4. Marco teórico
4.2 Innovar, reinventar o diversificar…
1. Sulphide ore is lighter, it is not wetted by water and floats with the froth.
2. The process of extraction of metal from its ore by heating the ore with a suitable reducing agent is known as pyrometallurgy.
3. Zinc scrap would be preferred because zinc is more electropositive than iron.
4. Electropositive metals which cannot be reduced by chemical reduction are easily reduced electrolytically.
5. The naturally occurring chemical substance present in the earth’s crust which can be obtained by mining are called minerals. Those minerals from which metals can be extracted economically are called ores.
6. This is because the sulphide ore particles are lighter and preferentially wetted by oil whereas gangue particles are preferentially wetted by water.
7. If either the ore or the gangue particles are capable of being attracted by magnetic field, then such ore is concentrated by magnetic separation method.
8. Graphite anode is used in the electrometallurgy of aluminium from alumina, Al 2 O 3 , Oxygen liberated at high temperature reacts with graphite to form both CO 2 and CO gases and prevent the liberation of O 2 gas at the anode which may react with Al metal to give Al 2 O 3 again.
At anode : C (s) + O 2– (melt) ® CO (g) + 2e – C (s) + 2O 2– (melt) ® CO 2(g) + 4e –
9. Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria. The solution containing Cu 2+ is treated with scrap iron or H 2 . Cu 2+ (aq) + H 2(g) ® Cu (s) + 2H + (aq) .
10. Collectors increase the natural hydrophobicity of the surface, increasing the separability of the hydrophobic and hydrophilic particles.
11. The chief ores of zinc are (i) Zinc blende, ZnS (ii) Calamine, ZnCO 3 (iii) Zincite, ZnO.
Extraction of zinc : From zinc blende zinc is extracted by roasting followed by reduction with coke.
(a) Roasting : The concentrated ore is heated with oxygen at 900°C in reverberatory furnace to convert zinc sulphide to zinc oxide.
2 2
(Zinc blende) (Zinc oxide)
2 ZnS+3O ®2 ZnO+ 2SO
(b) Reduction : The reduction of zinc oxide is done using coke.
673 K
ZnO+C¾¾¾®Zn+ CO
The metal is distilled off and collected by rapid chilling.
12. (i) Froth floatation process : This method is based on the preferential wetting properties with the frothing agent and water.
The powdered ore is added to water containing pine oil or turpentine oil (Frothing agent).
23. Explain the role of each of the following in the extraction of metal from their ores :
(i) CO in the extraction of nickel (ii) Zinc in the extraction of silver
(iii) Silica in the extraction of copper [Delhi SetII 2011]
24. Describe the principle controlling each of the following processes:
(i) Vapour phase refining of titanium metal.
(ii) Froth floatation method of concentration of a sulphide
ore. [AI SetI 2011]
25. Describe the principle controlling each of the following processes:
(i) Zone refining of metals
(ii) Electrolytic refining of metals [AI SetII 2011]
26. Describe the principle controlling each of the following process :
(i) Preparation of cast iron from pig iron.
(ii) Preparation of pure alumina (Al 2 O 3 ) from bauxite ore.
[AI SetIII 2011]
27. Write the reactions involved in the following processes:
(i) Leaching of bauxite ore to prepare pure alumina.
(ii) Refining of zirconium by van Arkel method.
(iii) Recovery of gold after gold ore has been leached with NaCN solution. [Foreign SetII 2011]
28. Describe the principle involved in each of the following processes.
(i) Mond process for refining of nickel.
(ii) Column chromatography for purification of rare
elements. [Delhi SetI 2012]
29. Which methods are usually employed for purifying the following metals?
(i) Nickel (ii) Germanium
Mention the principle behind each one of them.
[AI SetI 2012]
30. Explain the role of each of the following.
(i) NaCN in the extraction of silver.
(ii) SiO 2 in the extraction of copper. [AI SetIII 2012]
a n s w e r s
A stream of air is passed which agitates the mixture and produces froth. The sulphide particles of the ore stick to the oil droplets and rise to the surface and floats with air bubbles. Water wets the gangue particles which sink to bottom.
Water
Concentrated ore Gangue
Air
Ore + Water + Pine oil + Sodium ethyl
xanthate
Foam carrying ore particles
Water
Fig. 6.3
This method is used for concentration of sulphide ores.
e.g. Copper pyrite (ore of copper) argentite (ore of silver) etc.
(ii) This method 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 the impure metal. The molten zone moves along with the heater which is moved forward. As the heater moved forward, the pure metal crystallises out of the melt and the impurities pass on into the adjacent molten zone. The process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated. This end is cut off. This method is very useful for producing semiconductor and other metals of very high purity, e.g., germanium, silicon, boron, gallium and indium.
Noblegas atmosphere
Inductioncoil heaters moving as shown Metal rod
Molten zone
Zone refining process
(iii) Refining by liquation : In this method a low melting metal like tin is heated on a sloping surface of a reverberatory furnace. Metal melts and flows down, impurities leave behind.
13. (i) Thermodynamic factor helps us in choosing a suitable reducing agent for the reduction of a particular metal oxide to the metallic state.
(ii) Any metal will reduce the oxides of other metals ifD f G°
of the oxide of the element is less thanD f G° of the metal oxide to be reduced. Thus both Al and Zn can reduce Fe 2 O 3 and Mg can reduce Al 2 O 3 to Al.
14. (i) The role of depressant in froth floatation process is to prevent some of the sulphide from forming froth.
e.g., NaCN is used as depressant in the purification of PbS in presence of ZnS.
(ii) During the metallurgy of copper from copper pyrites, CuFeS 2 , its roasting gives FeO besides Cu 2 O and SO 2 , respectively.
2( ) 2( ) ( ) 2 ( ) 2( )
(Copper pyrites)
4 CuFeS s +11O g ®4 FeOs +2 Cu Os + 8SO g To remove FeO, SiO 2 is added to form slag.
( ) 2( ) 3( )
(Basic oxide) (Acidic oxide) Slag
FeOs + SiO s ® FeSiO l
(iii) Cryolite, Na 3 AlF 6 , dissolves alumina, Al 2 O 3 , lowers the melting temperature of electrolyte and also increase its conductivity.
15. (i) Cryolite (Na 3 AlF 6 ) is added as flux in the electrolytic reduction of alumina. It decreases the melting point of the alumina and increases its conductivity.
(ii) Carbon monoxide combines with nickel forming a volatile complex n ickel tetr acar bon yl wh ich decomposes on further heating to give pure nickel.
16. (i) NaCN combines with argentite ore of silver forming a soluble complex.
2 2 2
Sodium dicyano argentate
Ag S + 4 NaCN® 2 Na[Ag(CN) ]+ Na S
This complex displaced Ag when more reactive metal is added to it.
(ii) Refer Ans. 15 (i).
17. (i) Electrolytic refining : In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic ones go to the anode mud.
Anode : M ® M n+ + ne – Cathode : M n+ + ne – ® M
Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result of electrolysis is the transfer of copper in pure form from the anode to the cathode:
Anode : Cu ® Cu 2+ + 2e – Cathode : Cu 2+ + 2e – ® Cu
Impurities from the blister copper deposit as anode mud which contains antimony, selenium, tellurium, silver, gold and platinum; recovery of these elements may meet the cost of refining.
Zinc may also be refined this way.
(ii) Vapour phase refining : In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.
So, the two requirements are :
(a) The metal should form a volatile compound with an available reagent.
(b) The volatile compound should be easily decomposable, so that the recovery is easy.
Following example will illustrate this technique.
Mond process for Refining Nickel : In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl :
330 350 K
Ni + 4 CO ¾¾¾¾¾® - Ni(CO) 4
The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal :
450 470 K
Ni(CO)4 ¾¾¾¾¾®- Ni+ 4 CO
18. (i) Alumina is dissolved in cryolite to lower the melting point and improve its electrical conductivity.
(ii) Carbon is suitable reducing agent for reduction of zinc oxide. Reduction of Cr 2 O 3 by car bon is n ot thermodynamically favourable.
(iii) Free energy change for the reduction of copper sulphide to copper by carbon is positive.
Where as D r G° for the reduction of copper oxide to copper by carbon is negative and hence feasible.
19. (i) Pig iron is mixed with scarp iron and heated in a furnace. After removal of impurities required quantity of spiegel is added to make steel.
(ii) Zinc oxide is mixed with coke and clay and brickettes are made. The brickettes are heated at 673 K to give metallic zinc
673 K
( ) ( ) ( ) ( )
ZnOs + Cs ¾¾¾® Zn s + CO g
(iii) Van Arkel de Boer process : Impure titatinum is heated with iodine to form volatile complex TiI 4 which on further heating at higher temperature decomposes to give pure titanium.
( ) 2( ) 4 ( ) 2
Impure Pure
Tis + 2 I s ®TiI ¾¾D ® Ti s + 2 I
20. (i) Refer Ans. 16(i).
(ii) SiO 2 is added in the extraction of copper from copper matte to remove remaining FeS and FeO as silicate (slag)
2FeS + 3O 2 ® 2FeO + 2SO 2(g)
2FeO + SiO 2 ® 2FeSiO 3 (slag)
(iii) Iodine forms a volatile compound with titanium which on further heating decomposes to give pure titanium.
Ti + 2I 2(s) ® TiI 4 ¾¾D ® Ti + 2I 2 .
(Impure) (Pure)
21. (i) Refer Ans. 16 (i).
(ii) Refer Ans. 20 (iii).
(iii) Reger Ans. 14 (iii).
22. (i) Refer Ans. 17(ii).
(ii) Refer Ans. 17(i).
(iii) Recovery of silver after silver ore was leached with NaCN : During leaching Ag is oxidised to Ag + which
then combines with CN – ions to form soluble complex.
[Ag(CN) 2 ] – . Silver is then recovered from this complex by displacement method using more electropositive zinc metal.
2[Ag(CN) 2 ] – (aq) + Zn (s) ® 2Ag (s) + [Zn(CN) 4 ] 2– (aq)
23. (i) CO in the extraction of nickel : The role of CO is to convert impure nickel to volatile nickel tetracarbonyl whereas the impurities are left behind in the solid state.
The vapour of nickel tetracarbonyl on heating at 450
470 K decomposes to give pure nickel metal and carbon monoxide.
Ni + 4CO
Impure Nickel Nickel tetracarbonyl
350 K
330 Ni(CO) 4
Pure nickel Ni + 4CO ( ) g (ii) Zinc in the extraction of silver : Role of zinc is to recover silver from the complex by displacement reaction. Silver ore is leached with dilute solution of NaCN in the presence of air or oxygen to form a soluble complex.
4Ag + 8CN( )s – (aq) + 2H O + O 2 ( )l 2 ( ) g
4[Ag(CN) ] + 4OH 2 – – (aq ) Soluble Complex
2[Ag(CN) 2 ] – + Zn [Zn(CN) 4 ] 2– + 2Ag
(iii) Silica in the extraction of copper : Role of silica in the extraction of Cu is to remove ferrous impurities as slag during smelting and bessemerisation. The impurity ferrous sulphide is oxidised to ferrous oxide which is then reacted with silica (flux) to form ferrous silicate (slag).
FeO + SiO FeSiO 2 3
Flux Ferrous Silicate (Slag)
24. (i) In this method, the metal is converted into its volatile compoun d and collected elsewhere. It is th en decomposed to give pure metal.
Ti + 2I 2 500 K TiI 4 Impure
metal
Ti + 2I 2 700 K
TiI 4 Ti
Pure metal
(ii) This method of concentration of ore is based upon the principle that sulphide ore is lighter and its surface is preferentially wetted by oils while that of a gangue is preferentially wetted by water.
25. (i) Refer Ans. 12(ii).
(ii) Refer Ans. 17(ii).
26. (i) Pig iron is melted with scrap iron and coke using hot air blast. Due to this, impurities such as C, S and P present in the pig iron are removed as CO 2 , SO 2 and P 2 O 5 and carbon reduced to about 3%.
(ii) Al 2 O 3 present in bauxite is soluble in concentrated
NaOH solution whereas impurities are not, Al 2 O 3 is reprecipitated from the solution.
27. (i) Leaching of bauxite ore to prepare pure alumina:
Al 2 O 3 + 2NaOH (aq) + 3H 2 O (l) ® 2Na[Al(OH) 4 ] (aq) 2Na[Al(OH) 4 ] (aq) + CO 2(g) ® Al 2 O 3 ∙xH 2 O (s) + 2NaHCO 3(aq)
Al O ∙ H O Al O2 3x 2 ( )s 1470 K 2 3( ) s + H O x 2 ( ) g
(ii) Refining of zirconium by van Arkel de Boer method.
Zr (s) + 2I 2(s) D ZnI 4(g)
ZrI 4(g) Tungsten filament 2075 K Zr (s) + 2I 2(s)
Pure
(iii) Extraction of gold:
4Au (s) + 8CN – (aq) + 2H 2 O (l) + O 2(g) ®
4[AuCN) 2 ] – (aq) + 4OH – (aq) Gold is recovered from [Au(CN) 2 ] – complex by displacement method using a more electropositive metal zinc.
2[Au(CN) 2 ] – (aq) + Zn (s) ® 2Au (s) + [Zn(CN) 4 ] 2– (aq) 28. (i) Mond’s process for refining of nickel : Nickel is refined
by Mond’s process. In this process, Nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl.
330 350 K
Ni+4 CO¾¾¾¾® - Ni(CO) 4
The carbonyl is subjected to higher temperature so that it is decomposed giving the pure nickel metal.
450 470 K
4 Pure metal
Nickel tetracarbonyl
Ni(CO) ¾¾¾¾®- Ni+ 4 CO
(ii) Chromatographic method : This method is based on the extent of adsorption of different components on an adsorbent.
Mixture of compounds Stationary
phase
Coloured bands
Column chromatography
For example, the solid of Al 2 O 3 is packed in a glass tube and moving medium containing a solution of the components in liquid form is run down.
This method is very useful for purification of the elements which are available in small quantities and the impurities are not very different in chemical properties from the element to be purified.
29. (i) Refer Ans. 28(i).
(ii) Refer Ans. 12(ii).
30. (i) Refer Ans. 16(i).
(ii) Refer Ans. 23(iii).
JJJ
VERY SHORT ANSWER TYPE QUESTIONS (1 MARK) 1. Which is a stronger acid in aqueous solutions HF or HCl?
[2008]
2. In which one of the two structure NO 2 + and NO 2 – the bond angle has a higher value? [Delhi 2008]
3. Why is the bond angle in PH 3 molecule lesser than that in
NH 3 Molecule? [AI 2008]
4. Wh y is dinitrogen ver y unreactive as compar ed to
phosphorus? [AI 2008]
5. When HCl reacts with finely powdered iron, it forms ferrous chloride and not ferric chloride. Why? [AI 2008]
6. Fluorine exhibits only – 1 oxidation state in its compounds whereas other halogens exhibit many other oxidation states.
Why? [AI 2008 C]
7. Why is Bi (V) a stronger oxidant than Sb(V). ?
[Delhi 2009]
8. Why does NO 2 dimerise? [AI 2010]
9. Draw the structure of O 3 molecule. [Delhi 2010]
10. Nitrogen is relatively inert as compared to phosphorus. Why?
[AI 2010]
11. Draw the structure of XeF 2 molecule. [Delhi SetI 2011]
12. Draw the structure of XeF 4 molecule. [Delhi SetII 2011]
13. Draw the structure of BrF 3 molecule. [Delhi SetIII 2011]
14. Which one of PCl 4 + and PCl 4 – is not likely to exist and why?
[Delhi SetI 2012]
15. Of PH 3 and H 2 S which is more acidic and why?
[Delhi SetIII 2012]
16. Which is a stronger reducing agent, SbH 3 or BiH 3 and why?
[AI SetI 2012]
17. What is the basicity of H 3 PO 2 acid and why? [AI SetIII 2012]
SHORT ANSWER TYPE QUESTIONS (2 OR 3 MARKS) 18. Draw the structure of the following molecules :
(i) SF 4 (ii) XeF 4 [Delhi 2008]
19. Write chemical equations for the following processes:
(i) Chlorine reacts with a hot concentrated solution of sodium hydroxide.
(ii) Orthophosphorus acid is heated.
(iii) PtF 6 and xenon are mixed together. [Delhi 2008]
20. Complete the following chemical equations : (i) Ca 3 P 2(s) + H 2 O (l) ® ––––––
(ii) Cu 2+ (aq) + NH 3(aq) ® – – – – – –
(excess)
(iii) F 2(g) + H 2 O (l) ® –––––– [Delhi 2008]
21. Give reason for the following :
(a) Oxygen molecule has the formula O 2 while sulphur is S 8 . (b) H 2 S is less acidic than H 2 Te. [Delhi 2008]
22. Answer the following :
(i) Which neutral molecule would be isoelectronic with ClO – ?
(ii) Of Bi (V) and Sb (V) which may be a stronger oxidising
agent and why? [AI 2008]
23. Assign a reason for each of the following statements : (i) Phosphorus shows marked tendency for catenation but
nitrogen shows little tendency for catenation.
(ii) The electron gain enthalpy with negative sign for oxygen [– 141 kJ mol –1 ] is less than that for sulphur (– 200 kJ mol –1 ). [AI 2008]
24. (a) How are interhalogen compounds formulated and how are they prepared?
(b) With which neutral molecule of this group is ClO –
isoelectronic? [AI 2008 C]
25. (i) Draw the structure of SO 2 molecule. Comment on the nature of two S – O bonds formed in it. Are the two S – O bonds in this molecule equal?
(ii) What happens when white phosphorus is heated with conc. NaOH solution in an inert gas atmosphere?
[AI 2008 C]
26. Draw the structures of the following molecules :
(i) XeF 4 (ii) BrF 3 [Delhi 2009]
27. Complete the following chemical reaction equations : (i) P 4(s) + NaOH (aq) + H 2 O (l) ®
(ii) I – (aq) + H 2 O (l) + O 3(g) ® [Delhi 2009]
28. Why is red phosphorus less reactive than white phosphorus?
[Delhi 2009]