[3 Mark Question Answer]

Question 13 Marks

Metal X is found in nature as its sulphides XS. It is used in the galvanization of iron articles. Identify the metal X. How will you convert this sulphide ore into the metal? Explain with equations.

Answer

X = Zinc
Extraction of Zinc from Zinc Sulphide:
(i) Enrichment of ore - Removal of gangue particles
(ii) Roasting - Heating of ore in the presence of oxygen
2ZnS +3O₂→ 2ZnO + 2SO₂
(iii) Reduction by carbon at high temperature (Smelting)
$ZnO + C \xrightarrow{\Delta} Zn + CO$
(iv) Electrolytic refining of Zn.

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Question 23 Marks

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

Answer

Ores of aluminium
(i) Bauxite ; [Al₂O₃.2H₂O] ; Hydrated aluminium oxide
(ii) Cryolite ; [Na₃AlF₆] ; Sodium aluminium fluoride
Ores of zinc
(i) Zincite ; [ZnO] ; Zinc Oxide
Zinc blende ; (ZnS] ; Zinc sulphide.
Ores of Iron
(i) Haematite ; [Fe₂O₃] ; Iron (III) oxide
(ii) Magnetite ; [Fe₃O₄] ; Iron (II, III) oxide.

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Question 33 Marks

(a) What do you understand by the term concentration of ore?
(b) Name three processes used for the concentration of ore and name one specific ore which can be concentrated by the named process.

Answer

(a) Concentration of ore : The various processes which help in the removal of gangue from the dressed ore, thereby increasing the percentage of metal in the ore are called concentration of ore.
(b) (i) Magnetic separation : used for concentrating iron ores.
(ii) Gravity process or hydraulic washing : used for concentrating zinc oxide ore.
(iii) Froth floatation process : used for concentrating lead sulphide ore.

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Question 43 Marks

By stating reason give one use of aluminium.
(i) as a metal (ii) in powder form (iii) in the form of foil

Answer

(i) As a metal it is used for making utensils as it is a good conductor of heat.
(ii) In powder form, it is mixed with linseed oil and is used as paint in protecting iron from rusting, as it does not get oxidised quickly.
(iii) Aluminium foil is used in packing and food processing industry as it is highly malleable.

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Question 53 Marks

Here are two metals obtained from extraction processes: Zn and Pb. What would be the refining processes applicable to purify them? Give the related principle of each.

Answer

Zinc: Distillation. This process is employed for more volatile metals (having low boiling point) In this process, the impure metal is strongly heated in the absence of air. The metal vaporises, leaving behind impurities.
Lead: Liquation. This process is employed for metals having low melting points. The solid impure metal is placed in a hearth having a sloping floor. When the hearth is heated, metal melts and flows on the sloping floor.

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Question 63 Marks

The following is an extract from Metals in the Service of Man. Alexander and Street/Pelican 1976.
'Alumina (aluminium oxide) has a very high melting point of over 2000°C so that it cannot readily be liquefied. However, conversion of alumina to aluminium and oxygen by electrolysis can occur when it is dissolved in some other substance.
(a) Which solution is used to react with bauxite as a first step in obtaining pure aluminium oxide?
(b) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. Write the equation for this reaction.
(c) Name the element which serves both as the anode and the cathode in the extraction of aluminium.

Answer

(a) Sodium hydroxide solution. (b) $2 Al ( OH )_3(s) \xrightarrow[1100^{\circ} C ]{\text { heat }} Al _2 O _3(s)+3 H _2 O ( l )$ (c) Carbon

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Question 73 Marks

How will you convert impure bauxite to pure aluminium by the use of conc. sodium hydroxide?

Answer

Impure bauxite is digested with conc. NaOH for 2 hours when the aluminium oxide dissolves to form sodium aluminate, but the impurities like iron oxide and sand settle down.$
Al_2 O_3 \cdot 2 H_2 O+2 NaOH \xrightarrow{150^{\circ}-200^{\circ} C} 2 NaAlO_2+3 H_2 O
$(Bauxite)
The reaction mixture is filtered so as to obtain a clear solution. To this solution is added excess of water, which hydrolyses sodium aluminate to insoluble aluminium hydroxide.
$NaAlO _2+2 H _2 O \xrightarrow{50^{\circ}-60^{\circ} C } Al ( OH )_3+ NaOH$
The aluminium hydroxide is filtered, washed with hot water and then strongly heated to obtain pure alumina.
$2 Al ( OH )_3 \xrightarrow[1100^{\circ} C ]{\text { heat }} Al _2 O _3+3 H _2 O$ (Steam)

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Question 83 Marks

The following questions relate to the extraction of aluminium by electrolysis :
(i) Name the other aluminium containing compound added to alumina and state its 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]
The addition of cryolite lowers the melting point of alumina from 2050°C to 660°C. Furthermore, it increases the electrical conductivity of the molten alumina.
(ii) Al₃+ 3e^-→ Al
(iii) The anode (which is made of carbon) is attacked by nascent oxygen formed due to the discharge of O^2- ions and changes to carbon dioxide. As the anode is gradually consumed, it is periodically renewed.

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Question 93 Marks

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) The caustic alkali dissolves aluminium oxide to form sodium aluminate.
(ii) Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
(iii) Fluorspar is added. It helps in increasing the conductivity of the mixture.

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Question 103 Marks

Answer the following questions :
(i) Name a metal which is found abundantly in the earth's crust.
(ii) Write the chemical formulae of one main ore of iron and aluminium.
(iii) Write the constituents of electrolyte for the extraction of aluminium.

Answer

(i) Aluminium metal is formed abundantly in the earth's crust.
(ii) Iron - Haematite; Aluminium - Bauxite
(iii) • Fused alumina (Al₂O₃) 1 part by weight,
• Cryolite fused (Na₃AlF₆) 3 part by weight,
• Fluorspar (CaF₂) 1 part by weight

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Question 113 Marks

In order to obtain ½ tonne of aluminium, the following inputs are required :
Bauxite (2 tonnes); sodium hydroxide (75 kg); graphite (300 kg).
The compound of aluminium in bauxite is aluminium oxide and the main impurity is iron (III) oxide. The aluminium is obtained by the electrolysis of aluminium oxide in dissolved cryolite.
(a) When bauxite is treated with sodium hydroxide solution, what happens to the :
(i) aluminium oxide (ii) iron (III) oxide
(b) (i) Name the process used for the purification of bauxite.
(ii) Write the action of heat on aluminium hydroxide.

Answer

(a) (i) Aluminium oxide dissolves in sodium hydroxide solution to form water soluble sodium aluminate.
(ii) The iron (III) oxide does not react with sodium hydroxide and hence settles down as precipitate.
(b) (i) The process used for the purification of bauxite is called Baeyer's process
(ii) $2 Al ( OH )_3 \xrightarrow[1100^{\circ} C ]{\text { heat }} Al _2 O _3+3 H _2 O$

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Question 123 Marks

(a) Fill in the blanks using the most appropriate words :
(i) The ore from which aluminium is extracted must first be treated with ________ so that pure aluminium oxide can be obtained.
(ii) Pure aluminium oxide is dissolved in ________ to make it a conducting solution.
(b) Write the formula of Cryolite.

Answer

(a) (i) Sodium hydroxide solution.
(ii) Cryolite.
(b) Formula of Cryolite : Na₃AlF₆.

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