Question 1

Q1. What is the role of depressants in froth floatation process?

Accepted Answer

Depressants are chemical reagents used in the froth floatation process to selectively prevent certain sulphide ores from coming into the froth with the desired ore. They achieve this by selectively adsorbing on the surface of one of the sulphide components, making it hydrophilic and preventing it from forming froth. This allows the other sulphide ore to be preferentially floated.

**Example:** In the separation of galena (PbS) from zinc blende (ZnS), sodium cyanide (NaCN) is used as a depressant. NaCN forms a soluble complex, sodium tetracyanozincate(II) (Na₂[Zn(CN)₄]), with ZnS on the surface of zinc blende, thereby preventing it from coming into the froth. This allows PbS to come with the froth.

Question 2

Q2. Why is the reduction of a metal oxide easier if the metal formed is in liquid state at the temperature of reduction?

Accepted Answer

The reduction of a metal oxide becomes easier (more thermodynamically favorable) if the metal formed is in the liquid state at the temperature of reduction due to a significant increase in the entropy of the system.

According to the Gibbs-Helmholtz equation: ΔG = ΔH - TΔS

For a reduction process to be spontaneous and easier, the Gibbs free energy change (ΔG) must be negative. When a metal is formed in the liquid state from its solid oxide, there is a substantial increase in the randomness or disorder (entropy, ΔS) compared to forming a solid metal. The phase transition from solid to liquid (M(s) → M(l)) involves a positive change in entropy.

For example, consider the reduction of a metal oxide MₓOᵧ(s) by carbon:
MₓOᵧ(s) + C(s) → xM(l) + CO(g) (or CO₂(g))

In this reaction, the formation of liquid metal (M(l)) and gaseous carbon monoxide/dioxide (CO(g)/CO₂(g)) from solid reactants (MₓOᵧ(s) and C(s)) leads to a large positive change in entropy (ΔS > 0). A larger positive ΔS value makes the term -TΔS more negative, which in turn makes ΔG more negative (or less positive) at higher temperatures (T). This favors the spontaneity and ease of reduction.

Question 3

Q3. What is the role of depressant in froth floatation process?

Accepted Answer

In the froth floatation process, a depressant is a chemical substance added to selectively prevent certain minerals from coming with the froth. Its primary role is to separate two sulphide ores by selectively depressing the floatation of one of them. This allows the other sulphide ore to float and be collected.

For example, when galena (PbS) and zinc blend (ZnS) are present together in an ore, sodium cyanide (NaCN) is used as a depressant. NaCN selectively reacts with ZnS to form a complex, sodium tetracyanozincate(II) (Na₂[Zn(CN)₄]), which makes ZnS hydrophilic and prevents it from coming to the froth. As a result, only PbS floats with the froth, allowing for their effective separation.

**Reaction:**
ZnS + 4NaCN → Na₂[Zn(CN)₄] + Na₂S

Question 4

Q4. What is the significance of leaching in the extraction of aluminium?

Accepted Answer

Leaching is a crucial and significant step in the extraction of aluminium, primarily for the purification of bauxite ore (which contains aluminium oxide, Al₂O₃). This purification process is commonly known as Bayer's process.

The significance of leaching in aluminium extraction is as follows:
1.  **Purification of Bauxite Ore:** Bauxite ore typically contains various impurities such as iron oxides (Fe₂O₃), silica (SiO₂), and titanium dioxide (TiO₂). Leaching helps in effectively removing these impurities.
2.  **Selective Dissolution of Alumina:** In Bayer's process, powdered bauxite ore is treated with a concentrated solution of sodium hydroxide (NaOH) at elevated temperatures (473-523 K) and pressure (35-36 bar). Aluminium oxide, being amphoteric, selectively dissolves in the strong base to form soluble sodium meta-aluminate:
    Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)
3.  **Separation of Impurities:** The impurities like Fe₂O₃ and TiO₂ are basic or inert and do not dissolve in NaOH, remaining as insoluble 'red mud'. Silica (SiO₂) also reacts with NaOH to form sodium silicate (Na₂SiO₃), which is less soluble and can be removed later or precipitated out.
4.  **Recovery of Pure Alumina:** The clear solution of sodium meta-aluminate is filtered to remove the insoluble impurities. It is then diluted and seeded with freshly prepared hydrated alumina (Al₂O₃·xH₂O) to induce the precipitation of pure hydrated aluminium oxide:
    2Na[Al(OH)₄](aq) + CO₂(g) → Al₂O₃·xH₂O(s) + 2NaHCO₃(aq) (or by simply cooling and seeding)
    The precipitated hydrated alumina is then filtered, washed, and calcined (heated strongly) at 1470 K to obtain pure anhydrous alumina (Al₂O₃), which is then used for the electrolytic reduction (Hall-Héroult process).

Thus, leaching ensures the removal of impurities from bauxite, providing pure alumina for the subsequent electrolytic reduction, which is essential for obtaining high-purity aluminium.

Question 5

Q5. The reaction, Cr₂O₃ + 2Al → Al₂O₃ + 2Cr (ΔG° = – 421 kJ) is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?

Accepted Answer

The reaction Cr₂O₃ + 2Al → Al₂O₃ + 2Cr has a highly negative Gibbs free energy change (ΔG° = – 421 kJ). A negative ΔG° value indicates that the reaction is thermodynamically feasible and spontaneous. However, thermodynamic feasibility only tells us whether a reaction *can* occur, not *how fast* it will occur.

The reason this reaction does not take place at room temperature is due to a high **activation energy barrier**.

1.  **Kinetic Barrier:** For the reaction to proceed, the existing chemical bonds in the reactants (Cr₂O₃ and Al) must be broken, and new bonds must be formed to create the products (Al₂O₃ and Cr). This process requires a certain minimum amount of energy, known as the activation energy.
2.  **Insufficient Kinetic Energy:** At room temperature, the reactant molecules do not possess sufficient kinetic energy to overcome this activation energy barrier. The collision energy between reactant particles is too low to initiate the bond breaking and forming processes.
3.  **Initiation Required:** The reaction needs to be initiated by providing external heat (e.g., by using an ignition mixture like barium peroxide and magnesium powder). Once initiated, the reaction is highly exothermic (as indicated by the large negative ΔG°), and the heat released sustains the reaction, allowing it to proceed vigorously. This is the principle behind the thermite process, where aluminium acts as a strong reducing agent.

Question 6

Q6. What is the role of flux in metallurgical processes?

Accepted Answer

In metallurgical processes, a flux is a substance added to the ore during smelting or refining to remove non-fusible impurities (gangue) present in the ore. The flux reacts with the gangue at high temperatures to form a fusible product called slag, which is lighter than the molten metal and can be easily separated from it.

The primary roles of flux are:
1.  **Removal of Gangue:** It converts the non-fusible gangue into a fusible slag, facilitating its removal.
2.  **Protection of Molten Metal:** The molten slag forms a protective layer over the molten metal, preventing its oxidation by the atmosphere.

The choice of flux depends on the chemical nature of the gangue:
*   **Acidic Gangue:** If the gangue is acidic (e.g., SiO₂), a basic flux like calcium oxide (CaO) or magnesium oxide (MgO) is used.
    **Example:** SiO₂ (acidic gangue) + CaO (basic flux) → CaSiO₃ (calcium silicate slag)
*   **Basic Gangue:** If the gangue is basic (e.g., FeO, MnO), an acidic flux like silica (SiO₂) is used.
    **Example:** FeO (basic gangue) + SiO₂ (acidic flux) → FeSiO₃ (ferrous silicate slag)

Question 7

Q7. How is wrought iron different from cast iron?

Accepted Answer

Wrought iron and cast iron are two important commercial forms of iron, differing primarily in their carbon content and consequently in their physical and mechanical properties. Here's a detailed comparison:

Comparison between Wrought Iron and Cast Iron

Feature	Wrought Iron	Cast Iron
Carbon Content	Very low (0.1-0.25%). It is the purest form of commercial iron.	High (2.5-4.5%).
Other Impurities	Contains very small amounts of impurities like S, P, Si, Mn (typically less than 0.3%). Often contains slag inclusions (fibrous structure).	Contains significant amounts of impurities like S, P, Si, Mn (typically 0.5-3.0%), which contribute to its brittleness.
Nature/Structure	Fibrous structure due to uniformly distributed slag particles.	Crystalline structure, often containing graphite flakes (grey cast iron) or cementite (white cast iron).
Hardness	Soft and tough.	Hard and brittle.
Malleability & Ductility	Highly malleable and ductile; can be hammered, rolled, and drawn into wires.	Not malleable or ductile; cannot be hammered or drawn. It breaks on hammering.
Melting Point	High melting point (approx. 1500°C).	Lower melting point (approx. 1200°C).
Weldability	Easily weldable.	Cannot be welded easily; difficult to join.
Corrosion Resistance	Good corrosion resistance due to its purity and slag inclusions.	Less corrosion resistant than wrought iron due to higher impurity content.
Magnetism	Easily magnetized and demagnetized (used for electromagnets).	Retains magnetism (used for permanent magnets).
Uses	Used for making chains, anchors, railway couplings, agricultural implements, wires, bolts, nuts, ornamental ironwork, and electromagnets.	Used for casting machine parts, pipes, stoves, radiators, sanitary fittings, engine blocks, and manhole covers.

Question 8

Q8. What is meant by the term 'chromatography'? Give an example.

Accepted Answer

Chromatography is a powerful analytical and preparative technique used for the separation, identification, and purification of components of a mixture. The principle behind chromatography is the differential distribution (or differential movement) of components between two phases: a stationary phase and a mobile phase.

Components of the mixture travel at different speeds through the stationary phase, carried by the mobile phase, due to differences in their affinities (adsorption, solubility, or partition coefficients) for the two phases. This differential movement leads to their separation into distinct bands or zones.

**Key principles:**
*   **Stationary Phase:** A fixed phase (solid or liquid supported on a solid) through which the mixture passes.
*   **Mobile Phase:** A moving phase (liquid or gas) that carries the mixture through the stationary phase.

**Example: Column Chromatography**
In column chromatography, the stationary phase is a solid adsorbent (e.g., alumina, silica gel) packed in a vertical glass column. The mixture to be separated is loaded onto the top of the column. A suitable liquid solvent (mobile phase), called the eluent, is then passed through the column. Components with lower adsorption to the stationary phase and higher solubility in the mobile phase move faster down the column, while those with higher adsorption move slower. This differential movement causes the components to separate into distinct bands, which can then be collected individually as they elute from the column.

Question 9

Q9. What is the role of depressants in froth floatation process?

Accepted Answer

Depressants are chemical reagents used in the froth floatation process to selectively prevent one type of sulphide ore particle from coming with the froth, while allowing another type to float. Their main role is to achieve selective separation of different sulphide ores present in a mixture.

**Example:** In the separation of lead sulphide (PbS) and zinc sulphide (ZnS) ores, sodium cyanide (NaCN) is used as a depressant. NaCN selectively reacts with ZnS to form a soluble complex, sodium tetracyanozincate(II) (Na₂[Zn(CN)₄]), which prevents ZnS from coming into the froth. PbS, however, remains unaffected and floats with the froth.

Question 10

Q10. Why is it that only sulphide ores are concentrated by froth floatation process?

Accepted Answer

The froth floatation process is primarily used for the concentration of sulphide ores due to their unique surface properties, specifically their differential wettability.

**Principle:**
1.  **Preferential Wettability:** Sulphide ores are preferentially wetted by oils (like pine oil, eucalyptus oil) and are hydrophobic (repelled by water). This means they tend to stick to oil droplets and air bubbles.
2.  **Gangue Wettability:** The gangue (impurities) particles, on the other hand, are preferentially wetted by water and are hydrophilic (attracted to water). They tend to sink in water.
3.  **Froth Formation:** When air is blown through the mixture of powdered ore, water, oil, and frothing agents, the oil-coated sulphide ore particles get attached to the air bubbles and rise to the surface as a froth, which can be skimmed off.
4.  **Separation:** The gangue particles, being heavier and wetted by water, settle down at the bottom.

This distinct difference in surface properties (hydrophobicity of sulphide ores vs. hydrophilicity of gangue) makes the froth floatation process highly effective and specific for the concentration of sulphide ores.

Question 11

Q11. What is the significance of leaching in the extraction of aluminium?

Accepted Answer

Leaching is a crucial step in the extraction of aluminium, specifically in the concentration of bauxite ore (which is the primary ore of aluminium). The most common leaching process for bauxite is the **Bayer's process**.

**Significance of Leaching (Bayer's Process):**
1.  **Concentration of Alumina:** Bauxite ore contains 30-60% alumina (Al₂O₃) along with significant impurities like silica (SiO₂), iron oxides (Fe₂O₃), and titanium oxide (TiO₂). Leaching selectively dissolves alumina, separating it from these insoluble impurities.
2.  **Removal of Impurities:**
    *   **Alumina (Al₂O₃)** is amphoteric and reacts with hot concentrated sodium hydroxide (NaOH) solution to form soluble sodium meta-aluminate:
        Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)
    *   **Silica (SiO₂)**, being acidic, also reacts with NaOH to form soluble sodium silicate:
        SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l)
    *   **Iron oxides (Fe₂O₃) and Titanium oxide (TiO₂)** are basic/amphoteric but do not react with NaOH under these conditions and remain insoluble. They are filtered off as 'red mud'.
3.  **Purification of Alumina:** The soluble sodium meta-aluminate solution is then diluted with water and seeded with freshly prepared hydrated alumina. This causes the precipitation of pure hydrated alumina, while sodium silicate remains in the solution.
    2Na[Al(OH)₄](aq) + (seed) → Al₂O₃·xH₂O(s) + 2NaOH(aq)
    The precipitated hydrated alumina is then filtered, washed, and calcined (heated) at 1470 K to obtain pure anhydrous alumina (Al₂O₃).
    Al₂O₃·xH₂O(s) → Al₂O₃(s) + xH₂O(g)

In essence, leaching in aluminium extraction serves to purify and concentrate alumina from its ore, making it suitable for subsequent electrolytic reduction.

Question 12

Q12. The reaction, Cr₂O₃ + 2Al → Al₂O₃ + 2Cr (ΔG° = – 421 kJ) is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?

Accepted Answer

The reaction Cr₂O₃ + 2Al → Al₂O₃ + 2Cr has a highly negative Gibbs free energy change (ΔG° = – 421 kJ), which indicates that the reaction is thermodynamically feasible and spontaneous. However, thermodynamic feasibility only tells us about the spontaneity of a reaction under given conditions, not about its rate.

**Reason for not taking place at room temperature:**
1.  **High Activation Energy:** For the reaction to occur, the bonds in the reactants (Cr₂O₃ and Al) must be broken, and new bonds in the products (Al₂O₃ and Cr) must be formed. This process requires a certain minimum amount of energy, known as the activation energy. At room temperature, the kinetic energy of the reactant molecules is insufficient to overcome this high activation energy barrier.
2.  **Kinetic Barrier:** Even though the products (Al₂O₃ and Cr) are thermodynamically more stable than the reactants, the reaction has a kinetic barrier that prevents it from proceeding at a noticeable rate at room temperature.
3.  **Initiation:** The reaction, known as the aluminothermic process or Goldschmidt process, requires an initial input of heat (e.g., by igniting with a magnesium ribbon and barium peroxide mixture) to provide the necessary activation energy. Once initiated, the reaction is highly exothermic (releases a large amount of heat), which sustains the reaction and allows it to proceed rapidly.

Question 13

Q13. Can magnesium be reduced by carbon reduction method? If not, why not?

Accepted Answer

No, magnesium (Mg) cannot be effectively reduced from its oxide (MgO) by the carbon reduction method (using carbon or carbon monoxide) under typical industrial conditions.

**Reason (based on Ellingham Diagram principles):**
1.  **Thermodynamic Stability of MgO:** Magnesium is a highly electropositive metal, and its oxide, MgO, is exceptionally stable. This stability is reflected in its very negative standard Gibbs free energy of formation (ΔG°f) and its position in the Ellingham diagram.
2.  **Ellingham Diagram Comparison:** In the Ellingham diagram, which plots ΔG° vs. T for the formation of oxides, the line for the formation of MgO (Mg + ½O₂ → MgO) lies significantly below the line for the formation of CO (C + ½O₂ → CO) over a wide range of temperatures. This indicates that MgO is thermodynamically much more stable than CO (or CO₂) at temperatures where carbon reduction is typically carried out.
3.  **Reducing Power:** For carbon to reduce MgO, the Gibbs free energy change for the overall reaction (MgO + C → Mg + CO) must be negative. This would require the carbon oxidation line to be below the magnesium oxidation line at the reduction temperature. However, this is not the case for magnesium. Carbon is a weaker reducing agent than magnesium at the temperatures required for the reduction of MgO.
4.  **High Temperature Requirement and Carbide Formation:** While theoretically, at extremely high temperatures (above approximately 2000 °C), the carbon line might cross the magnesium line, such temperatures are impractical. Furthermore, at these very high temperatures, magnesium would react with carbon to form magnesium carbide (MgC₂), rather than free magnesium.

Therefore, due to the high thermodynamic stability of MgO and the relatively weaker reducing power of carbon compared to magnesium, carbon reduction is not a viable method for extracting magnesium from its oxide.

Question 14

Q14. Write down the reactions taking place in different zones in the blast furnace during the extraction of iron.

Accepted Answer

The extraction of iron from its oxide ore (haematite, Fe₂O₃) is carried out in a blast furnace. The charge, consisting of iron ore, coke, and limestone (CaCO₃) in the approximate ratio 8:4:1, is introduced from the top. A blast of hot air is blown from the bottom. The reactions occur in different temperature zones as the charge moves down.

**1. Zone of Combustion (Bottom of the furnace)**
Temperature: ~2200 K
In this zone, coke burns in the presence of the hot air blast to produce carbon dioxide, releasing a large amount of heat.
C(s) + O₂(g) → CO₂(g); ΔH = -393.5 kJ/mol
The CO₂ produced moves upwards and reacts with more hot coke to form carbon monoxide, which is the main reducing agent.
CO₂(g) + C(s) → 2CO(g); ΔH = +172.5 kJ/mol

**2. Zone of Reduction (Upper part of the furnace)**
Temperature: 500 K - 800 K
Here, the iron oxides are reduced by carbon monoxide.
3Fe₂O₃(s) + CO(g) → 2Fe₃O₄(s) + CO₂(g)
Fe₃O₄(s) + 4CO(g) → 3Fe(s) + 4CO₂(g)
Fe₂O₃(s) + CO(g) → 2FeO(s) + CO₂(g)

**3. Zone of Reduction (Middle part of the furnace)**
Temperature: 900 K - 1500 K
In this higher temperature zone, the remaining iron oxides are reduced by both carbon monoxide and carbon.
FeO(s) + CO(g) → Fe(s) + CO₂(g)
Also, the limestone decomposes to form calcium oxide (quicklime) and carbon dioxide.
CaCO₃(s) → CaO(s) + CO₂(g)

**4. Zone of Fusion (Lower part of the furnace)**
Temperature: >1500 K
In this zone, the spongy iron produced in the upper zones melts. The calcium oxide (CaO), a basic flux, reacts with the acidic impurity, silica (SiO₂), present in the ore to form a fusible slag of calcium silicate.
CaO(s) + SiO₂(s) → CaSiO₃(l) (Slag)
The molten slag is lighter than the molten iron and floats on top. This prevents the re-oxidation of the molten iron. The molten iron (called pig iron) and slag are tapped off separately from the bottom of the furnace.

Question 15

Q15. Write chemical reactions taking place in the extraction of zinc from zinc blende.

Accepted Answer

The principal ore of zinc is zinc blende (ZnS). The extraction of zinc from zinc blende involves the following steps:

**1. Concentration of the Ore**
The ore is concentrated by the froth flotation process. This method is used for sulphide ores. The powdered ore is mixed with water and a frothing agent (like pine oil) and a collector (like sodium ethyl xanthate). Air is bubbled through the mixture. The sulphide ore particles are preferentially wetted by the oil and rise to the surface with the froth, while the gangue particles are wetted by water and settle down.

**2. Roasting (Conversion to Oxide)**
The concentrated ore is then heated strongly in a reverberatory furnace in a regular supply of air at a temperature below the melting point of zinc. This process is called roasting. Zinc sulphide is converted into zinc oxide.
2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g)
The sulphur dioxide (SO₂) gas produced can be used in the manufacture of sulphuric acid (H₂SO₄).

**3. Reduction of Zinc Oxide (Smelting)**
Zinc oxide is reduced to zinc metal by heating it with crushed coke at about 1673 K in a fire clay retort. Carbon acts as the reducing agent.
ZnO(s) + C(s) → Zn(s) + CO(g)
Since the boiling point of zinc is low (1180 K), the metal is formed as a vapour. The zinc vapour is then condensed to obtain solid zinc, known as spelter zinc.

**4. Purification of Zinc**
The impure zinc metal (spelter) can be purified by fractional distillation or, more commonly, by electrolytic refining. In electrolytic refining, impure zinc is made the anode, a strip of pure zinc is the cathode, and an acidified solution of zinc sulphate (ZnSO₄) is the electrolyte. On passing an electric current, pure zinc gets deposited at the cathode.

Question 16

Q16. State the role of silica in the metallurgy of copper.

Accepted Answer

In the metallurgy of copper, the ore (like copper pyrites, CuFeS₂) contains iron sulphide (FeS) as a major impurity. During roasting and subsequent smelting in a reverberatory furnace, the iron sulphide is oxidized to iron(II) oxide (FeO).

2FeS(s) + 3O₂(g) → 2FeO(s) + 2SO₂(g)

This iron(II) oxide (FeO) is a basic impurity. To remove it, an acidic flux, silica (silicon dioxide, SiO₂), is added to the furnace. Silica combines with the iron(II) oxide at high temperatures to form a molten slag of iron(II) silicate (FeSiO₃).

FeO(s) + SiO₂(s) → FeSiO₃(l)
(Basic Impurity) + (Acidic Flux) → (Slag)

The molten slag is lighter than the molten copper matte (a mixture of Cu₂S and FeS) and immiscible with it. It floats over the molten matte and can be easily skimmed off. Thus, the role of silica is to act as an acidic flux to remove the basic impurity of iron oxide as slag.

Question 17

Q17. What is meant by the term “chromatography”?

Accepted Answer

Chromatography is a modern physical technique used for the separation, identification, and purification of components in a mixture. The term is derived from Greek words 'chroma' meaning colour and 'graphein' meaning to write, as it was first used to separate coloured plant pigments.

The underlying principle of chromatography is the differential distribution of the components of a mixture between two phases: a stationary phase and a mobile phase.

*   **Stationary Phase:** This is a fixed phase which can be a solid or a liquid supported on a solid. It has a large surface area.
*   **Mobile Phase:** This is a phase that moves over or through the stationary phase, carrying the components of the mixture with it. It can be a liquid or a gas.

During the process, the components of the mixture are adsorbed on the stationary phase to different extents. The component that is more strongly adsorbed moves slower, while the component that is less adsorbed moves faster with the mobile phase. This difference in movement causes the components to separate.

In metallurgy, chromatographic methods are particularly useful for the purification of elements that are available in minute quantities and have very similar chemical properties, making their separation by other methods difficult (e.g., separation of lanthanoids).

Question 18

Q18. What criterion is followed for the selection of the stationary phase in chromatography?

Accepted Answer

The primary criterion for selecting a stationary phase in chromatography is based on the principle of **differential adsorption** and the **nature of the substances to be separated**.

The key criteria are:

1.  **Selective Adsorption:** The stationary phase must be an adsorbent that shows different affinities for the different components of the mixture. It should adsorb one component more strongly than the others. This difference in the extent of adsorption is what leads to separation, as the weakly adsorbed component travels faster with the mobile phase than the strongly adsorbed one.

2.  **Inertness:** The stationary phase must be chemically inert. It should not react with the components of the mixture being separated or with the mobile phase (solvent).

3.  **Physical Properties:** It should be physically stable, insoluble in the mobile phase, and have a suitable particle size and surface area to allow for efficient separation and a good flow rate of the mobile phase.

4.  **Nature of Components:** The choice also depends on the polarity of the components to be separated. For example, for separating polar compounds, a polar stationary phase like silica gel or alumina is often used (normal-phase chromatography). For non-polar compounds, a non-polar stationary phase might be chosen (reverse-phase chromatography).

Common examples of stationary phases used in column chromatography are silica gel (SiO₂) and alumina (Al₂O₃).

Question 19

Q19. What is the role of depressants in froth floatation process?

Accepted Answer

In the froth floatation process, depressants are used to selectively prevent one or more components of the ore from coming with the froth. They achieve this by interacting with the surface of specific mineral particles, making them hydrophilic (water-wetting) and thus preventing them from attaching to the oil froth. This allows for the selective separation of two sulphide ores present in a mixture.

For example, in the separation of galena (PbS) and zinc blende (ZnS) ores, sodium cyanide (NaCN) is used as a depressant. NaCN selectively reacts with ZnS to form a soluble complex, sodium tetracyanozincate(II) (Na₂[Zn(CN)₄]), which prevents ZnS from coming into the froth. PbS, on the other hand, is not affected by NaCN and rises with the froth, allowing for its separation from ZnS.

Question 20

Q20. Why is the reduction of a metal oxide easier if the metal formed is in liquid state at the temperature of reduction?

Accepted Answer

The ease of reduction of a metal oxide can be understood in terms of the Gibbs free energy change (ΔG) for the reduction reaction. A reaction is spontaneous and easier to carry out if ΔG is negative.

The Gibbs free energy change is given by the equation: ΔG = ΔH - TΔS, where ΔH is the enthalpy change, T is the absolute temperature, and ΔS is the entropy change.

When the metal formed during reduction is in the liquid state at the temperature of reduction, there is a significant increase in entropy (ΔS) compared to when the metal is formed in the solid state. This is because the liquid state has a higher degree of disorder and randomness (higher entropy) than the solid state.

A larger positive value of ΔS (due to the formation of liquid metal from a solid oxide) makes the term -TΔS more negative. Consequently, this contributes to a more negative ΔG, making the reduction reaction thermodynamically more favourable and thus easier to carry out at that temperature.

Question 21

Q21. Name the two main ores of iron. When are they concentrated?

Accepted Answer

The two main ores of iron are:
1.  **Haematite** (Fe₂O₃)
2.  **Magnetite** (Fe₃O₄)

Both haematite and magnetite are concentrated using **magnetic separation** because they are ferromagnetic substances. In this process, the finely crushed ore is passed over a magnetic roller. The magnetic particles (ore) are attracted to the roller and fall in a separate heap closer to the roller, while the non-magnetic impurities (gangue) fall further away.

Additionally, haematite, being a heavy oxide ore, can also be concentrated by **gravity separation** (or hydraulic washing). In this method, the powdered ore is washed with a stream of water. The lighter gangue particles are washed away, while the heavier ore particles settle down.

Chapter 6: General Principles of Isolation of Elements