Class 12 Chemistry Chapter 6 Notes: General Principles and Processes of Isolation of Elements | Important Questions & PYQs

 Class 12 Chemistry – Chapter 6 General Principles and Processes of Isolation of Elements Notes

1. Occurrence of Metals

• Metals occur in nature as minerals

• Ores are those minerals from which metals can be economically extracted

• All ores are minerals, but all minerals are not ores

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2. Concentration of Ores

Removal of gangue (earthy impurities) from ore.

Main Methods

1. Hydraulic washing – density difference

2. Magnetic separation – magnetic ores

   • Example: Fe₂O₃, Fe₃O₄

3. Froth flotation – sulphide ores

4. Leaching – chemical method

Role of Depressant

• Prevents one sulphide ore from frothing

• Example: NaCN separates PbS from ZnS

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3. Extraction of Crude Metal

After concentration, ore is converted to oxide, then reduced.

Calcination

• Heating in absence or limited air

• Used for carbonates & hydroxides

• Example:

$$ZnC{O}_3\to ZnO+C{O}_2$$

Roasting

• Heating in excess air

• Used for sulphide ores

• Example:

$$2ZnS+3O_2\to 2ZnO+2S{O}_2$$

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4. Thermodynamic Principles of Metallurgy

Ellingham Diagram

• Plot of ΔG° vs Temperature

• Lower line → more stable oxide

• A metal can reduce oxides of metals above it

Key Points

• Reduction feasible only if ΔG° is negative

• Choice of reducing agent depends on temperature

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5. Electrochemical Principles of Metallurgy

• Highly reactive metals (Na, K, Al, Mg) are extracted by electrolysis

• Reduction not possible by carbon

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6. Oxidation–Reduction in Metallurgy

• Extraction involves redox reactions

• Metal oxide reduced, reducing agent oxidised

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7. Refining of Metals

Removal of remaining impurities to get pure metal

Important Methods

(A) Electrolytic Refining

• Anode: impure metal

• Cathode: pure metal

• Impurities settle as anode mud

• Used for: Cu, Ag, Au, Zn

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(B) Zone Refining

• Based on higher solubility of impurities in molten metal

• Used for semiconductors

• Metals: Si, Ge, B

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(C) Vapour Phase Refining

Mond’s Process (Ni)

$$Ni+4CO\to Ni(CO{)}_4\to Ni+4CO$$

Van Arkel Method (Zr, Ti)

• Formation & decomposition of metal iodide

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8. Metallurgy of Important Metals

(A) Aluminium

• Ore: Bauxite

• Concentration: Leaching (Bayer’s process)

• Cryolite:

  • Lowers melting point

  • Increases conductivity

• Extracted by electrolysis

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(B) Copper

• Ore: Copper pyrites (CuFeS₂)

• Silica acts as acidic flux

• Impurities removed as slag (FeSiO₃)

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(C) Zinc

• Ore: Zinc blende (ZnS)

• Steps:

  1. Froth flotation

  2. Roasting → ZnO

  3. Reduction with coke

  4. Electrolytic refining

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(D) Iron

• Extracted in blast furnace

• Different reactions occur in different zones

• Cast iron vs Pig iron differ in carbon content

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9. Flux & Slag

• Flux removes impurities

• Slag = flux + impurity
  Example:

$$FeO+Si{O}_2\to FeSi{O}_3$$

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10. Chromatography

• Technique for separation of components

• Based on differential adsorption

• Types:

  • Column chromatography

  • Paper chromatography

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11. Important Definitions

• Minerals: Natural compounds of metals

• Ores: Minerals used for extraction

• Anode mud: Insoluble impurities

• Leaching: Chemical dissolution of ore

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12. Very Important Exam Points

✔ Ellingham diagram decides reducing agent
✔ Cryolite is essential in Al metallurgy
✔ CO poisons catalyst in Haber process
✔ Zn cannot be reduced by CO
✔ Zone refining → ultra-pure metals

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13. Uses of Metals

• Aluminium → aircraft, cables

• Copper → electrical wiring

• Zinc → galvanisation

• Iron → construction, machinery

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are minerals and ores?
   Minerals are naturally occurring compounds of metals, while ores are those minerals from which metals can be economically extracted.

2. Why are all minerals not ores?
   Because all minerals do not allow economical extraction of metals.

3. What is concentration of ore?
   It is the process of removing gangue (earthy impurities) from ore.

4. Name the method used for concentration of sulphide ores.
   Froth flotation method.

5. What is calcination?
   Heating of ore in absence or limited supply of air.

6. What is roasting?
   Heating of ore in excess of air.

7. What is Ellingham diagram?
   A graph of ΔG° versus temperature for formation of metal oxides.

8. Why is electrolysis used for extraction of aluminium?
   Because aluminium is highly reactive and cannot be reduced by carbon.

9. What is flux?
   A substance added to remove impurities during metallurgy.

10. What is slag?
    Product formed by combination of flux and impurity.

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📝 Long Answer Questions

1. Explain different methods of concentration of ores.

Concentration of ore is done to remove gangue.

Main methods:

• Hydraulic washing:
  Based on difference in densities of ore and gangue.

• Magnetic separation:
  Used when ore or impurity is magnetic (e.g., Fe₂O₃, Fe₃O₄).

• Froth flotation:
  Used for sulphide ores; ore particles float with froth.

• Leaching:
  Chemical method in which ore dissolves in suitable reagent.

Role of depressant:
Prevents one sulphide ore from forming froth.
Example: NaCN separates PbS from ZnS.

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2. Explain roasting and calcination with examples.

Calcination

• Heating in absence or limited air

• Used for carbonates and hydroxides

• Example:
  ZnCO₃ → ZnO + CO₂

Roasting

• Heating in excess of air

• Used for sulphide ores

• Example:
  2ZnS + 3O₂ → 2ZnO + 2SO₂

Both processes convert ores into oxides.

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3. Describe refining of metals.

Refining is the process of removing impurities from crude metal.

Important methods:

• Electrolytic refining

  • Anode: impure metal

  • Cathode: pure metal

  • Impurities collect as anode mud

  • Used for Cu, Ag, Au, Zn

• Zone refining

  • Based on higher solubility of impurities in molten metal

  • Used for Si, Ge, B

• Vapour phase refining

  • Mond’s process for Ni

  • Van Arkel method for Zr and Ti

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📝 PYQs (Previous Years’ Questions – CBSE Board Oriented)

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🔹 1 Mark PYQs

1. What are ores?

2. What is gangue?

3. Define calcination.

4. Define roasting.

5. What is flux?

6. What is slag?

7. Name the ore of aluminium.

8. Which process is used for concentration of sulphide ores?

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🔹 2 Mark PYQs

1. Differentiate between minerals and ores.

2. Why is concentration of ore necessary?

3. What is the role of depressant in froth flotation process?

4. Why is aluminium extracted by electrolysis and not by reduction with carbon?

5. What is Ellingham diagram? State one use.

6. Why is cryolite added during extraction of aluminium?

7. What is electrolytic refining?

8. Write the reaction involved in calcination of zinc carbonate.

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🔹 3 Mark PYQs

1. Explain any three methods used for concentration of ores.

2. Describe froth flotation process with principle.

3. Explain roasting and calcination with suitable examples.

4. Explain Ellingham diagram and its significance.

5. Describe electrolytic refining of metals with labelled diagram (theory).

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🔹 5 Mark PYQs

1. (a) What is metallurgy?
   (b) Explain different methods of concentration of ores.

2. (a) What is Ellingham diagram?
   (b) How is it useful in extraction of metals?

3. (a) Explain extraction of aluminium from bauxite.
   (b) Write role of cryolite.

4. (a) Explain refining of metals.
   (b) Describe electrolytic refining and zone refining.

5. (a) Explain extraction of zinc from zinc blende.
   (b) Write chemical reactions involved.

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🔢 Numerical / Reasoning PYQs (Frequently Asked)

1. Why is ZnO not reduced by CO?

2. Why are sulphide ores first converted into oxides?

3. Why is carbon not suitable reducing agent for Al₂O₃?

4. Why is leaching preferred for bauxite?

5. Why is zone refining used for Ge and Si?

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ Froth flotation + depressant
✔ Roasting vs calcination
✔ Ellingham diagram (logic-based questions)
✔ Aluminium metallurgy + cryolite
✔ Electrolytic refining
✔ Zone refining

✔ Flux & slag 

❓ FAQs (Concept Clarity)

1. Why is Ellingham diagram important in metallurgy?
   It helps in selecting suitable reducing agent and temperature.

2. Why is cryolite added in extraction of aluminium?
   To lower melting point and increase electrical conductivity.

3. Why can ZnO not be reduced by CO?
   Because ZnO is more stable than CO at that temperature.

4. Why is zone refining used for semiconductors?
   Because it produces ultra-pure metals.

5. Why is froth flotation suitable for sulphide ores?
   Because sulphide ores are preferentially wetted by oil.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

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