Plus Two Chemistry Chapter Wise Previous Questions Chapter 6 General Principle and Processes of Isolation of Elements

Kerala State Board New Syllabus Plus Two Chemistry Chapter Wise Previous Questions and Answers Chapter 6 General Principle and Processes of Isolation of Elements.

Kerala Plus Two Chemistry Chapter Wise Previous Questions Chapter 6 General Principle and Processes of Isolation of Elements

Question 1.
Analyse the table given below: (March – 2010)

Metal	Ore
Copper	Copper pyrites, Copper glance, Cuprite
Zinc	Zinc blende, Calamine, Zincite
Aluminium	Bauxite, Diaspore
Iron	Haematite, Magnetite, Iron pyrites

a) Which of the ores mentioned in the above table can be concentrated by magnetic separation method? Justify your answer.
b) Identify the ores that can be concentrated by leaching.
c) What do you mean by leaching?
Answer:
a) Haematite, Magnetite, Iron pyrites Magnetic separation is based on difference in magnetic properties of the ore components. If either the ore or the gangue (one of these two) is capable of being attraced by a magnetic field, then such separations are carried out. In the case of iron ores mentioned above the ore particles are magnetic while the impurities are non-magnetic. Thus, when the ground ore is carried on a conveyer belt which passes over a magnetic roller, the ore particles are attracted towards the magnetic roller while the non-magnetic particles are collected away from the magnetic roller,

b) Bauxite

c) It is a chemical method used for the concentration of ore. For example, the ore of Al, bauxite is concentrated by leaching (Baeyer’s process). Bauxite is heated with NaOH solution. As a result of this reaction, sodium meta aluminate is formed. The aluminate solution is neutralised by passing CO₂ solution and hydrated Al₂O₃ is precipitated by seeding with freshly prepared samples of hydrated Al2O₃. Hydrated alumina is filtered, dried and heated to obtain pure Al2O₃.

Question 2.
You are provided with samples of some impure metals such as Titanium and Nickel. (Say – 2010)
a) Which method would you recommend for the purification of each of these metals?
b) Briefly explain each method.
Answer:
a) Ti – van Arkel method
Ni – Monds process

b) The crude metal is heated in an evacuated vessel with iodine. The metal iodide is formed. The metal iodide is decomposed on a tungsten filament, at about 1800 K. The pure metal is ’ deposited on the filament.

Mond process: In this method impure Ni is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl.

The carbonyl is subjected to higher temperature so that it is decomposed to give pure metal.

Question 3.
The concept of AG° of coupled reactions are used to explain reductions in metallurgy. (March – 2011)
a) Explain the above statement.
b) In the blast furnace for manufacturing iron, most of the reduction is carried out by CO rather than C(Coke). How can you account for this?
Answer:
a) The reduction of a metal oxide which is not feasible (\(\Delta G^{\ominus}\) positive) is coupled with the oxidation of a suitable reducing agent, which is usually a highly feasible reaction (\(\Delta G^{\ominus}\) highly negative) so that the \(\Delta G^{\ominus}\) of the overall reaction (coupled reaction) becomes negative and the reduction process occurs spontaneously.This is in accordance with Ellingham diagram. The \(\Delta G^{\ominus}\) value should be negative to make the reaction feasible.

b) CO is a good reducing agent at low temperature than coke. This is because, in the Ellingham diagram, at low temperature, the CO → CO₂ line is below the C → CO line. Thus, oxidation of CO is more feasible than that of coke. Hence, in the blast furnace, CO reduces Fe₂O₃ even at the lower tem perature range. At high-temperature coke reacts with CO₂ to form carbon monoxide. Thus CO is the actual reducing agent.

Question 4.
Bauxite, Al₂O₃, xH₂O, is an important ore of aluminium. It is concentrated by leaching. Explain the method. (Say – 2011)
Answer:
Leaching of alumina from bauxite (Baeyer’s process) The powered bauxite ore is diagested with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. AI₂O₃ is leached out as sodium aluminate along with SiO₂ as sodium silicate. Other impurities like iron oxides and TiO₂ are left behind.

AI₂O_3(s) + 2NaOH_(aq) + 3H₂O_(l) → 2Na[AI(OH)₄]_(aq)

The solidum aluminate in solution is neutralised by passing CO₂ gas and hydrated Al₂O₃ is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al₂O₃ which induces the precipitation of hydrated Al₂O₃.

2Na [AI(OH)₄]_(aq)+ CO_2(g) → AI₂O₃. XH₂O_(s)+ 2NaHCO_3(aq)

The sodium silicate remains in the solution and hy-drated alumina is filtered, dried and heated to give back pure Al₂O₃.

\(\mathrm{Al}_{2} \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O}_{(\mathrm{s})} \stackrel{1470 \mathrm{~K}}{\longrightarrow} \mathrm{Al}_{2} \mathrm{O}_{3(\mathrm{~s})}+\mathrm{xH}_{2} \mathrm{O}\)

Question 5.
a) All ores are minerals, but all minerals are not ores. Why? (March – 2012)
b) Carbonate ores are usually subjected to calcination, while sulphide ores are subjected to roasting. Comment on the statement.
Answer:
a) The naturally occuring materials in which the metals are present either in the native or in the combined state are called minerals.

The minerals from which the metals can be extracted economically are called ores.

Hence all ores are minerals but all minerals are not ores.

b) Calcination is the proœss of heating the ore in a limited supply of air below its metting point. This removes the volatile impurities and moisture from the ore. Oxygen is not used up during calcination.

\(\mathrm{ZnCO}_{3(\mathrm{~s})} \stackrel{\Delta}{\longrightarrow} \mathrm{ZnO}_{(\mathrm{s})}+\mathrm{O}_{2(g)}\)

Roasting is the process of heating the concentrated ore in a regular supply of air in a furnace below the melting point of the metal. It is usually employed in the concentration of sulphide ores.

2 Zns + 3O₂ → 2ZnO + 2SO₂
2 PbS + 3O₂ → 2PbO + 2SO₂
2 Cu2S + 3O₂ → 2Cu₂O + 2SO₂

Question 6.
Concentrated ore of iron, coke and limestone are fed into a blast furnace from the top. (Say – 2012)

i) Write down the reason for adding limestone along with the concentrated ore of iron.
ii) Write down the reactions taking place at the higher temperature range in the blast furnace.
OR
Metals are extracted from their chief ore.
i) Name the pencil pal ore of aluminium.
ii) Write the equations for the reactions taking place at the anode and at the cathode during the extraction of aluminium by the electrolytic process.
Answer:
i) Lime stone is added to blast furnace morder to remove acidic impurities (gangue) like silica (SiO₂). At high temperature, lime stone decomposed to form calcium oxide, which acts as a basic flux and removes acidic silica gaunge as calcium silicate slag.

\(\begin{array}{l}
\mathrm{CaCO}_{3} \stackrel{\text { Heat }}{\longrightarrow} \mathrm{CaO}+\mathrm{CO}_{2} \\
\mathrm{CaO}+\mathrm{SiO}_{2} \rightarrow \mathrm{CaSiO}_{3}
\end{array}\)

ii) The following reaction take place at the higher temperature range (900 K – 1500 K) in the blast furnace:

\(\mathrm{C}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}, \mathrm{FeO}+\mathrm{CO} \rightarrow \mathrm{Fe}+\mathrm{CO}_{2}\)

i) The principal ore of Al is Bauxite (Al₂O₃2H₂O). In Hall-Heroult process for the electrolytic extration of Al, purified Al₂O₃ mixed with Na₃ALF₆ or CaF₂ acts as the electrolyte, steel cathode and graphite anode. The following reactions take place during electrolysis:

At cathode: Al₃(melt) + 3e → 4 Al(l)

At anode : The oxygen liberated at anode reacts with the carbon of anode producing CO and CO₂.

\(\begin{array}{l}
\mathrm{C}_{(\mathrm{s})}+\mathrm{O}_{(\mathrm{melt})}^{2 .} \rightarrow \mathrm{CO}_{(9)}+2 \mathrm{e} \\
\mathrm{C}_{(\mathrm{s})}+2 \mathrm{O}_{(\mathrm{melt})}^{2 .} \rightarrow \mathrm{CO}_{2(\mathrm{~g})}+4 \mathrm{e}^{-}
\end{array}\)

The overall reaction is

\(2 \mathrm{Al}_{2} \mathrm{O}_{3}+3 \mathrm{C} \rightarrow 4 \mathrm{Al}+3 \mathrm{CO}_{2}\)

Question 7.
a) Match the items of Column I with hems of Column II. (March – 2013)

Column 1	Column II
i) Aluminium	a) Malachite
ii) Iron	b) Bauxite
iii) Copper	c) Limestone
iv) Zinc	d) Haematite
	e) Calamine

b) The reduction of the metal oxide is easier if the metal formed is in liquid state, at the temperature of reduction. Give reason.
Answer:
a)

Column 1	Column II
i) Aluminium	Bauxite
ii) Iron	Haematite
iii) Copper	Malachite
iv) Zinc	Calamine

b) The entropy ¡s higher if the metal is in liquid state than when it is in solid state. So the value of entropy change or the reduction process will be more positive. Thus, the value of AGe becomes more negative and the reduction becomes easier.

Question 8.
The scientific and technological processes used for isolation of the metal from its ore is known as metallurgy. (Say – 2013)
a) Name the method used for removing gangue from suiphide ores.
b) Explain the above method.
c) Give two examples for alloy steel.
Answer:
a) Froth floatation method

b) It is used for removing gangue from suiphide ores. The mineral particles become wet by oils while the gangue particles by water. Finely powdered ore is agitated with water containing little frothing agent (e.g. pine oil) and froth stabilizers (e.g. cresols) by passing a forceful current of air. Heavier gangue particles are left to the bottom. The froth is skimmed off and then dried.

Stainless steel (Fe-74%, Cr-18%, Ni-8%)
Nickel steel (Fe-96%, Ni-4%)

Question 9.
a) Calcination and roasting are pre-treatments in metallurgy before metal extraction. Differentiate between calcination and roasting. (March – 2014)
b) Match the items of Column I with items of Column II.

Column I	Column II
i) Distillation	a) Ge
ii) Liquation	b) Ni
iii) Zone refining	c) Cu
iv) Vapour phase refining	d) Zn
	e) Sn

a) Calcination is the process of heating the ore in the absence or limited supply of air when the volatile matter escapes leaving behind the metal oxide. Here oxygen ¡s not consumed, It is applied to hydrated oxides, hydroxides and carbonates.

Roasting is the process of heating the ore in a regular supply of air in a furnace at a temperature below the melting point of the metal. Here oxygen is consumed. it is applied to suiphide ores.

Column I	Column II
i) Distillation	d) Zn
ii) Liquation	e) Sn
iii) Zone refining	a) Ge
iv) Vapourphasereflning	b) Ni

Question 10.
Sulphide ores are concentrated by froth floatation process. (Say – 2014)
a) Write the name or formula of any two sulphide ores of copper.
b) Explain froth floatation process.
Answer:
a)

Copper pyrites (CuFeS₂)
Copper glance (Cu₂S)

b) This method is used for removing gangue from sulphide ores and is based on the principle of preferential wetting of solid surface by vanousliq ulds. i.e., the mineral particles become wet by oils while the gangue particles by water. Finely powered ore is agitated with water containing collectors (e.g. pine oils, fatty acids, xanthates, etc.) and froth stabilizers (e.g. cresols, aniline) by passing a forceful current of air. The collectors enhance non-wettability of the mineral particles and froth stabilisers stabilise the froth. The froth which is formed at the surface of water carries up lighter ore particles and heavier gangue particles are left to the bottom. The froth is skimmed off and then dried for recovery of the ore particles.

Question 1.
a) Name two metals which can be refined by van Arkel Method. (March – 2015)
b) Match the items of Column I with items of Column II.

Column I	Column II
i) Bauxite	a) Zinc
ii) Malachite	b) Iron
iii) Calamine	c) Copper
iv) Magnetite	d) Aluminium
	e) Lead

Answer:
a) Zirconium (Zr) or Titanium (Ti)
b) i – d;
ii – c;
iii – a;
iv – b

Question 11.
The process involved in metallurgy are the concentration of the ore, isolation of the metal from its concentrated ore and purification of the metal. (Say – 2015)
a) Froth floatation method is an ore concentration method. What is the principle behind the process?
b) What is the role of limestone (CaCO₃) in the extraction of iron?
c) Monds process is used for refining of Ni and Van Arkel method is used for refining Zr (Zirconium). Write one similarity between these processes.
Answer:
a) The principle behind froth floatation is adsorption. The ore particles are preferentially wetted by the oil and are carried to the surface by the froth. The gangue material wetted by water sinks to of the tank. The froth is light and is skimmed off.

b) Inside the blastfumace lime stone is decomposed to CaO which acts as a basic flux and removes the silicate impurity (acidic gangue) of the ore as calcium silicate slag. The slag is in molten state and separates our from iron.

\(\begin{array}{l}
\mathrm{CaCO}_{3} \stackrel{\text { Heat }}{\longrightarrow} \mathrm{CaO}+\mathrm{CO}_{2} \\
\mathrm{CaO}+\mathrm{SiO}_{2} \rightarrow \mathrm{CaSiO}_{3}
\end{array}\)

c) Vapour phase refining techniques. The metal is converted into its volatile compound and collected elsewhere. It is then deèomposed to give pure metal.

Question 12.
a) Which of the following is the ore of zinc? (March – 2016)
a) Bauxite
b) Magnetite
c) Malachite
d) Calamine

b) There are several methods for refining metals. Explain a method for refining Zirconium.
Answer:
a) d) Calamine
b) Van Arkel method. The crude metal is heated in an evacuated vessel with iodine. The metal iodide is formed.

Zr+ 2l₂ → Zrl₄

The metal iodide is decomposed on a tungsten filament, at 1800 K. Pure metal is deposited on the filament.

Zrl₄ → Zr + 2l₂

Question 13.
Metals are extracted from their ores (Say – 2016)
a) Among the following which metal is extracted from bauxite:
i) Zinc
ii) Iron
iii) Aluminium
iv) Copper

b) Suiphide ores are subjected to roasting while carbonate ores are subjected to calcination. Comment on the statement.
Answer:
a) iii) Aluminium

b) In roasting, the ore is heated in a regular supply of air in a furnace at a temperature below the melting point of the metal. Here, oxygen is consumed. The sulphide ores need to be converted to oxides. Hence these are subjected to roasting.

e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂

In calcination, the ore ¡s heated in a limited supply fair below its melting point morder to remove he volatile matter. Here, oxygen is not consumed. Hence, the carbonate ores are subjected to calcination.

e.g. ZnCO₃(s) → ZnO(s) + CO₂(g)

Question 14.
Leaching is a process of concentration of ores. Explain the leaching of alumina from bauxite. (March – 2017)
Answer:
Bauxite usually contains silica (SiO₂), iron oxides and titanium oxide (TiO₂) as impurities. The powdered ore is digested with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. Al₂O₃ is leached out as sodium aluminate leaving the impurities behind. (SiO₂ is also leached as sodium silicate).

Al₂O₃(s) + 2NaOH_(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)

The aluminate in solution is neutralised by passing CO₂ gas and hydrated Al₂O₃ is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al₂O₃ to induce the precipitation.

2Na[Al(OH)₄](aq) + CO₂(g) → Al₂O₃xH₂O + 2NaHCO₃(aq)

The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al₂O₃.

\(\mathrm{AI}_{2} \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O} \stackrel{1470 \mathrm{~K}}{\longrightarrow} \mathrm{Al}_{2} \mathrm{O}_{3}(\mathrm{~s})+\mathrm{xH}_{2} \mathrm{O}(\mathrm{g})\)

Question 15.
a) Which of the following is not an Ore of Iron? (Say – 2017)
i) Haematite
ii) Magnetite
iii) Malachite
iv) Sidenote

b) Explain froth floatation process for the concentration of Ore.
Answer:
a) i) Haematite

b) It is used for removing gangue from sulphide ores. The mineral particles become wet by oils while the gangue particles by water. The finely powdered ore is agitated with water containing little frothing agent (e.g. pine oil) and froth stabilizers (e.g. cresols) by passing a forceful current of air. Heavier gangue particles are left to the bottom. The froth is skimmed off and then dried.