April 07, 2012

p - Block Elements - II Five Marks


1. Describe in detail how noble gases are isolated by Dewar's process. Or How are noble gases separated by Dewar's method?
1. Principle:
The mixture of noble gases obtained by the Ramsay - Rayleigh's method is separated into individual constituents by the use of coconut charcoal which adsorbs different gases at different temperatures.
2.

Separation of noble gases (Dewar’s method)
The mixture of noble gases is passed into a double-walled bulb containing coconut charcoal and placed in a low temperature bath at 173K. It is allowed to remain in contact with the charcoal for about half an hour.
3. At 173K, only argon, krypton and xenon are adsorbed by the charcoal while
     helium and neon remain unadsorbed. These are pumped out and collected.
The mixture of helium and neon is kept in contact with coconut charcoal
4. at 93K which completely adsorbs neon
     leaving free helium.
The charcoal at 173K containing argon, krypton and xenon is placed in contact with another charcoal
       5. at the temperature of the liquid air (temp.77K) when argon diffuse into the other charcoal.
The temperature of the first charcoal (temp.173K) still containing krypton and xenon is
6. raised to 183K when krypton is set free while
    xenon remain adsorbed in the charcoal. When it is heated, xenon is recovered.
2. Describe in detail how noble gases are isolated from air by Ramsay - Rayleigh's method.
A mixture of air and oxygen is constantly admitted into a glass globe of about 50 litres capacity. Two platinum electrodes are introduced and a discharge from a transformer of about 6000 - 8000 volts is passed by the action of which nitrogen and oxygen rapidly combine to form oxides of nitrogen. The oxides are dissolved out in a solution of sodium hydroxide continuously circulated through the flask.
N2 + O2             2 NO
2 NO + O2        2NO2
2NO2 + 2NaOH NaNO3 + NaNO2 + H2O
Oxygen if any is removed by introducing alkaline pyrogallol in the globe. The supply of air and electric discharge is shut after some time and the remaining mixture of noble gases is pumped out.

Chemical method for isolation of noble gases
3. Discuss the structure of interhalogen compounds of AX and AX3 type.
Type AX. As expected, the compounds of the type AX are linear. Thus ClF, BrF, BrCl, ΙBr and ΙCl are all linear in structure.
Electronic structure of Chlorine atom, in the ground state and hybridized state is represented as

Linear structure of the interhalogen compounds of the type AX
Although the spatial arrangement of the four electron pairs (bp = 1 and lps = 3) around the central chlorine atom is tetrahedral, due to the presence of three lone pairs of electrons in three hybrid orbitals, the shape of AX molecule gets distorted and become linear.
Type AX3 Compounds of the type AX3 have trigonal bipyramidal structure, Fig. 3.5 for the ClF3 molecule.
Bipyramidal structure arises out of sp3d hybridisation involved in the formation of this compound, as illustrated in the Fig.3.6. The three dotted arrows indicate electrons contributed by the three fluorine atoms (without lone pair it is T-shaped).

Fig. 3.5 Bi pyramidal structure of CIF3 molecule
Fig. 3.6 sp3d hybridisation involved in the formation of ClF3 molecule
 Theoretically, we can come up with three possible arrangements for the three bonds and two lone pairs for the ClF3 molecule (Figure 7). The stable structure is the one that puts the lone pairs in equatorial locations, giving a T-shaped molecular structure.





Figure 7. (a) In a trigonal bipyramid, the two axial positions are located directly across from one another, whereas the three equatorial positions are located in a triangular arrangement. (b–d) The two lone pairs (red lines) in ClF3 have several possible arrangements, but the T-shaped molecular structure (b) is the one actually observed, consistent with the larger lone pairs both occupying equatorial positions.  
4. Explain the extraction of lead from its ore. Or How is lead extracted from its ore?
 Extraction: Lead is mainly extracted from the sulphide ore galena. Galena contains lead sulphide and small quantities of silver.
1. Concentration: Froth floatation process.
2. Smelting in a Reverberatory furnace: The concentrated ore is roasted in a reverberatory furnace at a moderate temperature. The temperature of furnace is controlled by regulating the air supply. During roasting, galena is partly oxidized to lead monoxide and partly to lead sulphate.
A reverberatory furnace
2PbS + 3O2  2 PbO + 2SO2
PbS + 2O2  PbSO4
More of galena is then added. The temperature is raised and simultaneously the air supply is reduced. Lead sulphide reacts with the two oxidised products giving lead.
PbS + 2PbO 3Pb + SO2
PbS + PbSO4  2Pb + 2SO2
Thus in this process roasting and smelting are carried out in the same furnace, at two different temperatures.
About 90% of lead is obtained as metal, the rest passes into slag. Lead is recovered from the slag by heating with lime and powdered coke.

Purification of Lead
Lead extracted by the above method contains impurities such as silver, copper, tin, bismuth, gold and iron. It is refined by the following processes.
a. Liquation
The impure metal is heated on a sloping hearth. Lead melts and flows down the slope. The infusible impurities remain on the hearth.
b. Desilverisation
Silver is removed by either Pattinson’s process or Park’s process.
c. Electrolytic refining
Anode – Impure lead
Cathode – Very pure lead
Electrolyte – Lead fluosilicate (PbSiF6) + Hydrofluosilicic Acid (H2SiF6)
The metallic impurities which are more electropositive than lead, such as iron and tin, go into the solution while the rest of the impurities are thrown down as anode mud.
5. Explain the following: i) dehydrating property of P2O5 ii) oxidizing power of fluorine.
i) Phosphorus pentoxide extracts water from many inorganic compound including sulphuric acid, nitric acid and several organic compounds. It is therefore, used as a powerful dehydrating agent.
            P4O10
H2SO4  SO3
           - H2O
            P4O10
2HNO3  N2O5
           - H2O
               P4O10
RCONH2  RCN
                      -H2O
ii) An important feature of the halogen is their oxidizing property which is due to high electron affinity of halogen atoms. The oxidizing power decreases from fluorine to iodine. Fluorine is the strongest oxidising agent. It oxidises other halide ions to halogens in solution or when dry.
F2 + 2X  2F + X2 (X = Cl–, Br, I)
Halogen of low atomic number oxidises the halide ion of higher atomic number.
6. How are noble gases isolated from air?
                 Answer: Q.No. 1 & Q.No. 2
7. How does Fluorine differ from other halogens? Or Write the anomalous nature of fluorine.
1. Fluorine is the most reactive element among halogen. This is due to the minimum value of F–F bond dissociation energy.
2. Fluorine decomposes cold dilute alkalies liberating OF2 and with conc. alkali, O2 is liberated. Under similar conditions, the other halogens will give rise to the hypohalites and halates respectively.
3. It has the greatest affinity for hydrogen, forming HF which is associated due to the hydrogen bonding. Hydrofluoric acid is a weak acid whereas the other hydrohalic acids are strong acids.
...... H– F...... H– F..... H–F.
4. It differs markedly from the other halogens in that it can form two types of salts with metals. NaF and NaHF2.
5. The salts of HF differ from the corresponding salts of other hydracids. AgF is soluble in water while the other AgX are insoluble.
6. Being strongly electronegative it can have only a negative oxidation state while the other halogens can have negative as well as positive oxidation state.
7. HF attacks glass while others do not.
8. Fluorine, because of the absence of d-orbitals in its valence shell does not form any polyhalides. Thus we have Ι3, Br3, Cl3 ions but no F3 ion.
 OR

S. No.
Fluorine
Other halogens
1
Fluorine is the most reactive element among halogen. This is due to the minimum value of F–F bond dissociation energy.
 Other halogens are less reactive.
2
Fluorine decomposes cold dilute alkalies liberating OF2 and with conc. alkali, O2 is liberated.
Under similar conditions, the other halogens will give rise to the hypohalites and halates respectively.
3
F has the greatest affinity for hydrogen, forming HF which is associated due to the hydrogen bonding. Hydrofluoric acid is a weak acid. .....H–F......H–F.....H–F.
Other hydrohalic acids are strong acids.
4
F differs markedly from the other halogens in that it can form two types of salts with metals.
Example: NaF and NaHF2.
Other halogens form only one type of salt with metals.
Example: NaCl, NaBr, NaI
5
The salts of HF differ from the corresponding salts of other hydracids. AgF is soluble in water.
Other AgX are insoluble in water.
6
Being strongly electronegative F can have only a negative oxidation state.
Other halogens can have negative as well as positive oxidation state.
7
HF attacks glass.
Other hydrogenhalides do not attack glass.
8
Fluorine, because of the absence of d-orbitals in its valence shell does not form any polyhalides. Thus we have no F3 ion
Form polyhalides. Thus we have Ι3, Br3, Cl3 ions.

8. How is fluorine isolated from their fluorides by Dennis method?
In Dennis’ Method fluorine is prepared by the electrolysis of fused sodium or potassium
hydrogen fluoride (perfectly dry). Electrolysis is carried out between graphite electrodes in a V-shaped electrically heated copper tube. The ends of the tube are covered with copper caps into which the graphite electrodes are fixed with bakelite cement. The copper tube is thickly lagged to prevent loss of heat.
KHF2  KF + HF
HF H+ + F¯
2H+ + 2e  H2       (At cathode)
2F  – 2e  F2        (At anode)
Preparation of fluorine
Fluorine liberated at the anode is passed through the U-tube containing sodium fluoride. This removes the hydrogen fluoride vapours coming with fluorine.
NaF + HF NaHF2
9. Illustrate i) tribasic nature of orthophosphoric acid ii) reducing property of Phosphorus acid.
i) Orthophosphoric acid is a tribasic acid. It combines with alkalies like NaOH to form three series of salts.
H3PO4 + NaOH NaH2PO4 + H2O
                                Sodium Di hydrogen Phosphate
H3PO4 + 2NaOH Na2HPO4 + 2H2O
                                  Disodium hydrogen Phosphate
H3PO4 + 3NaOH Na3PO4 + 3H2O
                                   Sodium Phosphate
ii) Phosphorus acid is a powerful reducing agent because it has P-H bond. It reduces silver nitrate solution into silver.
2AgNO3 + H3PO3 + H2O → 2Ag + H3PO4 + 2HNO3
10. Mention the uses of silicones.
1. Excellent insulators for electric motors and other appliances
2. Straight chain polymers of 20 to 500 units are used as silicone fluids. These polymers are used in waterproofing textiles, as lubricants and as polish.
3. Silicone rubbers are mixed with paints to make them damp-resistant.
4. Silicone resins are used as non-stick coating for pans and are used in paints and varnish.
5. Silicone oils are used for high temperature oil bath, high vacuum pump etc.
11. Write any five uses of Fluorine
1. Fluorine is used in the manufacture of freons. These non-toxic, non-combustible and volatile liquids are used as refrigerants in refrigerators, deep freezers and air conditioners.
2. CaF2 is used as flux in metallurgy.

3. NaF is used as a preservative to prevent fermentation and also for preventing dental cavities.

4. SF6 is used as an insulating material in high voltage equipment.
5. Teflon is used as container to store hydrofluoric acid.
6. UF6 is used in the separation of U235 from U238. 

12. Write a short note on Etching on glass
Industrially, Hydrogen fluoride is obtained by heating fluorspar (CaF2) with conc. H2SO4 in a lead vessel.
CaF2 + H2SO4→ CaSO4 + 2HF
HF distils over and the vapours are condensed in water in a lead receiver. Aqueous HF thus obtained is stored in wax bottles. It cannot be stored in glass or silica bottles as it attacks silicates and silica.
Na2 SiO3 + 6HF → Na2SiF6 + 3H2O
SiO2 + 4HF → SiF4 + 2H2O
The action of hydrofluoric acid on silica and silicates is used for etching glass. The glass article is first covered with a film on wax. The design to be etched is now drawn on the waxed surface and is then exposed to the action of hydrofluoric acid. Now the glass can be very soon etched. The wax is finally washed off with turpentine. 

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