Chemistry - Surface Chemistry & General principles and
process of Isolation of elements
[Time: 90Mins]
SOLUTION OF TEST
[Max. Marks: 35]
General Instructions
1. All questions are compulsory.
2. Question nos. 1 to 5 are very short answer questions and carry 1 mark each.
3. Question nos. 6 to 10 are short answer questions and carry 2 marks each.
4. Question nos. 11 to 15 are also short answer questions and carry 3 marks each
5. Question nos. 16 and 17 are long answer questions and carry 5 marks each
6. Use log tables if necessary, use of calculators is not allowed.
7. Surface Chemistry & General principles and process of Isolation of elements chapters
chapter Weight-age total marks
4+3=7 resp.
1.
How does the catalyst work?
[1 MARK]
Sol:
It provides an alternate path involving lower activation energy for the reactants.
[1 MARK]
2.
What is the cause of Brownian movement observed in colloidal solution?
solution
[1 MARK]
Sol:
Brownian movement is observed in colloidal solution due to collision
ision between colloidal particles and the
particles of dispersion medium.
[1 MARK]
3.
How does chemical adsorption of a gas on a solid vary with temperature?
Sol:
Chemical adsorption of a gas on solid first increases and then decreases with increase in temperature.
[1 MARK]
[1 MARK]
4.
What types of ores are roasted??
[1 MARK]
Sol:
Sulphide ores are roasted to convert them into their oxides.
[1 MARK]
5.
What is the principle of zone refining?
refining
[1 MARK]
Sol:
When the molten solution of an impure metal is allowed to cool, the pure metal crystallizes out while the
impurities remain in solution.
[1 MARK]
6.
Describe the method of refining of nickel.
Sol:
Mond process: In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex,
[2 MARK]
nickel tetracarbonyl:
The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal:
[2 MARK]
7.
What is electro-refining
refining ? Explain with the help of example.
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[2 MARK]
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Sol:
Metals like Cu, Ag, Au, Al are purified by electrolytic refining. In this process, impure metal acts as anode,
pure metal acts as cathode. Soluble salt of metal acts as electrolyte. When electric current is passed, impure
metal changes into ions which are attracted towards cathode and change into pure metal.
At anode Cu→Cu2+ + 2e-
At cathode Cu2+ + 2e-→Cu
[2 MARK]
8.
Why does physisorption decrease with increase of temperature?
temperature
[2 MARK]
Sol:
Physisorption is an exothermic process:
According to Le Chatelier's principle, if we increase the temperature, equilibrium will shift in the backward
direction, i.e., gas is released from the adsorbed surface.
[2 MARK]
9.
What are emulsions? Write two application of emulsification.
[2 MARK]
Sol:
When liquid is dispersed in liquid, such
su a system is called emulsion.
[1 MARK]
Applications:
(i)
The cleansing action of soap is based upon the formation of oil in water emulsion soap emulsifies
grease and takes it away in water
wate along with the dirt.
(ii)
Digestion of fats in the intestine is aided by emulsification.
[1 MARK]
10.
Why is the ester hydrolysis slow in the beginning and becomes faster after some time?
time
Sol:
The ester hydrolysis takes place as follows:
[2 MARK]
The acid produced in the reaction acts as catalyst (autocatalyst) for the reaction. Hence, the reaction
becomes faster after some time.
[2 MARK]
11.
Sol:
S.No
What is the difference between physisorption and chemisorption?
[3 MARK]
1
[ /2 MARK FOR EACH ONE POINT]
Physisorption
Chemisorptions
1.
It arises because of van der Waals' forces.
It is caused by chemical bond formation.
2.
It is not specific in nature.
It is highly specific in nature.
3.
It is reversible in nature.
It is irreversible.
4.
It depends on the nature of gas. More easily It also depends on the nature of gas. Gases which can
liquefiable gases are adsorbed readily.
react with the adsorbent show chemisorption.
5.
Enthalpy of adsorption is low (20-40
(20
kJ mol- ) in this Enthalpy of adsorption is high (80-240
(80
kJ mol-1 ) in
case.
this case.
6.
Low temperature is favourable for adsorption. It High temperature is favourable for adsorption. It
decreases with increase of temperature.
increases with the increase of temperature.
7.
No appreciable activation energy is needed.
8.
It results into multimolecular layers on adsorbent It results into unimolecular layer.
surface under high pressure.
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High activation energy is sometimes needed.
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12.
Explain each term in detail.(ANY THREE)
(a) Peptization, (b) Brownian movement , (c) Tyndall effect , (d) Electrophoresis
Sol:
[3 MARK]
(a) Peptization: It is a process of converting a fresh precipitate into colloidal particles by shaking it with the
dispersion medium in the presence of a small amount of a suitable electrolyte. The electrolyte added is
called peptizing agent.
[1 MARK]
(b) Brownian movement: - It may be defined as continuous zig-zag movement of the colloidal particles in a
colloidal sol.
Brownian movement does not depend upon the nature of the collide but depends upon the size of the
colloidal particles and viscosity of the sol. Smaller the size and lesser the viscosity, faster is the movement of
the particles.
[1 MARK]
Brownian movement is responsible for the stability of the colloidal solution.
(c) Tyndall effect: - It is defined as the scattering of light by the colloidal particles present in a colloidal sol.
Tyndall effect is observed only when the following two conditions are satisfied :
i)
The diameter of the dispersed particles is not much smaller as compared to the wavelength of the light
used, and
ii) There is a large difference in the refractive indices of the dispersed phase and the dispersion medium.
[1 MARK]
(d) Electrophoresis: - The movement of colloidal particles under the influence of an electric field is called
electrophoresis.
Electrophoresis can be used to find out the nature of the charge that the colloidal particles carry. [1 MARK]
13.
What is the role of silica in the metallurgy of copper?
[3 MARK]
Sol:
During roasting copper pyrites are converted into a mixture of FeO and Cu2O,
∆
2 CuFe2S2 + O2 
→ Cu2S + 2 FeS + SO2
[1 MARK]
Copper pyrites
∆
2 CuS2 + 3O2 
→ 2Cu2O + 2SO2 and 2 FeS + 3 O2 → 2FeO + 2 SO2
[1 MARK]
To Remove FeO (Basic), an acidic flux silica is added during smelting. FeO then combines with SiO2 to form
ferrous silicate (FeSiO3) slag which floats over molten matte and hence can be easily removed
FeO + SiO2 → FeSiO3
Flux
Slag
Thus, the role of silica in the metallurgy of copper is to remove ferrous oxide as slag.
14.
Explain the following processes with example:
(a) Van-Arkel Method
Sol:
[1 MARK]
(b) Magnetic separation.
[3 MARK]
(a) Van-Arkel Method: It is the process in which metal is reacted with suitable reagent to form volatile
compound which readily changes into metal, e.g., Ti and Zr are purified by this method.
[1 1/ 2 MARK]
(b) Magnetic Separation: This method is based on magnetic and non-magnetic properties of two
components of the ore (pure, impure). The ground ore is carried on a travelling band which passes over
magnetic roller. The ore-leaves the band and falls from it. The particles are attracted by the magnetic field
form a separated pile. This method is used to separate tungsten ore particles from cassiterite (SnO2). This
method is also used for separating other transition metal ores such as magnetite (Fe3O4), Chromite (FeO.
Cr2C3) and pyrolusite (MnO2) from unwanted gangue.
[1 1/ 2 MARK]
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15.
Name the chief ore of silver. Describe with chemical equations the extraction of silver from this ore.
Explain Vapour– phase refining?
[3 MARK]
Sol:
The chief ore of silver is argentite or silver glance (Ag2S).
The extraction of silver from argentite: Argentite ore is treated with dilute solution of NaCN in presence of
oxygen to form complex.
2Ag2S + 8CN- +O2 +2H2O → 4[Ag(CN)2] +2S + 4OH Zn acts as reducing agent and displaces silver from the
complex. 2[Ag(CN)2r +Zn → [Zn(CN)4]2- +2Ag The crude Ag metal thus obtained is refined by fusion with
borax or by electrolysis.
[1 1/ 2 MARK]
Vapour – phase refining: In this method, the crude metal is freed from impurities by first converting it into a
suitable volatile compound by heating it with a specific reagent at a lower temperature and then
decomposing the volatile compound at some higher temperature to give the pure metal. Thus, the two
requirement are:
i) the metal should form a volatile compound with a suitable reagent.
ii) the volatile compound should be easily decomposable so that the recovery is easy.
16.
What are the different methods employed for the preparation of lyophobic colloids?
Sol:
The methods of preparation may be classified into two types.
[1 1/ 2 MARK]
(1)
Dispersion methods in which large particles are broken down to colloidal sizes.
(2)
Aggregation methods in which the molecules or ions are united to form colloidal particles.
(1)
Dispersion Methods:
(2)
[5 MARK]
(a)
Peptization: This is the reverse of coagulation. Shaking a precipitate with charged ions make
it colloidal
[1 MARK]
(b)
Heating: Lyophobic sol can be prepared by heating together suitable substances. Gelatin,
agar – agar and starch sols are prepared by mixing them with hot water.
[1 MARK]
Aggregation Methods:
(a)
Reduction: Metal sols are prepared by reducing their salts. Such sols are generally coloured.
For example, gold sol is prepared by reducing 0.0005 per cent gold (III) chloride solution with
0.1% tannium solution.
[1 MARK]
(b)
Oxidation: Colloidal sulphur is obtained by oxidation of H2S with nitric acid.
H2S + 2HNO3 → S + 2H2O + 2NO2
(c)
Hydrolysis: Ferric hydroxide sol can be prepared by adding conc. Ferric chloride solution into
hot water.
FeCl3 + 3H2O → Fe (OH) + 3HCl
[1 MARK]
(d)
Double decomposition: As2O3 sol is prepared by mixing equal volumes of 1% solution of
As2O3 in water and saturated sol of H2S.
(e)
Bredig’s Arc method: Metal sols are prepared by striking an electric are between metal
electrodes immersed in a medium.
[1 MARK]
OR
17.
What are the main ores of Al? How can impure aluminium is purified? Explain the purification of bauxite
with chemical equations.
[5 MARK]
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Sol:
Main ores of Al:
1.
Bauxite
—
Al2O3.2H2O
2.
Felspar
—
K2O, A12O3.6H2O
3.
Cryolyte
—
Na3AlF6
[1 MARK]
Electrolytic refining of aluminium (Hoope's electrolytic process): [1 MARK FOR DIAGRAM]
Aluminium is further purified by Hoope's process. Process is carried out in an iron cell lined inside with
carbon. The cell consists of three layers which differ in specific gravities. The lowest layer acting as anode
consists of impure aluminium. A mixture
mixture of fluorides of Al, Ba and Na forms middle layer and acts as
electrolyte. The uppermost layer is of pure aluminium and it acts as anode.
On electrolysis, aluminum ions of the electrolyte are discharged at cathode and pure aluminum collects as
the uppermost
most layer. An equivalent amount of aluminum is taken up by the middle layer from the bottom
layer. Thus, aluminum is transferred from bottom layer to the upper layer through the middle layer while
impurities are left behind. Crude aluminum is added from time
time to time to continue the process. [1 MARK]
Baeyer's process applied for the purification of bauxite ore containing ferric oxide as chief impurity. This ore
is usually red in colour and hence called red bauxite.
The powdered ore is roasted first to convert ferrous oxide. If any, to ferric oxide and then digested with a
concentrated solution of sodium hydroxide under pressure at 150°C for several hours. Alumina dissolves in
sodium hydroxide forming soluble sodium meta aluminates (NaA102) while ferric oxide and silica being
insoluble settles down and are removed by filtration.
Al2O 3 .2H2O + 2NaOH → 2NaAlO2 + 3H2O
Sodium meta
aluminate
(soluble)
[1 MARK]
Sodium meta aluminates is then agitated with freshly precipitated Al(OH)3 for 36 hours. As a result sodium
meta aluminates gets hydrolyzed forming precipitate of aluminium hydroxide.
NaAlO 2 + 2H 2O → NaOH + Al(OH) 3
precipitat e
The precipitate is washed, dried and ignited to get pure alumina while solution of NaOH is concentrated and
used again.
ignite
2 Al(OH)3 
→ Al 2 O 3 + 3H2 O
Pure
alu min a
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[1 MARK]
5
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