Ncert Books Class 12 Chemistry Chapter 6 Free Download

For a 3-mark question, you can expect a detailed explanation of a specific refining method. Important topics include:

• Vapour Phase Refining: Explain the principles of the Mond process for nickel and the van Arkel method for zirconium, including the chemical reactions involved at different temperatures.

• Electrolytic Refining: Describe the process with a well-labelled diagram, clearly mentioning the materials used for the anode, cathode, and electrolyte, using copper as a key example.

• Zone Refining: Explain the principle and its application for producing high-purity semiconductors like silicon and germanium. A simple diagram illustrating the movement of the molten zone is often expected.

A 5-mark question on the blast furnace typically requires a comprehensive explanation covering different temperature zones and the reactions within them. Key reactions to focus on for the exam are:

• Zone of Combustion (Bottom, ~2200K): C(s) + O₂(g) → CO₂(g), followed by CO₂(g) + C(s) → 2CO(g). This produces the main reducing agent, CO.

• Zone of Reduction (Middle, 900K - 1500K): This is the most crucial part. Include the reduction of iron oxides by carbon monoxide: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂; Fe₃O₄ + CO → 3FeO + CO₂; FeO + CO → Fe + CO₂.

• Zone of Slag Formation (Middle, ~1200K): Decomposition of limestone (CaCO₃ → CaO + CO₂) and the reaction of the flux (CaO) with silica impurity (SiO₂) to form slag (CaO + SiO₂ → CaSiO₃).

• Zone of Fusion (Lower Middle): Molten iron and slag are formed and collect at the bottom.

An Ellingham diagram is crucial because it graphically represents the thermodynamic feasibility of a reduction process. It plots the standard Gibbs free energy of formation (ΔG°) of oxides against temperature. A metal oxide can be reduced by another element if, at a given temperature, the ΔG° of the reducing element's oxide formation is more negative than that of the metal oxide being reduced. In the diagram, any element can reduce the oxides of other elements that lie above it. HOTS (Higher Order Thinking Skills) questions might ask you to:

• Predict the feasibility of a reduction at a specific temperature not explicitly given.

• Explain the sudden change in slope for a reaction like 2Mg + O₂ → 2MgO, linking it to the change in the physical state (melting or boiling) of the metal.

• Justify why carbon becomes a better reducing agent than carbon monoxide at very high temperatures.

Both are important ore concentration methods, but they are based on different principles:

• Froth Flotation: This method is based on the differential wetting properties of the ore and gangue particles. It is primarily used for the concentration of sulphide ores (like ZnS, PbS). The mineral particles are wetted by oil (pine oil) and rise to the surface with the froth, while the gangue particles are wetted by water and settle down.

• Magnetic Separation: This method is based on the differences in the magnetic properties of the ore and gangue. It is used when either the ore or the impurity is magnetic. For example, it is used to separate magnetic Wolframite impurities from non-magnetic Cassiterite (tin ore, SnO₂).

A key difference to highlight in an exam is that froth flotation uses surface chemistry, while magnetic separation uses physical magnetic properties.

This is a common conceptual question. Carbon is not used to reduce alumina for two main reasons:

1. Thermodynamic Unfeasibility: Aluminium is a highly reactive metal and forms a very stable oxide. The reduction of alumina with carbon requires an extremely high temperature (around 2000°C), which is not economically viable. At this temperature, aluminium can react with carbon to form aluminium carbide (Al₄C₃), an undesirable impurity.

2. Alternative Process: The Hall-Héroult process, involving electrolytic reduction of alumina dissolved in molten cryolite, is a more efficient and feasible method that operates at a lower temperature (around 950°C).

For the extraction of copper from copper pyrites (CuFeS₂), the following steps and reactions are important from an examination perspective:

• Roasting: In a reverberatory furnace, the ore is heated in a supply of air to convert iron sulphides to oxides: 2CuFeS₂(s) + O₂(g) → Cu₂S(s) + 2FeS(s) + SO₂(g). Some sulphides of iron and copper are also oxidised.

• Smelting: The roasted ore is mixed with silica (flux) and heated. Iron oxide forms a slag (FeSiO₃) which is removed: FeO(s) + SiO₂(s) → FeSiO₃(l) (slag). This results in 'copper matte' containing Cu₂S and some FeS.

• Bessemerisation: The molten matte is put into a Bessemer converter. Hot air is blown through it, leading to auto-reduction: 2Cu₂S(l) + 3O₂(g) → 2Cu₂O(l) + 2SO₂(g), followed by 2Cu₂O(l) + Cu₂S(l) → 6Cu(l) + SO₂(g). The resulting copper is called blister copper.

For the past few academic sessions, this chapter has been removed from the official CBSE Class 12 board examination syllabus. However, students must always verify the latest syllabus for the academic year 2025-26 directly from the official CBSE website once it is released. While it may not be tested in boards, the principles of metallurgy, such as using Ellingham diagrams and understanding reduction/refining processes, are highly important for competitive exams like JEE and NEET. Therefore, a foundational understanding of the chapter remains beneficial.