CBSE Class 12 Chemistry General Principles Process of Isolation of Elements Assignment Set 01

Question. An alkaline earth metal.
(A) Potassium
(B) Calcium
(C) Lead
(D) Copper
Answer: B

Question. Which of the following metallic oxides cannot be reduced by normal reducing agents ?
(A) Magnesium oxide
(B) Copper(II) oxide
(C) Zinc oxide
(D) Iron(III) oxide
Answer: A

Question. When a metal atom becomes an ion
(A) it loses electrons and is oxidized
(B) it gains electrons and is reduced
(C) it gains electrons and is oxidized
(D) it loses electrons and is reduced
(Choose the correct answer from the choices A, B, C and D)
Answer: A

Question. Brass is an alloy of :
(A) Copper and tin
(B) Copper and zinc
(C) Zinc and lead
(D) Lead and tin
Answer: B

Question. The metal oxide which can react with acid as well as alkali is :
(A) Silver oxide
(B) Copper (II) oxide
(C) Aluminium oxide
(D) Calcium oxide
Answer: C

Question. Select the correct answer from A, B, C and D – The property which is true for metals.
(A) Metals are good conductors of electricity
(B) Metals are malleable and ductile
(C) Non-polar covalent compounds are formed from metals.
(D) Metals have 1, 2 or 3 valence electrons.
Answer: C

Question. The amphoteric metallic oxide is
(A) Calcium oxide
(B) Barium oxide
(C) Zinc oxide
(D) Copper (II) oxide
Answer: C

Question. The metals zinc and tin are present in the alloy :
(A) Solder
(B) Brass
(C) Bronze
(D) Duralumin.
Answer: C

Question. Heating an ore in a limited supply of air or in the absence of air at a temperature just below its melting point is known as :
(A) smelting
(B) ore dressing
(C) calcination
(D) bessemerisation
Answer: C

Question. Aluminium powder is used in thermite welding because,
(A) it is a strong reducing agent
(B) it is a strong oxidising agent
(C) it is corrosion resistant
(D) it is a good conductor of heat.
Answer: A

Question. The main ore used for the extraction of iron is :
(A) Haematite
(B) Calamine
(C) Bauxite
(D) Cryolite
Answer: A

Question. Select which is not an alloy of copper :
(A) Brass
(B) Bronze
(C) Solder
(D) Duralumin.
Answer: C

Question. Select the correct answer from A, B, C and D :kThe two main metals in Bronze are :
(A) Copper and zinc
(B) Copper and lead
(C) Copper and nickel
(D) Copper and tin
Answer: D

General Principles and Processes of isolation of elements

LEVEL-1 QUESTION

Question 1. What is a slag?
Answer: Flux + Impurity = Slag
In simple words: Slag is a fusible mass formed when flux combines with non-fusible impurities during smelting.

Exam Tip: Always state that slag is formed by the combination of flux and gangue (impurity).

Question 2. State the role of silica in the metallurgy of copper.
Answer: Act as acidic flux.
In simple words: Silica acts as an acidic flux to remove basic impurities like iron oxide present in copper matte.

Exam Tip: Mention that silica removes basic ferrous oxide (\(\text{FeO}\)) as iron silicate slag (\(\text{FeSiO}_3\)).

Question 3. Write the principle involved in zone refining.
Answer: Impurities are more soluble in melt than in the solid state.
In simple words: Impurities prefer to stay in the molten zone rather than the pure solid metal crystal as the heater moves along the rod.

Exam Tip: This method is used for obtaining ultrapure semiconductors like silicon and germanium.

Question 4. What is the thermodynamics consideration of selecting a reducing agent in metallurgy?
Answer: \(\Delta G\) should be negative.
In simple words: A reducing agent is suitable only if the overall Gibbs free energy change (\(\Delta G\)) for the coupled reaction is negative.

Exam Tip: Refer to Ellingham diagrams to check whether a metal oxide line lies below another at a given temperature.

Question 5. Why froth floatation process is used for concentration sulphide ores
Answer: It is based on the preferential wetting of the ore particles by oil and the gangue particles by water.
In simple words: Sulphide ore particles get wetted and floated by pine oil, whereas gangue particles are wetted and sunk by water.

Exam Tip: Mention both preferential wetting properties (hydrophobic ore, hydrophilic gangue) to score full marks.

Question 6. What is depressant and give one example?
Answer: They are used to separate 2 sulphide ores in froth floatation process. Ex: \(\text{NaCN}\) acts as depressant to separate \(\text{PbS}\) from \(\text{ZnS}\).
In simple words: A depressant selectively prevents one of the sulphide ores from coming into the froth, allowing selective separation.

Exam Tip: In the separation of \(\text{PbS}\) and \(\text{ZnS}\), \(\text{NaCN}\) selectively depresses \(\text{ZnS}\) by forming a soluble complex \(\text{Na}_2[\text{Zn}(\text{CN})_4]\).

Question 7. Explain the following terms: 
(i) Liquation 
(ii) Electrolytic refining
Answer: (i) It is used for low melting metals. Ex: \(\text{Sn}\)
(ii) Anode: impure metal, Cathode: pure metallic strip; Electrolyte: metallic salt solution; by passing electric current pure metals gets deposited on the cathode.
In simple words: Liquation melts low-melting metals on a sloping hearth, while electrolytic refining transfers pure metal from anode to cathode through electricity.

Exam Tip: For liquation, give the example of tin (\(\text{Sn}\)) or bismuth (\(\text{Bi}\)); for electrolytic refining, clearly state the anode and cathode reactions.

Question 8. Define leaching. How is this process used in the benefaction of silver and gold ores?
Answer: Ore is treated with suitable reagent to form soluble complex not the impurities. In extraction of gold and silver \(\text{NaCN}\) forms soluble complex with respective metals. By displacement reaction using \(\text{Zn}\) silver and gold can be separated out.
In simple words: Leaching selectively dissolves the metal ore in a chemical reagent, leaving insoluble impurities behind, and the metal is later recovered by displacement.

Exam Tip: Mention the name of the complex formed (e.g., sodium dicyanoargentate(I)) and use zinc powder as the reducing precipitant.

Question 9. Distinguish the following: 
(i) Roasting and calcination 
(ii) Electrolytic reduction and chemical reduction
Answer: (i) Roasting: heating the ore in presence of air.
Calcination: heating the ore in absence of air.
(ii) Electrolytic reduction: reduction in the presence of electric current.
Chemical reduction: reduction with the help of reductant.
In simple words: Roasting uses excess air for sulphide ores, calcination uses limited or no air for carbonate ores, while electrolytic reduction uses electricity and chemical reduction uses reducing agents like carbon.

Exam Tip: Clearly differentiate based on the presence or absence of air and the type of ore processed (sulphide vs. carbonate/hydrate).

Question 10. Explain the following: 
(i) Van Arkel method 
(ii) Hydraulic washing
Answer: (i) Metal is converted in volatile compound which can be decomposed at high temperature to give pure metal. For example:
\(\text{Zr} + 2\text{I}_2 \xrightarrow{870\text{ K}} \text{ZrI}_4 \xrightarrow{2075\text{ K}} \text{Zr} + 2\text{I}_2\)
(ii) It is based on the difference of densities of ore particles and impurities.
In simple words: Van Arkel purifies metals via volatile iodide decomposition, while hydraulic washing uses a water stream to wash away lighter impurities from heavier ore particles.

Exam Tip: Van Arkel method is specifically important for obtaining ultra-pure titanium (\(\text{Ti}\)) and zirconium (\(\text{Zr}\)).

LEVEL-2 QUESTIONS

Question 1. Name the method used for the refining of Nickel metal
Answer: Mond's Process
In simple words: Nickel is refined by converting it into a volatile carbonyl compound which is then decomposed.

Exam Tip: Remember the equations: nickel reacts with carbon monoxide at \(330-350\text{ K}\) to form volatile nickel tetracarbonyl.

Question 2. (a) Which solution is used for leaching of silver metal in the presence of air in the metallurgy of silver? 
(b) Out of C and \(\text{CO}\), which is a better reducing agent at a lower temperature range in the blast furnace to extract iron from the oxide ore.
Answer: (a) Sodium cyanide (\(\text{NaCN}\))
(b) \(\text{CO}\) is a better reducing agent at the lower temperature because it does not decompose into carbon and oxygen rather gets dioxidized to carbon dioxide, and reduce the metal oxide to metal.
In simple words: Sodium cyanide leaches silver, and carbon monoxide is more effective at lower blast furnace temperatures because its oxidation line lies lower in the Ellingham diagram.

Exam Tip: Specify the exact temperature zone in the blast furnace when explaining carbon monoxide reduction.

Question 3. Explain the following: 
i) NaCN acts as a depressant in preventing ZnS from forming froth. 
ii) Role of cryolite in the metallurgy of aluminium
Answer: i) \(\text{NaCN}\) combines with \(\text{ZnS}\) to form a complex sodiumtetracyanozincate(II) on the surface of \(\text{ZnS}\) and thus prevents it from forming the froth.
\(\text{ZnS} + 4\text{NaCN} \rightarrow \text{Na}_2[\text{Zn}(\text{CN})_4] + \text{Na}_2\text{S}\)
ii) The role of cryolite is two fold:
- It lowers the melting point of the mixture to about \(1250\text{ K}\)
- It increases the electrical conductivity of the mixture.
In simple words: NaCN coats ZnS to stop it from floating, while cryolite makes alumina molten and conductive at lower temperatures.

Exam Tip: Cryolite formula is \(\text{Na}_3\text{AlF}_6\). Always write both functions (melting point lowering and conductivity) for complete credit.

Question 4. The value \(\Delta G^\circ\) for formation of \(\text{Cr}_2\text{O}_3\) is \(-540\text{ kJ/mol}\) and that of \(\text{Al}_2\text{O}_3\) is \(-827\text{ kJ/mol}\). Is the reduction of \(\text{Cr}_2\text{O}_3\) possible with Al?
Answer: Yes, as \(\Delta G\) is more negative.
In simple words: Aluminium can reduce chromium(III) oxide because aluminum oxide is thermodynamically more stable with a more negative free energy of formation.

Exam Tip: A metal with a more negative \(\Delta G^\circ\) of oxide formation can reduce the oxide of a metal having a less negative \(\Delta G^\circ\).

Question 5. Name the chief ore of tin. Write its formula.
Answer: Cassiterite - \(\text{SnO}_2\)
In simple words: The primary mineral source for extracting tin metal is cassiterite, chemically known as tin(IV) oxide.

Exam Tip: Memorize common ores and their formulas (e.g., Bauxite: \(\text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O}\), Haematite: \(\text{Fe}_2\text{O}_3\)).

LEVEL-3 QUESTIONS

Question 1. Why is it advantageous to roast a sulphide ore to oxide before reduction?
Answer: Oxides can be more easily reduced to the respective metals using carbon or hydrogen as reducing agent. On the other hand sulphides cannot be reduced by carbon or hydrogen because \(\Delta_r G\) for such reactions is positive and hence these reactions are not feasible.
In simple words: Metal oxides are thermodynamically much easier to reduce to free metals than metal sulphides using standard reducing agents.

Exam Tip: Mention the thermodynamic feasibility based on Ellingham diagram lines for sulphides versus oxides.

Question 2. Write the chemical reactions involved in the extraction of metallic silver from argentite.
Answer:
\(\text{Ag}_2\text{S} + 4\text{NaCN} \rightarrow 2\text{Na}[\text{Ag}(\text{CN})_2] + \text{Na}_2\text{S}\)
\(4\text{Na}_2\text{S} + 5\text{O}_2 \rightarrow 2\text{Na}_2\text{SO}_4 + 4\text{NaOH} + 2\text{S} \downarrow\)
\(2\text{Na}[\text{Ag}(\text{CN})_2] + \text{Zn} \rightarrow \text{Na}_2[\text{Zn}(\text{CN})_4] + 2\text{Ag} \downarrow\)
Sod. tetracyanozincate (II)
In simple words: Silver is first leached with sodium cyanide in the presence of air to form a soluble complex, from which silver is displaced using zinc dust.

Exam Tip: Practice balancing all three equations (leaching, oxidation of sodium sulphide, and zinc displacement) as they frequently appear together.

Question 3. Explain the role of 
(i) Cryolite in the electrolytic reduction of alumina 
(ii) Carbon monoxide in the purification of nickel.
Answer: (i) Cryolite lowers the melting point of mixture. Or it act as electrolyte.
(ii) Carbon monoxide forms a volatile complex with nickel which on heating decomposes to give pure nickel.
In simple words: Cryolite facilitates smoother electrolysis by lowering the melting point, while carbon monoxide purifies nickel via Mond's process.

Exam Tip: Clearly state that cryolite (\(\text{Na}_3\text{AlF}_6\)) improves conductivity and fluidity of the electrolytic bath.

Question 4. Describe how the following change are brought about: 
(i) Pig iron into steel. 
(ii) Zinc oxide into metallic Zinc. 
(iii) Impure titanium into pure titanium.
Answer: (i) Pig iron converted into steel by adding carbon and some other elements.
(ii) Metallic zinc is obtained from zinc oxide by reduction by coke.
\(\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}\)
(iii) Impure titanium is heated with iodine to form volatile complex \(\text{TiI}_4\) which on further heating to higher temperature decomposes to give pure titanium.
In simple words: Steel is made by adjusting carbon in pig iron, zinc is extracted by carbon reduction, and titanium is purified using the Van Arkel iodine method.

Exam Tip: Mention specific temperatures or additives where applicable, such as using scrap steel or transition metal additives for steel production.

Question 5. Describe the role of 
(i) \(\text{NaCN}\) in the extraction of gold from gold ore. 
(ii) \(\text{SiO}_2\) in the extraction of copper from copper matte. 
(iii) Iodine in refining of zirconium. 
Write chemical equations for the involved reactions.
Answer: (i) Role of \(\text{NaCN}\) in the extraction of gold is to do the leaching of gold ore in the presence of the air from which gold is obtained later by replacement.
\(4\text{Au}(\text{s}) + 8\text{CN}^-(\text{aq}) + 2\text{H}_2\text{O} + \text{O}_2(\text{g}) \rightarrow 4[\text{Au}(\text{CN})_2]^- + 4\text{OH}^-\)
(ii) \(\text{SiO}_2\) is added in copper matte to convert the remaining \(\text{FeS}, \text{FeO}\) to slag.
\(\text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3(\text{slag})\)
(iii) Iodine is heated with Zirconium to form a volatile compound which on further heating decompose to give pure zirconium as shown:
\(\text{Zr} + 2\text{I}_2 \rightarrow \text{ZrI}_4\)
\(\text{ZrI}_4 \rightarrow \text{Zr} + 2\text{I}_2\)
In simple words: NaCN dissolves gold into a soluble complex, silica removes iron impurities as slag, and iodine purifies zirconium via volatile iodide formation.

Exam Tip: Write all phases (s, aq, g) in chemical equations to ensure complete accuracy in metallurgical questions.

Question 7. State briefly the principles which serve as basis for the following operations in metallurgy. 
(i) Froth-floatation process 
(ii) Zone refining 
(iii) Refining by liquation
Answer: (i) Froth floatation process: This process is used for the concentration of sulphide ores. For example, ores of lead, zinc and copper, because of the fact that metallic sulphides are more wetted by certain oils (pine oil) and less by water. The finely crushed ore is put into a water containing a small quantity of oil (pine oil). The mixture is then agitated by passing a blast of air through it. The froth is formed which carries the ore particles along with it to the surface leaving the impurities behind. The froth scummed off. It is then dried for the recovery of the ore particle. In this way the ore is concentrated by froth floatation process.
(ii) Zone Refining: This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed at one end of a rod of impure metal. The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystallizes out the melt and the impurities pass on into the adjacent molten zone. The process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated. This end is cut off. This method is very useful for producing semi-conductor and other metals of very high purity e.g., germanium, silicon, boron, gallium and indium.
(iii) Refining by liquation: In this method a low melting metal like tin (\(\text{Sn}\)) can be made to flow on sloping surface. In this way it is separated from higher melting impurities.
In simple words: Froth floatation separates based on wetting differences with oil and water, zone refining relies on impurity solubility differences in solid vs. liquid states, and liquation uses melting point differences.

Exam Tip: When asked for principles of metallurgical operations, write a comprehensive paragraph covering both the physical principle and the step-by-step mechanism.