NCERT Solutions for Class 12 Chemistry Part 1 Chapter 6
General Principles and Processes of Isolation of Elements Class 12
Chapter 6 General Principles and Processes of Isolation of Elements Exercise Solutions
In text : Solutions of Questions on Page Number : 150
Which of the ores mentioned in Table 6.1 can be concentrated by magnetic separation method?
What is the significance of leaching in the extraction of aluminium?
In the extraction of aluminium, the significance of leaching is to concentrate pure alumina (Al2O3) from bauxite ore.
The impurities are then filtered and the solution is neutralized by passing CO2 gas. In this process, hydrated
Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina (Al2O3).
Answer :
The change in Gibbs energy is related to the equilibrium constant, K as
.
At room temperature, all reactants and products of the given reaction are in the solid state. As a result, equilibrium does not exist between the reactants and the products. Hence, the reaction does not take place at room temperature. However, at a higher temperature, chromium melts and the reaction takes place.
Increasing the temperature increases the value of making the value of more and more negative. Therefore, the reaction becomes more and more feasible as the temperature is increased.
Answer :
The change in Gibbs energy is related to the equilibrium constant, K as
.
At room temperature, all reactants and products of the given reaction are in the solid state. As a result, equilibrium does not exist between the reactants and the products. Hence, the reaction does not take place at room temperature. However, at a higher temperature, chromium melts and the reaction takes place.
Exercise : Solutions of Questions on Page Number : 163
Copper can be extracted by hydrometallurgy but not zinc. Explain.
But to displace zinc, more reactive metals i.e., metals having lower reduction potentials than zinc such as Mg, Ca, K, etc. are required. But all these metals react with water with the evolution of H2 gas.
As a result, these metals cannot be used in hydrometallurgy to extract zinc. Hence, copper can be extracted by hydrometallurgy but not zinc.
What is the role of depressant in froth floatation process?
In the froth floatation process, the role of the depressants is to separate two sulphide ores by selectively preventing one ore from forming froth. For example, to separate two sulphide ores (ZnS and Pbs), NaCN is used as a depressant which selectively allows PbS to come with froth, but prevents ZnS from coming to froth. This happens because NaCN reacts with ZnS to form Na2[Zn(CN)4].
Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?
The Gibbs free energy of formation (ΔfG) of Cu2S is less than that of and . Therefore, H2 and C cannot reduce Cu2S to Cu.
On the other hand, the Gibbs free energy of formation of is greater than that of . Hence, C can reduce Cu2O to Cu.
Hence, the extraction of copper from its pyrite ore is difficult than from its oxide ore through reduction.
Explain: (i) Zone refining (ii) Column chromatography.
(i) Zone refining:
Column chromatography is a technique used to separate different components of a mixture. It is a very useful technique used for the purification of elements available in minute quantities. It is also used to remove the impurities that are not very different in chemical properties from the element to be purified. Chromatography is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. In chromatography, there are two phases: mobile phase and stationary phase. The stationary phase is immobile and immiscible. Al2O3 column is usually used as the stationary phase in column chromatography. The mobile phase may be a gas, liquid, or supercritical fluid in which the sample extract is dissolved. Then, the mobile phase is forced to move through the stationary phase. The component that is more strongly adsorbed on the column takes a longer time to travel through it than the component that is weakly adsorbed. The adsorbed components are then removed (eluted) using a suitable solvent (eluant).
Out of C and CO, which is a better reducing agent at 673 K?
Name the common elements present in the anode mud in electrolytic refining of copper. Why are they so present ?
In electrolytic refining of copper, the common elements present in anode mud are selenium, tellurium, silver, gold, platinum, and antimony.
Write down the reactions taking place in different zones in the blast furnace during the extraction of iron.
During the extraction of iron, the reduction of iron oxides takes place in the blast furnace. In this process, hot air is blown from the bottom of the furnace and coke is burnt to raise the temperature up to 2200 K in the lower portion itself. The temperature is lower in the upper part. Thus, it is the lower part where the reduction of iron oxides (Fe2O3 and Fe3O4) takes place.
The reactions taking place in the lower temperature range (500 - 800 K) in the blast furnace are:
Write chemical reactions taking place in the extraction of zinc from zinc blende.
The different steps involved in the extraction of zinc from zinc blende (ZnS) are given below:
Sulphide ore is converted into oxide by the process of roasting. In this process, ZnS is heated in a regular supply of air in a furnace at a temperature, which is below the melting point of Zn.
State the role of silica in the metallurgy of copper.
During the roasting of pyrite ore, a mixture of FeO and Cu2O is obtained.
The role of silica in the metallurgy of copper is to remove the iron oxide obtained during the process of roasting as 'slag'. If the sulphide ore of copper contains iron, then silica (SiO2) is added as flux before roasting. Then, FeO combines with silica to form iron silicate, FeSiO3 (slag).
What is meant by the term "chromatography"?
What criterion is followed for the selection of the stationary phase in chromatography?
How is leaching carried out in case of low grade copper ores?
The resulting solution is treated with scrap iron or H2 to get metallic copper.
Why is zinc not extracted from zinc oxide through reduction using CO?
The standard Gibbs free energy of formation of ZnO from Zn
The value of for formation of Cr2O3 is - 540 kJmol-1 and that of Al2 O3 is - 827 kJmol-1. Is the reduction of Cr2O3 possible with Al?
The value of for the formation of Cr2O3 from Cr (-540 kJmol-1) is higher than that of Al2O3 from Al (-827 kJmol-1). Therefore, Al can reduce Cr2O3 to Cr. Hence, the reduction of Cr2O3 with Al is possible.
As for the reduction reaction of Cr2O3 by Al is negative, this reaction is possible.
Out of C and CO, which is a better reducing agent for ZnO ?
Reduction of ZnO to Zn is usually carried out at 1673 K. From the above figure, it can be observed that above 1073 K, the Gibbs free energy of formation of CO from C and above 1273 K, the Gibbs free energy of formation of
CO2 from C is lesser than the Gibbs free energy of formation of ZnO. Therefore, C can easily reduce ZnO to Zn.
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.
The above figure is a plot of Gibbs energy vs. T for formation of some oxides.
It can be observed from the above graph that a metal can reduce the oxide of other metals, if the standard free energy of formation of the oxide of the former is more negative than the latter. For example,
since is more negative than , Al can reduce Cu2O to Cu, but Cu cannot reduce Al2O3.
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?
At cathode: At anode:
At cathode:
What is the role of graphite rod in the electrometallurgy of aluminium?
If a metal is used instead of graphite as the anode, then O2will be liberated. This will not only oxidise the metal of the electrode, but also convert some of the Al liberated at the cathode back into Al2O3. Hence, graphite is used for preventing the formation of O2at the anode. Moreover, graphite is cheaper than other metals.
Outline the principles of refining of metals by the following methods:
(ii) Electrolytic refining
Answer :
This method is based on the principle that impurities are more soluble in the molten state of metal (the melt) than in the solid state. In the process of zone refining, a circular mobile heater is fixed at one end of a rod of impure metal.
Vapour phase refining is the process of refining metal by converting it into its volatile compound and then, decomposing it to obtain a pure metal. To carry out this process,
(i) the metal should form a volatile compound with an available reagent, and
(ii) the volatile compound should be easily decomposable so that the metal can be easily recovered. Nickel, zirconium, and titanium are refined using this method.
Predict conditions under which Al might be expected to reduce MgO.
Describe a method for refining nickel.
How can you separate alumina from silica in bauxite ore associated with silica? Give equations, if any.
Then, CO2 gas is passed through the resulting solution to neutralize the aluminate in the solution, which results in the precipitation of hydrated alumina. To induce precipitation, the solution is seeded with freshly prepared samples of hydrated alumina.
Giving examples, differentiate between 'roasting' and 'calcination'.
How is 'cast iron' different from 'pig iron"?
The iron obtained from blast furnaces is known as pig iron. It contains around 4% carbon and many impurities such as S, P, Si, Mn in smaller amounts.
Differentiate between "minerals" and "ores".
Minerals are naturally occurring chemical substances containing metals. They are found in the Earth's crust and are obtained by mining.
Ores are rocks and minerals viable to be used as a source of metal.
For example, there are many minerals containing zinc, but zinc cannot be extracted profitably (conveniently and economically) from all these minerals.
Zinc can be obtained from zinc blende (ZnS), calamine (ZnCO3), Zincite (ZnO) etc. Thus, these minerals are called ores of zinc.
Why copper matte is put in silica lined converter?
Copper matte contains Cu2S and FeS. Copper matte is put in a silica-lined converter to remove the remaining FeO and FeS present in the matte as slag (FeSiO3). Also, some silica is added to the silica-lined converter. Then, a hot air blast is blown. As a result, the remaining FeS and FeO are converted to iron silicate (FeSiO3) and Cu2S is converted into metallic copper.
What is the role of cryolite in the metallurgy of aluminium?
Cryolite (Na3AlF6) has two roles in the metallurgy of aluminium:
1. To decrease the melting point of the mixture from 2323 K to 1140 K.
2. To increase the electrical conductivity of Al2O3.