Chapter 6 General Principles and Processes of Isolation of Elements Solutions
Question - 21 : - What is the role of cryolite inthe metallurgy of aluminium?
Answer - 21 : -
Cryolite (Na3AlF6) has two roles in the metallurgy of aluminium:
1. To decrease the melting point of the mixture from 2323 K to1140 K.
2. To increase the electrical conductivity of Al2O3.
Question - 22 : - How is leaching carried out incase of low grade copper ores?
Answer - 22 : -
In case of low grade copper ores,leaching is carried out using acid or bacteria in the presence of air. In thisprocess, copper goes into the solution as Cu2+ ions.
The resulting solution is treatedwith scrap iron or H2 toget metallic copper.
Question - 23 : - Why is zinc not extracted fromzinc oxide through reduction using CO?
Answer - 23 : -
The standard Gibbs free energy offormation of ZnO from Zn
is lowerthan that of CO2 fromCO. Therefore, CO cannot reduce ZnO to Zn. Hence, Zn is not extracted from ZnOthrough reduction using CO.
Question - 24 : - 
Answer - 24 : - 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.
Alternatively,
Subtracting equation (ii) from(i), we have
As for the reduction reactionof Cr2O3 by Al is negative,this reaction is possible.
Question - 25 : - Out of C and CO, which is abetter reducing agent for ZnO ?
Answer - 25 : -
Reduction of ZnO to Zn is usuallycarried out at 1673 K. From the above figure, it can be observed that above1073 K, the Gibbs free energy of formation of CO from C and above 1273 K, theGibbs free energy of formation of CO2 from C is lesser than the Gibbs free energy offormation of ZnO. Therefore, C can easily reduce ZnO to Zn.
On the other hand, the Gibbs freeenergy of formation of CO2 fromCO is always higher than the Gibbs free energy of formation of ZnO. Therefore,CO cannot reduce ZnO. Hence, C is a better reducing agent than CO for reducingZnO.
Question - 26 : - The choice of a reducing agent ina particular case depends on thermodynamicfactor. How far do you agree with this statement? Support youropinion with two examples.
Answer - 26 : - The above figure is a plot of Gibbs energy 
vs. T for formation of some oxides.
Question - 27 : - Name the processes from whichchlorine is obtained as a by-product. What willhappen if an aqueous solution of NaCl is subjected to electrolysis?
Answer - 27 : -
In the electrolysis of moltenNaCl, Cl2 isobtained at the anode as a by product.
If an aqueous solution of NaCl iselectrolyzed, Cl2 willbe obtained at the anode but at the cathode, H2 will be obtained (instead of Na). This is becausethe standard reduction potential of Na (E°= − 2.71 V) is more negative thanthat of H2O (E° = −0.83 V). Hence, H2Owill get preference to get reduced at the cathode and as a result, H2 is evolved.
Question - 28 : - What is the role of graphite rodin the electrometallurgy of aluminium?
Answer - 28 : -
In the electrometallurgy ofaluminium, a fused mixture of purified alumina (Al2O3), cryolite (Na3AlF6) and fluorspar (CaF2) is electrolysed. In this electrolysis,graphite is used as the anode and graphite-lined iron is used as the cathode.During the electrolysis, Al is liberated at the cathode, while CO and CO2 are liberated at the anode, according tothe following equation.
If a metal is used instead ofgraphite as the anode, then O2 will be liberated. This will not onlyoxidise the metal of the electrode, but also convert some of the Al liberatedat the cathode back into Al2O3. Hence, graphite is used for preventing theformation of O2 atthe anode. Moreover, graphite is cheaper than other metals.
Question - 29 : - Outline the principles ofrefining of metals by the following methods:
Answer - 29 : -
(i) Zone refining
(ii) Electrolytic refining
(iii) Vapour phase refining
Answer
(i) Zone refining:
Thismethod is based on the principle that impurities are more soluble in the moltenstate of metal (the melt) than in the solid state. In the process of zonerefining, a circular mobile heater is fixed at one end of a rod of impuremetal. As the heater moves, the molten zone of the rod also moves along withit. As a result, pure metal crystallizes out of the melt and the impuritiespass to the adjacent molten zone. This process is repeated several times, whichleads to the segregation of impurities at one end of the rod. Then, the endwith the impurities is cut off. Silicon, boron, gallium, indium etc. can bepurified by this process.
(ii) Electrolyticrefining;
Electrolyticrefining is the process of refining impure metals by using electricity. In thisprocess, impure metal is made the anode and a strip of pure metal is made thecathode. A solution of a soluble salt of the same metal is taken as theelectrolyte. When an electric current is passed, metal ions from theelectrolyte are deposited at the cathode as pure metal and the impure metalfrom the anode dissolves into the electrolyte in the form of ions. Theimpurities present in the impure metal gets collected below the anode. This isknown as anode mud.
(iii) Vapour phaserefining
Vapourphase refining is the process of refining metal by converting it into itsvolatile compound and then, decomposing it to obtain a pure metal. To carry outthis process,
(i) the metal should form a volatile compoundwith an available reagent, and
(ii) the volatile compound should be easilydecomposable so that the metal can be easily recovered.
Nickel,zirconium, and titanium are refined using this method.
Predict conditions under which Almight be expected to reduce MgO.
Answer - 30 : -
Above 1350°C, the standard Gibbsfree energy of formation of Al2O3 from Al is lessthan that of MgO from Mg. Therefore, above 1350°C, Al can reduce MgO.