GSEB Class 12 Chemistry Solutions Chapter 6 General Principles and Processes of Isolation

Gseb Class 12 Chemistry General Principles And Processes Of Isolation Of Elements Intext Questions And Answers

Question 1. Which of the ores mentioned in Table 6.1 can be concentrated by magnetic separation method?
Answer: Magnetic separation is a technique employed to distinguish magnetic ores from their non-magnetic impurities. For example, magnetic ores (such as anion ore) are effectively separated from non-magnetic components like silica and other contaminants using this specific method.
In simple words: This method utilizes magnets to remove magnetic ore particles from non-magnetic impurities.

🎯 Exam Tip: Understanding the physical properties of ores, particularly their magnetic nature, is key to applying the correct concentration technique. Magnetic separation is effective for ferromagnetic ores.

Question 2. What is the significance of leaching in the extraction of aluminium?
Answer: Leaching plays a crucial role in removing insoluble impurities, such as silica and iron oxides. In this process, alumina (aluminum oxide) becomes soluble in a sodium hydroxide (NaOH) solution, while these impurities remain insoluble, allowing for their efficient separation.
In simple words: Leaching helps clean aluminum ore by dissolving only the useful aluminum compound, leaving unwanted dirt behind.

🎯 Exam Tip: Leaching is a chemical concentration method, vital for ores where physical separation is inefficient due to similar physical properties between the ore and impurities.

Question 3. The reaction, Cr₂O₃ + 2 Al \( \to \) Al₂O₃ + 2Cr (\( \Delta G^\circ \) – 421 kJ) is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?
Answer: At room temperature, all the reactants and products involved in this reaction are in a solid state. Consequently, the reaction does not proceed under ambient conditions. However, when temperatures are elevated, the reactants melt, enabling the reaction to occur effectively.
In simple words: The reaction needs high heat because all materials are solid at room temperature and need to melt to mix and react.

🎯 Exam Tip: Even thermodynamically favorable reactions may require an activation energy or elevated temperature to overcome kinetic barriers, especially when reactants are solid.

Question 4. Is it true that under certain conditions, Mg can reduce Al₂O₃ and Al can reduce MgO? What are those conditions?
Answer: Yes, this statement is accurate. According to the Ellingham diagram, the plots for the oxidation of aluminum (Al) and magnesium (Mg) intersect at approximately 1350°C. This indicates that below 1350°C, magnesium (Mg) is capable of reducing aluminum oxide (Al₂O₃), whereas above 1350°C, aluminum (Al) can effectively reduce magnesium oxide (MgO).
In simple words: Yes, at different high temperatures (above or below 1350°C), either magnesium can reduce aluminum oxide or aluminum can reduce magnesium oxide.

🎯 Exam Tip: Ellingham diagrams are crucial for predicting the thermodynamic feasibility of reduction reactions at different temperatures, indicating which metal can reduce the oxide of another.

Gseb Class 12 Chemistry General Principles And Processes Of Isolation Of Elements Text Book Questions And Answers

Question 1. Copper can be extracted by hydrometallurgy but not zinc. Explain?
Answer: Zinc is a significantly reactive metal. Consequently, it is challenging to displace it from an aqueous solution of ZnSO₄ through hydrometallurgy methods as easily as copper.
In simple words: Zinc is too reactive to be easily removed from its solution using hydrometallurgy, unlike copper.

🎯 Exam Tip: The reactivity series of metals plays a critical role in hydrometallurgy; less reactive metals are easier to displace from their salt solutions.

Question 2. What is the role of depressant in froth floatation process?
Answer: In the froth floatation process, a depressant is employed to selectively inhibit certain types of sulfide ore particles from forming froth with air bubbles. For instance, sodium cyanide (NaCN) acts as a depressant to facilitate the separation of lead sulfide (PbS) ore from zinc sulfide (ZnS) ore. NaCN achieves this by forming a zinc complex, Na₂\[Zn(CN)₄\], on the surface of ZnS, thereby preventing it from entering the froth.
In simple words: Depressants stop one specific ore from floating in froth flotation, allowing other ores to be separated.

🎯 Exam Tip: Depressants enhance the selectivity of the froth flotation process, making it possible to separate different sulfide ores effectively by preventing unwanted minerals from floating.

Question 3. Why is the extraction of copper from pyrites more difficult than that from its oxide ore through, reduction?
Answer: The Gibbs energies of formation for most sulfides are generally higher than that for CS₂. In fact, CS₂ itself is an endothermic compound. Therefore, it is standard practice to roast sulfide ores to convert them into their corresponding oxides before proceeding with reduction.
In simple words: Copper sulfide ores are harder to reduce directly than oxide ores because their formation is more stable, so they are usually turned into oxides first.

🎯 Exam Tip: Roasting sulfide ores to oxides before reduction is a common metallurgical strategy to make the subsequent reduction step more thermodynamically favorable.

Question 4. Explain:
1. Zone refining
2. Column chromatography.
Answer:**Zone Refining:** This technique is used to produce metals of exceptionally high purity. It operates on the principle that the melting point of a substance is lowered by the presence of impurities. Consequently, when an impure molten metal is gradually cooled, pure metal crystals solidify first, while the impurities tend to remain in the molten portion. The impure metal, typically in the form of a rod, is heated to create a narrow molten zone at one end. This molten zone is then slowly moved towards the other end by shifting the heat source. Impurities concentrate within the moving molten zone and are eventually swept to one end of the metal, where they are discarded. This method is utilized for refining semiconductors like Germanium (Ge), Silicon (Si), and Gallium (Ga).
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र जोन रिफाइनिंग की प्रक्रिया को दर्शाता है, जिसमें एक अशुद्ध धातु की छड़ को एक चलती भट्टी द्वारा धीरे-धीरे गर्म किया जाता है। भट्टी के नीचे एक संकरा पिघला हुआ क्षेत्र (molten zone) बनता है, जिसमें अशुद्धियाँ धातु के शुद्ध भाग से अलग होकर जमा हो जाती हैं, जिससे शुद्ध धातु एक छोर पर और अशुद्धियाँ दूसरे छोर पर केंद्रित हो जाती हैं। **Chromatographic Methods:** These methods are employed for purifying elements that are available in minute quantities and where the impurities possess chemical properties not significantly different from the element to be purified. The technique relies on the principle that various components of a mixture are adsorbed differently onto an adsorbent material. In column chromatography, an adsorbent (such as silica gel, Al₂O₃, etc.) is packed into a glass column. The mixture to be separated and purified, dissolved in a suitable solvent, is applied to the top of the column. As the solution passes through, the components of the mixture adsorb onto the column material. They are then eluted using an appropriate solvent (eluant). Components that are weakly adsorbed are eluted first, followed by those more strongly adsorbed.
In simple words: Zone refining purifies metals by moving a molten zone where impurities collect, while chromatography separates mixtures by how differently their parts stick to a material.

🎯 Exam Tip: Zone refining is highly effective for semiconductor purification, while chromatography is ideal for separating small quantities of substances with similar chemical properties. Both rely on differential affinities or solubilities.

Question 5. Out of C and CO, which is a better reducing agent at 673 K?
Answer: From a thermodynamic perspective, at temperatures below 673 K, both carbon (C) and carbon monoxide (CO) can function as reducing agents. However, CO is more readily oxidized to CO₂ compared to carbon being oxidized to CO₂. Therefore, below 673 K, carbon monoxide (CO) is a more effective reducing agent than carbon (C).
In simple words: Below 673 K, carbon monoxide is a better reducing agent than carbon because it oxidizes into carbon dioxide more easily.

🎯 Exam Tip: The relative reducing power of C and CO changes with temperature, a concept best understood by analyzing their respective positions on an Ellingham diagram.

Question 6. Name the common elements present in the anode mud in electrolytic refining of copper. Why are they so present?
Answer: The anode mud, formed during the electrolytic refining of copper, typically contains common elements such as silver (Ag), gold (Au), platinum (Pt), antimony (Sb), and selenium (Se). These elements are present because they are less reactive than copper and do not undergo oxidation or dissolution in the copper sulfate (CuSO₄) and sulfuric acid (H₂SO₄) solution used as the electrolyte. Consequently, they settle at the bottom near the anode as insoluble anode mud.
In simple words: Anode mud contains valuable but unreactive metals like silver and gold because they don't dissolve in the electrolyte and simply fall off the anode.

🎯 Exam Tip: Anode mud is a valuable byproduct of electrolytic refining, as it contains precious metals that are more noble than the metal being refined and do not oxidize at the anode.

Question 7. Write down the reactions taking place in different zones in the blast furnace during the extraction of iron?
Answer: The following reactions occur in various zones within the blast furnace during the extraction of iron: In combustion zone: C + O₂ \( \to \) CO₂ + heat In fusion zone: CO₂ + C \( \to \) 2CO - Heat In slag formation zone: CaCO₃ \( \to \) CaO + CO₂CaO + SiO₂ \( \to \) CaSiO₃In reduction zone: Fe₂O₃ + CO \( \to \) 2FeO + CO₂FeO + CO \( \to \) Fe + CO₂
In simple words: In a blast furnace, carbon burns to make carbon dioxide and carbon monoxide, limestone forms slag, and iron oxides are reduced to iron in different heated areas.

🎯 Exam Tip: Knowing the specific reactions and their locations (zones) within the blast furnace is crucial for understanding the overall process of iron extraction and the role of each reactant.

Question 8. Write chemical reactions taking place in the extraction of zinc from zinc blende?
Answer: The primary ores of zinc include zinc blende (Sphalerite, ZnS), calamine (ZnCO₃), and zincite (ZnO). According to the Ellingham diagram, the Zn - ZnO curve is positioned significantly above the C - CO curve at temperatures exceeding 1000K. This indicates that coke can effectively reduce ZnO above this temperature. The extraction process typically begins with the concentration of powdered ore using gravity separation, followed by froth flotation. The concentrated ore is then roasted at approximately 1173 K. The zinc oxide obtained is then molded into briquettes with coke and clay and heated in vertical retorts using producer gas at 1673 K. Zinc metal has a low boiling point (1180 K) and is purified through distillation. The zinc metal vaporizes and is collected by rapid cooling. Further purification can be achieved via fractional distillation. Uses of Zinc:
1. For galvanization of iron sheets, as zinc resists atmospheric corrosion by forming a basic carbonate on its surface.
2. As a component in various alloys, such as brass (Cu - 60%, Zn - 40%) and German silver (Cu - 25-30%, Zn - 25-30%, Ni - 40-50%).
3. Zinc dust serves as a reducing agent in the production of dyestuffs and paints.
4. In the extraction of silver and gold through the cyanide process.
5. As a cathode in dry cells.
In simple words: Zinc is extracted from its sulfide ore (zinc blende) by concentrating it, roasting it to zinc oxide, and then reducing it with coke at high temperatures, followed by purification.

🎯 Exam Tip: Understanding the Ellingham diagram is essential for determining the appropriate reducing agent and temperature for metal extraction. The process for zinc involves multiple steps including concentration, roasting, reduction, and refining.

Question 9. State the role of silica in the metallurgy of copper?
Answer: In the metallurgy of copper, silica (SiO₂) plays a crucial role by reacting with and removing any remaining iron (III) oxide (Fe₂O₃) present in the matte. It achieves this by forming iron silicate (FeSiO₃), which acts as a slag.
In simple words: Silica helps clean copper by reacting with iron impurities to form a removable waste product called slag.

🎯 Exam Tip: Fluxes like silica are essential in metallurgy to remove gangue (impurities) by forming a fusible slag, preventing them from contaminating the desired metal.

Question 10. What is meant by the term “chromatography"?
Answer: Chromatography is a versatile separation and purification technique based on the differing adsorption tendencies of a metal and its impurities onto a suitable adsorbent. This method relies on the principle that various compounds within a mixture are adsorbed to different extents by an adsorbent. The term "chromatography" originates from the Greek words 'Chroma' (meaning color) and 'graphy' (meaning writing), reflecting its initial use in separating colored plant pigments.
In simple words: Chromatography is a separation technique that uses different speeds of movement through a material to separate mixtures based on how strongly each part sticks to it.

🎯 Exam Tip: The principle of differential adsorption or partitioning is central to all chromatographic methods, making it a powerful tool for complex mixture separations.

Question 11. What criterion is followed for the selection of the stationary phase in chromatography?
Answer: The selection criterion for the stationary phase in chromatography is based on ensuring that impurities are either more strongly adsorbed or more soluble in the stationary phase compared to the element being purified. Under these conditions, during the elution of the column, the impurities will be retained by the stationary phase, allowing the pure component to be eluted easily.
In simple words: The stationary phase is chosen so that impurities stick to it more strongly or dissolve better in it than the substance we want to purify, letting the pure substance pass through.

🎯 Exam Tip: Effective separation in chromatography depends on a clear difference in the affinity of the components for the stationary phase, ensuring impurities are selectively held back.

Question 12. Describe a method for refining nickel?
Answer: **Mond Process:** This method is specifically used for refining nickel. Impure nickel is heated in a stream of carbon monoxide (CO) gas at temperatures between 330-350 K. Under these conditions, a volatile nickel carbonyl, Ni(CO)₄, is formed, while the impurities are left behind in solid form. The gaseous nickel carbonyl is then heated to a higher temperature (450-470 K), causing it to decompose and yield pure nickel. Ni + 4CO \(\xrightarrow{60-80^\circ C}\) Ni(CO)₄
Ni(CO)₄ \(\xrightarrow{180-200^\circ C}\) Ni + 4CO
In simple words: The Mond process purifies nickel by turning impure nickel into a gas with carbon monoxide, leaving dirt behind, then heating the gas to get pure nickel.

🎯 Exam Tip: The Mond process is an excellent example of vapor phase refining, where the formation and decomposition of a volatile compound are utilized for purification.

Question 13. eparate alumina from silica in a bauxite ore associated with silica? Give equations, if any?
Answer: This separation is achieved through a chemical method, specifically leaching. In this process, the ore is treated with a suitable reagent that dissolves the desired ore component but not the impurities. The undissolved impurities are subsequently removed via filtration. The pure ore is then recovered from the solution using another appropriate chemical method. Here are some key examples: **(a) Bauxite (Al₂O₃.2H₂O):** The powdered bauxite ore is treated with a concentrated sodium hydroxide (NaOH) solution under pressure at temperatures around 473-523 K. During this step, Al₂O₃ dissolves to form sodium meta-aluminate (NaAlO₂). Insoluble impurities like Fe₂O₃ and SiO₂ are separated by filtration. The filtrate, containing sodium aluminate, is then neutralized by bubbling CO₂ gas through it. This causes hydrated Al₂O₃ to precipitate. The precipitate is filtered, washed, dried, and then calcined at approximately 1473 K to yield pure alumina (Al₂O₃). Al₂O₃ + 3H₂O + 2NaOH \( \to \) 2Na\[Al(OH)₄\]
2Na\[Al(OH)₄\] + CO₂ \( \to \) Al₂O₃.xH₂O + 2NaHCO₃
2Al(OH)₃ \(\xrightarrow{1470K}\) Al₂O₃ + 3H₂O **(b) Silver ores and gold present in rocks:** These are leached with a dilute solution of sodium or potassium cyanide. Ag₂S + 4NaCN \( \to \) 2Na\[Ag(CN)₂\] + Na₂S
4Au + 8KCN + 2H₂O + O₂ \( \to \) 4K\[Au(CN)₂\] + 4KOH The metal is subsequently recovered by introducing zinc scrap into the solution. 2Na\[Ag(CN)₂\] + Zn \( \to \) 2Ag + Na₂\[Zn(CN)₄\]
2K\[Au(CN)₂\] + Zn \( \to \) 2Au + K₂\[Zn(CN)₄\]
3. **Calcination** is a process where the concentrated ore is strongly heated in the absence of air (oxygen). This process removes volatile impurities and converts decomposable oxy-salts into oxides. e.g., ZnCO₃ \( \to \) ZnO + CO₂
2Al(OH)₃ \( \to \) Al₂O₃ + 3H₂O **Roasting** is a process in which the ore (typically sulfide ores) is strongly heated in the presence of air. e.g., 2ZnS + 3O₂ \( \to \) 2ZnO + 2SO₂
In simple words: Alumina is separated from silica in bauxite by dissolving the alumina with strong caustic soda, then precipitating it as hydrated alumina, and finally heating it to get pure alumina. Roasting involves heating in air, while calcination is heating without air.

🎯 Exam Tip: The Baeyer's process for bauxite purification is a classic example of leaching, showcasing the selective dissolution of the desired metal compound from its impurities. Differentiate clearly between roasting (in air) and calcination (without air).

Question 2. Iron is extracted from its oxide ores
(i) Name two minerals of iron (Fe)
(ii) Do you think that all minerals of iron are used as ores of iron? Substantiate.
(iii) Name the purest form of iron.
(iv) How do you obtain the above form of iron?
Answer:(i) Two common minerals of iron are Hematite (Fe₂O₃) and Magnetite (Fe₃O₄). Siderite (FeCO₃) is another example. (ii) No, not all minerals containing iron are utilized as ores of iron. Only those minerals from which iron can be economically and efficiently extracted are considered viable ores. (iii) The purest form of iron is Wrought iron. (iv) Wrought iron is obtained by heating cast iron along with hematite.
In simple words: Iron is mainly extracted from hematite and magnetite. Not all iron minerals are ores, only those that are profitable to extract from. Wrought iron is the purest form and is made by heating cast iron with hematite.

🎯 Exam Tip: Distinguishing between a mineral and an ore is fundamental in metallurgy: an ore is an economic source of a metal, while a mineral is just a naturally occurring compound. Wrought iron, despite being pure, is less common in modern industrial applications compared to steel.

Question 3. The day today life of man is related with electronics and computers. The semiconducting materials which used in this field are refined by zone refining method?
1. Identify a semiconducting material which is refined by zone refining process.
2. Explain why we use this method to purify the semiconductors.
3. Write the principle behind zone refining.
4. Explain zone refining process using suitable diagram.
Answer:1. Common semiconducting materials refined by the zone refining process include Germanium (Ge), Silicon (Si), and Gallium (Ga). 2. Zone refining is employed to purify semiconductors because it produces materials with an exceptionally high degree of purity, which is critical for their performance in electronic devices. 3. The underlying principle of zone refining is that impurities are more soluble in the molten state of a substance than in its solid state. 4. In the zone refining process, the impure metal is prepared as a rod. A localized, narrow molten region is created at one end of this rod. This molten zone is then slowly moved along the rod towards the other end by a traversing heat source. As the molten zone moves, impurities preferentially dissolve in it and are swept along, accumulating at one end of the metal. The purified metal solidifies behind the moving zone, leaving the impurities concentrated at the discarded end of the rod.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र जोन रिफाइनिंग प्रक्रिया को दर्शाता है, जहाँ एक अशुद्ध धातु की छड़ को एक चलती भट्टी द्वारा गर्म किया जाता है। एक छोटा पिघला हुआ क्षेत्र (molten zone) बनता है, जिसमें अशुद्धियाँ जमा होती हैं। जैसे-जैसे भट्टी आगे बढ़ती है, पिघला हुआ क्षेत्र शुद्ध धातु को पीछे छोड़ते हुए अशुद्धियों को एक छोर पर ले जाता है।
In simple words: Zone refining purifies semiconductors like germanium by melting a small section and moving it; impurities stay in the melted part, leaving pure material behind.

🎯 Exam Tip: Zone refining's effectiveness in achieving ultra-high purity is essential for semiconductors, as even trace impurities can drastically alter their electrical properties.

Question 4. Como now date are given below.
(a) Identify the metal whose metallurgy is associated here.
(b) Draw a neat labelled diagram by using the above data and explain the extraction of that metal.
Answer:(a) The provided information is related to the extraction of Aluminum (Al). (b) **Electrolysis of Fused Alumina (Hall's Process):** Aluminum cannot be practically obtained from alumina (Al₂O₃) by reduction with coke because aluminum exhibits a greater affinity for oxygen than carbon. Therefore, aluminum is typically isolated from its primary ore, bauxite (Al₂O₃.2H₂O), through electrolysis. The powdered bauxite ore is first treated with concentrated NaOH solution. This causes aluminum to dissolve in the form of Na\[Al(OH)₄\]. Impurities are removed by filtration, and the filtrate is then neutralized by passing CO₂ gas, which precipitates hydrated Al₂O₃. Subsequent ignition of this precipitate yields pure Al₂O₃. Al₂O₃ + 3H₂O + 2NaOH \( \to \) 2Na\[Al(OH)₄\]
2Na\[Al(OH)₄\] + CO₂ \( \to \) Al₂O₃.xH₂O + 2NaHCO₃
Al₂O₃.xH₂O \(\xrightarrow{1470K}\) Al₂O₃ + xH₂O Pure alumina is dissolved in molten cryolite (Na₃AlF₆). Cryolite acts as an excellent conductor of electricity and also significantly lowers the melting point of alumina. The electrolytic cell consists of a steel box lined internally with a refractory material and a carbon lining, which serves as the cathode. Carbon rods suspended into the electrolyte function as the anodes. The operating temperature of the cell is maintained at 1273 K. During electrolysis, aluminum metal is deposited at the cathode, while oxygen gas is liberated at the anode. This nascent oxygen reacts with the carbon anodes to form CO and CO₂, which then escape through an outlet. Due to this consumption, the carbon anodes must be replaced periodically. The cell reactions are as follows: **Anode:** O^2- \( \to \) 1/2 O₂ + 2e^-
C + 1/2O₂ \( \to \) CO
C + O₂ \( \to \) CO₂**Cathode:** Al³⁺ + 3e^- \( \to \) Al The overall reaction for the Hall-Heroult process is: 2Al₂O₃ + 3C \( \to \) 4Al + 3CO₂This electrolytic process is known as the Hall-Heroult process. Aluminum is further purified electrolytically by Hoope's process and is widely used for: * Electrical transmission. * Production of various alloys such as duralumin and magnalium. * Manufacturing aluminum foils for food wrappers and chocolates. * Fine aluminum dust used in paints and lacquers.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एल्युमीनियम के उत्पादन की हॉल-हेरॉल्ट प्रक्रिया को दर्शाता है। इसमें एक स्टील का बर्तन होता है जिसकी भीतरी परत कार्बन की बनी होती है और वह कैथोड का कार्य करती है। कार्बन एनोड्स को पिघले हुए एल्युमिना और क्रायोलाइट के मिश्रण में डुबोया जाता है। इलेक्ट्रोलाइटिक प्रक्रिया के दौरान, शुद्ध पिघला हुआ एल्युमीनियम कैथोड पर जमा होता है और निचले आउटलेट से बाहर निकाला जाता है, जबकि एनोड्स पर कार्बन डाइऑक्साइड गैस निकलती है।
In simple words: Aluminum is extracted by the Hall-Heroult process, where purified alumina is dissolved in molten cryolite and electrolyzed. Aluminum metal deposits at the cathode, while oxygen reacts with carbon anodes.

🎯 Exam Tip: The Hall-Heroult process is a cornerstone of industrial aluminum production, emphasizing the role of cryolite in lowering melting points and enhancing conductivity, and the continuous replacement of carbon anodes due to oxidation.

 

Question.(a) Identify the metal whose metallurgy is associated here. (b) Draw a neat labelled diagram by using the above data and explain the extraction of that metal.
Answer:(a) The provided information pertains to the extraction of aluminium (Al). (b) The extraction of aluminium is primarily achieved through the electrolysis of fused alumina, a process famously known as Hall's process. Aluminium cannot be reduced from alumina (Al₂O₃) using coke because aluminium possesses a stronger affinity for oxygen compared to carbon. Thus, aluminium is typically isolated from bauxite, which is hydrated alumina \((\text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O})\), via electrolysis. The initial step involves treating the powdered bauxite ore with concentrated sodium hydroxide (NaOH) solution. This causes the aluminium to dissolve, forming soluble sodium tetrahydroxoaluminate(III) \((\text{Na}[\text{Al(OH)}_4])\). Insoluble impurities are then removed through filtration. The resulting filtrate is neutralized by bubbling carbon dioxide (CO₂) gas through it, which precipitates hydrated alumina \((\text{Al}_2\text{O}_3 \cdot \text{xH}_2\text{O})\). This precipitate, upon ignition (heating), yields pure alumina \((\text{Al}_2\text{O}_3)\). The relevant chemical reactions are: \[ \text{Al}_2\text{O}_3 \cdot 3\text{H}_2\text{O} + 2\text{NaOH} \rightarrow 2\text{Na}[\text{Al(OH)}_4] \] \[ 2\text{Na}[\text{Al(OH)}_4] + \text{CO}_2 \rightarrow \text{Al}_2\text{O}_3 \cdot \text{xH}_2\text{O} + 2\text{NaHCO}_3 \] \[ \text{Al}_2\text{O}_3 \cdot \text{xH}_2\text{O} \xrightarrow{1470\text{K}} \text{Al}_2\text{O}_3 + \text{xH}_2\text{O} \] Pure alumina is then dissolved in molten cryolite \((\text{Na}_3\text{AlF}_6)\). Cryolite serves two crucial purposes: it enhances the electrical conductivity of alumina and significantly lowers its melting point. The electrolytic cell is constructed from a steel box lined internally with a refractory material and a carbon layer, with the carbon acting as the cathode. Carbon rods are immersed in the electrolyte, serving as anodes. The operating temperature of the cell is maintained at approximately 1273 K. During electrolysis, aluminium metal is deposited at the cathode, while oxygen gas is liberated at the anode. This oxygen reacts with the carbon anodes, producing carbon monoxide (CO) and carbon dioxide (CO₂), which escape. Consequently, the carbon anodes are consumed and must be replaced periodically. The anode reactions are: \[ \text{O}^{2-} \rightarrow \frac{1}{2}\text{O}_2 + 2\text{e}^- \] \[ \text{C} + \frac{1}{2}\text{O}_2 \rightarrow \text{CO} \] \[ \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \] The cathode reaction is: \[ \text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al} \] The overall reaction for this process is: \[ 2\text{Al}_2\text{O}_3 + 3\text{C} \rightarrow 4\text{Al} + 3\text{CO}_2 \] This method of electrolysis is known as the Hall-Héroult process. For further purification, aluminium can be refined electrolytically by Hoope's process. Aluminium finds numerous applications, including:
(i) Electrical transmission due to its excellent conductivity.
(ii) Production of various alloys, such as duralumin and magnalium.
(iii) Manufacturing of aluminium foils for packaging, like chocolate wrappers.
(iv) Use of fine aluminium dust in paints and lacquers.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एल्यूमीनियम के उत्पादन के लिए हॉल-हेरौल्ट प्रक्रिया को दर्शाता है। इसमें एक स्टील बॉक्स होता है जिसमें कार्बन की परत (कैथोड) लगी होती है। कार्बन एनोड इलेक्ट्रोलाइट में निलंबित होते हैं, जिसमें पिघला हुआ एल्यूमिना और क्रायोलाइट होता है। पिघला हुआ एल्यूमीनियम कैथोड पर जमा होता है और निचले हिस्से से बाहर निकाला जाता है।
In simple words: Aluminium is extracted from its ore, bauxite, by dissolving it in concentrated NaOH, purifying the resulting solution, and then performing electrolysis on molten alumina mixed with cryolite. This process separates pure aluminium at the cathode.

🎯 Exam Tip: Remember the dual role of cryolite in Hall's process (lowering melting point and increasing conductivity). Also, be able to write the balanced chemical equations for each step and electrode reactions.

 

Alloy	Composition
Duralumin	Al(95%) + Cu(4%) + Mg(0.5%) + Mn (0.5%)
Aluminium Bronze	Al (90%) + Cu (10%)
Aleald	Duralumin coated with pure aluminium
Magnalium	Al(83%) + Mg (15%) + Ca(2%)
Alnico	Al (12%) + Fe (60%) + Ni (20%) + Co (5%)

 

Question 5.While adding the raw materials into the blast furnace for the extraction of iron, it is forgotten to mix limestone with the charge. (a) Predict the result of this mistake. (b) Give reason for this result. (c) Bessemer converter follows blast furnace in steel industry. Justify?
Answer:(a) If limestone is omitted from the blast furnace charge, both metal production and the effective removal of impurities will be unsatisfactory. (b) The absence of limestone leads to poor production of carbon dioxide (CO₂) and calcium oxide (CaO). Calcium oxide acts as a flux, which is crucial for removing acidic impurities like silica (SiO₂). Without it, silica will remain as an impurity, hindering the efficient extraction of pure iron. The reaction for slag formation is: \[ \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \] \((\text{flux}) \quad (\text{gangue}) \quad (\text{slag})\) \((\text{basic}) \quad (\text{acidic}) \quad (\text{neutral})\) (c) The blast furnace is primarily used to produce pig iron. This pig iron is subsequently remelted to form cast iron. Cast iron then serves as the raw material for the Bessemer converter, where it is further processed to produce steel. Therefore, the Bessemer converter indeed follows the blast furnace in the steel industry's production sequence.
In simple words: Forgetting limestone in a blast furnace means less iron is produced and more impurities remain because limestone helps remove acidic impurities by forming slag. The blast furnace makes pig iron, which is then refined into steel using processes like the Bessemer converter.

🎯 Exam Tip: Understand the specific role of each raw material in the blast furnace. Limestone acts as a flux to remove acidic impurities. Knowing the sequence of processes (blast furnace to Bessemer converter) for steel production is also important.

 

Question 6.At the region of Rameses- II (about 1300 BC), copper was so expensive that copper vessels were regarded as treasures in Egyptian temples, i.e., men in the prehistoric periods extracted copper. 1. Identify the ore from which copper is extracted today. 2. Explain the various steps involved in the extraction of copper. 3. Give the name and composition of any two alloys of copper.
Answer:1. Today, copper is primarily extracted from copper pyrites, which has the chemical formula CuFeS₂. 2. The extraction of copper involves several key steps:
(i) **Concentration:** The copper pyrites ore is first concentrated, often using froth flotation due to its sulphide nature.
(ii) **Roasting:** The concentrated ore is then roasted in the presence of air to convert some sulphides into oxides. This process removes sulfur as sulfur dioxide. \[ 2\text{CuFeS}_2 + 3\text{O}_2 \xrightarrow{\Delta} \text{Cu}_2\text{S} + 2\text{FeS} + 2\text{SO}_2 \]
(iii) **Smelting:** The roasted ore is mixed with coke and sand \((\text{SiO}_2)\) and heated in a blast furnace. Iron sulphide (FeS) present in the ore is oxidized to ferrous oxide (FeO), which then combines with the added silica to form a fusible slag (iron silicate, FeSiO₃). This slag, being lighter, floats and is removed. The molten product, known as 'matte,' mainly consists of cuprous sulphide \((\text{Cu}_2\text{S})\) and a small amount of FeS. \[ 2\text{FeS} + 3\text{O}_2 \rightarrow 2\text{FeO} + 2\text{SO}_2 \] \[ \text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3 \]
(iv) **Bessemerisation:** The molten matte is transferred to a Bessemer converter. Air, mixed with some sand, is blown through the molten matte. Any remaining FeS is oxidized to FeO, which again combines with silica to form slag. Cuprous sulphide is partially oxidized to cuprous oxide, which then reacts with the remaining cuprous sulphide to produce molten copper. \[ 2\text{Cu}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{Cu}_2\text{O} + 2\text{SO}_2 \] \[ \text{Cu}_2\text{S} + 2\text{Cu}_2\text{O} \rightarrow 6\text{Cu} + \text{SO}_2 \] The resulting molten copper is about 99% pure and is called 'blister copper' due to the rough, blistered surface caused by the escape of sulfur dioxide gas during solidification. 3. Two common copper alloys and their compositions are:
(i) **Brass:** Composed of approximately 60% Copper (Cu) and 40% Zinc (Zn).
(ii) **Bronze:** Composed of approximately 90% Copper (Cu) and 10% Tin (Sn).
(iii) **Gun metal:** Composed of 88% Copper (Cu), 2% Zinc (Zn), and 10% Tin (Sn).
(iv) **Bell metal:** Composed of 80% Copper (Cu) and 20% Tin (Sn).
In simple words: Copper is mainly extracted from copper pyrites by first heating it (roasting), then smelting it to remove iron impurities as slag, and finally converting the remaining copper sulphide to copper in a Bessemer converter. Common copper alloys include Brass (copper and zinc) and Bronze (copper and tin).

🎯 Exam Tip: Focus on the main ore of copper and the purpose of each step in its extraction (roasting, smelting, Bessemerisation), especially the chemical reactions involved in slag formation and copper auto-reduction. Also, know at least two important copper alloys and their constituents.