CBSE Class 12 Chemistry Metallurgy Important Questions and Answers

Question. Write down the word which correctly completes the following sentence : “By dissolving aluminium oxide in cryolite a __ [conducting / non-conducting] solution is produced.
State why is so much graphite is requires for this electrolytic process.
Write the equation for the reaction which takes place at the cathode.
Answer : “By dissolving aluminium oxide in cryolite a conducting solution is produced.
The graphite anodes are periodically replaced during electrolysis process of fused alumina.
At cathode : 2Al³⁺ + 6e^– → 2Al

Question. In construction work, state why the alloy duralumin is used rather than pure aluminium.
Answer : Duralumin is light, strong and corrosion resistant.

Question. Name an alloy of copper and zinc.
Answer : Brass

Question. Element X is a metal with a valency 2. Element Y is a nonmetal with a valency 3. Write equations to show how X and Y form ions.
Answer : Y + 3e^– → Y3^– (cations formed)

Question. Cations are formed by _____ (loss / gain) of electrons and anions are formed by _____ (loss / gain) of electrons.
Answer : Cations are formed by loss of electrons and anions are formed by gain of electrons.

Question. 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.
1. Write down the three letters each from the above list which are relevant to – Aluminium.
2. 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
3. Write the formula of Cryolite.
Answer : 1. Aluminium: Bauxite (A), Cryolite (C), Sodium hydroxide solution (F).
2. (a) Sodium Hydroxide Solution (b) Cryolite
3. Na₃AlF₆

Question. Calcium, Copper, Lead, Aluminium, Zinc, Chromium, Magnesium, Iron.
Choose the major metals from the list given above to make the following alloys :
1. Stainless steel
2. Brass.
Answer : 1. Iron, Chromium
2. Copper, Zinc

Question. Name the following :
1. A metal which is liquid at room temperature.
2. A compound added to lower the fusion temp, of the electrolytic bath in the extraction of Al.
3. The process of heating an ore to a high temperature in the presence of air.
Answer : 1. Mercury or Gallium
2. Cryolite [Na₃ (AlF₆)]
3. Roasting

Question. 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 finaky turn blue.
Answer : Silver Nitrate (AgNO₃)

Question. Correct the following statements. — Haematite is the chief ore of aluminium.
Answer : Haematite is the chief ore of iron.
                          Or
Bauxite is the chief ore of aluminium.

Question. State the property of the metal being utilized in the following :
1. Zinc in Galvanization
2. Aluminium in Thermite welding
Answer : Use of metal :
(a) Zinc in Galvanization
(b) Aluminium in Thermite welding
Property :
(a) Zinc forms a protective layer of zinc oxide which prevents rusting of iron.
(b) Strong affinity for oxygen.

Question. State the main constituent metal in each alloy :
1. Duralumin
2. Brass
3. Stainless steel
Answer : 1. Aluminium
2. Copper
3. Iron

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

Question. (i) Name a metal which is found abundantly in the earth’s crust.
Answer : Aluminium

(ii) Difference between calcination and roasting?
Answer : Roasting :
1. Ore is heated in the presence of air.
2. Used generally for sulphide ores. SO₂ gas is given off.
2ZnS+ 3O₂ → 2ZnO + 2SO₂
3. Volatile impurities are removed as oxides.
Calcination :
1. Ore is heated in the absence of air.
2. Hence Used generally for carbonate ores.
Hence, CO₂ gas is given off. ZnCO₃ → ZnO + CO_2.
3. Moisture, organic impurities and volatile impurities are removed.

(iii) Name the process used for the enrichment of sulphide ore.
Answer : Froth floatation process

(iv) Write the chemical formulae of one main ore – of iron and aluminium.
Answer : Iron : Haematite → Fe₂O₃
Aluminium : Bauxite → Al₂,O₃.2H₂O

(v) Write the constituents of electrolyte for – the extraction of aluminium.
Answer : 1. Fused Alumina (Al2O₃₎ 1 part by weight.
2. Cryolite fused (Na₃ AlF₆) 3 part by weight.
3. Fluorspar (CaF₂) 1 part by weight.

Question. Name the following metals :
1. A metal present in cryolite other than sodium.
2. A metal which is unaffected by dilute or concentrated acids.
Answer : 1. Aluminium
2. Platinum

Question. 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 O2-ions and changes to carbon dioxide. As the anode is gradually consumed, it is periodically renewed.

Question. Name the following :
1. The property possessed by metals by which they can be beaten into sheets.
2. A compound added to lower the fusion temperature of electrolytic bath in the extraction of aluminium.
3. The ore of zinc containing its sulphide.
Answer : 1. Melleability
2. Cryolite (Na3AlF6)
3. The ore of zinc containing sulphide ZnS (Zinc Blende).

Question. Give scientific reasons for : Zinc oxide can be reduced to zinc by using carbon monoxide, but aluminium oxide can – not be reduced by a reducing agent.
Answer : Zinc ion is lower in electrochemical series. Thus, carbon monoxide provides enough energy and hence reduces zinc oxide to zinc. Aluminium ion is higher in electrochemical series. The carbon monoxide does not produce enough energy and hence aluminium oxide is not reduced to aluminium.

Question. From the list of oxides — SO₂, SiO₂, Al₂O₃, MgO, CO, Na₂O — Select
1. A basic oxide
2. An amphoteric oxide.
Answer : 1. Na₂O
2. Al₂O₃

Question. (i) Describe the role played in the extraction of aluminium by each of the substances listed.
(a) Cryolite
(b) Sodium hydroxide
(c) Graphite
(ii) Explain why :
(a) In the electrolysis of alumina using the Hall Heroult’s Process the electrolyte is covered with powdered coke.
(b) During galvanization, iron sheets are coated with zinc during galvanization.
Answer : (i) (a) Cryolite lowers the melting point of alumina from 2050° to 950°C. This in turn increases the conductivity of electrolyte and saves on electricity, which is the main source of energy in the reduction of alumina.
(b) Sodium hydroxide dissolves the alumina (Al₂O₃), but not the impurities to form sodium aluminate. The impurities are filtered out and the sodium aluminate is treated with carbon dioxide to form pure aluminium hydroxide. Alumina is recovered from sodium hydroxide by strong heating.
(c) Graphite is used as cathode as it is a good conductor of electricity and is not attacked by the chemicals.
(ii) (a) It prevents burning of carbon anodes and prevents heat loss from the molten electrolyte.
(b) Zinc forms a protecting coating on the surface of iron and hence prevents it from rusting. Zinc by itself form a protective layer of zinc oxide and hence does not tarnish.

Question. Select from the following salts — AgCl, MgCl₂, NaHSO4, PbCO3, ZnCO₃, KNO₃, Ca(NO₃)₂ — the salt which on heating gives a yellow residue when hot and white when cold.
Answer : On heating, this salt gives a yellow residue when hot and a white residue when cold is ZnCO₃

Question. 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. Identify the term in the following :
“The method used to separate ore from gangue by preferential wetting.”
Answer : Froth flotation process

Question. From the list of terms given, choose the most appropriate term to match the given description: [calcination, roasting, pulverisation, smelting]
1. Crushing of the ore into a fine powder.
2. Heating of the ore in the absence of air to a high temperature.
Answer : 1. Pulverisation
2. Calcination

Question. Name the following :
1. An alloy of lead and tin that is used in electrical circuits.
2. An ore of zinc containing its sulphide.
3. A metal oxide that can be reduced by hydrogen.
Answer : 1. Fusible alloy
2. Zincite (ZnS)
3. Copper oxide (CuO)

Question. Answer the following questions with respect to the electrolytic process – in the extraction of aluminium.
1. Identify the components of the electrolyte other than pure alumina and the role played by each.
2. Explain why powdered coke is sprinkled over the electrolytic mixture.
Answer : 1. Cryolite (Na3AlF4) : It increases the conductivity of mixture and lowers the m.p. of electrolyte from 2050°C to 950°C.
2. Fluorspar (CaF₂) : It increases mobility of ions in the mixture as it acts as solvent for alumina and cryolite.
Powdered coke prevents (a) heat loss from the electrolyte (b) burning of electrodes projecting out of electrolyte.

Question. Complete the following by selecting the correct option from the choices given –
1. The metal whose oxide, which is amphoteric, is reduced to metal by carbon reduction _____ [Fe / Mg / Pb / Al]
2. The divalent metal whose oxide is reduced to metal by electrolysis of its fused salt is _____ [Al / Na / Mg / K]
Answer : 1. The metal whose oxide, which is amphoteric, is reduced to metal by carbon reduction Pb (Lead).
2. The divalent metal whose oxide is reduced to metal by electrolysis of its fused salt is Mg (Magnesium).
 

Additional Questions

Question. 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
1. halides
2. oxides
3. sulphides.
Answer : Gold and Platinum are the two metals that occur in the free or native state.
The metallic compounds which occur in :
1. Halides – Cryolite[Na₃AlF₆], Flurospar[CaF₂], Rock Salt[NaCl]
2. Oxides – Bauxite[Al₂O₃.2H₂O], Zincite[ZnO], Cuprite[Cu₂O]
3. Sulphides – Iron Pyrite[FeS₂], Zinc blende[ZnS], Gdlena[PbS]

Question. Differentiate between
1. mineral & ore
2. matrix & flux.
Answer : Differentiate between mineral & ore Mineral :
The compounds of various metal found in nature associated with their earthly impurities are called minerals. Ore :
The naturally occurring minerals from which metals can be extracted profitably and conveniently are called ores,
Differentiate between matrix & flux
1. Matrix : The rocky impurities including silica [SiO₂], mud etc. associated with the ore is called matrix or gangue.
2. Flux : The substance added to the ore to get rid of the matrix resulting in the formation of a fusible compound slag.

Question. In the stages involved in the extraction of metals in general – give reasons for the following.

(i) Dressing of the ore is an essential process in the extraction of metal from its ore.
Answer : The ores are found mixed with earthy impurities like sand, clay, lime stone etc.
These unwanted impurities in the ore are called gangue or matrix.
The process of removal of gangue from powdered ore is called concentration or ore dressing.
Hence, it is the essential process in the extraction of metal from its ore because it convert’s the impure ore to pure concentrated ore

(ii) An electromagnetic wheel is used in the magnetic separation process of ore from gangue.
Answer : An electromagnetic wheel is used in the magnetic separation process of ore from gangue as it seperate’s the magnetic particles from the non-magnetic particles.
The magnetic particles get attracted to the magnetic wheel and thus get seperated from the gangue.

(iii) In the froth floatation process, the ore floats on the top & the gangue settles down.
Answer : The impurities get wetted by water and remain behind in the tank. Since, the ore is lighter, it comes on the surface with the froth and the impurities(gatlgue) are left behind.

(iv) Magnetic separation is not used during the dressing of bauxite ore in the extraction of aluminium.
Answer : Bauxite is concentrated by Leaching (Baeyer’s process). The impure bauxite is treated with concentrated NaOH, Al2O3 and SiO2 dissolve, but Fe2O3 and other basic materials remain insoluble and are removed by filtration. Aluminium is highly reactive metal, belonging to the III A group of the periodic table. In nature, aluminium is found in the form of its oxide in its ore.
Hence, Magnetic separation is not used during the dressing of bauxite ore in the extraction of aluminium.

(v) Conversion of concentrated ore to its 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 : Conversion of concentrated ore to its 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 because in metallurgy of aluminium, the ore is already an oxide.

(vi) 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 : Roasting is a process of converting an ore into its oxide by heating strongly in
presence of excess air, so that oxygen gets added to form the corresponding
oxide. It is done on sulphide ores in order to remove sulphur as sulphur gets
escape in the form of gas.
2ZnsS (s) + 3O₂ → 2ZnO (s) + 2SO₂ (g)
Whereas in carbonates ores, one needs to drive out carbonate and moisture
impurities. So, ore is heated to a high temperature in absence of air. This process
is called calcination.
ZnCO₃ (s) → ZnO (s) + CO₂ (g)

(vii) 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.
While carbonate and hydrated ores are calcined, so CO₂ or water vapours are given off.

(viii) 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 : The method used to extract a metal from its ore depends on the position of the metal in the reactivity series.
(a) Metals higher up in the series need to be extracted using electricity.
(b) Metals lower in the series can be extracted by reduction with carbon.

(ix) 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 : Oxides are highly active metals like potassium, sodium, calcium, magnesium and aluminium have great affinity towards oxygen and so cannot be reduced by coke.

(x) Coke is not preferred as a reducing agent in the reduction of the oxide of mercury to its metal, but is preferred in the reduction of the oxide of zinc to its metal.
Answer : Coke is not preferred as a reducing agent in the reduction of the oxide of mercury to its metal because metals low in the activity series are very reactive and the oxides of the metals can be reduced to metals by heating in air.
The reducing nature of carbon plays an important role in metallurgy and helps extract metals from their ores.
For example :

Question. State the function of
1. NaOH
2. cryolite
3. fluorspar in the metallurgy of aluminium
Answer : Functions of :
(i) NaOH : Crushed and powdered bauxite is heated with a cone, solution of
NaOH at 150-200°C for 2 hours under pressure. The main impurities present in bauxite (Fe₂O₃ and SiO₂) remain unaffected with cone. NaOH since these are not amphoteric. Bauxite, being amphoteric reacts with the base to form sodium aluminate, which is soluble in water. Thus NaOH helps in the purification of the ore.
(ii) Cryolite :
(a) To increase the mobility of the fused mixture.
(b) To lower the fusion point of the mixture.
(c) To increase the electrical conductivity of the mixture.
(iii) Fluorspar :
(a) To increase the mobility of the fused mixture.
(b) To lower the fusion point of the mixture.

Question. Give reasons for the following – pertaining to Hall Herault’s process.
1. The fusion temperature of the electrolyte has to be lowered before conducting the electrolytic reduction.
2. The constituents of the electrolyte in addition to one part of fused alumina contains three parts of cryolite and one part of fluorspar.
3. A layer of powdered coke sprinkled over the electrolytic mixture, protects the carbon electrodes.
4. It is preferred to use a number of graphite electrodes as anode, instead of a single graphite electrode.
Answer : (i) The liberated A1 metal [m.p. 660°C] may also tend to volatize 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 :
(a) They lowers the fusion point of the mixture i. e. the mixture fuses [melts] 
around 950°C instead of 2050°C.
(b) They enhances 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.
(c) 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 because :
(a) It prevents burning of carbon electrodes in air at the emergence point from the bath.
(b) It minimizes or prevents heat loss by radiation.
(iv) The graphite[carbon] anodes are continously replaced during the electrolysis because :
(a) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
(b) The carbon anode is thus oxidised to carbon monoxide which either bums giving carbon dioxide or escapes out through an outlet.
2C + O₂ → 2CO [2CO + O₂ → 2CO₂]
(c) The carbon anode is hence consumed and renewed periodically after a certain period of usage,

Question. Define — (a) alloy, (b) amalgam
Answer : Alloy : An alloy is a homogenous mixture of two or more metals and nonmetals, out of which at least one is a metal.
For example, brass (Zn + Cu), bronze (Cu + Sn), solder (Sn +Pb), stainless steel (Fe + Cr + Ni + C) etc.
Amalgam : An alloy in which mercury is an essential constituent is called an amalgam.
For example, Dental amalgam (Hg + Ag + Sn), it is used for filling dental cavities.