12th Chemistry - Metallurgy- Answers (Unit -1)

 

I. Choose the correct answer.

1. Bauxite has the composition ………………
(a) Al₂O₃
(b) Al₂O₃.nH₂O
(c) Fe₂O₃.2H₂O
(d) None of these
Answer:
(b) Al₂O₃.nH₂O

2. Roasting of sulphide ore gives the gas (A). (A) is a colourless gas. Aqueous solution of (A) is acidic. The gas (A) is ………………
(a) CO₂
(b) SO₃
(c) SO₂
(d) H,S
Answer:
(c) SO₂

3. Which one of the following reaction represents calcination?
(a) 2Zn + O₂ → 2ZnO
(b) 2ZnS + 3O₂ → 2ZnO + 2SO₂
(c) MgCO₃ → MgO + CO₂
(d) Both (a) and (c)
Answer:
(c) MgCO₃ → MgO + CO₂

4. The metal oxide which cannot be reduced to metal by carbon is ………………
(a) PbO
(b) Al₂O₃
(C) ZnO
(d) FeO
Answer:
(b) Al₂O₃

 5. Which of the metal is extracted by Hall-Herold process?
(a) Al
(b) Ni
(c) Cu
(d) Zn
Answer:
(a) Al

6. Which of the following statements, about the advantage of roasting of sulphide ore before reduction is not true?
(a) ∆G_f° of sulphide is greater than those for CS₂ and H₂S.
(b) ∆G_r° is negative for roasting of sulphide ore to oxide.
(c) Roasting of the sulphide to its oxide is thermodynamically feasible.
(d) Carbon and hydrogen are suitable reducing agents for metal sulphides.
Answer:
(d) Carbon and hydrogen are suitable reducing agents for metal sulphides.

7. Match items in column -1 with the items of column – II and assign the correct code:


Answer:
(c) A – (iv), B – (ii), C – (iii), D – (i)

8. Wolframite ore is separated from tinstone by the process of ………………
(a) Smelting
(b) Calcination
(c) Roasting
(d) Electromagnetic separation
Answer:
(d) Electromagnetic separation

9. Which one of the following is not feasible?
(a) Zn(s) + Cu²⁺(aq) → Cu(s) + Zn²⁺(aq)
(b) Cu(s) + Zn²⁺+(aq) → Zn(s) + Cu²⁺(aq)
(c) Cu(s) + 2Ag⁺(aq) → Ag(s) + Cu²⁺(aq)
(d) Fe(s) + Cu²⁺(aq) → Cu(s) + Fe²⁺(aq)
Answer:
(b) Cu(s) + Zn²⁺+(aq) → Zn(s) + Cu²⁺(aq)

9. Electrochemical process is used to extract ………………
(a) Iron
(b) Lead
(c) Sodium
(d) Silver
Answer:
(c) Sodium

11. Flux is a substance which is used to convert ………………
(a) Mineral into silicate
(b) Infusible impurities to soluble impurities
(c) Soluble impurities to infusible impurities
(d) All of these
Answer:
(b) Infusible impurities to soluble impurities

12. Which one of the following ores is best concentrated by froth – floatation method?
(a) Magnetite
(b) Hematite
(c) Galena
(d) Cassiterite
Answer:
(c) Galena

13. In the extraction of aluminium from alumina by electrolysis, cryolite is added to ………………
(a) Lower the melting point of alumina
(b) Remove impurities from alumina
(c) Decrease the electrical conductivity
(d) Increase the rate of reduction
Answer:
(a) Lower the melting point of alumina

14. Zinc is obtained from ZnO by ………………
(a) Carbon reduction
(b) Reduction using silver
(c) Electrochemical process
(d) Acid leaching
Answer:
(a) Carbon reduction

15. Cupellation is a process used for the refining of ………………
(a) Silver
(b) Lead
(c) Copper
(d) Iron
Answer:
(a) Silver

16. Extraction of gold and silver involves leaching with cyanide ion. Silver is later recovered by ………………
(a) Distillation
(b) Zone refining
(c) Displacement with zinc
(d) liquation
Answer:
(c) Displacement with zinc

17. Considering Ellingham diagram, which of the following metals can be used to reduce alumina?
(a) Fe
(b) Cu
(c) Mg
(d) Zn
Answer:
(c) Mg

18.

 
(a) Liquation
(b) Van Arkel process
(c) Zone refining
(d) Monds process
Answer:
(b) Van Arkel process

19. Which of the following is used for concentrating ore in metallurgy?
(a) Leaching
(b) Roasting
(c) Froth floatation
(d) Both (a) and (c)
Answer:
(d) Both (a) and (c)

20. The incorrect statement among the following is ………………
(a) Nickel is refined by Monds process
(b) Titanium is refined by Van Arkels process
(c) ZinC blende is concentrated by froth floatation
(d) In the metallurgy of gold, the metal is leached with dilute sodium chloride solution
Answer:
(d) In the metallurgy of gold, the metal is leached with dilute sodium chloride solution

21. In the electrolytic refining of copper, which one of the following is used as anode?
(a) Pure copper
(b) Impure copper
(c) Carbon rod
(d) Platinum electrode
Answer:
(b) Impure copper

22. Which of the following plot gives Ellingham diagram?
(a) ∆S Vs T
(b) ∆G° Vs T
(c) ∆G° Vs
(d) ∆G° Vs T
Answer:
(b) ∆G° Vs T

23. In the Ellingham diagram, for the formation of carbon monoxide
(a) (△S0△T)is negative
(b) (△G0△T)is positive
(c) (△G0△T)is negative
(d) initially (△T△G0)is positive, after 700°C, (△G0△T)is negative
Answer:
(c) (△G0△T)is negative

24. Which of the following reduction is not thermodynamic ally feasible?
(a) Cr₂O₃ → Al₂O₃ + 2Cr
(b) Al₂O₃ → Cr₂O₃ + 2Al
(c) 3TiO₂ + 4Al → 2Al₂O₃ + 2Al
(d) none of these
Answer:
(b) Al₂O₃ → Cr₂O₃ + 2Al

25. Which of the following is not true with respect to Ellingham diagram?
(a) Free energy changes follow a straight line. Deviation occurs when there is a phase change.
(b) The graph for the formation of CO₂ is a straight line almost parallel to free energy axis.
(c) Negative slope of CO shows that it becomes more stable with increase in temperature.
(d) Positive slope of metal oxides shows that their stabilities decrease with increase in temperature.
Answer:
(b) The graph for the fonnation of CO₂ is a straight line almost parallel to free energy axis.

II. Answer the following questions:

1. What is the difference between minerals and ores?
Answer:
Minerals:

1. Minerals contain a low percentage of metal.
2. Metal cannot be extracted easily from minerals.
3. Clay Al₂O₃. SiO₂. 2H₂O is the mineral of aluminium.

Ores:

1. Ores contain a large percentage of metal.
2. Ores can be used for the extraction of metals on a large scale readily and economically.
3. Bauxite Al₂O₃. 2H₂O is the ore of aluminium.

2. What are the various steps involved in extraction of pure metals from their ores?
Answer:
The extraction of pure metals from the concentrated ores is carried out in two steps:

1. Conversion of the ore into oxides of the metal of interest.
2. Reduction of the metal oxides to elemental metals.

3. What is the role of Limestone in the extraction of Iron from its oxide Fe₂O₃?
Answer:
In the extraction of iron, a basic flux limestone is used. Limestone decomposes to form CaO which reacts with silica gangue present in the iron ore is acidic in nature to form calcium silicate (slag).

4. Which type of ores can be concentrated by froth floatation method? Give two examples for such ores.
Answer:
Sulphide ores can be concentrated by froth floatation method, e.g.,

1. Copper pyrites (CuFeS₂H₂)
2. Zinc blende (ZnS)
3. Galena (PbS)

5. Describe a method for refining nickel.

Answer:
The impure nickel is heated in a stream of carbon monoxide at around 350K. The nickel reacts with the CO to form a highly volatile nickel tetracarbonyl. The solid impurities are left behind.
Ni (s) + 4 CO (g) → Ni(CO)₄(g)

On heating the nickel tetracarbonyl around 460 K, the complex decomposes to give pure metal.
Ni(CO)₄ (g) → Ni (s) + 4 CO (g)

6. Explain zone refining process with an example using the Ellingham diagram given below.
Answer:
1. Zone Refining method is based on the principles of fractional crystallisation. When an impure metal is melted and allowed to solidify, the impurities will prefer to be in the molten region, i.e. impurities are more soluble in the melt than in the solid state metal.

2. In this process, the impure metal is taken in the form of a rod. One end of the rod is heated using a mobile induction heater which results in melting of the metal on that portion of the rod.

3. When the heater is slowly moved to the other end the pure metal crystallises while the impurities will move on to the adjacent molten zone formed due to the movement of the heater. As the heater moves further away, the molten zone containing impurities also moves along with it.

4. The process is repeated several times by moving the heater in the same direction again and again to achieve the desired purity level.

5. This process is carried out in an inert gas atmosphere to prevent the oxidation of metals.

6. Elements such as germanium (Ge), silicon (Si) and galium (Ga) that are used as semiconductor are refined using this process.

7. Using the Ellingham diagram,
(A) Predict the conditions under which
(i) Aluminium might be expected to reduce magnesia.
(ii) Magnesium could reduce alumina.
(B) it is possible to reduce Fe2 3 O by coke at a temperature around 1200K

Answer:
1. The conditions under which:
(a) Ellingham diagram is used to predict thermodynamic feasibility of reduction of oxides of one metal by another metal. Any metal can reduce the oxides of other metals that are located above it in the Ellingham diagram. In the Ellingham diagram, for the formation of magnesia (magnesium oxide) occupy lower position than aluminium oxide. Therefore aluminium cannot be used to reduce the oxides of magnesium (magnesia). Above 1623K, A1 can reduce MgO to Mg, so that ArG° becomes negative and the process becomes thermodynamically feasible.

3MgO +2A1  1623 K A1₂O_{3 +3Mg}

(b)

• (43)Al + O₂ → (23)Al₂O₃
• 2Mg + O₂ → 2MgO

At the point of intersection of the Al₂O₃ and MgO curves in Ellingham diagram. ∆G°
becomes zero for the reaction:

(23)Al₂O₃ → 2MgO + (43)Al
Below that point magnesium can reduce alumina.

2. From the Ellingham diagram, we find that at 983 K, the curves intersect.

The value of ∆G° for change of C to CO₂ is less than the value of ∆G° for change of CO to CO₂. Therefore, coke (C) is a better reducing agent than CO at 983K or above temperature. However, below this temperature (e.g., at 673K), CO is more effective reducing agent than C.

3. Yes, it is possible to reduce Fe₂O₃ by coke at a temperature around 1200 K. In the Ellingham diagram, carbon line cuts across the lines of many metal oxides and hence it can reduce all those metal oxides at sufficiently high temperature. Ellingham diagram for the formation of Fe₂O₃ and CO intersects around 1000 K.

Below this temperature, the carbon line lies above the iron line which indicates that Fe₂O₃ is more stable than CO and hence at this temperature range the reduction is not thermodynamically feasible. However above 1000 K carbon line lies below the iron line and hence we can use coke as a reducing agent around 1200 K. Around 1200 K, coke is better reducing agent because above 1000 K, Gibb’s free energy for the formation of Fe₂O₃ is more than the formation of CO₂ from C.

8. Give the uses of zinc.
Answer:
Applications of Zinc (Zn):

1. Metallic zinc is used in galvanizing metals such as iron and steel structures to protect them from rusting and corrosion.
2. Zinc is also used to produce die-castings in the automobile, electrical and hardware industries.
3. Zinc oxide is used in the manufacture of many products such as paints, rubber, cosmetics, pharmaceuticals, plastics, inks, batteries, textiles arid electrical equipment. Zinc sulphide is used in making luminous paints, fluorescent lights and x-ray screens.
4. Brass an alloy of zinc is used in water valves and communication equipment as it is highly resistant to corrosion.

10. Explain the electrometallurgy of aluminium.
Answer:
Electrochemical extraction of aluminium Hall-Herold process:
In this method, electrolysis is carried out in an iron tank lined with carbon, which acts as a cathode. The carbon blocks immersed in the electrolyte acts as a anode. A 20% solution of alumina, obtained from the bauxite ore is mixed with molten cryolite and is taken in the electrolysis chamber. About 10%, calcium chloride is also added to the solution. Here calcium chloride helps to lower the melting point of the mixture. The fused mixture is maintained at a temperature of above 1270 K. The chemical reactions involved in this process are as follows:

Ionisation of alumina:
Al₂O₃ → 2Al₃ + 3OO^2-

Reaction at cathode:
2Al³⁺ (melt) + 3e^– → Al_(l)

Reaction at anode:
2O^2- (melt) → O₂ + 3e^–

Since carbon acts as anode the following reaction also takes place on it.

• C (s) + O^2- (melt) → CO + 2e^–
• C (s) + 2O^2- (melt) → CO₂ + 4e^–

Due to the above two reactions, anodes are slowly consumed during the electrolysis. The pure aluminium is formed at the cathode and settles at the bottom. The net electrolysis reaction can be written as follows:
4Al³⁺ (melt) + 6O^2- (melt) + 3C_(s) → 4A_(l) + 3CO_2(g)

 

 

 

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