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Chapter 6

Metallurgy

Class 10 - Dalal Simplified ICSE Chemistry Solutions


Equation Worksheet

Question 1

Extraction of Metals — General details of stages involvedComplete and balance the equation
Step 1 : Dressing of the ore   
Step 2 : Conversion of conc. ore to its oxide   
Roasting of sulphide ore   
1. Roasting of zinc blendeZnS+O2800°C...............+...............[g]\text{ZnS} + \text{O}_2 \xrightarrow{800 \degree\text{C}} ............... + ............... [\text{g}]
2. Roasting of iron pyriteFeS2+O2...............+...............[g]\text{FeS}_2 + \text{O}_2 \rightarrow ............... + ............... [\text{g}]
Calcination of carbonate ore   
3. Calcination of zinc carbonateZnCO3<400°C...............+...............[g]\text{ZnCO}_3 \xrightarrow{\lt 400 \degree\text{C}} ............... + ............... [\text{g}]
4. Calcination of iron [II] carbonateFeCO3...............+...............[g]\text{FeCO}_3 \rightarrow ............... + ............... [\text{g}]
Step 3: Reduction of metallic oxide to its oxide 
Reduction by electrolysis 
5. Electrolytic reduction of pure aluminaAl2O3 ⇌ ............... + ...............
6. At cathode2Al3+ + 6e- ⟶ ...............
Reduction by reducing agents [C, CO, H2] 
7. Reduction of zinc oxide - by cokeZnO+CΔ...............+...............[g]\text{ZnO} + \text{C} \xrightarrow{\Delta} ............... + ............... [\text{g}]
8. Reduction of iron [III] oxide - by COFe2O3+COΔ...............+...............[g]\text{Fe}_2\text{O}_3 + \text{CO} \xrightarrow{\Delta} ............... + ............... [\text{g}]
9. Reduction of lead [II] oxide - by cokePbO+CΔ...............+...............[g]\text{PbO} + \text{C} \xrightarrow{\Delta} ............... + ............... [\text{g}]
10. Reduction of copper [II] oxide - by cokeCuO+CΔ...............+...............[g]\text{CuO} + \text{C} \xrightarrow{\Delta} ............... + ............... [\text{g}]
11. Reduction of copper [II] oxide - by hydrogenCuO+H2Δ...............+...............[g]\text{CuO} + \text{H}_2 \xrightarrow{\Delta} ............... + ............... [\text{g}]
Reduction by thermal decomposition
12. Thermal decomposition of mercury [II] oxideHgOΔ...............+...............[g]\text{HgO} \xrightarrow{\Delta} ............... + ............... [\text{g}]
13. Thermal decomposition of silver [I] oxideAg2OΔ...............+...............[g]\text{Ag}_2\text{O} \xrightarrow{\Delta} ............... + ............... [\text{g}]
Step 4: Electrorefining of impure metals 

Answer

Extraction of Metals — General details of stages involvedComplete and balance the equation
Step 1 : Dressing of the ore   
Step 2 : Conversion of conc. ore to its oxide   
Roasting of a sulphide ore   
1. Roasting of zinc blende2ZnS+3O2800°C2ZnO+2SO2[g]2\text{ZnS} + 3\text{O}_2 \xrightarrow{800 \degree\text{C}} \bold{2ZnO} + \bold{2SO}_\bold{2}[\text{g}]
2. Roasting of iron pyrite4FeS2+11O22Fe2O3+8SO2[g]4\text{FeS}_2 + 11\text{O}_2 \rightarrow \bold{2Fe}_\bold{2}\bold{O}_\bold{3} + \bold{8SO}_\bold{2} [\text{g}]
Calcination of a carbonate ore   
3. Calcination of zinc carbonateZnCO3<400°CZnO+CO2[g]\text{ZnCO}_3 \xrightarrow{\lt 400 \degree\text{C}} \bold{ZnO} + \bold{CO}_\bold{2} [\text{g}]
4. Calcination of iron [II] carbonateFeCO3FeO+CO2[g]\text{FeCO}_3 \rightarrow \bold{FeO} + \bold{CO}_\bold{2} [\text{g}]
Step 3: Reduction of metallic oxide to its oxide 
Reduction by electrolysis 
5. Electrolytic reduction of pure aluminaAl2O32Al3+ + 3O2-
6. At cathode2Al3+ + 6e-2Al
Reduction by reducing agents [C, CO, H2] 
7. Reduction of zinc oxide - by cokeZnO+CΔZn+CO[g]\text{ZnO} + \text{C} \xrightarrow{\Delta} \bold{Zn} + \bold{CO} [\text{g}]
8. Reduction of iron [III] oxide - by COFe2O3+3COΔ2Fe+3CO2[g]\text{Fe}_2\text{O}_3 + 3\text{CO} \xrightarrow{\Delta} \bold{2Fe} + \bold{3CO}_\bold{2} [\text{g}]
9. Reduction of lead [II] oxide - by coke2PbO+CΔ2Pb+CO2[g]2\text{PbO} + \text{C} \xrightarrow{\Delta} \bold{2Pb} + \bold{CO}_\bold{2} [\text{g}]
10. Reduction of copper [II] oxide - by cokeCuO+CΔCu+CO[g]\text{CuO} + \text{C}\xrightarrow{\Delta} \bold{Cu} + \bold{CO} [\text{g}]
11. Reduction of copper [II] oxide - by hydrogenCuO+H2ΔCu+H2O[g]\text{CuO} + \text{H}_2 \xrightarrow{\Delta} \bold{Cu} + \bold{H}_\bold{2}\bold{{O}} [\text{g}]
Reduction by thermal decomposition 
12. Thermal decomposition of mercury [II] oxide2HgOΔ2Hg+O2[g]2\text{HgO} \xrightarrow{\Delta} \bold{2Hg} + \bold{O}_\bold{2} [\text{g}]
13. Thermal decomposition of silver [I] oxide2Ag2OΔ4Ag+O2[g]2\text{Ag}_2\text{O} \xrightarrow{\Delta} \bold{4Ag} + \bold{O}_\bold{2} [\text{g}]
Step 4: Electrorefining of impure metals 

Question 2

Extraction of Aluminium 
Step 1 : Dressing of the ore chemical method 
Baeyer's process - Step 1, Step 2, Step 3 
14. Bauxite to - sodium aluminateAl2O3.2H2O+NaOH150200°C...............+...............\text{Al}_2\text{O}_3.2\text{H}_2\text{O} + \text{NaOH} \xrightarrow{150-200 \degree\text{C}} ............... + ...............
15. Sodium aluminate to - aluminium hydroxideNaAlO2+H2O5060°C...............+...............\text{NaAlO}_2 + \text{H}_2\text{O} \xrightarrow{50-60 \degree\text{C}} ............... + ............... \downarrow
16. Aluminium hydroxide to pure aluminaAl(OH)31100°C...............+...............\text{Al(OH)}_3 \xrightarrow{1100 \degree\text{C}} ............... + ...............
Step 2: Conversion of conc. ore to its oxide   
Step not necessary since the ore is already an oxide   
Step 3: Reduction of metallic oxide to its metal   
Hall Heroult's Process - Reduction by electrolysis   
17. Electrolytic reduction of pure aluminaAl2O3 ⇌ ............... + ...............
18. At cathode2Al3+ + 6e- ⟶ ...............
19. At anode3O2- - 6e- ⟶ 3[...............] ⟶ 3O2
Step 4: Electrorefining of aluminium [reference] 

Answer

Extraction of Aluminium 
Step 1 : Dressing of the ore chemical method 
Baeyer's process - Step 1, Step 2, Step 3 
14. Bauxite to - sodium aluminateAl2O3.2H2O+2NaOH150200°C2NaAlO2+3H2O\text{Al}_2\text{O}_3.2\text{H}_2\text{O} + \text{2NaOH} \xrightarrow{150-200 \degree\text{C}} \bold{2NaAlO}_\bold{2} + \bold{3H}_\bold{2}\bold{{O}}
15. Sodium aluminate to - aluminium hydroxideNaAlO2+2H2O5060°CNaOH+Al(OH)3\text{NaAlO}_2 + \text{2H}_2\text{O} \xrightarrow{50-60 \degree\text{C}} \bold{NaOH} + \bold{Al(OH)}_\bold{3}\downarrow
16. Aluminium hydroxide to pure alumina2Al(OH)31100°CAl2O3+3H2O\text{2Al(OH)}_3 \xrightarrow{1100 \degree\text{C}} \bold{Al}_\bold{2}\bold{O}_\bold{3} + \bold{3H}_\bold{2}\bold{{O}}
Step 2: Conversion of conc. ore to its oxide   
Step not necessary since the ore is already an oxide   
Step 3: Reduction of metallic oxide to its metal   
Hall Heroult's Process - Reduction by electrolysis   
17. Electrolytic reduction of - pure aluminaAl2O32Al3+ + 3O2-
18. At cathode2Al3+ + 6e-2Al
19. At anode3O2- - 6e- ⟶ 3[O] ⟶ 3O2
Step 4: Electrorefining of aluminium [reference] 

Questions

Question 1(2005)

A to F below relate to the source and extraction of either Zinc or aluminium

A : Bauxite,
B : Coke,
C : Cryolite,
D : Froth floatation,
E : Sodium hydroxide solution,
F : Zinc blende.

(i) Write down the three letters each from the above list which are relevant to – Aluminium.

(ii) Fill in the blanks using the most appropriate words from A to F :

(a) The ore from which aluminium is extracted must first be treated with ............... so that pure Aluminium oxide can be obtained.

(b) Pure Aluminium oxide is dissolved in ............... to make a conducting solution.

(iii) Write the formula of Cryolite.

Answer

(i) A : Bauxite and C : Cryolite are ores of aluminium. E : Sodium hydroxide solution is used in the dressing of the ore.

(ii) Fill in the blanks :

(a) The ore from which aluminium is extracted must first be treated with sodium hydroxide solution so that pure Aluminium oxide can be obtained.

(b) Pure Aluminium oxide is dissolved in Cryolite to make a conducting solution

(iii) Na3AlF6 — formula of Cryolite

Question 2(2005)

Calcium, Copper, Lead, Aluminium, Zinc, Chromium, Magnesium, Iron. Choose the major metals from the list given above to make the following alloys :

(i) Stainless steel

(ii) Brass.

Answer

(i) Iron and Chromium are used to make stainless steel

(ii) Copper and Zinc are used to make brass.

Question 1(2006)

Name the following :

(i) A metal which is liquid at room temperature.

(ii) A compound added to lower the fusion temp. of the electrolytic bath in the extraction of Al.

(iii) The process of heating an ore to a high temperature in the presence of air.

Answer

(i) Mercury is liquid at room temperature.

(ii) Cryolite [Na3AlF6] is added to lower the fusion temp, of the electrolytic bath in the extraction of Al

(iii) Roasting is the process of heating an ore to a high temperature in the presence of air.

Question 2(2006)

A strip of copper is placed in four different colourless salt solutions. They are KNO3, AgNO3, Zn(NO3)2, Ca(NO3)2. Which one of the solutions will finally turn blue.

Answer

Silver Nitrate (AgNO3)
Reason — Copper displaces silver from AgNO3 soln. forming the blue copper [II] nitrate soln.

Question 3(2006)

When a metal atom becomes an ion : [Choose the correct answer from the choices A, B, C and D]

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

Answer

it loses electrons and is oxidized

Reason — metals readily give the valence electron during reactions and are hence oxidised.

Question 1(2007)

In the extraction of aluminium :

(i) Which soln. is used to react with bauxite as a first step in obtaining pure aluminium oxide.

(ii) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. Write the equation for this reaction.

(iii) Name the element which serves as the anode and cathode in the extraction of aluminium.

(iv) Write equation for the reaction at the cathode during the extraction of aluminium by electrolysis.

(v) Give the equation for the reaction at the anode when aluminium is purified by electrolysis.

Answer

(i) Sodium hydroxide is used to react with bauxite as a first step in obtaining pure aluminium oxide.

(ii) Conversion of Al(OH)3 - to pure alumina by heating Al(OH)3

2Al(OH)3[aluminium hydroxide]1100°CAl2O3[pure alumina]+3H2O\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100\degree\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}

(iii) Element of Anode and Cathode are:

  • Cathode — Carbon lining [gas carbon] of the cell.
  • Anode — Thick carbon rods (graphite) attached to copper clamps dipping into fused electrolyte.

(iv) 2Al3+ + 6e- ⟶ 2Al

(v) Al - 3e- ⟶ Al3+ (Hoope's process - Electrorefining of Aluminium)

Question 1(2008)

Brass is an alloy of :

  1. Copper and tin
  2. Copper and zinc
  3. Zinc and lead
  4. Lead and tin

Answer

Copper and zinc

Question 2(2008)

The following is a sketch of an electrolytic cell used in the extraction of aluminium :

The following is a sketch of an electrolytic cell used in the extraction of aluminium. What is the substance of which the electrodes A and B are made? At which electrode (A or B) is the aluminium formed? What are the two aluminium compounds in the electrolyte C? Why is it necessary for electrode B to be continuously replaced? Metallurgy, Simplified Chemistry Dalal Solutions ICSE Class 10.

(i) What is the substance of which the electrodes A and B are made?

(ii) At which electrode (A or B) is the aluminium formed ?

(iii) What are the two aluminium compounds in the electrolyte C ?

(iv) Why is it necessary for electrode B to be continuously replaced ?

Answer

(i) Electrodes A and B are made of graphite (carbon).

(ii) Aluminium is formed at cathode (i.e., electrode A).

(iii) Fused alumina (Al2O3) and Cryolite (Na3AlF6) are the compounds in the electrolyte C.

(iv) The graphite [carbon] anodes (i.e electrode B) are continuously replaced during the electrolysis because :

  1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
  2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 2C + O2 ⟶ 2CO [2CO + O2 ⟶ 2CO2]
  3. The carbon anode is hence consumed and renewed periodically after a certain period of usage.

Question 1(2009)

The metal oxide which can react with acid as well as alkali is :

  1. Silver oxide
  2. Copper [II] oxide
  3. Aluminium oxide
  4. Calcium oxide

Answer

Aluminium oxide

Reason — Amphoteric oxides like aluminium oxide react with acids as well as alkalis giving salt and water.

Question 2(2009)

Correct the following statement — Haematite is the chief ore of aluminium.

Answer

Haematite is the chief ore of iron.

Or

Bauxite is the chief ore of aluminium.

Question 3(2009)

The sketch illustrates refining of aluminium by Hoope's process. [reference]

The sketch illustrates refining of aluminium by Hoope's process. Which of A and B is the cathode and which one is the anode? What is the electrolyte in the tank? What material is used for the cathode? Metallurgy, Simplified Chemistry Dalal Solutions ICSE Class 10.

(i) Which of A and B is the cathode and which one is the anode?

(ii) What is the electrolyte in the tank?

(iii) What material is used for the cathode?

Answer

(i) A is cathode and B is anode.

(ii) Mixture of cryolite, BaF2, AlF, CaF2 serves as the electrolyte

(iii) Pure molten aluminium with carbon electrodes serves as the cathode.

Question 4(2009)

State the property of the metal utilized in Aluminium in Thermite welding.

Answer

Aluminium is a strong reducing agent and has a strong affinity for oxygen. It reduces Fe2O3, to Fe which fills the gap. Hence it is used for thermite (Fe2O3) welding.

Question 1(2010)

State the main constituent metal in each alloy :

(i) Duralumin

(ii) Brass

(iii) Stainless steel

Answer

(i) Aluminium

(ii) Copper

(iii) Iron

Question 2(2010)

Select the correct property which is not true for metals:

  1. Metals are good conductors of electricity
  2. Metals are malleable and ductile
  3. Non-polar covalent compounds are formed from metals
  4. Metals have 1, 2 or 3 valence electrons.

Answer

Non-polar covalent compounds are formed from metals.

Reason — A non-polar covalent compound is formed when electrons are shared equally between two atoms and metals generally donate electrons and form bonds. Hence, they do not form non-polar covalent compounds

Question 1(2011)

Choose from the following list of the substances - Acetylene gas, aqua fortis, coke, brass, barium chloride, bronze, platinum. The substance which is an alloy of zinc, copper and tin.

Answer

Bronze is an alloy of zinc, copper and tin.

Question 2(2011)

  1. Name a metal found abundantly in the earth's crust.
  2. Difference between calcination and roasting?
  3. Name the process used for the enrichment of sulphide ore.
  4. Write the chemical formulae of one main ore each of iron and aluminium.
  5. Write the constituents of the electrolyte for the extraction of aluminium.

Answer

  1. Aluminium is found abundantly in the earth's crust.

  2. Difference between calcination and roasting is as follows:

CalcinationRoasting
It is the process of heating the concentrated ore in a limited supply or absence of air at a temperature not sufficient to melt the ore.It is the process of heating the concentrated ore in the presence of excess air to a high temperature before it is reduced to a metal.
Generally carbonate ores are calcinated and hence carbon dioxide is given outGenerally sulphide ores are roasted and hence sulphur dioxide is given out.
  1. Froth floatation process

  2. Chemical formula of ores are :
    Haematite ⟶ Fe2O3 : Ore of iron
    Bauxite ⟶ Al2O3.2H2O : Ore of aluminium

  3. Constituents of electrolyte are :
    Fused Alumina (Al2O3) — 1 part by weight.
    Cryolite (Na3AlF6) — 3 part by weight.
    Fluorspar (CaF2) —1 part by weight.

Question 1.1(2012)

An alkaline earth metal.

  1. Potassium
  2. Calcium
  3. Lead
  4. Copper

Answer

Calcium

Reason — It belongs to the group II A of the periodic table, hence it is an alkaline earth metal.

Question 1.2(2012)

The metallic oxide/s which cannot be reduced by normal reducing agents ?

  1. Magnesium oxide
  2. Copper (II) oxide
  3. Zinc oxide
  4. Iron (III) oxide

Answer

Magnesium oxide

Reason — They are most electropositive and have strong affinity for oxygen. Hence, they cannot be reduced by normal reducing agents

Question 2(2012)

Match the properties and uses of alloys in List 1 with the appropriate answer from List 2.

List 1:

  1. The alloy contains Cu and Zn, is hard and is used in decorative articles.
  2. It is stronger than Aluminium, light and is used in making light tools.
  3. It is lustrous, hard, corrosion resistant and used in surgical instruments.
  4. Tin lowers the melting point of the alloy and is used for soldering purpose.
  5. The alloy is hard, brittle, takes up polish and is used for making statues.

List 2 :

(A) Duralumin

(B) Brass

(C) Bronze

(D) Stainless steel

(E) Solder

Answer

  1. The alloy contains Cu and Zn, is hard and is used in decorative articles. — (B) Brass
  2. It is stronger than Aluminium, light and is used in making light tools. — (A) Duralumin
  3. It is lustrous, hard, corrosion resistant and used in surgical instruments. — (D) Stainless steel
  4. Tin lowers the melting point of the alloy and is used for soldering purpose. — (E) Solder
  5. The alloy is hard, brittle, take up polish and is used for making statues. — (C) Bronze

Question 3(2012)

Name a metal present in cryolite other than sodium.

Answer

Aluminium

Question 4(2012)

The following questions are relevant to the extraction of Aluminium :

(i) State the reason for addition of caustic alkali to bauxite ore during purification of bauxite.

(ii) Give a balanced chemical equation for the above reaction.

(iii) Along with cryolite and alumina, another substance is added to the electrolyte mixture. Name the substance and give one reason for the addition.

Answer

(i) Bauxite is reacted with a conc. solution of NaOH (caustic alkali) under pressure for 2 hrs as a first step in obtaining Al2O3.
The impurities present in bauxite mainly Fe2O3 and SiO2 remain unaffected with conc. NaOH as impurities are not amphoteric.
Bauxite, being amphoteric reacts with the base forming salt [sodium aluminate] and water. Hence, impurities are separated out. Therefore, caustic alkali is added to bauxite ore during purification of bauxite.

(ii) Balanced chemical equation for the conversion of impure bauxite to sodium aluminate is given below:

Al2O3.2H2[Impure bauxite]Amphoteric oxide+2NaOH [conc. soln]Base150200°C2NaAlO2[sodium aluminate]Salt+3H2OWater\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

(iii) Fluorspar is added to the electrolyte mixture as it lowers the fusion point of the mixture i.e., the mixture fuses around 950°C instead of 2050°C.

Question 1(2013)

The amphoteric metallic oxide is :

  1. Calcium oxide
  2. Barium oxide
  3. Zinc oxide
  4. Copper (II) oxide

Answer

Zinc oxide is an amphoteric oxide. It reacts with acids and alkalis giving salt and water.

Question 2(2013)

The metals zinc and tin are present in the alloy :

  1. Solder
  2. Brass
  3. Bronze
  4. Duralumin.

Answer

Bronze

Question 3(2013)

Using the information below about X and Y – complete the following questions :

X-2,8,7Y-2,8,2
Dissolves in water and turns blue litmus redVery low solubility in water. Dissolves in HCl
Tends to oxidise elements and compoundsTends to act as a reducing agent
Poor electrical and thermal conductivityGood electrical and thermal conductivity
No tendency to form alloysForms alloys

(i) ............... is the metallic element.

(ii) Metal atoms tend to have a maximum of ............... electrons in the outermost energy level.

(iii) Non-metallic elements tend to form ............... oxides while metals tend to form ............... oxides.

(iv) Non-metallic elements tend to be ............... conductors of heat and electricity.

(v) Metals tend to ............... electrons and act as ............... agents in their reactions with elements and compounds.

Answer

(i) Y is the metallic element.

(ii) Metal atoms tend to have a maximum of three electrons in the outermost energy level.

(iii) Non-metallic elements tend to form acidic oxides while metals tend to form basic oxides.

(iv) Non-metallic elements tend to be bad conductors of heat and electricity.

(v) Metals tend to lose electrons and act as reducing agents in their reactions with elements and compounds.

Question 4(2013)

The following questions relate to the extraction of aluminium by electrolysis :

(i) Name the other aluminium containing compound added to alumina and state it's significance.

(ii) Give the equation for the reaction that takes place at the cathode.

(iii) Explain why is it necessary to renew the anode periodically.

Answer

(i) The compound is cryolite [Na3AlF6]. It —

  1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
  2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
  3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(ii) 2Al3+ + 6e- ⟶ 2Al

(iii) The anodes are continuously replaced during the electrolysis because :

  1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
  2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 2C + O2 ⟶ 2CO [2CO + O2 ⟶ 2CO2]
  3. The carbon anode is hence consumed and renewed periodically after a certain period of usage.

Question 1.1(2014)

Heating an ore in a limited supply of air or in the absence of air at a temperature just below it's melting point is known as :

  1. Smelting
  2. Ore dressing
  3. Calcination
  4. Bessemerisation

Answer

Calcination

Question 1.2(2014)

Aluminium powder is used in thermite welding because :

  1. it is strong reducing agent
  2. it is strong oxidizing agent
  3. it is corrosion resistant
  4. it is a good conductor of heat.

Answer

it is strong reducing agent.

Reason — Aluminium is a strong reducing agent, it reduces Fe2O3, to Fe which fills the gap. Hence it is used for thermite (Fe2O3) welding.

Question 2(2014)

State the main components of the following alloys :

(i) Brass

(ii) Duralumin

(iii) Bronze.

Answer

(i) Brass — Cu [60-80%], Zn [40-20%]

(ii) Duralumin — Al [95%], Mg [0.5%], Mn [0.5%], Cu [4%]

(iii) Bronze — Cu [80%], Zn [1%], Sn [19%]

Question 3(2014)

Name the following :

(i) The property possessed by metals by which they can be beaten into sheets.

(ii) A compound added to lower the fusion temperature of electrolytic bath in the extraction of aluminium.

(iii) The ore of zinc containing its sulphide.

Answer

(i) Malleability

(ii) Cryolite (Na3AlF6)

(iii) Zinc Blende (ZnS).

Question 1(2015)

Select which is not an alloy of copper :

  1. Brass
  2. Bronze
  3. Solder
  4. Duralumin.

Answer

Solder

Reason — Solder is an alloy of lead and tin.

Question 2(2015)

Give scientific reasons for: Zinc oxide can be reduced to zinc by using carbon monoxide, but aluminium oxide cannot be reduced by a reducing agent.

Answer

Metals like Zinc are less electropositive and have less affinity for oxygen, hence their oxides like ZnO can be reduced by reducing agents like CO. Metal like Aluminium on the other hand are most electropositive and have strong affinity for oxygen. Hence, their oxides like aluminium oxide cannot be reduced by a reducing agent.

Question 3(2015)

From the oxides — SO2, SiO2, Al2O3, MgO, CO, Na2O — Select

(i) A basic oxide

(ii) An amphoteric oxide.

Answer

(i) Na2O

(ii) Al2O3

Question 4(2015)

A metal 'X' has a valency 2 and a non-metal 'Y' has a valency 3. Write an equation to show how 'Y' forms an ion.

Answer

3X - 6e- ⟶ 3X2+

2Y + 6e- ⟶ 2Y3-

Question 5.1(2015)

Describe the role played in the extraction of aluminium by each of the substances listed.

  1. Cryolite
  2. Sodium hydroxide
  3. Graphite

Answer

  1. Addition of Cryolite :
    (i) lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
    (ii) enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
    (iii) enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.
  2. Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of NaOH under pressure for 2 hrs as a first step in obtaining Al2O3. The impurities present in bauxite mainly Fe2O3 and SiO2 remain unaffected with conc. NaOH as impurities are not amphoteric. Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.
  3. Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.

Question 5.2(2015)

Explain why in the electrolysis of alumina using the Hall Heroult's process the electrolyte is covered with powdered coke.

Answer

The layer of powdered coke is sprinkled over the electrolytic mixture as :

  1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
  2. it minimizes or prevents heat loss by radiation.

Question 1(2016)

Fill in the blanks from the choices given :

Metals are good ............... [oxidizing agents / reducing agents] because they are electron ............... [acceptors/donors].

Answer

Metals are good reducing agents because they are electron donors.

Question 2(2016)

Select the correct answer from A, B, C and D : The two main metals in Bronze are :

(A) Copper and zinc
(B) Copper and lead
(C) Copper and nickel
(D) Copper and tin

Answer

Copper and tin

Question 3(2016)

Identify the term in the following :

'The method used to separate ore from gangue by preferential wetting.'

Answer

Froth flotation process

Question 4(2016)

(i) Name the solution used to react with Bauxite as a first step in obtaining pure aluminium oxide, in the Baeyer's process.

(ii) Write the equation for the reaction where the aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide.

(iii) Name the compound added to pure alumina to lower the fusion temperature during the electrolytic reduction of alumina.

(iv) Write the equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis.

(v) Explain why it is preferable to use a number of graphite electrodes as anode instead of a single electrode, during the above electrolysis.

Answer

(i) Sodium hydroxide

(ii) Conversion of Al(OH)3 - to pure alumina - heat on Al(OH)3

2Al(OH)3[aluminium hydroxide]1100°CAl2O3[pure alumina]+3H2O\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100\degree\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}

(iii) Cryolite (Na3AlF6)

(iv) 2Al3+ + 6e- ⟶ 2Al

(v) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O2 ⟶ 2CO; 2CO + O2 ⟶ 2CO2] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.

Question 1(2017)

From the list of terms given, choose the most appropriate term to match the given description:
[calcination, roasting, pulverisation, smelting] -

(i) Crushing of the ore into a fine powder.

(ii) Heating of the ore in the absence of air to a high temperature.

Answer

(i) Pulverisation

(ii) Calcination

Question 2(2017)

Name the following :

(i) An alloy of lead and tin that is used in electrical circuits.

(ii) An ore of zinc containing its sulphide.

(iii) A metal oxide that can be reduced by hydrogen.

Answer

(i) Solder [fuse metal]

(ii) Zinc blende [ZnS]

(iii) Copper oxide (CuO)

Question 3(2017)

Answer the following questions with respect to the electrolytic process – in the extraction of aluminium.

(i) Identify the components of the electrolyte other than pure alumina and the role played by each.

(ii) Explain why powdered coke is sprinkled over the electrolytic mixture.

Answer

(i) Cryolite and fluorspar are components of the electrolyte. Addition of cryolite enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity. Addition of both cryolite & fluorspar lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.

(ii) The layer of powdered coke is sprinkled over the electrolytic mixture as :

  1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
  2. it minimizes or prevents heat loss by radiation.

Question 4(2017)

Complete the following by selecting the correct option from the choices given –

(i) The metal whose oxide, which is amphoteric, is reduced to metal by carbon reduction ............... [Fe/Mg/Pb/Al]

(ii) The divalent metal whose oxide is reduced to metal by electrolysis of its fused salt is ............... [Al/Na/Mg/K]

Answer

(i) The metal whose oxide, which is amphoteric, is reduced to metal by carbon reduction — Pb.

(ii) The divalent metal whose oxide is reduced to metal by electrolysis of it's fused salt is — Mg

Question 1(2018)

Give a reason for : Alkali metals are good reducing agents.

Answer

Alkali metals have one electron in the outermost shell and hence during reactions they readily give the valence electron and get themselves oxidised. Hence, they are good reducing agents.

Question 2(2018)

Give one word or a phrase for the statement : The process by which sulphide ore is concentrated.

Answer

Froth flotation process

Reason — Sulphide ores like ZnS and PbS are lighter than the impurities present. They are concentrated by froth flotation process.

Question 3(2018)

Fill in the blank with the correct choice given in the bracket - The most common ore of iron is ...............[calamine/haematite]

Answer

The most common ore of iron is haematite

Question 4(2018)

Name the main component of the alloy -

(i) Brass

(ii) Duralumin

Answer

(i) Brass — Copper & Zinc

(ii) Duralumin — Aluminium & Copper.

Question 5(2018)

Name the most common ore of the metal aluminium from which the metal is extracted. Write the chemical formula of the ore.

Answer

Bauxite [Al2O3.2H2O]

Question 6(2018)

Name the process by which impure ore of aluminium gets purified, by a conc. soln. of an alkali.

Answer

Baeyer's Process

Question 7(2018)

Write a balanced chemical equation for -

(i) The formation of aluminium at the cathode, during the electrolysis of alumina

(ii) Action of heat on aluminium hydroxide.

Answer

(i) At cathode : 2Al3+ + 6e- ⟶ 2Al

(ii) Conversion of Al(OH)3 - to pure alumina - heat on Al(OH)3

2Al(OH)3[aluminium hydroxide]1100°CAl2O3[pure alumina]+3H2O\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100\degree\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}

Question 1(2019)

Choose the correct answer from the options A, B, C and D given. The reason for using aluminium in the alloy duralumin is:

A: Aluminium is brittle
B: Aluminium gives strength
C: Aluminium brings lightness
D: Aluminium lowers melting point

Answer

Aluminium brings lightness

Question 2(2019)

Write balanced chemical equation for the following reaction: Reduction of copper[II] oxide by hydrogen.

Answer

CuO+H2ΔCu+H2O[g]\text{CuO} + \text{H}_2 \xrightarrow{\Delta} \text{Cu} + \text{H}_2\text{O} [\text{g}]

Question 3(2019)

Give the appropriate term defined by the statement given : The process by which certain ores, specially carbonates are converted to oxides in the absence of air.

Answer

Calcination

Question 4(2019)

Arrange the following according to the instructions given in the bracket: K, Pb, Ca, Zn
[In the increasing order of reactivity]

Answer

Pb < Zn < Ca < K

Question 5(2019)

Give the chemical formula of :

(i) Bauxite

(ii) Cryolite

(iii) Sodium aluminate

Answer

(i) Al2O3.2H2O — Bauxite

(ii) Na3AlF6 — Cryolite

(iii) NaAlO2— Sodium aluminate

Question 6(2019)

Answer the following questions based on the extraction of aluminium from alumina by Hall- Heroult's Process.

(i) What is the function of cryolite used along with alumina as the electrolyte.

(ii) Why is the powdered coke sprinkled on top of the electrolyte.

(iii) Name the electrode from which aluminium is collected.

Answer

(i) Addition of cryolite —

  1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
  2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
  3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(ii) The layer of powdered coke is sprinkled over the electrolytic mixture as :

  1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
  2. it minimizes or prevents heat loss by radiation.

(iii) Cathode

Question 7(2019)

Match the alloys given in column I with the uses given in column II

Column IColumn II
(i) DuraluminA: Electrical fuse
(ii) SolderB: Surgical instruments
(iii) BrassC: Aircraft body
(iv) Stainless steelD: Decorative articles

Answer

Column IColumn II
(i) DuraluminC: Aircraft body
(ii) SolderA: Electrical fuse
(iii) BrassD: Decorative articles
(iv) Stainless steelB: Surgical instruments

Question 8(2019)

Name the gas evolved when : Alumina undergoes electrolytic reduction.

Answer

Oxygen gas

Question 1(2020)

State one relevant observation for the reaction : Sulphide ore is added to a tank containing oil and water and then stirred or agitated with air.

Answer

The ore is wetted by the oil and gets separated from the gangue in the form of froth. The impurities gets wetted by water and remain behind in the sink.
Since the ore is lighter, it comes on the surface with the froth and impurities are left behind.

Question 2(2020)

Give one word or a phrase for the statement : The substance prepared by adding other metals to a base metal in appropriate proportions to obtain certain desirable properties.

Answer

Alloy

Question 3(2020)

Fill in the blank from the choices given in bracket: An alloy used to make statues is .............. [bronze,brass,fuse metal]

Answer

An alloy used to make statues is bronze.

Question 4(2020)

Baeyer's process is used to concentrate bauxite ore to alumina. Give balanced chemical equations for the reaction taking place for it's conversion from bauxite to alumina.

Answer

Conversion of impure bauxite to sodium aluminate:

Al2O3.2H2[Impure bauxite]Amphoteric oxide+2NaOH [conc. soln]Base150200°C2NaAlO2[sodium aluminate]Salt+3H2OWater\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

Conversion of sodium aluminate to aluminium hydroxide:

NaAlO2[sodium aluminate]+2H2O5060°CNaOH[sodium hydroxide]+Al(OH)3[aluminium hydroxide]\underset{\text{[sodium aluminate]}}{\text{NaAlO}_2} + 2\text{H}_2\text{O} \xrightarrow{50 - 60 \degree\text{C}} \underset{\text{[sodium hydroxide]}}{\text{NaOH}} + \underset{\text{[aluminium hydroxide]}}{\text{Al(OH)}_3}

Conversion of Al(OH)3 - to pure alumina - heat on Al(OH)3:

2Al(OH)3[aluminium hydroxide]1100°CAl2O3[pure alumina]+3H2O\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100\degree\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}

Precipitated Al(OH)3 is ignited to give pure alumina [Al2O3]

Question 5(2020)

Differentiate between the following pairs based on the information given in the bracket.

Cations and anions [formation from an atom]

Answer

CationsAnions
Metallic atoms having 1, 2, 3 valence electrons tend to donate the valence electron on ionization and become positive ions called cations.Non-metallic atoms having 4, 5, 6 or 7 valence electrons tend to accept electrons on ionization and become negative ions called anions.

Question 6(2020)

Name the following elements :

(i) A trivalent metal used to make light tools.

(ii) A monovalent non-metal present in fluorspar.

Answer

(i) Aluminium

(ii) Fluorine

Additional Questions

Question 1

State how the physical and chemical property differences between metals and non-metals are related to their basic atomic structure.

Define a metal with particular emphasis on :

(i) ionization

(ii) valency

(iii) formation of oxides.

State the position [group] in the periodic table to which the following metals belong :

(i) Na – alkali metal

(ii) Mg – alkaline earth metal

(iii) Fe and Zn transition elements

(iv) inner transition elements (metals)

(v) Al – post transition element.

Answer

Physical and chemical property differences between metals and non-metals:
Metal atoms have larger atomic size so their valence electrons are held less tightly by the nucleus. This allows the valence electrons to move freely making metals good conductors of heat and electricity. Metals have 1, 2 or 3 valence electrons and tend to lose one or more electrons when supplied with energy and form cations. Thus, they are electropositive in nature and strong reducing agents.

Non-metal atoms have relatively smaller atomic size. So, their valence electrons are held tightly by the nucleus, they are not free to move. This makes non-metals bad conductors of heat and electricity. Non-metals have 5, 6 or 7 electrons in the valence shells and tend to gain electrons to form anions. Thus, non-metals are electronegative in nature and strong oxidizing agents.

Definition of Metal:
A Metal is an element which ionizes by losing electrons when energy is supplied to it and is hence an electron donor. Metals have 1, 2 or 3 valence electrons (positive valency of 1, 2 or 3) and tend to form positive ions (cations) on ionization. Thus, Metals form electrovalent compounds. Metals form at least one basic oxide. For example — K2O, Na2O, CaO, MgO, CuO, etc.

Position [group] in the periodic table:

(i) Na — alkali metal — Group I (IA)

(ii) Mg — alkaline earth metal — Group 2 (IIA)

(iii) Fe and Zn transition elements — Group 8 (VIII), Group 12 (IIB)

(iv) Inner transition elements — Group 3(IIIB) but are shown separately as f-block of the periodic table.

(v) Al — post transition element — Group 13 (III A)

Question 2

Metals occur in the free state and in the combined state. Name two metals which occur in the free or native state. In the combined state metals occur in the form of compounds. Name two different metallic compounds in each case which occur as (i) halides (ii) oxides (iii) sulphides

Answer

Gold and Platinum occur in the free metallic form.

The metallic compounds in combined form are :

(i) halides — Cryolite[Na3AlF6], Fluorspar[CaF2]

(ii) oxides — Bauxite[AlO3.2H2O]

(iii) sulphide — Iron Pyrite[FeS2], Zinc blende[ZnS]

Question 3

Differentiate between

(i) mineral & ore

(ii) matrix & flux.

Answer

(i) Difference between mineral & ore

MineralOre
The compounds of various metals found in nature associated with their earthly impurities are called minerals.The naturally occurring minerals from which metals can be extracted profitably and conveniently are called ores.

(i) Difference between matrix and flux

MatrixFlux
The rocky impurities including silica [SiO2], mud etc. associated with the ore is called matrix or gangue.The substance added to the ore to get rid of the matrix resulting in the formation of a fusible compound slag.

Question 4

Give the (i) common (ii) chemical name (iii) formula of two common ores each of aluminium, zinc and iron.

Answer

MetalCommon name
of ore
Chemical name
of ore
Formula
of ore
AluminiumBauxiteHydrated aluminium oxideAl2O3.2H2O
   CryoliteSodium aluminium fluorideNa3AlF6
ZincZinc blendeZinc sulphideZnS
   CalamineZinc carbonateZnCO3
IronHaematiteIron [III] oxideFe2O3
   MagnetiteTri iron tetra oxideFe3O4

Question 5.1

Give reasons for the following:
Dressing of the ore is an essential process in the extraction of metal from it's ore.

Answer

Dressing of the ore converts the impure ore to pure concentrated ore. Hence, it is an essential process in the extraction of metal from it's ore.

Question 5.2

Give reasons for the following:
An electromagnetic wheel is used in the magnetic separation process of ore from gangue.

Answer

Magnetic separation process is used to separate magnetic ore from non-magnetic gangue or magnetic gangue from non-magnetic ore. Electromagnetic wheel is used in magnetic separation so that magnetic particles get attracted to the magnetic wheel and separate from the non-magnetic particles.

Question 5.3

Give reasons for the following:
In the froth floatation process, the ore floats on the top & the gangue settles down.

Answer

As the ore is lighter, it comes on the surface with the froth. Gangue is heavier so it settles down.

Question 5.4

Give reasons for the following:
Magnetic separation is not used during the dressing of bauxite ore in the extraction of aluminium.

Answer

In bauxite ore, the gangue minerals are fine grained and intimately locked with the ore making magnetic separation extremely difficult. On the other hand, Aluminium is highly reactive metal. Dressing of bauxite ore through chemical leaching by Baeyer's process is much easier. Hence, magnetic separation is not used during the dressing of bauxite ore in the extraction of aluminium.

Question 5.5

Give reasons for the following:
Conversion of concentrated ore to it's oxide is an essential step in the extraction of metals from the ore, even then the step is not necessary in the metallurgy of aluminium.

Answer

The step is not necessary as the main ore of Aluminium is Bauxite (Al2O3.2H2O) and it is already an oxide.

Question 5.6

Give reasons for the following:
Roasting of the concentrated ore is carried out in the presence of excess air, while calcination of the concentrated ore in the absence or limited supply of air.

Answer

Calcination is carried out on carbonates and hydrated ores, where water of hydration and other volatile impurities get removed. Calcination of carbonate ores gives out carbon dioxide. Hence, the ore is heated in absence of air. On the other hand, sulphide ores are generally roasted so that sulphur present as impurity in the ore is given out as sulphur dioxide. Hence, roasting is carried out in the presence of excess air.

Question 5.7

Give reasons for the following:
Roasting the ore generally results in evolution of sulphur dioxide gas, while calcination of the ore evolves carbon dioxide gas.

Answer

Generally, sulphide ores are roasted, so SO2 is given off. Calcination is carried on carbonates and hydrated ores , so CO2 or water vapours is given off.

Question 5.8

Give reasons for the following:
Reduction of metallic oxides to metal in the extraction of metals from the ores is based on the position of the metal in the activity series.

Answer

Activity series is a series of metals arranged according to their decreasing reactivity.

The metal at the top of the series

  1. is most easily oxidized
  2. is most electropositive and most reactive
  3. can displace the metal below from it's salt soln.

Due to these reasons, the extraction of metals from the ores is based on the position of the metal in the activity series.

Question 5.9

Give reasons for the following:
Oxides or halides of highly electropositive metals e.g. K, Na, Ca, Al are reduced to metals by electrolysis and not by reduction with coke.

Answer

Metals from K to Al, are most electropositive, have strong affinity for oxygen hence, they cannot be reduced by reducing agents like coke. They can be reduced by electrolysis only.

Question 5.10

Give reasons for the following:
Coke is not preferred as a reducing agent in the reduction of the oxide of mercury to it's metal, but is preferred in the reduction of the oxide of zinc to it's metal.

Answer

Mercury is least electropositive and have least affinity for oxygen hence, its oxide can be reduced by heat alone whereas Zinc is more electropositive than Mercury and have more affinity for oxygen as compared to mercury hence, reduction of the oxide of zinc to it's metal can be done by coke.

Question 6

In the extraction of aluminium from bauxite, the first step is the dressing of the ore by Baeyer's process. Give balanced equations for the conversion of impure bauxite to pure alumina using a concentrated solution of NaOH.

Answer

Conversion of impure bauxite to sodium aluminate:

Al2O3.2H2[Impure bauxite]Amphoteric oxide+2NaOH [conc. soln]Base150200°C2NaAlO2[sodium aluminate]Salt+3H2OWater\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

Conversion of sodium aluminate to aluminium hydroxide:

NaAlO2[sodium aluminate]+2H2O5060°CNaOH[sodium hydroxide]+Al(OH)3[aluminium hydroxide]\underset{\text{[sodium aluminate]}}{\text{NaAlO}_2} + 2\text{H}_2\text{O} \xrightarrow{50 - 60 \degree\text{C}} \underset{\text{[sodium hydroxide]}}{\text{NaOH}} + \underset{\text{[aluminium hydroxide]}}{\text{Al(OH)}_3}

Conversion of Al(OH)3 - to pure alumina - heat on Al(OH)3:

2Al(OH)3[aluminium hydroxide]1100°CAl2O3[pure alumina]+3H2O\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100\degree\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}

Question 7

In the electrolytic reduction of pure alumina to pure aluminium by Hall Herault's process, give the electrolytic reactions involved in the same, resulting in formation of aluminium at the cathode.

Answer

In the electrolytic reduction of pure alumina to pure aluminium by Hall Herault's process, the constitutents of electrolyte are cryolite (Na3AlF6) and fluorspar (CaF2) and alumina (Al2O3)

The electrolytic reactions are as follows :

Cryolite : Na3AlF6 ⇌ 3Na1+ + Al3+ + 6F1-

Fluorspar : CaF2 ⇌ Ca2+ + 2F1-

Alumina : Al2O3 ⇌ 2Al3+ + 3O2-

At cathode : 2Al3+ + 6e- ⟶ 2Al

At anode : 3O2- - 6e- ⟶ 3[O] ⟶ 3O2

Al3+ are discharged in preference to Na1+ and Ca2+ ions due to it's lower position in the electrochemical series.

Question 8

State the function of (i) Sodium hydroxide (ii) Cryolite (iii) Fluorspar in the metallurgy of aluminium in Hall Herault's process.

Answer

(i) Function of Sodium hydroxide — Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of Sodium hydroxide under pressure for 2 hours as a first step in obtaining Al2O3.

The impurities present in bauxite mainly Fe2O3 and SiO2 remain unaffected with conc. NaOH as impurities are not amphoteric.

Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.

Al2O3.2H2[Impure bauxite]Amphoteric oxide+2NaOH [conc. soln]Base150200°C2NaAlO2[sodium aluminate]Salt+3H2OWater\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

(ii) Addition of Cryolite & Fluorspar

  1. Lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
  2. Enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
  3. Addition of Cryolite enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

Question 9

Give reasons for the following pertaining to Hall Herault's process.

(i) The fusion temperature of the electrolyte has to be lowered before conducting the electrolytic reduction.

(ii) The constituents of the electrolyte in addition to one part of fused alumina contains three parts of cryolite and one part of fluorspar.

(iii) A layer of powdered coke sprinkled over the electrolytic mixture, protects the carbon electrodes.

(iv) It is preferred to use a number of graphite electrodes as anode, instead of a single graphite electrode.

Answer

(i) Fused alumina has a melting point around 2050°C. Such high temperatures, create the below difficulties:

  1. A large amount of electrical energy is required to liberate Al2O3 and maintain the electrolyte in molten state at that temperature.
  2. The liberated Al metal [m.p. 660°C] may also tend to volatilize out and get wasted.

Hence the fusion temperature of the electrolytic mixture has to be lowered.

(ii) The reasons for addition of mainly Cryolite [in a higher ratio] and Fluorspar to the electrolytic mixture are:

  1. They lower the fusion point of the mixture i.e., the mixture fuses [melts] around 950°C instead of 2050°C.
  2. They enhance the mobility of the fused mixture by acting as a solvent for the electrolytic mixture. Thus cryolite in the molten state of subdivision dissolves aluminium oxide.
  3. Addition of cryolite enhances the conductivity of the mixture. Since pure alumina is almost a non-conductor of electricity.

(iii) The layer of powdered coke is sprinkled over the electrolytic mixture as:

  1. It prevents burning of carbon electrodes in air at the emergence point from the bath.
  2. It minimizes or prevents heat loss by radiation.

(iv) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O2 ⟶ 2CO; 2CO + O2 ⟶ 2CO2] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.

Question 10

Define (a) alloy (b) amalgam

Answer

(a) Alloy — Alloy is a substance prepared by adding other metals [or non-metals] to a base metal in appropriate proportions to obtain certain desirable properties. It is usually made by melting the components together and solidifying the mixture.

(b) Amalgam — Amalgam is an alloy in which the base metal is mercury. Examples are Na/Hg [liquid amalgam], Zn/Hg amalgam [used in voltaic cells].

Question 11

State (i) composition (ii) reason for alloying (iii) one use of each of the following alloys.

(i) Brass

(ii) Bronze

(iii) Duralumin

(iv) Solder [fuse metal]

(v) Stainless steel

Answer

AlloyCompositionReason for alloyingUse
1.BrassCu [60 - 80%], Zn [40 - 20%]Hard, Lustrous, easily cast, malleable, ductileElectrical fittings, decorative articles
2.BronzeCu [80%], Zn [1%], Sn [19%]Hard, brittle, takes up polishStatues, medals, coins
3.DuraluminAl [95%], Mg [0.5%], Mn [0.5%], Cu [4%]Al - imparts lightness, Mg - imparts strengthAircrafts, light tools, cooker
4.Solder [fuse metal]Pb [50%], Sn [50%]Sn - lowers melting point of alloyElectrical fuse, Soldering purposes
5.Stainless steelFe [75%], Ni [10%], Cr [15%], C [.5%]Ni, Cr - imparts lustre, C - imparts hardnessUtensils, cutlery, surgical instruments

Unit Test Paper 6 — Metallurgy

Question 1

Loss of electrons from an element 'X' is represented by : X - 3e- ⟶ X3+ [X ⟶ X3+ + 3e-)

  1. X is a ............... [metal/non-metal] and will form ............... [electrovalent/covalent] compounds only.

  2. X3+ formed is a ............... [cation/anion] and element 'X' has a valency of ............... [+2/+3/-3].

  3. If X3+ combines with oxygen the formula of the product is ............... [X2O3 / XO / X3O2]

  4. If the above product in the ............... [solid/molten] state is electrolysed the ion X3+ will get discharged at the ............... (anode/cathode).

  5. The ion X3+ ............... [accepts/loses] electrons and gets ............... [oxidized/reduced] to neutral ............... [ions/atoms/molecules].

Answer

  1. X is a metal and will form electrovalent compounds only.

  2. X3+ formed is a cation and element 'X' has a valency of +3.

  3. If X3+ combines with oxygen the formula of the product is X2O3.

  4. If the above product in the molten state is electrolysed the ion X3+ will get discharged at the cathode.

  5. The ion X3+ accepts electrons and gets reduced to neutral atoms.

Question 2.1

The oxide of the metal which reacts with both acids and alkalis to give salt & water.

  1. MgO
  2. CuO
  3. Al2O3
  4. K2O

Answer

Al2O3

Question 2.2

The common name of the ore of iron whose chemical formula is Fe3O4.

  1. Iron pyrites
  2. Magnetite
  3. Haematite
  4. Spathic iron ore

Answer

Magnetite

Question 2.3

The chemical name of the main ore of aluminium.

  1. Aluminium fluoride
  2. Aluminium oxide
  3. Sodium aluminium fluoride
  4. Hydrated aluminium oxide

Answer

Hydrated aluminium oxide

Question 2.4

The process of dressing of the ore which involves separation of ore & gangue due to preferential wetting.

  1. Magnetic separation
  2. Hydrolytic method
  3. Froth flotation method
  4. Chemical method

Answer

Froth flotation method

Question 2.5

The metallic oxide reduced to metal generally by thermal decomposition.

  1. ZnO
  2. MgO
  3. HgO
  4. Al2O3

Answer

HgO

Question 3

Name the substance A to E, reacted with each reactant below to give the respective product/s.

  1. Conc. iron pyritesAIron III oxide+Sulphur dioxide\text{Conc. iron pyrites} \xrightarrow{\text{A}} \text{Iron III oxide} + \text{Sulphur dioxide}

  2. Zinc oxideBZinc+Carbon monooxide\text{Zinc oxide} \xrightarrow{\text{B}} \text{Zinc} + \text{Carbon monooxide}

  3. Copper [II] oxideCCopper+Water\text{Copper [II] oxide} \xrightarrow{\text{C}} \text{Copper} + \text{Water}

  4. Impure bauxiteDSodium aluminate\text{Impure bauxite} \xrightarrow{\text{D}} \text{Sodium aluminate}

  5. Sodium aluminateEAluminium hydroxide\text{Sodium aluminate} \xrightarrow{\text{E}} \text{Aluminium hydroxide}

Answer

  1. A → Oxygen
  2. B → Carbon
  3. C → Hydrogen
  4. D → Sodium hydroxide
  5. E → H2O

Question 4

Name the following :

(i) A metallic ore converted to it's oxide – iron [II] oxide on heating the concentrated ore.

(ii) A black metallic oxide reduced to metal on heating with coke.

(iii) The compound which on ignition at elevated temperatures gives pure alumina.

(iv) The non-metal which forms the anode during electrolytic reduction of fused alumina in Hall Heroult's process.

(v) A metal other than manganese, present in duralumin but not in magnalium.

Answer

(i) Spathic Iron Ore (Iron [II] Carbonate, FeCO3)

(ii) CuO

(iii) Aluminium hydroxide

(iv) Carbon (Graphite)

(v) Copper (Cu)

Question 5

Match the properties & uses of alloys in List 1 with the correct answer from List 2.

List 1List 2
1. Contains Pb & Sn, has a low melting point & is used in electrical fuseA. Duralumin
2. Contains Cu, Zn, & Sn is hard, take up polish & used in statues and medals.B. Stainless steel
3. Contains Cu & Zn is hard, malleable & ductile & is used in electrical fittings.C. Solder
4. Contains Al & Cu is light & is used in aircrafts and pressure cooker.D. Brass
5. Contains Fe, Ni & Cr is corrosion resistant and is used in cutlery.E. Bronze

Answer

List 1List 2
1. Contains Pb & Sn, has a low melting point & is used in electrical fuseC. Solder
2. Contains Cu, Zn, & Sn is hard, take up polish & used in statues and medals.E. Bronze
3. Contains Cu & Zn is hard, malleable & ductile & is used in electrical fittings.D. Brass
4. Contains Al & Cu is light & is used in aircrafts and pressure cooker.A. Duralumin
5. Contains Fe, Ni & Cr is corrosion resistant and is used in cutlery.B. Stainless steel
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