Chapter 6 General Principles and Processes of Isolation of Elements Solutions
Question - 1 : - What is the significance ofleaching in the extraction of aluminium?
Answer - 1 : -
In the extraction of aluminium,the significance of leaching is to concentrate pure alumina (Al2O3) from bauxite ore.
Bauxiteusually contains silica, iron oxide, and titanium oxide as impurities. In theprocess of leaching, alumina is concentrated by digesting the powdered ore witha concentrated solution of NaOH at 473-523 K and 35-36 bar. Under theseconditions, alumina (Al2O3) dissolves as sodiummeta-aluminate and silica (SiO2)dissolves as sodium silicate leaving the impurities behind.
The impurities are then filteredand the solution is neutralized by passing CO2 gas. In this process, hydrated Al2O3 gets precipitated and sodium silicate remains inthe solution. Precipitation is induced by seeding the solution with freshlyprepared samples of hydrated Al2O3.
Hydrated alumina thus obtained isfiltered, dried, and heated to give back pure alumina (Al2O3).
Question - 2 : - The reaction,
Answer - 2 : -
is thermodynamically feasible asis apparent from the Gibbs energy value. Why doesit not take place at room temperature?
Answer
The change in Gibbs energy isrelated to the equilibrium constant, K as
At room temperature, allreactants and products of the given reaction are in the solid state. As aresult, equilibrium does not exist between the reactants and the products.Hence, the reaction does not take place at room temperature. However, at ahigher temperature, chromium melts and the reaction takes place.
We alsoknow that according to the equation
Increasing the temperatureincreases the value of 
making the value of 
more and more negative.Therefore, the reaction becomes more and more feasible as the temperature isincreased.
Question - 3 : - Is it true that under certainconditions, Mg can reduce SiO2 andSi can reduce MgO? What are those conditions?
Answer - 3 : - 
The temperature range in which 
is lesser than
, Mg can reduce SiO2 to Si. On the other hand, the temperatures range inwhich is less than, Si can reduce MgO to Mg. The temperature at which ΔfG curvesof these two substances intersect is 1966 K. Thus, at temperatures less than1966 K, Mg can reduce SiO2 andabove 1966 K, Si can reduce MgO.
Question - 4 : - Copper can be extracted byhydrometallurgy but not zinc. Explain.
Answer - 4 : -
The reduction potentials of zincand iron are lower than that of copper. In hydrometallurgy, zinc and iron canbe used to displace copper from their solution.
But to displace zinc, morereactive metals i.e., metals having lower reduction potentials than zinc suchas Mg, Ca, K, etc. are required. But all these metals react with water with theevolution of H2 gas.
As a result, these metals cannotbe used in hydrometallurgy to extract zinc.
Hence,copper can be extracted by hydrometallurgy but not zinc.
Question - 5 : - What is the role of depressant infroth floatation process?
Answer - 5 : -
In the froth floatation process,the role of the depressants is to separate two sulphide ores by selectivelypreventing one ore from forming froth. For example, to separate two sulphideores (ZnS and Pbs), NaCN is used as a depressant which selectively allows PbSto come with froth, but prevents ZnS from coming to froth. This happens becauseNaCN reacts with ZnS to form Na2[Zn(CN)4].
Question - 6 : - Why is the extraction of copperfrom pyrites more difficult than that from its oxide orethrough reduction?
Answer - 6 : - The Gibbs free energy of formation (ΔfG)of Cu2S is less thanthat of 
and
. Therefore, H2 and C cannot reduceCu2S to Cu.
Onthe 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 copperfrom its pyrite ore is difficult than from its oxide ore through reduction.
Explain: (i) Zone refining (ii) Column chromatography.
Answer - 7 : -
(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 with it. As aresult, pure metal crystallizes out of the melt and the impurities pass ontothe adjacent molten zone. This process is repeated several times, which leadsto the segregation of impurities at one end of the rod. Then, the end with theimpurities is cut off. Silicon, boron, gallium, indium etc. can be purified bythis process.
(ii) Columnchromatography:
Columnchromatography is a technique used to separate different components of amixture. It is a very useful technique used for the purification of elementsavailable in minute quantities. It is also used to remove the impurities thatare not very different in chemical properties from the element to be purified.Chromatography is based on the principle that different components of a mixtureare differently adsorbed on an adsorbent. In chromatography, there are twophases: mobile phase and stationary phase. The stationary phase is immobile andimmiscible. Al2O3 column is usuallyused as the stationary phase in column chromatography. The mobile phase may bea gas, liquid, or supercritical fluid in which the sample extract is dissolved.Then, the mobile phase is forced to move through the stationary phase. Thecomponent that is more strongly adsorbed on the column takes a longer time totravel through it than the component that is weakly adsorbed. The adsorbedcomponents are then removed (eluted) using a suitable solvent (eluant).
Question - 8 : - Out of C and CO, which is abetter reducing agent at 673 K?
Answer - 8 : - 
Question - 9 : - Name the common elements presentin the anode mud in electrolytic refining ofcopper. Why are they so present ?
Answer - 9 : -
In electrolytic refining ofcopper, the common elements present in anode mud are selenium, tellurium,silver, gold, platinum, and antimony.
Theseelements are very less reactive and are not affected during the purificationprocess. Hence, they settle down below the anode as anode mud.
Question - 10 : - Write down the reactions takingplace in different zones in the blast furnace duringthe extraction of iron.
Answer - 10 : -
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 thetemperature up to 2200 K in the lower portion itself. The temperature is lowerin the upper part. Thus, it is the lower part where the reduction of ironoxides (Fe2O3 and Fe3O4) takes place.
Thereactions taking place in the lower temperature range (500 − 800 K) in theblast furnace are:
