12th Chemistry Guide 
Unit 1 - Metallurgy
Book Back Question and Answers 

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 colorless gas. An aqueous solution of (A) is acidic. The gas (A) is
a) CO2
b) SO3
c) SO2
d)H2S
Answer:
c) SO2

3. Which one of the following reaction represents calcinations?
a) 2Zn + O2 → 2ZnO
b) 2ZnS + 3O2 → 2ZnO + 2SO2
c) MgCO3 → MgO + CO2
d) Both (a) and (c)
Answer:
c) MgCOa → 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 – Heroult 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 the reduction is not true?
a) ΔG°f of sulphide is greater than those for CS2 and H2S
b) ΔG°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 I – with the items of Column – II and assign the correct code.


Answer:
c) (iv) (ii) (iii) (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)

10. 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) Heamatite
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. Extraction of gold and silver involves leaching with cyanide ion. silver is later recovered by (NEET – 2017)
a) Distillation
b) Zone refining
c) Displacement with zinc
d) liquation
Answer:
c) Displacement with zinc

16. Considering the Ellingham diagram, which of the following metals can be used to reduce alumina? (NEET – 2018)
a) Fe
b) Cu
c) Mg
d) Zn
Answer:
c) Mg

17. The following set of reactions are used in refining Zirconium

This method is known as
a) Liquation
b) Van Arkel process
c) Zone refining
d) Mond’s process
Answer:
b) Van Arkel process

18. 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)

19. The incorrect statement among the following is
a) Nickel is refined by Mond’s process
b) Titanium is refined by Van Arkel’s process
c) Zinc blende is concentrated by froth floatation
d) In the metallurgy of gold, the metal is leached with a dilute sodium chloride solution
Answer:
d) In the metallurgy of gold, the metal is leached with a dilute sodium chloride solution

20. 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

21. Which of the following plot gives Ellingham diagram
a) ΔS VsT
b) ΔG° VsT
c) ΔG° Vs1/T
d) ΔG° VsT²
Answer:
b) ΔG°VsT

22. In the Ellingham diagram, for the formation of carbon monoxide

Answer: c)

23. Which of the following reduction is not thermodynamically feasible?
a) Cr2O3 + 2Al → Al2O3 + 2Cr
b) Al2O3 + 2Cr → Cr2O3 + 2Al
c) 3TiO2 + 4Al → 2Al2O3 + 3Ti
d) None of these
Answer:
b) Al2O3 + 2Cr → Cr2O3 + 2Al

24. Which of the following is not true with respect to the Ellingham diagram?
a) Free energy changes follow a straight line. The deviation occurs when there is a phase change.
b) The graph for the formation of CO2is a straight line almost parallel to the free energy axis.
c) Negative slope of CO shows that it becomes more stable with an increase in temperature.
d) Positive slope of metal oxides shows that their stabilities decrease with an increase in temperature.
Answer:
b) The graph for the formation of CO2 is a straight line almost parallel to the free energy axis.

II. Answer the following questions

1.What is the difference between minerals and ores?

Answer:

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

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

2.What are the various steps involved in the extraction of pure metals from their ores?

Answer:
Steps involved in the extraction of pure metals from their ores are
Concentration of the ore
Extraction of the crude metal.
Refining of the crude metal.

3.What is the role of Limestone in the extraction of iron from its oxide Fe2O3?

Answer:
Limestone (CaO) is used as a flux in the extraction of iron from its oxide Fe2O3.
Flux is a chemical substance that forms an easily fusible slag with gangue.
Oxide of iron can be reduced by carbon monoxide as follows
Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) ↑
In this extraction, a basic flux quick lime (or) lime (CaO) reacts with acidic gangue silica to form the slag calcium silicate.

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 the froth floatation method.
e.g.,
Copper pyrites (CuFeS2H2)
Zinc blende (ZnS)
Galena (PbS)

5.Describe a method for refining nickel Mond process for refining nickel: 

Answer:
Impure nickel is heated in a stream of carbon monoxide at around 350K. Nickel reacts with CO to form a highly volatile nickel tetracarbonyl. The solid impurities are left behind.
Ni(S) + 4CO(g) → Ni(Co)4(g)
On heating nickel tetra carbonyl around 460K, the complex decomposes to give a pure nickel.
Ni(CO)4(g) → Ni(S)+ 4CO(g)

6.Explain the zone refining process with an example. (PTA – 6 MARCH 2020)

Answer:

• 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.
• 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 the melting of the metal on that portion of the rod.
• 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.
• The process is repeated several times by moving the heater in the same direction again and again to achieve the desired purity level.
• This process is carried out in an inert gas atmosphere to prevent the oxidation of metals.
• Elements such as germanium (Ge), silicon (Si) and gallium (Ga) that are used as semiconductors 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 Fe2O3 by coke at a temperature around 1200K

Answer:
A) i) Ellingham diagram for the formation of Al2O3 and MgO intersects around 1600K. Above this temperature aluminium line lies below the magnesium line. Hence we can use aluminium to reduce magnesia above 1600K.
ii) In Ellingham diagram below 1600K magnesium line lies below aluminium line. Hence below 1600K magnesium can reduce alumina.
B) In Ellingham diagram above 1000K carbon line lies below the iron line. Hence it is possible to reduce Fe2O3 by coke at a temperature around 1200K.

8.Give the uses of zinc. (PTA – 4)

Answer:

• Metallic zinc is used in galvanisation to protect iron and steel structures from rusting and corrosion.
• Zinc is used to produce die – castings in the automobile, electrical and hardware industries.
• Zinc oxide is used in the manufacture of paints, rubber, cosmetics, pharmaceuticals, plastics, inks, batteries, textiles and electrical equipment.
• Zinc sulphide is used in making luminous paints, fluorescent lights and x-ray screens.
• Brass an alloy of zinc which is highly resistant to corrosion is used in water valves and communication equipment.

9.Explain the electrometallurgy of aluminium.

Answer:
Hall – Heroult Process
Cathode: Iron tanked lined with carbon
Anode: Carbon blocks
Electrolyte: 20% solution of alumina obtained from bauxite + Molten Cryolite +10 % calcium chloride (lowers the melting point of the mixture)
Temperature: Above 1270K
Ionisation of Alumina Al2O3 → +2Al3+ + 3O2-
Reaction at cathode: Al3+ (melt) + 3e– → All
Reaction at anode : 2O2- (melt) → O2 (melt) + 4e–
Since carbon acts as anode the following reaction also takes place on it.
C(s) + O2 (melt) → CO + 2e–
C(s) + 2O2 (melt) → CO2 + 4e–
During electrolysis, anodes are slowly consumed due to the above two reactions. Pure aluminium is formed at the cathode and settles at the bottom.
Net electrolysis reaction is
4Al3+ (melt) + 6O2- (melt) + 3C(s) → 4Al(l) + 3CO2(g)

10.Explain the following terms with suitable examples, i) Gangue ii) Slag (PAT – 2)

Answer:
i) Gangue:
The non-metallic impurities, rocky materials and siliceous matter present in the ores are called gangue.
(eg): SiO2 is the gangue present in the iron ore Fe2O3.
ii) Slag:
Slag is a fusible chemical substance formed by the reaction of gangue with a flux.

11.Give the basic requirement for vapour phase refining.

Answer:

• The metal is treated with a suitable reagent to form a volatile compound.
• Then the volatile compound is decomposed to give the pure metal.

12.Describe the role of the following in the process mentioned.

i) Silica in the extraction of copper.
ii) Cryolite in the extraction of aluminium.
iii) Iodine in the refining of Zirconium.
iv) Sodium cyanide in froth floatation.
Answer:
i) In the extraction of copper silica acts as an acidic flux to remove FeO as slag FeSiO3.

ii) As Al2O2 is a poor conductor cryolite improves the electrical conductivity.

In addition, crvolite serves as an added impurity and lowers the melting point of the electrolyte.

iii) First Iodine forms a Volatile tetraiodide with impure metal, which decomposes to give pure metal. Impure zirconium metal is heated in an evacuated vessel with iodine to form the volatile zirconium tetraiodide (Zrl4). The impurities are left behind, as they do not react with iodine.

Zr(S) + 2I2(S) → Zrl4(Vapour)

On passing volatile zirconium tetraiodide vapour over a tungsten filament, it is decomposed to give pure zirconium.

Zrl4(Vapour) → Zrl(S) + 2I2(S)

iv) Sodium cyanide acts as a depressing agent in froth floatation process. It prevent other metal sulphides from coming to the froth.

eg: NaCN depresses the floatation property ZnS present in Galena (PbS) by forming a layer of Zinc complex Na2[Zn(CN)4] on the surface of Zinc sulphide.

13.Explain the principle of electrolytic refining with an example. (PTA – 5)

Answer:

• Crude metal is refined by electrolysis carried out in an electrolytic cell.
• Cathode: Thin strips of pure metal.
• Anode: Impure metal to be refined.
• Electrolyte: Aqueous solution of the salt of the metal with dilute acid.
• As the current is passed, the metal of interest dissolves from the anode and passes into the electrolytic solution.
• At the same time, same amount of metal ions from the electrolytic solution will be deposited at the cathode.
• Less electro positive impurities in the anode settle down as anode mud.

eg: Electrorefining of silver:

Cathode: Pure silver Anode: Impure silver rods.

Electrolyte: Acidified aqueous solution of silver nitrate.

• On passing current the following reactions will take place.
• Reaction at anode: Ag(s) → Ag+(aq) + e–
• Reaction at cathode: Ag+(aq) + e– → Ag(s)
• At anode silver atoms lose electrons and enter the solution. From the solution silver ions migrate towards the cathode. At cathode silver ions get discharged by gaining electrons and deposited on the cathode.

14.The selection of reducing agent depends on the thermodynamic factor: Explain with an example.

Answer:

• A suitable reducing agent is selected based on the thermodynamic considerations.
• For a spontaneous reaction AG should be negative.
• Thermodynamically, the reduction of metal oxide with a given reducing agent can occur if AG for the coupled reaction is negative.
• Hence the reducing agent is selected in such a way that it provides a large negative value for the coupled reaction.
• 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 diagram.
• Ellingham diagram for the formation of FeO and CO intersects around 1000K. Below this temperature the carbon line lies above the iron line.
• Hence FeO is more stable than CO and the reduction is not thermodynamically feasible.
• However above 1000K carbon line lies below the iron line. Hence at this condition, FeO is less stable than CO and the reduction is thermodynamically feasible. So coke can be used as a reducing agent above this temperature.
• Following free energy calculation also confirm that the reduction is thermo¬dynamically favoured.

From theEllingham diagram at 1500K

2Fe(s) + O2(g) → 2FeO(g) = 350 KJmol-1 …………….. 1

2C(s) + 022(g) → 2CO(g) = 480 KJmol-1 ……………… 2

Reverse the reaction 1

2FeO(s) → 2Fe(s) + O2(g) = 350 KJmol-1 ……………… 3

Couple the reactions 2 and 3

2FeO(s) + 2C(s) → 2Fe(s)+ 2CO(g) = 130 KJmol-1 ……………… 4

• The standard tree energy change for the reduction of one mole of FeO is  = -65 KJmol-1

15.Give the limitations of Ellingham diagram.

Answer:

• Ellingham diagram is constructed based only on thermodynamic considerations.
• It gives information about the thermodynamic feasibility of a reaction.
• It does not tell anything about the rate of the reaction.
• Moreover, it does not: give an idea about the possibility of other reactions that might be taking place.
• The interpreparation of G is based on the assumption that the reactants are in equilibrium with the product which is not always true.

16.Write a short note on electrochemical principles of metallurgy.

Answer:

• Reduction of oxides of active metals such as sodium, potassium etc. by carbon is thermodynamically not feasible.
• Such metals are extracted from their ores by using electrochemical methods.
• In this method the metal salts are taken infused form or in solution form.
• The metal ion present can be reduced by treating the solution with a suitable reducing agent or by electrolysis.

Gibbs free energy change for the electrolysis is ∆G° = nFE°
n = number of electrons involved in the reduction
F = Faraday = 96500 coulombs
E° = electrode potential of the redox couple.
If E° is positive, is negative and the reduction is spontaneous.

• Hence a redox reaction is planned in such a way that the e.mi of the net redox reaction is positive.

A more reactive metal displaces a less reactive metal from its salt solution.

eg;Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)

• Zinc is more reactive than copper and displaces copper from its salt solution.