NCERT Solutions for Class 12 Chemistry Chapter 6 - General Principles and Processes of Isolation of Elements

Question 1:

Which of the ores mentioned in Table 6.1, can be concentrated by magnetic separation method?

Answer:

Iron ores:

  1. Haematite (Fe2O3)
  2. Magnetite (Fe3O4)
  3. Iron pyrites (FeS2)
  4. Siderite (FeCO3)
Question 2:

What is the significance of leaching in the extraction of aluminium?

Answer:

The principal ore of aluminium is bauxite (Al2O3). It contains SiO2, iron oxides and titanium oxide as impurities. The significance of leaching in the extraction of aluminium from bauxite is to remove the impurities from the ore.

Question 3:

The reaction :
Cr2O3 + 2Al Al2O3 + 2Cr ; G° = – 421 kJ
is thermodynamically feasible as is apparent from Gibbs energy value. Why does it not take place at room temperature?

Answer:

The reaction is thermodynamically favourable. Since in the given redox reaction, all the reactants and the products are solids at room temperature, it does not occur at room temperature. However, at high temperature when chromium melts, the reaction proceeds rapidly. Thus, heating is required for this reaction to start.

Question 4:

Is it true that under certain conditions Mg can reduce Al2O3 and Al can reduce MgO? What are those conditions?

Answer:

In the Ellingham diagram, the Al2O3 and MgO curves intersect at a point corresponding to 1623K. At the point of intersection of Al2O3 and MgO curves (1623K), the rG° becomes zero for the reaction,

Below 1623K, the fG° curve for the formation of Al2O3 lies above the fG° curve for MgO. Therefore, below 1623 K, Mg can reduce Al2O3 to Al.

However, above 1623 K, the fG° curve for MgO lies above fG° curve for Al2O3 and therefore, at temperatures above 1623 K, Al can reduce MgO to Mg.

Question 5:

Copper can be extracted by hydrometallurgy but not zinc. Explain.

Answer:

Zinc is more electropositive (E° = – 0.76V) and, therefore, is highly reactive metal. Hence, it cannot be easily displaced from its solution of ZnSO4. On the other hand, copper is less electropositive (E° = + 0.34 V) and can be readily displaced from its solution by some more active metal such as zinc.
Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Question 6:

What is the role of depressant in froth floatation process?

Answer:

The depressants are used to prevent certain types of particles from forming the froth with bubbles in froth floatation process. This helps to separate two sulphide ores. For example, in case of an ore containing zinc sulphide (ZnS) and lead sulphide (PbS), sodium cyanide (NaCN) is used as a depressant. It forms a layer of zinc complex Na2[Zn(CN)4] with ZnS on the surface of ZnS and, therefore, prevents it from forming the froth. Therefore, it acts as a depressant.

However, NaCN does not prevent PbS from forming the froth and allows it to come with the froth.

Question 7:

Why is the extraction of copper from pyrite difficult than that from its oxide through reduction?

Answer:

The graph of rG° vs T in Ellingham diagram for the formation of oxides shows that the copper–copper oxide line is almost at the top. Therefore, it is very easy to reduce oxide ores of copper directly to metal by heating with coke. This is because, G° vs T lines for CO has negative slope at higher temperature and therefore, can easily reduce Cu2O to copper. However, the Gibbs energies of formation of most sulphides are greater than that for CS2. In fact, CS2 is an endothermic compound. Moreover, there is no CS analogous to CO for which G° vs T graphs slope downward. Therefore, extraction of copper from pyrite ore (Cu2S) is difficult.

Question 8:

Explain (i) zone refining and (ii) column chromatography.

Answer:

(i) Zone refining. This method is used for metals which are required in very high purity such as silicon, germanium, boron, gallium and indium. This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. Therefore, an impure metal on solidification will deposit crystals of pure metal and the impurities will remain behind in the molten part of the metal. In this method, the impure metal is cast into a thin bar. A circular mobile heater is fixed at one end of the rod of impure metal. One zone of the bar is melted by a circular mobile heater in the atmosphere of a noble gas like argon. At the heated zone, the metal melts. As the heater moves slowly, the impurities also move into the adjacent molten part. In this way, the impurities are made to move into one end which is finally cut off and discarded. The molten metal present at the colder region solidifies in the mean time since it is away from the heater. Thus, we get completely pure metal by this method. This method is specially useful for producing semiconductors of very high purity.

(ii) Column chromatography. This is a modern method of separation or purification and is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. The mixture is put in a liquid or gaseous medium (called moving phase) which is moved through a porous medium (adsorbent called stationary phase).

Different components of the mixture are desorbed at different levels in the column. After the separation, the different adsorbed components are extracted from the adsorbent with suitable solvent. This process of recovery of extraction of the adsorbed components is called elution. The stationary phase is either a solid or a liquid, while the moving phase may be a liquid or gas. The mobile phase and the stationary phase are chosen such that components of the sample have different solubilities in the two phases. A component which is quite soluble in the stationary phase takes longer time to travel through it than a component which is not very soluble in the stationary phase but very soluble in the mobile phase. Thus, components of a sample are separated from each other as they move through the stationary phase. Depending upon the physical states of two phases and also on the process of passage of moving medium, the chromatographic technique is given different names such as column chromatography, thin layer chromatography, paper chromatography and gas chromatography.

Question 9:

Out of C and CO, which is a better reducing agent at 673 K?

Answer:

At 673 K, the G° vs T line for CO, CO2 is lower than that of C, CO line. Therefore, CO is a better reducing agent at 673 K.

Question 10:

Name the common elements present in the anode mud in electrolytic refining of copper. Why are they so present?

Answer:

The anode mud in the electrolytic refining of copper contains antimony, selenium, tellurium, silver, gold and platinum. These are present as impurities in blister copper. These are less reactive and are not affected by CuSO4 – H2SO4 solution and hence settle down under anode as anode mud.

Question 11:

Write down the reactions taking place in different zones in the blast furnace during the extraction of iron.

Answer:

In blast furnace, reduction of iron oxides takes place in different temperature ranges. The lower part of the blast furnace has high temperature of the order of 2200 K (called combustion zone) and the top of the furnace has low temperature of the order of 500–800 K (called reduction zone). The reduction occurring in the lower temperature range (upper part) is by carbon and in the higher temperature range (lower part) is by carbon monoxide.
At lower temperature range (500–800 K) in upper part of furnace the reactions occurring are :

In the middle portion (at about 1270 K), limestone decomposes to give lime (CaO) and CO2. Lime acts as a flux and combines with silicate impurity to form slag.

Slag is in the molten state and separates out from iron.

Question 12:

Write chemical reactions taking place in the extraction of zinc from zinc blende.

Answer:
  1. The concentrated zinc blende ore (ZnS) is roasted in the presence of excess air at about 1200 K to convert it to zinc oxide.

  2. Zinc oxide is reduced to zinc by heating with crushed coke at 1673 K.

  3. The impure copper is refined by electrorefining method. In this method, the impure zinc is made anode and a plate of pure zinc is made cathode in an electrolytic bath containing zinc sulphate and a small amount of dilute H2SO4. On passing current, the following reactions occur:

The zinc gets deposited on cathode and is collected.

Question 13:

State the role of silica in the metallurgy of copper.

Answer:

During roasting, the copper pyrites are converted into a mixture of FeO and Cu2O.

Copper pyrites

To remove FeO (basic), the roasted ore is mixed with silica and heated. Silica acts as a flux and combines with ferrous oxide present to form fusible slag of iron silicate.

The slag being lighter floats and forms the upper layer and is removed through slag hole. Therefore, silica helps to remove FeO in the metallurgy of copper.

Question 14:

Which method of refining may be suitable if element is obtained in minute quantity?

Answer:

Chromatography. It is a technique for the separation and purification based on the differences in the adsorbing tendencies of the metal and its impurities on a suitable adsorbent. It is based on the principle that different components of a mixture are differently adsorbed on an adsorbent.

Question 15:

Describe a method for refining nickel.

Answer:

For refining nickel, nickel is heated in a stream of carbon monoxide forming volatile complex (nickel tetracarbonyl).

The carbonyl is subjected to high temperature so that the complex decomposes to give the pure metal.

This process is called Mond process.

Question 16:

How can you separate alumina from silica in a bauxite ore associated with silica? Give equations if any.

Answer:

Question 17:

Giving examples, differentiate between roasting and calcination.

Answer:

Question 18:

How is cast iron different from pig iron?

Answer:

The iron obtained from blast furnace is pig iron. It contains about 4% of carbon and many impurities in smaller amount (e.g. S, P, Si, Mn, etc.).
Cast iron is obtained by melting pig iron with scrap iron and coke using hot air blast. It contains slightly lower carbon content (about 3%) and is extremely hard and brittle.

Question 19:

Differentiate between “minerals” and “ores”.

Answer:

Minerals are naturally occurring chemical substances in the earth’s crust obtainable by mining.
The minerals from which a metal can be economically and conveniently extracted is ore.

Al can be conveniently and economically extracted from bauxite. Therefore, bauxite is an ore of Al.

Question 20:

Why is copper matte put in silica lined converter?

Answer:

Question 21:

What is the role of cryolite in the metallurgy of aluminium?

Answer:

Cryolite is added to bauxite ore before electrolysis because of the following reasons:

  1. It acts as a solvent.
  2. It lowers the melting point of alumina to about 1173 K.
  3. Addition of cryolite to alumina increases the electrical conductivity.
Question 22:

How is leaching carried out in case of low grade copper ores?

Answer:

Question 23:

Why is zinc not extracted from zinc oxide through reduction using CO?

Answer:

Question 24:

Answer:

Question 25:

Out of C and CO which is a better reducing agent for ZnO?

Answer:

C is a better reducing agent for ZnO.

Question 26:

The choice of a reducing agent in a particular case depends on thermodynamic factor. How far do you agree with this statement? Support your opinion with two examples.

Answer:

The thermodynamic factor helps us in choosing a suitable reducing agent for the reduction of a particular metal oxide to metal. The feasibility of thermal reduction can be predicted on the basis of fG° vs T plots for the formation of oxides, known as Ellingham diagram. From the diagram, it can be predicted that metals for which the standard free energy of formation of their oxides is more negative can reduce those metal oxides for which the standard free energy of formation of their respective oxides is less negative. In other words, a metal will reduce the oxides of other metals which lie above it in Ellingham diagram because the standard free energy change, (rG°), of the combined redox reaction will be –ve by an amount equal to the difference in fG° of the two metal oxides. For example, both Al and Zn can reduce FeO to Fe, but Fe cannot reduce Al2O3 to Al or ZnO to Zn. Similarly, C can reduce ZnO to Zn but not CO. Thus, the choice of a particular reducing agent depends on thermodynamic factor.

Question 27:

Name the processes from which chlorine is obtained as a by-product. What will happen if an aqueous solution of NaCl is subjected to electrolysis?

Answer:
  1. Chlorine is obtained as a by-product during electrolysis of sodium chloride by Down’s process for the manufacture of sodium.

  2. Electrolysis of brine solution in Castner– Kellner cell for manufacturing sodium hydroxide (caustic soda).
Question 28:

What is the role of graphite rod in the electrometallurgy of aluminium?

Answer:

The graphite rod is useful in the electrometallurgy of aluminium for reduction of alumina to aluminium.

Question 29:

Out line the principles of refining of metals by the following methods:

  1. Electrolytic refining
  2. Vapour phase refining
Answer:

(ii) Electrolytic refining. This method is based upon the phenomenon of electrolysis. 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. Both anode and cathode are placed in a suitable electrolytic bath containing soluble salt of the same metal. On passing the current, metal ions from the electrolyte are deposited at the cathode in the form of pure metal, while equivalent amount of metal dissolves from the anode into the electrolyte in the form of metal ions. The impurities fall down below the anode as anode mud. The reactions occurring at the electrodes are :

Copper is refined using an electrolytic method as shown in figure. In this method, crude copper is made anode, a thin sheet of pure copper is made cathode and acidified solution of copper sulphate is used as an electrolyte. On passing the electric current, metal ions from the electrolyte are deposited at the cathode in the form of pure metal. On the other hand, an equivalent amount of metal dissolves from the anode into the electrolyte in the form of metal ions. The reactions occurring at electrodes are :

The impurities settle down below the anode in the form of anode mud.
The anode mud may have impurities of antimony, selenium, tellurium, silver, gold and platinum. The recovery of these elements may meet the cost of refining.

(iii) Vapour phase refining. This method is based on the fact that certain metals are converted to their volatile compounds while the impurities are not affected during compound formation. The compound formed decomposes on heating to give pure metal.
For example, nickel is refined by this technique and the method is known as Mond process. In this method, nickel is heated in a steam of carbon monoxide to form volatile nickel carbonyl Ni(CO)4.
The carbonyl vapours when subjected to still higher temperature (450–470 K) undergo thermal decomposition giving pure nickel.

Question 30:

Predict conditions under which Al might be expected to reduce MgO.

Answer:

Question 31:

Why is an external emf of more than 2.2V required for the extraction of Cl2 from brine ?

Answer:

For the reaction of extraction of Cl2 from brine solution

Therefore, extraction of Cl2 from brine solution will require an external e.m.f. more than 2.20 V for electrolysis process.

Question 32:

At temperatures above 1073 K coke can be used to reduce FeO to Fe. How can you justify this reduction with Ellingham diagram?

Answer:

From the Ellingham diagram, it is clear that at temperature above 1073 K, G(C, CO) is less than G (Fe, FeO). Therefore, coke can reduce FeO to Fe.

Question 33:

Wrought iron is the purest form of iron. Write a reaction used for the preparation of wrought iron from cast iron. How can the impurities of sulphur, silicon and phosphorus be removed from cast iron ?

Answer:

The reaction is :

Limestone is added as flux and the impurities of sulphur, silicon and phosphorus change to their oxides and pass into slag.

Question 34:

How is copper extracted from low grade copper ores ?

Answer:

Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria. The solution containing copper ions (Cu2+) is treated with scrap iron, zinc or H2 as :

In this way, copper is obtained.

Question 35:

Write two basic requirements for refining of a metal by Mond process and by van Arkel method.

Answer:

The basic requirements for refining a metal by Mond process and by van Arkel method are :

  1. The metal should form a volatile compound with an available reagent.
  2. The volatile compound should be easily decomposable, so that metal can be easily recovered.
Question 36:

Although carbon and hydrogen are better reducing agents, they are not used to reduce metallic oxides at high temperatures. Why ?

Answer:

This is because at high temperature carbon and hydrogen react with metals to form carbides and hydrides respectively.

Question 37:

How do we separate two sulphide ores by Froth Floatation Method ? Explain with an example.

Answer:

Two sulphide ores can be separated by adjusting proportion of oil to water or by using depressants. For example, in the case of an ore containing ZnS and PbS, the depressant NaCN is used. It forms a layer of zinc complex Na2[Zn(CN)4] on the surface of ZnS and therefore, prevents it from forming the froth. Therefore, it acts as a depressant.

Question 38:

The purest form of iron is prepared by oxidising impurities from cast iron in a reverberatory furnace. Which iron ore is used to line the furnace ? Explain by giving reaction.

Answer:

Question 39:

The mixture of compounds A and B is passed through a column of Al2O3 by using alcohol as eluant. Compound A is eluted in preference to compound B. Which of the compounds A or B, is more readily adsorbed on the column ?

Answer:

Since compound ‘A’ comes out before compound ‘B’, compound ‘B’ is more readily adsorbed on column.

Question 40:

Why is sulphide ore of copper heated in a furnace after mixing with silica ?

Answer:

Iron oxide present as impurity in sulphide ore of copper forms slag which is iron silicate and copper is produced in the form of copper matte.

Question 41:

Why are sulphide ores converted to oxide before reduction ?

Answer:

Sulphides are not reduced easily but oxides are easily reduced.

Question 42:

Which method is used for refining Zr and Ti ? Explain with equation.

Answer:

van Arkel method is used for refining Zr and Ti. In this method, crude metal is heated with iodine to form volatile unstable compound. The compound is then decomposed to get pure metal.

Question 43:

What should be the considerations during the extraction of metals by electrochemical method ?

Answer:

During extraction of metals by electrochemical method, the following are considered so that proper precautions can be taken :

  1. reactivity of metal produced
  2. suitability of electrodes
Question 44:

What is the role of flux in metallurgical processes?

Answer:

Flux is used for making the molten mass more conducting. Flux combines with impurities to form easily feasible product known as slag.

Question 45:

How are metals used as semiconductors refined? What is the principle of the method used?

Answer:

Semiconducting metal is produced by zone refining method which is based on the principle that the impurities are more soluble in melt than in the solid state of metals.

Question 46:

Write down the reactions taking place in blast furnace related to the metallurgy of iron in the temperature range 500–800 K.

Answer:

The following reactions take place in blast furnance during the metallurgy of iron in the temperature range 500–800 K.

Question 47:

Give two requirements for vapour phase refining.

Answer:
  1. The metal should form a volatile compound with available reagent.
  2. The volatile compound should be unstable and easily decomposable so that the recovery is easy.
Question 48:

Write the chemical reactions involved in the extraction of gold by cyanide process. Also give the role of zinc in the extraction.

Answer:

The following reactions occur :

In this reaction, zinc acts as a reducing agent.

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