Students get through AP Inter 2nd Year Chemistry Important Questions 5th Lesson General Principles of Metallurgy which are most likely to be asked in the exam.
AP Inter 2nd Year Chemistry Important Questions 5th Lesson General Principles of Metallurgy
Very Short Answer Questions
Question 1.
What is the role of depressant in froth floatation?
Answer:
By using depressants in froth floatation process, it is possible to separate a mixture of two sulphide ores.
Eg: In the ore containing ZnS and PbS, the depressant used is NaCN. It prevents ZnS from coming to the froth but allows PbS to come with the froth.
Question 2.
Explain “poling”. (AP Mar. ’16, ’15; IPE ’16, 15 (AP))
Answer:
When the metals are having the metal oxides as impurities this method is employed. The impure metal is. melted and is then covered by carbon powder. Then it is stirred with green wood poles. The reducing gases formed from the green wood and the carbon, reduce the oxides to the metal.
Eg : Cu & Sn metals are refined by this method.
Question 3.
Decribe a method for the refining of nickel.
Answer:
Mond’s process:
- In Mond’s process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetra carbonyl.
- Nickel tetra carbonyl is strongly heated to decompose and gives the pure Nickel.
Question 4.
What is the role of cryolite in the metallurgy of aluminium ? (IPE 2015 (TS), BMP, 2016 (TS))
Answer:
By adding the cryolite to the pure Alumina, the melting point of pure Alumina is lowered (which is very high 2324K) and electrical conductivity of pure alumina is increased.
Question 5.
Give the composition of the following alloys (IPE ’16, ’14 (TS)) (AP & TS Mar. ’17)
a) Brass
b) Bronze
c) German Silver
Answer:
a) Composition of Brass : 60 – 80% Cu, 20 – 40% Zn
b) Composition of Bronze : 75 – 90% Cu, 10 – 25% Sn
c) Composition of German silver : 50 – 60% Cu, 10 – 30% Ni, 20 – 30% Zn.
Question 6.
What is matte ? Give its composition.
Answer:
During the extraction of ’Cu’ from copper pyrites the product of the blast furnace consists mostly of Cu2S and a little of FeS. This product is known as “Matte”. It is collected from the outlet at the bottom of the furnace.
Question 7.
What is flux ? Give an example.
Answer:
Flux : An outside substance added to. ore to lower its melting point is known as flux.
- Flux combines with gangue and forms easily fusible slag.
gangue + flux → slag
Question 8.
How is aluminium useful in the extraction of chromium and manganese from their oxides ?
Answer:
- ‘Al’ is used as reducing agent.
- By Alumino thermite process Cr, Mn are extracted from their oxides.
- The reactions are highly exothermic.
Cr2O3 + 2Al → 2Cr + Al2O3
3Mn3O4 + 8Al → 4Al2O3 + 9Mn
Question 9.
What is a mineral ?
Answer:
The chemical compound from which a metal can be extracted is called a mineral. Bauxite, crayolite are the minerals of aluminium.
Question 10.
What is an ore ?
Answer:
An ore is a mineral from which the metal can be extracted easily and economically. Bauxite is the ore of aluminium.
Question 11.
What is a ore. Give the ores of Al, Zn, Fe, Cu.
Answer:
The mineral from which metal can be extracted economically is called ore.
Aluminium ores : Bauxite = Al2 O3. 2H2O; Cryolite = Na3Al F6
Zinc ores : Zinc blende = Zns; Calamine = ZnCO3
Copper ores : Cuprite: CuO; Copper pyrites = Cu2S
Iron ores : Haematite = Fe2O3 Magnetite = Fe3O4
Question 12.
What is the role of SiO2 in the extraction of copper.
Answer:
In the extraction of copper SiO2 acts as flux. It combines with FeO and removes as slag.
FeO + SiO2 (flux) → FeSiO3 (Slag)
Short Answer Questions
Question 1.
Outline the principles of refining of metals by the following methods.
(a) Zone refining
(b) Electrolytic refining
(c) Poling
(d) Vapour phase refining.
Answer:
a) Zone refining :
- Zone refining is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.
- A circular mobile heater is fixed at one end of a rod of impure metal.
- The molten zone moves along with the heater moves forward the pure metal crystallises out of the melt and the impurities pass into the adjacent molten zone.
- The above process is repeated several times and the heater is moved in the same direction form one end to the other end. At one end impurities get concentrated. This end is cut off.
- This method is very useful for producing semiconductor grade metals of very high purity.
Eg : Ge, Si, B, Ga etc…
b) Electrolytic refining: This process is used for less reactive metals like Cu, Ag, AZ, Au etc.
- In this process anode is made by impure metal and a thin strip of pure metal acts as cathode.
- On electrolysis metal dissolves from anode and pure metal gets deposited at cathode.
Impurities settle down below anode in the form of anode mud.
c) Poling : When the metals are having the metal oxides as impurities this method is employed. The impure metal is melted and is then covered by carbon powder. Then it is stirred with green wood poles. The reducing gases formed from the green wood and the carbon, reduce the oxides to the metal. Eg : Cu & Sn metals are refined by this method.
d) Vapour phase refining: In this method the metal is converted into its volatile compound and coll Ted. It is then decomposed to give pure metal.
- The metal should form a volatile compound with an available reagent.
- The volatile compound should be easily decomposable. So the recovery is easy.
E.g : Mond’s process :- In Mond’s process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetra carbonyl,
- Nickel tetra carbonyl is strongly heated to decompose and gives the pure Nickel.
- In Mond’s process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetra carbonyl,
Question 2.
Explain the purification of sulphide ore by froth floatation method. (Mar ’15) (A.P. Mar. ’17)
Answer:
Froth floatation method :
- This method is used to concentrate sulphide ores.
- In this process a suspension of the powdered ore is made with water.
- A rotating paddle is used to agitate the suspension and air is blown into the suspension in presence of an oil.
- Froth is formed as a result of blown of air, which carries the mineral particles.
- To the above slurry froth collectors and stabilizers are added.
- Collectors like pine oil enhance non-wettability of the mineral particles.
- Froth stabilizers like cresol stabilize the froth.
- The mineral particles wet by oil and gangue particles wet by water.
- The broth is light and is skimmed off. The ore particles are then obtained from the froth. By using depressants in froth floatation process, it is possible to separate a mixture of two suplhide ores. Eg: In the ore containing ZnS and PbS, the depressant used is NaCN. It prevents ZnS from coming to the froth but allows PbS to come with the froth.
Question 3.
What is Ellingham diagram ? What information can be known from this in the reduction of oxides ?
Answer:
The graphical representation of Gibbs energy which provides a sound basis for considering the choice of reducing agent in the reduction of oxides. This graphical representation is known as Ellingham diagram.
This diagram helps us in predicting the feasibility of thermal reduction of an ore.
- The Criterion of feasibility of a reaction is that at a given temperature, Gibbs energy of the reaction must be negative.
- Ellingham diagram normally consists plots of ΔG° vs T for formation of oxides of elements.
- The graph indicates whether a reaction is possible or not, i.e., the tendency of reduction with a reducing agent is indicated.
- The reducing agent forms its oxide when the metal oxide is reduced. The role of reducing agent is to make the sum of ΔG° values of the two reactions negative.
- Out of C and CO, Carbon monoxide (CO) is a better reducing agent at 673K.
- At 983K (or) above coke(C) is better reducing agent.
- The above observations are from Ellingham diagram.
Zinc is not extracted from zinc oxide through reduction by using CO.
Explanation :
2Zn + O2 → 2ZnO, ΔG° = -650 kJ
2CO + O2 → 2CO2, ΔG° = -450 kJ
2ZnO + 2CO → 2Zn, 2CO2, ΔG° = 200 kJ
ΔG° = Positive indicates that the reaction is not feasible.
The above fact is explained on the basis of Ellingham diagram.
Question 4.
Give examples to differentiate roasting and calcination. (IPE ’14, B.M.P. 2016) (A.P. & T.S. Mar. ’16, ’15)
Answer:
Roasting: Removal of the volatile components of a mineral by heating mineral either alone (or) mixed with some other substances to a high temperature in the presence of air is called Roasting.
- It is applied to the sulphide ores.
- SO2 gas is producted along with metal oxide.
Calcination: Removed of the volatile components of a mineral by heating in the absence of air is called calcination.
- It is applied to carbonates and bicarbonates.
- CO2 gas is produced along with metal oxide.
Question 5.
How is copper extracted from copper pyrites ?
Answer:
Extraction of copper from copper pyrites :
Copper pyrite is the main source of copper metal. Various steps involved in the extraction of copper are discussed below.
Step -I:
Concentration of ore by froth floatation process :
The ore is first crushed in ball mills. The finely divided ore is suspended in water. A little pine oil is added and the mixture is vigorously’ agitated by a current of air. The froth formed carries the ore particles almost completely. The gangue sinks to the bottom of the tank. The froth is separated and about 95% concentrated ore is obtained.
Step -II:
Roasting : To remove the volatile impurities like As (or) Sb, the ore is roasted in a free supply of air. A mixture of sulphides of copper and iron are obtained and these are partially oxidised to respective oxides.
Cu2S. Fe2S3 + O2 → Cu2S + 2FeS + SO2
2Cu2S + 3O2 → 2Cu2O + 2SO2
2FeS + 3O2 → 2FeO + 2SO2
Step -III: .
The roasted ore is mixed with a little coke and sand (Silica) and smelted in a blast furnace and fused. A blast of air, necessary for the combustion of coke, is blown through the tuyeres present at the base of the furnace.’ The oxidation of the sulphides of copper and iron will be completed further. A slag of iron silicate is formed according to the reactions given below :
Step -IV :
After smelting the copper ore in blast furnace, the product of the blast furnace consists mostly of Cu2S and a little of ferrous sulphide. This product is known as “Matte.” It is collected from the outlet at the bottom of the furnace. After then the following processes are carried out for getting the pure copper.
Bessemerization : The matte is charged into a Bessemer converter. A bessemer converter is a pear-shaped furnace. It is made of steel plates. The furnace is given a basic lining with lime or magnesium oxide (obtained from dolomite or magnesite). The converter is held in position by trunnions and can be tilted in any position. A hot blast of air and sand is blown through the tuyeres present near the bottom. Molten metal, the product in the furnace, collects at the bottom of the converter.
Reactions that took place in blast furnace go to completion. Almost all of iron is eliminated slag. Cuprous oxide combines with cuprous sulphide and forms Cu metal.
2Cu2O + Cu2S → 6Cu + SO2
The molten metal is cooled in sand moulds. SO2 escapes. The impure copper metal is known as ‘Blister copper” and is about 98% pure.
Step -V:
Refining: The Blister copper is purified by electrolysis. The impure copper metal is made into plates. They are
suspended into lead — lined tanks containing Copper (II) Sulphate solution. Thin plates of pure copper serve as
cathode. The cathode plates are coated with graphite. On electrolysis, pure copper is deposited at the cathode. The
copper obtained is almost 100% pure Cu.
Question 6.
Explain the process of leacing of aluminium from bauxite.
Answer:
Extraction of Aluminium from Bauxite:
For the purpose of extraction of Al, Bauxite is the best source.
Purification of Bauxite: Bauxite containing Fe2O3 as impurity is known as Red Bauxite.
Bauxite containing SiO2 as impurity is known as White Bauxite and can be purified by “Set-peck’s Process. Red Bauxite is purified by Bayer’s process and Hall’s process.
Bayer’s Process: Red bauxite is roasted and digested in concentrated NaOH at 423 K. Bauxite dissolves in NaOH to form sodium meta aluminate while impurity Fe2O3 does not dissolve which can be removed by filtration.
Al2O3.2H2O + 2NaOH → 2NaAlO2 + 3H2O
The solution which contains sodium meta aluminate is diluted and crystals of Al(OH)3, are added which serves as seeding orgent. Sodium meta aluminate undergoes hydrolysis to precipitate Al(OH)3.
2NaAlO2 + 4H2O → 2NaOH + 2Al(OH)3↓
Al(OH)3 is filtered and ignited at 1200°C to get anhydrous alumina.
Halls’ Process: Red Bauxite is fused with sodium carbonate to form sodium meta aluminate
which is extracted with water. The impurity Fe2O3 is filtered out.
Al2O3 + Na2CO3 → 2NaAlO2 + CO2
Into the solution of sodium meta aluminate, CO2 gas is passed to precipitate Al(OH)3.
The precipitated Al(OH)3 is ignited at 1200°C to get anhydrous alumina.
2Al(OH)3 → Al2O3 + 3H2O
Serpeck’s Process: Powdered bauxite is mixed with coke and heated to 2075 K in a current of nitrogen gas. Aluminium Nitride is formed while SiO2 is reduced to Si which escapes out.
Aluminium nitride is hydrolysed to get aluminium hydroxide which on ignition gives anhydrous alumina.
Electrolytic Reduction of Alumina: Pure Alumina (Al2O3) is a bad conductor of electricity and it has high melting point (2050°C). So it cannot be electrolysed. Alumina is electrolysed by dissolving in fused cryolite to increase the conductivity and small amount of Fluorspar is added to reduce its melting point. Thus the electrolyte is a fused mixture of Alumina, Cryolite and Fluorspar.
Electrolysis is carried out in an iron tank lined inside with graphite (carbon) which functions as cathode. A number of carbon rods (or) copper rods suspended in the electrolyte functions as anode.
An electric current of 100 amperes at 6 to 7 volts is passed through the electrolyte. Heat produced by the current keeps the mass in fused state at 1175 to 1225K. The following reactions take place in the electrolytic cell under these conditions.
Na3AlF6 → 3NaF + AlF3
Cryolite
4AlF3 → 4Al3+ + 12F–
At cathode : 4Al3+ + 12e– → 4Al
At anode : 12F– → 6F2 + 12e–
F2 formed at the anode reacts with alumina and forms Aluminium fluoride.
2Al2O3 + 6F2 → 4AlF3 + 3O2
Aluminium, produced at the cathode, sinks to the bottom of the cell. It is removed from time to time through topping hole.
Purification of Aluminium: (Hoope’s Process)
The impurities present are Si, Cu, Mn etc.,
The electrolytic cell used for refining of aluminium consists of iron tank lined inside with
carbon. This acts as anode. The tank contains three layers of fused masses. The bottom layer
contains impure aluminium. Middle layer contains mixture of AlF3, NaF and BaF2 saturated with Al2O3. Top layer contains pure aluminium and graphite rods kept in it act as cathode.
On passing current aluminium ions from the middle layer are discharged at the cathode as pure aluminium. Equivalent amount of aluminium from the bottom layer passes into middle layer.
Question 7.
Explain the reactions occuring in the blast furnace in the extraction of iron.
Answer:
In the Blast furnace, reduction of iron oxides takes place in different temperature ranges. Hot air is blown from the bottom of the furnace and coke is burnt to give temperature upto about 2200K in the lower portion itself, The burning of coke therfore supplies most of the heat required in the process. The CO and heat moves to upper part of the furnace. In upper part, the temperature is lower and the iron oxides (Fe2O3 and Fe3O4) coming from the top are reduced in steps to FeO. Thus, the reduction reactions taking place in the lower temperature range and in the higher temperature range, depend on the points of corresponding intersections in the ΔrGθ vs T plots. These reactions can be summarised as follows:
At 500 — 800 K (lower temperature range in the blast furnace)
3 Fe2O3 + CO → 2 Fe3O4 + CO2
Fe3O4 + 4CO → 3 Fe + 4 CO2.
Fe2O3 + CO → 2 FeO + CO2
At 900- 1500 K (higher temperature range in the blast furnace)
C + CO2 → 2 CO
FeO + CO → Fe + CO2
Lime stone is also decomposed to CaO which removes silicate impurity of the ore as CaSiO3 slag. The slag is in molten state and separåtes out from iron.
The iron obtained from blast furnace contains about 4% carbon and many impurities in smaller amount (e.g., S, P, Si, Mn). This is known as pig iron. Cast iron is different from and is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3%) and is extremely hard and brittle.
Fe2O3 + 3 C → 2 Fe + 3 CO