magnesium reacts with hot water, zinc with steam, iron requires red-hot steam.

c. Less reactive metals: Cu

Characteristics:

  • Do not react with water or steam
  • Do not react with dilute acids (except oxidizing acids)
  • Found in native state sometimes
  • Extracted by roasting/reduction of sulphides/oxides
  • Good conductors, corrosion resistant

Additional less reactive metals: Ag, Au, Pt (noble metals)

Complete Reactivity Series (for reference):

Reactivity Metals Extraction Method Reaction with Water Reaction with Dilute Acid
Most reactive K, Na, Li, Ca Electrolysis Cold water vigorously Explosive (avoid!)
Very reactive Mg, Al Electrolysis/thermite Steam/hot water Vigorous
Moderately reactive Zn, Fe, Sn, Pb Reduction with C/CO Steam when hot Moderate to slow
Less reactive Cu, Hg, Ag Roasting/reduction No reaction No reaction (except oxidizing acids)
Least reactive Au, Pt Native/washing No reaction No reaction (except aqua regia)

Metallurgy: Key Processes Summary

Process Purpose Example Key Reaction/Principle
Froth Flotation Concentrate sulphide ores Cu from CuFeS₂ Pine oil makes ore hydrophobic, attaches to bubbles
Leaching (Bayer's) Purify bauxite (Al ore) Al from bauxite Al₂O₃ + 2NaOH → 2NaAlO₂ (amphoteric nature)
Calcination Remove water/CO₂ from ores Zn from ZnCO₃ ZnCO₃ → ZnO + CO₂ (thermal decomposition)
Roasting Convert sulphide to oxide Pb from PbS 2PbS + 3O₂ → 2PbO + 2SO₂ (oxidation)
Smelting Reduce oxide to metal Fe from Fe₂O₃ Fe₂O₃ + 3CO → 2Fe + 3CO₂ (reduction)
Electrorefining Purify crude metal Cu purification Anode: Cu → Cu²⁺ + 2e⁻; Cathode: Cu²⁺ + 2e⁻ → Cu
Hall-Héroult Extract Al from Al₂O₃ Al production 2Al₂O₃ → 4Al + 3O₂ (electrolysis in cryolite)