CBSE Important Questions Class 12 Chemistry Chapter 13 – General Principles and Processes of Isolation of Elements

Class 12 Chemistry Chapter 13 General Principles and Processes of Isolation of Elements is part of the CBSE Class 12 Chemistry syllabus. It explains how metals are taken out of their ores. Students learn the steps of metallurgy — concentration of the ore, roasting and calcination, reduction to the free metal, and refining — along with the use of Ellingham diagrams and the extraction of iron, copper, zinc and aluminium.

These CBSE important questions for Class 12 Chemistry Chapter 13 cover using ΔG values to show that roasting is spontaneous, zone refining and column chromatography, the impurity in iron oxide and how slag is formed, the definition of metallurgy, a case study on galvanising and electrical wires, froth flotation, Ellingham diagrams and the role of cryolite. Every question has a clear step-by-step solution in simple English, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. Using the following ΔrG° values at 1073 K, show that the roasting of zinc sulphide to zinc oxide is spontaneous.
S₂(s) + 2O₂(g) → 2SO₂(g); ΔrG° = −544 kJ/mol
2Zn(s) + O₂(g) → 2ZnO(s); ΔrG° = −480 kJ/mol
2Zn(s) + S₂(s) → 2ZnS(s); ΔrG° = −293 kJ/mol

Answer: We want the reaction 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g).
Step 1: Reverse the third equation, which changes the sign of ΔG:
2ZnS(s) → 2Zn(s) + S₂(s); ΔG = +293 kJ/mol
Step 2: Add this to the first two equations:
S₂ + 2O₂ → 2SO₂  (−544)
2Zn + O₂ → 2ZnO  (−480)
2ZnS → 2Zn + S₂  (+293)
Adding: 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g)
ΔrG° = −544 − 480 + 293 = −731 kJ/mol
Since ΔG is negative, the roasting of zinc sulphide to zinc oxide is a spontaneous process.

Q2. Explain (i) zone refining and (ii) column chromatography.

Answer: (i) Zone refining: This is based on the principle that impurities are more soluble in the molten metal than in the solid metal.
A circular mobile heater is moved slowly along a rod of impure metal. A narrow molten zone travels with the heater. As the zone moves forward, pure metal crystallises out behind it while the impurities stay in the molten part and move ahead.
After repeating this several times, all the impurities collect at one end of the rod, which is then cut off.
Used for: silicon, germanium, boron, gallium and indium — elements needed in very pure form for semiconductors.
(ii) Column chromatography: This is based on the principle that different components of a mixture are adsorbed to different extents on an adsorbent.
There are two phases: a stationary phase (usually an Al₂O₃ column) and a mobile phase (a solvent). The mixture is put on top of the column and the solvent is allowed to flow down.
A component that is strongly adsorbed moves slowly, while a weakly adsorbed component moves quickly. They separate into bands and are then washed out (eluted) one by one.
Used for: purifying elements available in very small quantities, and for separating impurities whose chemical properties are very close to the element itself.

Q3. What is the impurity present in iron oxide during reduction? How is it removed?

Answer: The main impurity in iron oxide ore is silica (SiO₂), which is acidic.
How it is removed: Limestone (CaCO₃) is added as a flux in the blast furnace. On heating it decomposes:
CaCO₃ → CaO + CO₂
The calcium oxide (a basic flux) then combines with the acidic silica to form calcium silicate:
CaO + SiO₂ → CaSiO₃
This calcium silicate is the slag. Being lighter than molten iron, it floats on top and is drawn off separately, leaving the iron behind.

Q4. Define metallurgy. Name the main steps involved in it.

Answer: Metallurgy is the whole process of extracting a metal from its ore in a pure, usable form.
Main steps:
1. Crushing and grinding of the ore into a fine powder.
2. Concentration (dressing) of the ore to remove the gangue, by hydraulic washing, magnetic separation, froth flotation or leaching.
3. Conversion to oxide by roasting (for sulphide ores) or calcination (for carbonate ores).
4. Reduction of the oxide to the free metal, usually with carbon, carbon monoxide or another metal.
5. Refining of the crude metal, by electrolysis, distillation, zone refining, vapour phase refining or chromatography.

Q5. Read the information and answer the questions.
X: a metal used for galvanising iron.
Y: a metal used to make electrical wires.
Z: a non-metal extracted by oxidation.
(a) Identify X, Y and Z.
(b) Between roasting and calcination, which is used to extract Y from its sulphide ore? Write the equation.
(c) In which process is Z formed as a by-product?
(d) Which method is used to obtain pure X?

Answer: (a) X = zinc (used for galvanising iron to prevent rusting)
Y = copper (used for electrical wires because of its high conductivity)
Z = sulphur (a non-metal obtained by oxidation)
(b) Roasting is used, because copper occurs as the sulphide ore copper pyrites. Roasting heats the ore strongly in the presence of air:
2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
(Calcination is used for carbonate ores and is done in the absence of air.)
(c) Sulphur is obtained as a by-product during the roasting of sulphide ores, where the SO₂ released is collected and used to make sulphuric acid.
(d) Pure zinc is obtained by electrolytic refining, and also by distillation since zinc has a low boiling point.

Q6. What is the difference between roasting and calcination?

Roasting Calcination
Ore is heated strongly in the presence of excess air Ore is heated in the absence or limited supply of air
Used for sulphide ores Used for carbonate and hydrated ores
SO₂ gas is released CO₂ gas or water vapour is released
2ZnS + 3O₂ → 2ZnO + 2SO₂ ZnCO₃ → ZnO + CO₂

Both processes convert the ore into the oxide, because oxides are much easier to reduce to the metal than sulphides or carbonates.

Q7. What is froth flotation? Which type of ore is concentrated by it?

Answer: Froth flotation is a method of concentrating an ore that works on the difference in wetting behaviour of the ore and the gangue.
The powdered ore is mixed with water in a tank, and pine oil (a collector) and a stabiliser are added. Air is blown through to make a froth.
The ore particles are wetted by the oil and stick to the air bubbles, rising to the top as a froth which is skimmed off. The gangue particles are wetted by water and sink to the bottom.
Type of ore: This method is used mainly for sulphide ores, such as copper pyrites, zinc blende and galena.

Q8. What is an Ellingham diagram? State one of its limitations.

Answer: An Ellingham diagram is a plot of ΔG° for the formation of oxides against temperature.
Its use: It tells us at what temperature a given reducing agent will work. A metal whose line lies lower on the diagram can reduce the oxide of a metal whose line lies above it. This is how we decide whether carbon, CO or another metal should be used as the reducing agent, and at what temperature.
Limitation: The diagram tells us only whether a reduction is thermodynamically feasible, that is whether ΔG is negative. It says nothing about the rate of the reaction. A reaction may be perfectly feasible on paper but so slow in practice that it is useless.

Q9. What is the role of silica in the extraction of copper?

Answer: In the extraction of copper from copper pyrites, the roasted ore contains iron oxide (FeO) as an unwanted impurity along with the copper.
Silica (SiO₂) is added as an acidic flux. It combines with the basic FeO to form iron silicate:
FeO + SiO₂ → FeSiO₃ (slag)
This iron silicate forms the slag, which floats on top of the molten mixture and is removed.
So the role of silica is to remove the iron oxide impurity, leaving behind the copper matte (Cu₂S with some FeS).

Q10. Explain vapour phase refining with the example of the Mond process.

Answer: In vapour phase refining, the metal is converted into a volatile compound which is then decomposed to give the pure metal. Two conditions must be met: the metal must form a volatile compound easily, and that compound must decompose easily to give back the metal.
Mond process (for nickel):
Step 1: Impure nickel is heated with carbon monoxide at about 330 K. It forms the volatile nickel tetracarbonyl, leaving the impurities behind:
Ni + 4CO → Ni(CO)₄
Step 2: The vapour is then heated to a higher temperature of about 450-470 K, where it decomposes to give pure nickel:
Ni(CO)₄ → Ni + 4CO
The carbon monoxide is recycled, making the process efficient.

Q11. What is the role of cryolite in the electrolytic extraction of aluminium?

Answer: Pure alumina (Al₂O₃) has a very high melting point of about 2323 K, which makes direct electrolysis far too expensive.
Cryolite (Na₃AlF₆) is added for two reasons:
1. It lowers the melting point of the mixture to about 1140 K, so much less energy is needed.
2. It increases the electrical conductivity of the melt, which makes the electrolysis more efficient.
A small amount of fluorspar (CaF₂) is added as well for the same purpose. This whole arrangement is called the Hall-Héroult process.

Q12. Why is zinc not extracted from zinc oxide through reduction using carbon monoxide?

Answer: The reduction of zinc oxide by carbon monoxide
ZnO + CO → Zn + CO₂
has a positive ΔG at the temperatures used, so it is not spontaneous and does not happen to any useful extent.
Looking at the Ellingham diagram, the CO-to-CO₂ line lies above the Zn-to-ZnO line, so CO cannot reduce ZnO.
Instead, zinc oxide is reduced using coke (carbon) at a high temperature of about 1673 K:
ZnO + C → Zn + CO
At this high temperature the C-to-CO line falls below the Zn-to-ZnO line, so carbon works as a reducing agent while carbon monoxide does not.

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