General Principles And Processes Of Isolation Of Elements - Class 12 Medical Chemistry - Extra Questions

Zinc is extracted from ores like :

Ore	Chemical formulae
(i) Calamite	Zinc carbonate, $$ZnC{O}_{3}$$
(ii) Zinc blende	Zinc sulphide, $$ZnS$$

Zinc: The important ores of Zinc are
Zinc blende, $$ZnS$$
Calamine, $$ZnCO_3$$
Zincite, $$ZnO$$
The extraction of zinc blende involves the following steps
(i) Concentration: The ore is crushed and then concentrated by froth-floatation process.
(ii) Roasting: The concentrated ore is then roasted in the presence of excess of air at about 1200K.
$$2ZnS + 3O_2 \xrightarrow{\Delta} 2ZnO + 2SO_2 \uparrow$$
(iii) Reduction: Zinc oxide is mixed with powdered coke and heated to 1673 K in a fire clay retort, in which ZnO is reduced to zinc metal.
$$ZnO + C \xrightarrow{1673 K} Zn + CO \uparrow$$
(iv) Purification: Zinc is purified by electrolytic refining. Where
Anode is impuric zinc
Cathode is a thin sheet of pure zinc
Electrolyte is Zinc sulphate solution + a little of dil. $$H_2SO_4$$
On passing electric current, pure zinc get deposited at the cathode.

The chief Ore of zinc is zinc blende. The extraction of zinc from zinc blende involves the following steps.

1. Concentration: The ore is crushed and then concentrated by froth-floatation Process.
2. Roasting: The concentrated ore is then roasted in the presence of excess of air at about $$1200k$$: $$2ZnS+3O_{2}\overset{\Delta }{\rightarrow}2ZnO+2SO_{2}$$
3. Reduction: Zinc oxide is mixed with powdered coke and heated to $$1673k$$ in a fire clay retort in which $$ZnO$$ is reduced to zinc metal. $$ZnO+C\overset{1673K}{\rightarrow}Zn+CO$$
   
4. Purification: Zinc is purified by electrolytic refining. In this process, impure zinc is anode and cathode is of pure thin sheet of zinc. The electrolyte is $$ZnSO_{4}$$ solution containing a little of dil.$$H_{2}SO_{4}$$. On passing electric current pure zinc gets deposited at the cathode.

A metal M found in nature as sulphide ore ($$M_2S$$) is one of the good conductors of heat and electricity and used in making electric wires. The metal M is copper ($$Cu$$) and its sulfide is $$Cu_2S$$ (Copper pyrite).

As we know, Zn is more reactive than Cu so in second case Zn will react with $$CuSO_4$$ and  $$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$$

$$\begin{matrix} & Oxidising\, agent  & Reducing\, agent\\ (a) & O_2 (Itself\, reduced\, to\, O^{2-}) & S^{-2}\, (in\, Cu_2S)\, oxidised\, to\, SO_2.\\ (b)& Cu_2O\, oxidizes\, S^{2-}\, of\, Cu_2S\, to\,SO_2 & Cu_2S, reduces\, Cu_2O\, to\, Cu,\, and\, itself\, also\, reduced\, to\, Cu.\end{matrix}$$

As $$CO_2$$ & CO are more stable than $$CS_2$$ so 

 $$2ZnS\,+\, 3O_2\, \rightarrow\, 2ZnO\, +\, 2SO_2$$

     $$ZnO\,+\, C\, \rightarrow\, Zn \, +\, CO$$

is much more spontaneous reaction than B.

And as we know,

$$\Delta G = \Delta H - T\Delta S$$

As A is more spontaneous reaction so it will have higher negative value of G.

So, B have higher positive value of G.

Extraction of Copper:
Copper is mainly extracted from copper pyrites. After the concentration of its ore by froth flotation process, the ore is roasted in a current of air to remove arsenic, antimony and much of sulphur.
The ore is then mixed with a little coke and sand and smelted in a water-jacketed blast furnace. The minor reactions that occurred during roasting continue here. Ferrous oxide combines with sand to form a fusible slag. Cuprous oxide formed combines with ferrous sulphide to give ferrous oxide and cuprous sulphide. This is because iron has more affinity for oxygen than copper.
Molten mass collected from the bottom of furnace contains largely cuprous sulphide and a little ferrous sulphide. This molten mass is known as matte.
The molten matte is finally transferred to Bessemer converter. A blast of sand and air is blown in the converter through tuyeres which are situated a little above the bottom. This causes removal of S and As oxides and ferrous oxide as slag. 
Finally, copper may be refined electrolytically (electrolyte; copper sulphate : an anode; impure copper and cathode; pure copper).
So in such extraction, only two steps are followed.

Gun metal and American coins are generally made by copper alloys.

Chemical formula of Copper Glance $$ - Cu_2S \rightarrow$$ Sulphide Ore

In the Ellinggam diagram (graph of $$\displaystyle\Delta _r G^o $$ vs T) for the formation of oxides, copper-copper oxide line is almost at the top. Hence, oxide ores of copper can be easily reduced to copper by heating with coke. The $$\displaystyle \Delta G^o$$ vs T lines for CO show negative slope at high temperature. Hence, it can easily reduce copper oxide to copper. However, $$\displaystyle \Delta G_f$$ for sulphide ores are higher than that for $$\displaystyle CS_2$$ (which is an endothermic compound). Hence, the extraction of copper from pyrite ore is difficult.

Iron scraps or hydrogen is employed to get copper from the leached low grade copper ore.

(i) In the extraction of copper from copper matte, $$\displaystyle  SiO_2$$ reacts with iron oxide to form slag of ferrous silicate and is removed.
$$\displaystyle  FeO + SiO_2 \rightarrow FeSiO_3$$
(ii) In froth floatation process, to separate PbS and ZnS from ores, NaCN is used as depressant. It selectively prevents ZnS from coming to the froth and PbS is removed along with the froth.

Ores of Iron-

Zinc can displace $$Cu$$ from $$CuSO_4$$ solution but cannot displace $$Mg$$ from $$MgSO_4$$ solution. As the oxidation potential of magnesium is higher than the oxidation potential of zinc.

Electro refining of zinc:
Electrolyte used: $$ZnSO_{4}$$ solution containing a little $$H_{2}SO_{4}$$
Anode : Block of impure zinc
Cathode : Thin sheet of pure zinc.
On passing current pure zinc is obtained at cathode.
Anode reaction: $$\underset {impure}{Zn} - 2e^{-}\rightarrow Zn_{aq}^{2+}$$
Cathode reaction: $$Zn_{aq}^{2+} + 2e^{-}\rightarrow \underset {pure\ metal}{Zn}$$

Chemical Reaction:
Deduction
      $$2ZnS+3{ O }_{ 2 }\longrightarrow 2ZnO+2S{ O }_{ 2 }\uparrow $$
      $$ZnS+2{ O }_{ 2 }\longrightarrow ZnS{ O }_{ 4 }$$
      $$2ZnS{ O }_{ 4 }\underrightarrow { \quad 900\quad  } 2ZnO+2S{ O }_{ 2 }\uparrow +{ O }_{ 2 }\uparrow $$
Reduction
      $$ZnO+C\underrightarrow { \quad 1400-1450\quad  } Zn+CO$$

Sulphide ore of copper is copper glance $$Cu_2S$$.

Process by which sulphide ore is concentrated is called Froth Flotation method.

a) Roasting of Copper (I)  Sulphide ore:

Metal $$'M'$$ is copper $$(Cu)$$ which is one of the best conductor of heat and electricity used in making electric wires is formed in nature as $${ Cu }_{ 2 }S$$.

$$CuFeS_x$$ Name given in the question is copper pyrite.

Extraction of $$Ag$$ from Argentite,

The reaction involved in the extraction of copper from copper pyrite using a limited supply of air in the reverberatory furnace is as follow:

Galvanization is the process of applying a protective zinc coating to steel or iron. The purpose of galvanizing is to prevent rusting.

Calcination $$\rightarrow$$ The process of conversion of metals into their oxides at high temperature in absence of air or limited supply of air to remove the volatile impurities is known calcination.

a

In the given metals only Cu and Ag can be obtained by electrolytic reduction of an aqueous solution of their salts because standard reduction potential of Cu and Ag is greater than H$$_2$$ and standard reduction potential of Na and Al is less than H$$_2$$. 

Iron is the most abundant element on Earth, it accounts for only 5% of the Earth's crust, thus being only the fourth most abundant element.

 The main copper sulphide minerals are chalcopyrite $$(CuFeS_2)$$ chalcocite $$(Cu_2S)$$, covellite $$(CuS)$$, bornite $$(Cu_5FeS_4)$$, tetrahedrite $$((Cu,Fe)_{12}Sb_4S_{13})$$ and enargite $$(Cu_3AsS_4)$$. The largest source of copper is from porphyry ore deposits in which one or a combination of the aforementioned minerals occurs.

By adding iron turnings to aqueous solution of $$CuSO_4$$,
we get a deposition of copper metal.
$$CuSO_4+Fe \to Cu+FeSO_4$$

Silica acts as a flux to remove impurity, iron oxide $$(FeO)$$ as slag in the metallurgy of copper.

Distillation, as zinc is a low boiling metal.

The reduction of zinc oxide is zone using coke, as a reducing agent. For the purpose of heating, the oxide is made into brickettes with coke and clay.

(a) $$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$$
(b) $$2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$$
(c) electrolytic refining reaction involed :

(a) Iron (II) oxide.

(i) 1. Zinc: Froth Flotation, Zinc Blende, Coke
 2. Aluminium: Bauxite, Cryolite, Sodium hydroxide solution
(ii) 1. Sodium hydroxide.

Wrought iron is prepared by heating cast iron with hematite.

Wrought iron the purest form of iron.

Metal production and removal of impurities are not satisfactory.

(a) Copper Pyrites $$[CuFeS_{2}]$$
(b) Cuprite $$(Cu_{2}O]$$.

Copper is refined using an electrolytic method. 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 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 electrode are:
At cathode : $$Cu^{2+} + 2e^{-}\rightarrow Cu$$
At anode : $$Cu \rightarrow Cu^{2+} + 2e^{-}$$
The impurities settle down below the anode in the form of anode mud.

(a) Zinc blende $$(Zn)$$
(b) Zincite $$(ZnO)$$.

Ores are minerals from which metals can be extracted economically. Iron can be extracted economically from Haematite & Magnetite.

In low grade copper, copper is present in very less amount. It can be extracted by using acid. In presence of acid copper goes into solution in the form of $$Cu^{2+}$$ ions.
$$2Cu_{(s)} + 2H_{2}SO_{4(aq)} + O_{2(g)} \rightarrow 2CuSO_{4(aq)} + 2H_{2}O_{(l)}$$
Or
$$Cu_{(s)} + 2H_{(aq)}^{+} + \dfrac {1}{2} O_{2(g)} \rightarrow Cu^{2+}_{(aq)} + H_{2}O_{(l)}$$.

Because silica lines act as flux so it is used in Bessemer convertor in reduction of copper oxide. Chemical reactions which take place in Bessemer convertor are as follows:
$$CaO + SiO_{2} + CaSiO_{3}$$
$$\underset {Gangue}{FeO} + \underset {Flux}{SiO_{2}}\rightarrow \underset {Slag}{FeSiO_{3}}$$
$$2Cu_{2}S + 3O_{2} \rightarrow 2Cu_{2}O + 2SO_{2}\uparrow$$
$$Cu_{2}S + 2Cu_{2}O \rightarrow \underset {Blistered\ copper}{6Cu} + SO_{2}\uparrow$$ (Self reduction).

(i) $$2Cu_{2}O + Cu_{2}S \rightarrow 6Cu + SO_{2}$$
(ii) $$Ag_{2}S + 4NaCN + 2O_{2}\rightarrow 2Na[Ag(CN)_{2}] + Na_{2}SO_{4}$$
(iii) $$Al_{2}O_{3}.2H_{2}O_{7} + NaOH \rightarrow 2NaAlO_{2} + 3H_{2}O$$
(iv) $$CuFeS_{2} + O_{2} = \rightarrow Cu_{2}S + 2FeS + SO_{2}\downarrow$$
(v) $$Cu_{2}S + 2Cu_{2}O \rightarrow 6Cu + SO_{2}\downarrow$$.

Copper , Zinc and Lead are found in sulphide ores.

In the concentrated ore, small quantity of silica is added and heated in the presence of excess of air in reverberatory furnace. During this, moisture and other volatile impurities are repelled in the form of oxide.
Following chemical changes occur in the furance:
$$2CuFeS_{2} + 2O_{2} Cu_{2}S + 2FeS + 2SO_{2}\downarrow$$
$$2FeS + 3O_{2} \rightarrow 2FeO + 2SO_{2}\downarrow$$
$$2Cu_{2}S + 3O_{2} \rightarrow 2Cu_{2}O + 2SO_{2}\downarrow$$
$$\underset {Gangue}{FeO} + \underset {Flux}{SiO_{2}} \rightarrow \underset {Slag}{FeSiO_{3}}$$
$$Cu_{2}O + FeS \rightarrow Cu_{2}S + FeO$$
This mixture $$Cu, S$$ and $$Cu$$, thus obtained is know copper matte

The ore is crushed into small lumps and then each out the soluble impurities in running water. Then it undergoes roasting. Thus impurities like sulfur , arsenic , moisture etc. removed. The roasted ore is mixed with coke and limestone and the mixture is changed into a blast furnace.
Reaction in blast furnace:

At high temperature , $$ CaCO_3$$ (limestone) decomposes to form $$CaO$$.
$$ CaO_2 (s) \rightarrow CaO(s) + CO_2(g) $$ 
This $$ CaO$$ combines with acidic gangue, $$ SiO_2$$ forming slag.
$$ CaO(s) + SiO_2(g) \rightarrow CaSiO_3(s)$$

Coke reacts with oxygen and form $$CO_2$$
$$ C(s) + O_2(g) \rightarrow CO_2(g) $$ 

$$CO_2$$ combines with more carbon and produces $$CO$$.
$$CO_2(g) + C_2(s) + Heat \rightarrow 2CO(g)$$
This $$CO$$ acts as reducing agent.

$$Fe_2O_3$$ is reduced to form $$Fe$$.
$$ Fe_2O_3 (s) + 3CO (g) \rightarrow 2Fe(s) + 3CO_2(g) $$ 

Ore of Iron	Haematite , Magnetite
Raw materials fed into  the blast furnace	Roasted haematite , lime stone , coke
The compounds used for  reducing haematite	carbon monoxide
Gangue	Silica $$(SiO_2)$$
Flux	CaO
Slag	$$CaSiO_3$$
Equation of formation of  slag	$$ CaO + SiO_2 \rightarrow CaSiO_3$$

Iron Ore is $$\mathrm{Fe_2O_3}$$ and name of this ore is Haematite.

a. Haematite
b. Levigation
c. Roasting
d. Impurities with less density are removed.
e. Moisture content is removed. Sulfur, Arsenic, Phosphorous, etc. are converted into oxides. As they are in gaseous          state, these are also removed.

$$Fe$$ because of its high nuclear binding energy is very stable and hence is an abundent element on the earth. As the atomic number increase, the neutron/proton, i.e., $$n/p$$ ratio increases. As a result, the nuclear binding energy decreases and hence the abundance of elements with the high atomic numbers decrease.  

$$\mathrm{SiO_2}$$ is the gangue in the ore $$\mathrm{Fe_2O_3}$$ (Haematite).

Due to its very high nuclear binding energy.

$$CuFeS_2\,+\,2H_2SO_4\,\rightarrow\,CuSO_4\,+\,FeSO_4\,+\, 2H_2S$$

$$Fe^{2+}\,\equiv\,MnO^{-}_{4}$$

10 mL of $$Fe^{2+}\,\equiv\,10 \,mL \,of \,0.02 \,M\, MnO^{-}_{4}$$

$$\equiv\,10 \,ml \,of \,0.1 \,N\, MnO^{-}_{4}$$

$$\therefore\, Fe^{2+}\,=\,0.1N$$

$$=\,5.6\,g\,L^{-1}\,in\,10\,gL^{-1}\,mineral$$

= $$56\%\, Fe$$

The redox changes are as follows:

During the roasting of pyrite ore, a mixture of $$FeO$$ and $$Cu_2O$$ is obtained. The role of silica in the metallurgy of copper is to remove the iron oxide obtained during the process of extraction of pure copper from copper pyrite, $$SiO_2$$ acts as acidic flux which combines with iron oxide($$FeO$$) to form iron silicate ($$FeSiO_3$$).

Leaching is the process to extract pure copper from a low grade ore. Leaching is the process of extracting substances from a solid by dissolving them in a liquid, in this process the ore is dissolved in liquid but not impurities. Then the copper solution is reacted with iron to get pure copper (reduction process).

$$Cu^{2+}+H_2\rightarrow Cu(pure)+2H^{+}$$

Metals low in activity series like copper are very unreactive. The oxides of these metals can be reduced to metals by heating alone. Copper is found as $$Cu_2S$$ in nature. It can be formed just heating it in air.

The carbonate ores are changed into oxides by heating strongly in limited air. This process is called calcination. $$ZnCO_3(s) \xrightarrow {heat} ZnO(s) +CO_2 (g)$$
The metal oxides are then reduced to the corresponding metals by using suitable reagents such as carbon. For, example, when zinc oxide is heated with carbon, it is reduced to metallic zinc. 

Copper : Ore : Pyrite $$\left( CuFeS_2\right)$$

Low grade ore of copper are leached in the presence of dilute. $$H_2SO_4$$ solution.
$$CuO (s) + H_2SO_4 (aq) \rightarrow CuSO_4 (aq) + H_2O (l)$$

It provides the flux $$CaO$$ which removes the impurity $$SiO_{2}$$ present in the ore by forming fusible calcium silicate slag.

(i) Impure zinc contains impurities of cadmium $$(b.p.1073\ K)$$ lead $$(b.p.2024\ K)$$ and iron $$(b.p.3273\ K)$$. The boiling point of zinc is $$1180\ K$$. The impure metals is distilled when zinc and cadmium with low boiling points dist over leaving behind lead and iron. The boiling points of $$Zn$$ and $$Cd$$ differ by about $$100\ K$$, therefore, the mixture of $$Zn$$ and $$Cd$$ is again subjected to fractional distillation when low boiling $$Cd$$ distils over leaving behind zinc metals in the distillation flack.
(ii) The metal zinc may be further purified by electrolytic refining as explained on page $$10/17$$.
(iii) The metal copper is refined by poling.
(iv) The metal germanium may be purified by zone refining as explained on page.