Isolation of Elements

(A) Roasting of malachite

$ \mathrm{CuCO}_3 \mathrm{Cu}(\mathrm{OH})_2 \underset{+\mathrm{O}_2}{\stackrel{\Delta}{\longrightarrow}} \mathrm{CuO}+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 $

Roasting means that heating of substance in excess of oxygen. So cuprite $\mathrm{Cu}_2 \mathrm{O}$ is not produced. Hence, this statement is not correct.

(B) Calcination means heating in absence of air

$ \underset{\text { Calamine }}{\mathrm{ZnCO}_3} \stackrel{\Delta}{\longrightarrow} \underset{\text { Zincite }}{\mathrm{ZnO}}+\mathrm{CO}_2 $

$\therefore$ Statement is true.

(C)

$ \begin{aligned} \mathrm{CuFeS}_2+\mathrm{O}_2 \stackrel{\Delta}{\longrightarrow} \mathrm{Cu}_2 \mathrm{~S}+\mathrm{FeO}+\mathrm{SO}_2 \\\\ \mathrm{FeO}+\mathrm{SiO}_2 \longrightarrow \mathrm{FeSiO}_2 \end{aligned} $

$\therefore$ Statement is true.

(D)

$ \begin{aligned} \mathrm{Ag}+\mathrm{KCN}+\mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O} \longrightarrow & 4 \mathrm{~K}\left[\mathrm{Ag}(\mathrm{CN})_2\right] +4 \mathrm{KOH} \\\\ 2 \mathrm{~K}\left[\mathrm{Ag}(\mathrm{CN})_2\right]+\mathrm{Zn} \longrightarrow & \mathrm{K}_2\left[\mathrm{Zn}(\mathrm{N})_4\right]+2 \mathrm{Ag} \end{aligned} $

Silver is obtained by reaction with Zinc

So, the above statement is true.

(A) Limestone is added to remove silica as impurity.

(B) Pig iron obtained from blast furnace contains 4% carbon and many other impurities (eg. S, P, Si, Mn) in small amount.

(C) Coke (C) converts $\mathrm{CO}_2$ in $\mathrm{CO}$.

$ \mathrm{C}+\mathrm{CO}_2 \rightarrow 2 \mathrm{CO} $

(D) Exhaust gases consist of $\mathrm{CO}$ and $\mathrm{CO}_2$.

Hence, (A, B, C) are correct.

(A) The reduction of Al₂O₃ with coke (C) at a temperature > 2500$^\circ$C is not carried out due to the formation of carbides.

(B) It is correct as neutralisation of aluminate solution is done by passing CO₂ gas to precipitate hydrated alumina.

(C) Reaction of powdered one is carried out with hot concentrated NaOH at 473 K - 523 K and 35 - 36 bar pressure.

(D) Electrolysis of Al₂O₃ is done mixed with Na₃AlF₆ to produce Al and CO₂. It is a correct statement.

Statements (a), (c) and (d) are correct, whereas statement (b) is incorrect. All statements are explained as follows :

(a) A mixture PbO and PbS reacts as follows:

This reaction is known as self-reduction.

(b) Silica is added to remove impurities of Fe in the form of slag, FeSiO₃ .

(c) CuFeS₂ are in partially oxidised first by roasting and then self-reduction of Cu takes place to produce blister copper.

(d) Zn powder reacts with Na[Au(CN)₂ ] to form gold. This process is called cyanide process.

2Zn + 4Na[Au(CN)₂] → 2Na₂[Zn(CN)₄]+ 4Au

Pb(NO₃)₂, Bi(NO₃)₃ and Zn(NO₃)₂ give white precipitate with dil. NaOH and dil. HCl. Complete reactions are as follows:


Whereas, AgNO₃ and Hg(NO₃)₃ give ppt. of different colours.

$2Na[Al{(OH)_4}](aq) + C{O_2}\mathrel{\mathop{\kern0pt\longrightarrow} \limits_{}} N{a_2}C{O_3} + {H_2}O + 2Al{(OH)_3} \downarrow or\,A{l_2}{O_3}\,.\,2{H_2}O (ppt.)$

Hence, the statement (a) is true.

(b) The electrolysis of alumina by Hall-Heroult's process is carried by using a fused mixture of alumina and cryolite (Na₂AlF₆) along with minor quantities of aluminium fluoride and fluorspar. The addition of cryolite and fluorspar increases the electrical conductivity of alumina and lowers the fusion temperature. Hence, the statement (b) is true.

(c) At anode, alumina reacts with fluorine to liberate oxygen and evolved oxygen reacts with carbon to form carbon dioxide and hence, statement (c) is true.

(d) Steel cathode with carbon lining and graphite anode are used and hence, the statement (d) is true.

Cyanide process of gold extraction involves leaching out gold from its ore with CN^- in the presence of O₂ (Q) in water to form [Au(CN)₂]^- (R). When [Au(CN)₂]^- reacts with Zn (T), it forms [Zn(CN)₄]^2- (Z) and Au.

The corresponding reactions are as follows :


Hence, options (a, b, c) are correct.

(a) In the extraction of copper from copper pyrite (CuFeS₂), after crushing, concentration of ore is done by froth floatation process.

(b) Iron is removed as slag.

(c) Auto-reduction :

(d) Blister copper is finally purified by electrolytic refining.

(a) is wrong statement. Impure copper is set as anode where copper is oxidized to Cu²⁺ and goes into electrolytic solutions.

(b) CuSO₄ is used as an electrolyte in purification process.

(c) Pure copper is deposited at cathode as : Cu²⁺ + 2e^$-$ $\to$ Cu : (At cathode)

(d) Less active metals like Ag, Au etc. settle down as anode mud.

Extraction of various metals from their respective concentrated ores is carried out by different methods:

(i) Carbon Reduction of Hematite:

The process for the extraction of iron from hematite (Fe₂O₃) involves carbon reduction:

3 Fe2O3 + CO → 2 Fe3O4 + CO2(g)

Fe3O4 + 4 CO → 3 Fe(s) + 4 CO(g)

This reaction occurs at temperatures between 500-800 K.

FeO(s) + CO(g) → Fe(s) + CO2(g)

Carbon monoxide needed for the reduction is produced as follows:

C(s) + CO2(g) → 2 CO

The net reaction can be written as:

FeO(s) + C(s) → Fe(s) + CO(g)

Thus, carbon and its oxides are utilized for the reduction of iron ores such as Fe₂O₃.

(ii) Carbon Reduction of Cassiterite (SnO₂):

The reduction process involves carbon:

SnO2(s) + C(s) → Sn(s) + CO2(g)

Therefore, carbon is used for the reduction of cassiterite ore.

(iii) Electrolytic Refining of Aluminium (Al₂O₃):

Aluminium is extracted from bauxite using electrolysis. The reduction occurs at the cathode:

Al3+ + 3 e- → Al(s)

The reduction of Al³⁺ takes place at the cathode.

(iv) Dow's Process for Extraction of Magnesium from Dolomite (MgCO₃.CaCO₃):

Dolomite is treated with hydrochloric acid (HCl) to produce magnesium chloride (MgCl₂). Electrolysis of MgCl₂ reduces Mg²⁺ to magnesium metal at the cathode:

Mg2+(aq) + 2 e- → Mg

Magnesium metal is obtained by the reduction of Mg²⁺.

Tin is obtained by reducing the ore cassiterite with coal in a reverberatory furnace. Limestone is added to produce a slag with the impurities, which can be separated.

$ \mathrm{SnO}_2+2 \mathrm{C} \longrightarrow \mathrm{Sn}+2 \mathrm{CO} $

Crude tin obtained from this process is contaminated with iron, copper, lead, and other metals. To purify it, the crude tin is remelted in an inclined furnace—a process known as liquation. During liquation, the easily fusible tin melts away, leaving the less fusible impurities behind.

Finally, the molten tin is refined using green poles of wood in contact with air. This step, known as poling, helps to oxidize any remaining metal impurities. These impurities form a scum on the surface, which can be removed.

This step-by-step process ensures the extraction and purification of tin from cassiterite.