General Principles and Processes of Isolation of Elements

Multiple Choice Questions (MCQs)

1. In the extraction of chlorine by electrolysis of brine…… .

(a) oxidation of $Cl^{-}$ion to chlorine gas occurs

(b) reduction of $Cl^{-}$ion to chlorine gas occurs

(c) for overall reaction $\Delta G^{s}$ has negative value

(d) a displacement reaction takes place

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Answer

(a) Following reaction takes place in the extraction of chlorine by electrolysis

$$ 2 H_2 O(l)+2 Cl^{-}(aq) \rightarrow H_2(g)+Cl_2(g)+2 OH^{-}(aq) $$

From the above reaction it is very clear that the oxidation of $Cl^{-}$ion to chlorine gas occurs. Thus, option (a) is the correct answer.

$\Delta^{\circ} G$ for this reaction is $+422 kJ$. Therefore, thermodynamically reaction is not feasible. We know that

$$ \Delta^{\circ} G=-n F E^{\circ} $$

By using this equation value of $E^{\circ}$ can be calculated. Thus,

$$ E^{\circ}=-\frac{\Delta G^{\circ}}{n F}=-2.2 V $$

Therefore, for the occurrence of this reaction, external potential (emf) greater than $2.2 V$ is required. Due to this fact (c) is not the correct option.

  • (b) Reduction of $Cl^{-}$ ion to chlorine gas does not occur. Instead, oxidation of $Cl^{-}$ ion to chlorine gas occurs, as shown in the reaction: $$ 2 H_2 O(l)+2 Cl^{-}(aq) \rightarrow H_2(g)+Cl_2(g)+2 OH^{-}(aq) $$

  • (c) The overall reaction $\Delta G^{\circ}$ has a positive value ($+422 kJ$), indicating that the reaction is not thermodynamically feasible without external potential. Therefore, $\Delta G^{\circ}$ does not have a negative value.

  • (d) A displacement reaction does not take place in the electrolysis of brine. The process involves the oxidation of $Cl^{-}$ ions to chlorine gas and the reduction of water to hydrogen gas, not a displacement reaction.

2. When copper ore is mixed with silica, in a reverberatory furnace copper matte is produced. The copper matte contains

(a) sulphides of copper (II) and iron (II)

(b) sulphides of copper (II) and iron (III)

(c) sulphides of copper (I) and iron (II)

(d) sulphides of copper (I) and iron (III)

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Answer

(c) When copper ore is mixed with sillica in a reverberatory furnace copper matte is produced. The copper matte contains sulphide of copper (I) and iron (II).

Copper matte $\rightarrow Cu_2 S$ and $FeS$

  • (a) sulphides of copper (II) and iron (II): This option is incorrect because in the copper matte, copper is present as copper (I) sulfide ($Cu_2S$) and not as copper (II) sulfide ($CuS$). The iron is correctly identified as iron (II) sulfide ($FeS$).

  • (b) sulphides of copper (II) and iron (III): This option is incorrect because in the copper matte, copper is present as copper (I) sulfide ($Cu_2S$) and not as copper (II) sulfide ($CuS$). Additionally, iron is present as iron (II) sulfide ($FeS$) and not as iron (III) sulfide ($Fe_2S_3$).

  • (d) sulphides of copper (I) and iron (III): This option is incorrect because while copper is correctly identified as copper (I) sulfide ($Cu_2S$), iron is present as iron (II) sulfide ($FeS$) in the copper matte, not as iron (III) sulfide ($Fe_2S_3$).

3. Which of the following reactions is an example of autoreduction?

(a) $Fe_3 O_4+4 CO \longrightarrow 3 Fe+4 CO_2$

(b) $Cu_2 O+C \longrightarrow 2 Cu+CO$

(c) $Cu^{2+}(aq)+Fe(s) \longrightarrow Cu(s)+Fe^{2+}(aq)$

(d) $Cu_2 O+\frac{1}{2} Cu_2 S \longrightarrow 3 Cu+\frac{1}{2} SO_2$

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Answer

(d) $Cu_2 O+\frac{1}{2} Cu_2 S \longrightarrow 3 Cu+\frac{1}{2} SO_2$

This reaction includes reduction of copper (I) oxide by copper (I) sulphide. In this process, copper is reduced by itself hence this process is known as autoreduction and the solidified copper. So, obtained is known as blister copper.

  • (a) $Fe_3 O_4+4 CO \longrightarrow 3 Fe+4 CO_2$: This reaction involves the reduction of iron oxide by carbon monoxide, not by itself. Therefore, it is not an example of autoreduction.

  • (b) $Cu_2 O+C \longrightarrow 2 Cu+CO$: This reaction involves the reduction of copper(I) oxide by carbon, not by itself. Hence, it is not an example of autoreduction.

  • (c) $Cu^{2+}(aq)+Fe(s) \longrightarrow Cu(s)+Fe^{2+}(aq)$: This reaction involves the reduction of copper ions by iron, not by itself. Therefore, it is not an example of autoreduction.

4. A number of elements are available in earth’s crust but most abundant elements are…… .

(a) $Al$ and $Fe$

(b) $Al$ and $Cu$

(c) $Fe$ and $Cu$

(d) $Cu$ and $Ag$

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Answer

(a) Among a number of elements which are available in earth crust, the most abundant elements are aluminium and iron. Aluminium is third most abundant element in earth crust. i.e., $8.3 $% by weight while iron present in earth crust with $4.2 $% by weight. Copper and silver are also found in earth crust but their abundance percentage is low.

  • Option (b) is incorrect because copper (Cu) is not among the most abundant elements in the earth’s crust. While aluminum (Al) is indeed abundant, copper’s abundance percentage is much lower compared to iron (Fe).

  • Option (c) is incorrect because copper (Cu) is not among the most abundant elements in the earth’s crust. Iron (Fe) is abundant, but copper’s abundance percentage is significantly lower compared to aluminum (Al).

  • Option (d) is incorrect because neither copper (Cu) nor silver (Ag) are among the most abundant elements in the earth’s crust. Both elements have much lower abundance percentages compared to aluminum (Al) and iron (Fe).

5. Zone refining is based on the principle that…… .

(a) impurities of low boiling metals can be separated by distillation.

(b) impurities are more soluble in molten metal than in solid metal.

(c) different components of a mixture are differently adsorbed on an adsorbent.

(d) vapours of volatile compound can be decomposed in pure metal.

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Answer

(b) Zone refining is based on the principle that the impurities are more soluble in molten state than in solid state of the metal. A circular mobile heater fixed at one end of impure metal rod. The molten zone moves along with heater which is moved forward.

As the heater moves forward, the pure metal crystallises out of the melt and the impurities pass on into adjacent molten zone. The process is repeated several times and the heater is moved in the same direction.

At one end, impurities get concentrated. This end is cut off. e.g., germanium, silicon, gallium etc., are refined by this method.

  • (a) Impurities of low boiling metals can be separated by distillation: This option is incorrect because distillation is a process used to separate components based on differences in their boiling points, not on their solubility in molten versus solid states.

  • (c) Different components of a mixture are differently adsorbed on an adsorbent: This option is incorrect because adsorption is a surface phenomenon where different components adhere to the surface of an adsorbent material, which is not related to the solubility of impurities in molten and solid states.

  • (d) Vapours of volatile compound can be decomposed in pure metal: This option is incorrect because it describes a process involving the decomposition of volatile compounds, which is not related to the principle of solubility differences between molten and solid states in zone refining.

6. In the extraction of copper from its sulphide ore, the metal is formed by the reduction of $Cu_2 O$ with

(a) FeS

(b) $CO$

(c) $Cu_2 S$

(d) $SO_2$

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Answer

(c) In the extraction of copper from its sulphide ore, the metal is formed by the reduction of $Cu_2 O$ with $Cu_2 S$. This reaction completes with the process of autoreduction.

Chemical reaction occurring in this reaction is as follows

$$ Cu_2 O+\frac{1}{2} Cu_2 S \longrightarrow 3 Cu+\frac{1}{2} SO_2 $$

In this process, copper appears as blister copper.

  • (a) FeS: Iron sulfide (FeS) is not used in the reduction of $Cu_2 O$ to form copper. FeS does not participate in the autoreduction process that is specific to copper extraction.

  • (b) $CO$: Carbon monoxide (CO) is a reducing agent used in various metallurgical processes, but it is not used in the reduction of $Cu_2 O$ in the extraction of copper from its sulphide ore. The specific autoreduction process involves $Cu_2 S$.

  • (d) $SO_2$: Sulfur dioxide (SO_2) is a byproduct of the reduction reaction and not a reducing agent. It does not reduce $Cu_2 O$ to copper.

7. Brine is electrolysed by using inert electrodes. The reaction at anode is…… .

(a) $Cl^{-}(aq) \rightarrow \frac{1}{2} Cl_2(g)+e^{-}\quad \quad $; $E_{\text {Cell }}^{s}=1.36 V$

(b) $2 H_2 O(l) \rightarrow O_2(g)+4 H^{+}+4 e^{-} \quad \quad $; $E_{\text {Cell }}^{s}=1.23 V$

(c) $Na^{+}(aq)+e^{-} \rightarrow Na(s) \quad \quad $; $E_{\text {Cell }}^{s}=2.71 V$

(d) $H^{+}$(aq) $+e^{-} \rightarrow \frac{1}{2} H_2(g) \quad \quad $; $E_{\text {Cell }}^{s}=0.00 V$

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Answer

(a) Brine is electrolysed by using inert electrodes. The possible reactions occurring at anode are

$$ \begin{matrix} Cl^{-}(aq) \longrightarrow \frac{1}{2} Cl_2(g)+e^{-} ; & E_Cell^{s}=1.36 V \\ 2 H_2 O(l) \longrightarrow O_2(g)+4 H^{+}+4 e^{-} ; & E_Cell^{s}=1.23 V \end{matrix} $$

The reaction at anode with lower value of $E^{\circ}$ is preferred and therefore water should get oxidised in preference to $Cl^{-}(a q)$. However, $Cl_2$ is produced instead of $O_2$. This unexpected result is explained on the basis of the fact that water needs greater voltage for oxidation to $O_2$ (as it is kinetically slow process) than that needed for oxidation of $Cl^{-}$ions to $Cl_2$.

  • Option (b): Although the standard electrode potential for the oxidation of water to oxygen is lower ($E_{\text{Cell}}^{s} = 1.23 , \text{V}$) compared to the oxidation of chloride ions to chlorine ($E_{\text{Cell}}^{s} = 1.36 , \text{V}$), the oxidation of water is kinetically slow and requires a higher overpotential. Therefore, in practice, chloride ions are oxidized preferentially.

  • Option (c): This option describes the reduction of sodium ions to sodium metal, which occurs at the cathode, not the anode. The question specifically asks for the reaction at the anode.

  • Option (d): This option describes the reduction of hydrogen ions to hydrogen gas, which also occurs at the cathode, not the anode. The question specifically asks for the reaction at the anode.

8. In the metallurgy of aluminium…… .

(a) $Al^{3+}$ is oxidised to $Al(s)$.

(b) graphide anode is oxidised to carbon monoxide and carbon dioxide.

(c) oxidation state of oxygen changes in the reaction at anode.

(d) oxidation state of oxygen changes in the overall reaction involved in the process.

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Answer

(b) In the metallurgy of aluminium, electrolysis is performed in a steel vessel with lining of carbon acts as cathode and graphite acts as anode. During this process graphite anode is oxidised to $CO$ and $CO_2$.

Chemical reaction occurring in this process is as follows

$$ 2 Al_2 O_3+3 C \longrightarrow 4 Al+3 CO_2 $$

This process is known as Hall - Heroult process. The electrolytic reactions are

At cathode $Al^{3+}($ melt $)+3 e^{-} \longrightarrow Al(l)$

At anode $C(s)+O^{2-}($ melt $) \longrightarrow CO(g)+2 e^{-}$

$C(s)+2 O^{2-}$ (melt) $\longrightarrow CO_2(g)+4 e^{-}$

  • (a) $Al^{3+}$ is oxidised to $Al(s)$: This statement is incorrect because in the electrolysis process, $Al^{3+}$ ions are reduced, not oxidized. The reduction reaction at the cathode is $Al^{3+} + 3e^{-} \rightarrow Al(s)$.

  • (c) oxidation state of oxygen changes in the reaction at anode: This statement is incorrect because the oxidation state of oxygen does not change in the reaction at the anode. The anode reactions involve the oxidation of carbon to form carbon monoxide ($CO$) and carbon dioxide ($CO_2$), but the oxidation state of oxygen remains -2 in both products.

  • (d) oxidation state of oxygen changes in the overall reaction involved in the process: This statement is incorrect because the oxidation state of oxygen remains -2 throughout the overall reaction. The overall reaction is $2 Al_2 O_3 + 3 C \rightarrow 4 Al + 3 CO_2$, where oxygen is in the -2 oxidation state in both $Al_2 O_3$ and $CO_2$.

9. Electrolytic refining is used to purify which of the following metals?

(a) $Cu$ and $Zn$

(b) Ge and $Si$

(c) $Zr$ and $Ti$

(d) $Zn$ and $Hg$

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Answer

(a) Copper and zinc are two metals which are generally purified by using electrolyte refining. In this process, impure metal is used as anode and pure metal is used as a cathode. Impurities from the blister copper or impure zinc deposit as anode mud.

  • (b) Ge and Si: Germanium (Ge) and Silicon (Si) are typically purified using methods such as zone refining or chemical vapor deposition, not electrolytic refining.

  • (c) Zr and Ti: Zirconium (Zr) and Titanium (Ti) are usually purified through processes like the Kroll process or the Van Arkel process, rather than electrolytic refining.

  • (d) Zn and Hg: While zinc (Zn) can be purified by electrolytic refining, mercury (Hg) is not typically purified using this method. Mercury is usually purified through distillation.

10. Extraction of gold and silver involves leaching the metal with $CN^{-}$ion. The metal is recovered by…… .

(a) displacement of metal by some other metal from the complex ion.

(b) roasting of metal complex.

(c) calcination followed by roasting.

(d) thermal decomposition of metal complex.

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Answer

(a) Extraction of gold and silver involves leaching the metal with $CN^{-}$ion. The metal is recovered by displacement of metal by some other metal from the complex ion.

This is an oxidation reaction.

$$ 4 Au(s)+8 CN^{-}(aq)+2 H_2 O(aq)+O_2(g) \longrightarrow 4[Au(CN)_2]^{-}(aq)+4 OH^{-}(aq) $$

$$ 4[Au(CN_2)] (aq)+Zn(s) \longrightarrow 2Au(s) + [Zn(CN)_4]^{2-} (aq) $$

Here, $Zn$ acts as a reducing agent.

Direction (Q. Nos. 11-13) Answer the questions on the basis of figure

  • (b) Roasting of metal complex: Roasting is a process that involves heating the ore in the presence of oxygen. This method is typically used to convert sulfide ores into oxides, which can then be reduced to extract the metal. However, in the case of gold and silver extraction, the metal is already in a complex ion form with cyanide, and roasting is not applicable for recovering the metal from this complex.

  • (c) Calcination followed by roasting: Calcination is the process of heating the ore in the absence of air to remove volatile impurities, and roasting follows to convert the ore into an oxide. This method is generally used for carbonate and sulfide ores. Since gold and silver are leached with cyanide to form a complex ion, calcination and roasting are not relevant steps for recovering the metal from the cyanide complex.

  • (d) Thermal decomposition of metal complex: Thermal decomposition involves breaking down a compound by heating it. This method is not suitable for recovering gold and silver from their cyanide complexes because the complex ions are stable and do not decompose easily with heat. Instead, a displacement reaction with another metal, such as zinc, is used to recover the gold and silver.

11. Choose the correct option of temperature at which carbon reduces Fe0 to iron and produces $CO$.

(a) Below temperature at point $A$

(b) Approximately at the temperature corresponding to point $A$

(c) Above temperature at point $A$ but below temperature at point $D$

(d) Above temperature at point $A$

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Thinking Process

This problem is based on concept of Ellingham diagram which relates Gibbs free energy of reaction and temperature. The point above which greater the negative value of formation of compound is observed, compound will form at that temperature. At a particular temperature compound having higher negative value of Gibbs free energy will be formed first.

Answer

(d) In the figure shown above point $A$

$$ \Delta_{f} G_{(C, CO)}^{\circ}<\Delta_{f} G_{(Fe, FeO)}^{\circ} $$

Therefore, above point $A, C$ reduces $FeO$ to $Fe$ forming carbon monoxide. Hence, reduction of $FeO$ by carbon occurs above point $A$ only.

  • Option (a) Below temperature at point A: Below point A, the Gibbs free energy change for the formation of CO from C is not sufficiently negative compared to the Gibbs free energy change for the formation of FeO from Fe. Therefore, carbon cannot effectively reduce FeO to Fe below this temperature.

  • Option (b) Approximately at the temperature corresponding to point A: At the temperature corresponding to point A, the Gibbs free energy changes for the formation of CO from C and FeO from Fe are approximately equal. This means that the reduction reaction is not favored or is at equilibrium, so carbon cannot effectively reduce FeO to Fe at this temperature.

  • Option (c) Above temperature at point A but below temperature at point D: This option is partially correct but not specific enough. While it is true that carbon can reduce FeO to Fe above point A, the critical information is that the reduction starts just above point A. The range between point A and point D is too broad and does not precisely indicate the onset of the reduction reaction.

12. Below point ‘A’ Fe0 can…… .

(a) be reduced by carbon monoxide only.

(b) be reduced by both carbon monoxide and carbon.

(c) be reduced by carbon only.

(d) not be reduced by both carbon and carbon monoxide.

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Answer

(a) Below point $A$ Gibbs free energy change for the formation of $CO_2$ from $CO(\Delta G_{CO_2, CO_2}^{\circ})$ has lower value (more negative value) than Gibbs free energy change for the formation of $FeO(\Delta G_{Fe, FeO})$. Hence, $FeO$ will be reduced by $CO$ only below point $A$.

  • (b) FeO cannot be reduced by both carbon monoxide and carbon below point A because the Gibbs free energy change for the formation of CO₂ from CO is more negative than that for the formation of FeO, indicating that CO is a more effective reducing agent than carbon in this context.

  • (c) FeO cannot be reduced by carbon only below point A because the Gibbs free energy change for the formation of CO₂ from CO is more negative than that for the formation of FeO, making CO the preferred reducing agent over carbon.

  • (d) FeO can be reduced by carbon monoxide below point A, as indicated by the more negative Gibbs free energy change for the formation of CO₂ from CO compared to the formation of FeO. Therefore, it is incorrect to say that FeO cannot be reduced by both carbon and carbon monoxide.

13. For the reduction of $Fe 0$ at the temperature corresponding to point $D$, which of the following statements is correct?

(a) $\Delta G$ value for the overall reduction reaction with carbon monoxide is zero.

(b) $\Delta G$ value for the overall reduction reaction with a mixture of $1 mol$ carbon and 1 mol oxygen is positive.

(c) $\Delta G$ value for the overall reduction reaction with a mixture of $2 mol$ carbon and 1 mol oxygen will be positive.

(d) $\Delta G$ value for the overall reduction reaction with carbon monoxide is negative.

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Answer

(a) At point $D, \Delta G$ curve for formation of $CO_2$ from $CO$ and $FeO$ from $Fe$ intersect each other therefore, overall reduction of $FeO$ with $CO$ is zero.

Hence, (a) is the correct choice.

  • (b) $\Delta G$ value for the overall reduction reaction with a mixture of $1 mol$ carbon and 1 mol oxygen is positive.

    The statement is incorrect because the mixture of 1 mol carbon and 1 mol oxygen would form carbon dioxide ($CO_2$), which is a highly exothermic reaction. This would result in a negative $\Delta G$ value, not a positive one.

  • (c) $\Delta G$ value for the overall reduction reaction with a mixture of $2 mol$ carbon and 1 mol oxygen will be positive.

    The statement is incorrect because the mixture of 2 mol carbon and 1 mol oxygen would form carbon monoxide ($CO$), which is also an exothermic reaction. This would result in a negative $\Delta G$ value, not a positive one.

  • (d) $\Delta G$ value for the overall reduction reaction with carbon monoxide is negative.

    The statement is incorrect because at point $D$, the $\Delta G$ value for the overall reduction reaction with carbon monoxide is zero, not negative. This is due to the intersection of the $\Delta G$ curves for the formation of $CO_2$ from $CO$ and $FeO$ from $Fe$.

Multiple Choice Questions (More Than One Options)

14. At the temperature corresponding to which of the points in Fig. Fe0 will be reduced to $Fe$ by coupling the reaction $2 Fe 0 \longrightarrow 2 Fe+O_2$ with all of the following reactions?

  1. $C+O \longrightarrow CO_2$
  2. $2 C+O_2 \longrightarrow 2 CO$
  3. $2 CO+O_2 \longrightarrow 2 CO_2$

(a) Point $A$

(b) Point $B$

(c) Point $D$

(d) Point $E$

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Answer

$(b, d)$

Below point $B$ and $E, FeO$ will be reduced to $Fe$ by all the three reactions shown above in the question. $\Delta G_{(C, CO_2)}^{\circ}, \Delta G_{(C, CO)}^{\circ}, \Delta G_{(CO_2, CO_2)}^{\circ}$ lie below $\Delta f G_{(Fe, FeO)}^{\circ}$ curve at point $B$ and $E$. Therefore, $FeO$ will be reduced by all three reactions.

Hence, options (b) and (d) are correct choice.

  • Option (a) Point A: At point A, the Gibbs free energy changes for the reactions involving carbon and carbon monoxide do not lie below the Gibbs free energy change for the reduction of FeO to Fe. Therefore, FeO will not be reduced by all three reactions at this point.

  • Option (c) Point D: At point D, the Gibbs free energy changes for the reactions involving carbon and carbon monoxide do not lie below the Gibbs free energy change for the reduction of FeO to Fe. Therefore, FeO will not be reduced by all three reactions at this point.

15. Which of the following options are correct?

(a) Cast iron is obtained by remelting pig iron with scrap iron and coke using hot air blast.

(b) In extraction of silver, silver is extracted as cationic complex.

(c) Nickel is purified by zone refining.

(d) $Zr$ and $Ti$ are purified by van Arkel method.

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Thinking Process

This process is based on concept of purification techniques and formation of cast iron.

Answer

$(a, d)$

Correct statements are

(a) Cast iron is obtained by remelting pig iron with scarp iron and coke using hot air blast.

(d) $Zr$ and $Ti$ are purified by van Arkel method as

$$ \underset{\text { Impure }}{Zr}+2 I_2 \longrightarrow \underset{\text { Volatile }}{ZrI_4} \stackrel{\Delta}{\longrightarrow} \underset{\text { Pure }}{Zr+2 I_2} $$

(b) and (c) can be correctly stated as

(b)In extraction of silver, silver is extracted as anionic complex $[Ag(CN)_2]^{-}$

(c)Nickel is purified by vapour phase refining method.

$$ Ni+4 CO \rightarrow Ni(CO)_4 \xrightarrow{450-470 K} Ni+4 CO $$

  • (b) In extraction of silver, silver is extracted as anionic complex $[Ag(CN)_2]^{-}$: The statement is incorrect because silver is not extracted as a cationic complex but as an anionic complex, specifically $[Ag(CN)_2]^{-}$.

  • (c) Nickel is purified by zone refining: The statement is incorrect because nickel is not purified by zone refining. Instead, it is purified by the vapour phase refining method, where nickel reacts with carbon monoxide to form a volatile complex, nickel tetracarbonyl, which decomposes upon heating to yield pure nickel.

16. In the extraction of aluminium by Hall-Heroult process, purified $Al_2 O_3$ is mixed with $CaF_2$ to

(a) lower the melting point of $Al_2 O_3$

(b) increase the conductivity of molten mixture.

(c) reduce $Al^{3+}$ into $Al(s)$

(d) acts as catalyst

Show Answer

Thinking Process

This problem is based on extraction of aluminium using Hall-Heroult process.

Answer

$(a, b)$

In the metallurgy of aluminium $Al_2 O_3$ is mixed with $Na_3 AlF_6$ and $CaF_2$ which causes following affects.

(i) Lower the melting point of $Al_2 O_3$ (ii) Increase the conductivity of molten mixture

  • (c) $CaF_2$ does not reduce $Al^{3+}$ into $Al(s)$. The reduction of $Al^{3+}$ to $Al(s)$ is achieved through electrolysis, where an electric current is passed through the molten mixture to facilitate the reduction process.
  • (d) $CaF_2$ does not act as a catalyst in the Hall-Heroult process. Its primary roles are to lower the melting point of $Al_2 O_3$ and increase the conductivity of the molten mixture.

17. Which of the following statements is correct about the role of substances added in the froth floatation process?

(a) Collectors enhance the non-wettability of the mineral particles.

(b) Collectors enhance the wettability of gangue particles.

(c) By using depressants in the process two sulphide ores can be separated.

(d) Froth stabilisers decrease wettability of gangue.

Show Answer

Thinking Process

This problem is based on the method involved in froth floatation method and function of collector and depressants.

Answer

$(a, c)$

Froth floatation process is used to extract metal from sulphide ore. This method utilises collectors and depressants whose functions are as follows

(i) Collectors enhance the non-wettability of the mineral particles.

(ii) By using depressants in the process two sulphide ores can be separated. e.g., sodium cyanide is used as a depressant to separate lead sulphide ore from zinc sulphide ore.

  • Collectors do not enhance the wettability of gangue particles; they enhance the non-wettability of mineral particles, making option (b) incorrect.
  • Froth stabilisers do not decrease the wettability of gangue; they help in stabilizing the froth formed during the process, making option (d) incorrect.

18. In the froth floatation process, zinc sulphide and lead sulphide can be separated by…… .

(a) using collectors

(b) adjusting the proportion of oil to water

(c) using depressant

(d) using froth stabilisers

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Answer

$(b, c)$

Froth floatation method is used to extract metal from sulphide ore. ZnS and PbS can be separated by using depressant and adjusting the proportion of oil to water. Depressant used for this purpose is $NaCN$. It selectively prevents $ZnS$ from coming to the froth.

Hence, (b) and (c) are correct choices.

  • (a) using collectors: Collectors are reagents that increase the non-wettability of the mineral particles, making them attach to air bubbles and rise to the froth. However, collectors do not specifically help in separating zinc sulphide (ZnS) from lead sulphide (PbS) as they generally enhance the floatability of both types of sulphide minerals.

  • (d) using froth stabilisers: Froth stabilisers are used to maintain the froth formed during the floatation process, ensuring that it is stable enough to carry the mineral particles to the surface. They do not play a role in selectively separating ZnS from PbS.

19. Common impurities present in bauxite are…… .

(a) $CuO$

(b) $ZnO$

(c) $Fe_2 O_3$

(d) $SiO_2$

Show Answer

Answer

$(c, d)$

Bauxite is an ore of aluminium which contain $Fe_2 O_3$ and $SiO_2$ as common impurities.

  • (a) $CuO$: Copper oxide ($CuO$) is not a common impurity in bauxite. Bauxite primarily contains impurities such as iron oxides and silica, not copper compounds.

  • (b) $ZnO$: Zinc oxide ($ZnO$) is not typically found in bauxite. The common impurities in bauxite are iron oxides and silica, rather than zinc compounds.

20. Which of the following ores are concentrated by froth floatation?

(a) Haematite

(b) Galena

(c) Copper pyrites

(d) Magnetite

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Answer

$(b, c)$

Haematite $(Fe_2 O_3)$ and magnetite $(Fe_3 O_4)$ are oxide ores while galena ( $.PbS)$ and copper pyrites $(CuFeS_2)$ are sulphide ores. As we know sulphide ores are extracted by using froth floatation method. Hence, (b) and (c) are the correct choices.

  • Haematite $(Fe_2O_3)$ and magnetite $(Fe_3O_4)$ are oxide ores, and froth flotation is not typically used for concentrating oxide ores.

21. Which of the following reactions occur during calcination?

(a) $CaCO_3 \longrightarrow CaO+CO_2$

(b) $2 FeS_2+\frac{11}{2} O_2 \longrightarrow Fe_2 O_3+4 SO_2$

(c) $Al_2 O_3 \cdot x H_2 O \longrightarrow Al_2 O_3+x H_2 O$

(d) $ZnS+\frac{3}{2} O_2 \longrightarrow ZnO+SO_2$

Show Answer

Thinking Process

This process is based on concept of calcination.

Answer

$(a, c)$

Calcination involves heating of the ore below its melting point in the absence of air or in limited supply of air. Oxygen containing ores like oxide, hydroxides and carbonates are calcined. Thus, the following reactions occur during calcination.

$$ \begin{aligned} & CaCO_3 \stackrel{\Delta}{\longrightarrow} CaO+CO_2 \\ & Al_2 O_3 \cdot x H_2 O \stackrel{\Delta}{\longrightarrow} Al_2 O_3+x H_2 O \end{aligned} $$

  • Option (b): The reaction $2 FeS_2+\frac{11}{2} O_2 \longrightarrow Fe_2 O_3+4 SO_2$ involves the oxidation of iron sulfide (FeS₂) in the presence of oxygen, which is a roasting process, not calcination. Calcination occurs in the absence or limited supply of air, whereas roasting requires a significant amount of air or oxygen.

  • Option (d): The reaction $ZnS+\frac{3}{2} O_2 \longrightarrow ZnO+SO_2$ also involves the oxidation of zinc sulfide (ZnS) in the presence of oxygen, which is another example of roasting. As with option (b), this process requires a significant amount of air or oxygen, which is not characteristic of calcination.

22. For the metallurgical process of which of the ores calcined ore can be reduced by carbon?

(a) Haematite

(b) Calamine

(c) Iron pyrites

(d) Sphalerite

Show Answer

Answer

$(a, b)$

In the metallurgical process, the oxide ores are reduced by carbon. Sulphide ore cannot be reduced by carbon. Here, haematite $(Fe_2 O_3)$ and calamine $(ZnO)$ are oxide ores of iron and zinc respectively while iron pyrites $(FeS_2)$ and sphalerite $(ZnS)$ are sulphide ores of iron and zinc respectively.

Therefore, haematite and calamine can be reduced by carbon.

  • Iron pyrites (FeS₂) is a sulphide ore and cannot be reduced by carbon.
  • Sphalerite (ZnS) is a sulphide ore and cannot be reduced by carbon.

23. The main reactions occurring in blast furnace during extraction of iron from haematite ore…… .

(a) $Fe_2 O_3+3 CO \longrightarrow 2 Fe+3 CO_2$

(b) $FeO+SiO_2 \longrightarrow FeSiO_3$

(c) $Fe_2 O_3+3 C \longrightarrow 2 Fe+3 CO$

(d) $CaO+SiO_2 \longrightarrow CaSiO_3$

Show Answer

Answer

$(a, d)$

In extraction of iron from haematite ore following reactions take place.

(i) $Fe_2 O_3+3 CO \longrightarrow 2 Fe+3 CO_2$

This reaction represents reduction of $Fe_2 O_3$ to $Fe$.

(ii) $\underset{\text { Formation of slag }}{CaO}+{SiO_2} \longrightarrow \underset{Slag}{CaSiO_3}$

  • Option (b) $FeO + SiO_2 \longrightarrow FeSiO_3$ is incorrect because this reaction does not occur in the blast furnace. Instead, $FeO$ is reduced to $Fe$ by carbon monoxide ($CO$) or carbon ($C$), and $SiO_2$ reacts with $CaO$ to form slag ($CaSiO_3$).

  • Option (c) $Fe_2 O_3 + 3 C \longrightarrow 2 Fe + 3 CO$ is incorrect because, in the blast furnace, carbon monoxide ($CO$) is the primary reducing agent for $Fe_2 O_3$, not carbon ($C$). The reaction involving carbon would require much higher temperatures and is not the main pathway in the blast furnace process.

24. In which of the following method of purification, metal is converted to its volatile compound which is decomposed to give pure metal?

(a) Heating with stream of carbon monoxide

(b) Heating with iodine

(c) Liquation

(d) Distillation

Show Answer

Thinking Process

This problem is based on purification of metal using vapour phase refining method.

Answer

( $a, b$ )

Vapour phase refining method includes

(i) Heating of metal with stream of $CO$

$Ni+4 CO \rightarrow Ni(CO)_4 {\xrightarrow{450-470 K}} Ni+4 CO$ (Mond’s process)

(ii) Heating with iodine

$$ Zr+2 I_2 \xrightarrow{870 K} ZrI_4 \xrightarrow[\substack{\text { Tungsten } \\ \text { filament }}]{2075 K} Zr+2 I_2 \text { (van Arkel method) } $$

  • Liquation: This method involves melting the metal and allowing it to flow away from impurities which have higher melting points. It does not involve the conversion of the metal to a volatile compound.

  • Distillation: This method involves heating the metal to its boiling point and then condensing the vapor to obtain the pure metal. It does not involve the formation of a volatile compound that decomposes to give the pure metal.

25. Which of the following statements are correct?

(a) A depressant prevents certain type of particle to come to the froth.

(b) Copper matte contains $Cu_2 S$ and $ZnS$.

(c) The solidified copper obtained from reverberatory furnace has blistered appearance due to evolution of $SO_2$ during the extraction.

(d) Zinc can be extracted by self-reduction.

Show Answer

Answer

$(a, c)$

Correct statements are

(i) A depressant prevents certain type of particle to come to the froth. e.g., $NaCN$ is added as a depressant during separation of $PbS$ and $ZnS$.

(ii) The solidified copper obtained from reverberatory furnace has blistered appearance due to evolution of $SO_2$ during the extraction.

(b) and (d) are incorrect statements, and can be correctly stated as

(iii) Copper matte contains $Cu_2 S$ and $FeS$.

(iv) Zinc can be extracted by reduction of $ZnO$ with carbon.

  • (b) Copper matte contains (Cu_2S) and (FeS), not (ZnS).
  • (d) Zinc is extracted by reduction of (ZnO) with carbon, not by self-reduction.

26. In the extraction of chlorine from brine…… .

(a) $\Delta G^{s}$ for the overall reaction is negative.

(b) $\Delta G^{s}$ for the overall reaction is positive.

(c) $E^{s}$ for the overall reaction has negative value.

(d) $E^{s}$ for the overall reaction has positive value.

Show Answer

Answer

$(b, c)$

Electrolysis of brine solution is used to extract chlorine. Overall chemical reaction occurring in this process and value of $\Delta G^{\circ}$ can be shown as

$$ 2 H_2 O(l)+2 Cl^{-}(aq) \longrightarrow H_2(g)+Cl_2(g)+2 OH^{-}(aq) $$

For the given reaction, value of $\Delta G^{\circ}$ is $422 kJ$

Using $\Delta G^{\circ}=-n F E^{\circ}$, the value of $E^{\circ}=E^{\circ}-2.2 V$.

Therefore, $\Delta G^{\circ}$ for the overall reaction is positive and $E^{\circ}$ has negative value.

  • Option (a) is incorrect because the value of $\Delta G^{\circ}$ for the overall reaction is positive, not negative. A positive $\Delta G^{\circ}$ indicates that the reaction is non-spontaneous under standard conditions.

  • Option (d) is incorrect because the value of $E^{\circ}$ for the overall reaction is negative, not positive. A negative $E^{\circ}$ indicates that the reaction is non-spontaneous under standard conditions.

Short Answer Type Questions

27. Why is an external emf of more than $2.2 V$ required for the extraction of $Cl_2$ from brine?

Show Answer

Answer

$2 Cl^{-}(aq)+2 H_2 O(l) \longrightarrow 2 OH^{-}(aq)+H_2(g)+Cl_2(g)$

For the given reaction, value of $\Delta G^{\circ}$ is $+422 kJ$.

Using $\Delta G^{\circ}=-n F E^{\circ}$, the value of $E^{\circ}=-2.2 V$.

Therefore, an external emf greater than $2.2 V$ is required for the extraction of $Cl_2$ from brine.

28. At temperature above 1073 K, coke can be used to reduce $Fe 0$ to $Fe$. How can you justify this reduction with Ellingham diagram?

Show Answer

Answer

Using Ellingham diagram, we observe that at temperature greater than $1073 K$; $\Delta G_{(C, CO)}<\Delta G_{(Fe, FeO)}$. We know that according to Ellingham diagram, compound having lower $\Delta_{t} G^{s}$ undergo its formation.

Hence, coke can reduce $FeO$ to $Fe$.

29. Wrought iron is the purest form of iron. Write a reaction used for the preparation of wrought iron from cast iron. How can the impurities of sulphur, silicon and phosphorus be removed from cast iron?

Show Answer

Answer

(a)

$\underset{\substack{\text { Haematite } \\ \text { lining }}}{Fe_2 O_3}+\underset{\begin{matrix} \text { Impurities } \\ \text { (present in cast iron) }\end{matrix} }{3 C} \longrightarrow \underset{\begin{matrix} \text { Wrought } \\ \text { iron }\end{matrix} }{2 Fe}+3 CO$

This reaction takes place in reverberatory furnace lined with haematite.

(b) The haematite oxidises $S$ to $SO_2$, $Si$ to $SiO_2$ and $P$ to $P_4 O_{10}$. Sometimes limestone is added as flux. Impurities of $S, Si$ and $P$ oxidise and pass into slag. The metal is removed and freed from slag by passing through rollers.

30. How is copper extracted from low grade copper ores?

Show Answer

Answer

Copper is extracted from low grade copper ore using hydrometallurgy method. For this purpose, ore is leached out using bacteria. The solution containing $Cu^{2+}$ is treated with scrap iron and $H_2$.

$$ Cu^{2+}(aq)+H_2(g) \longrightarrow Cu(s)+2 H^{+}(aq) $$

31. Write two basic requirements for refining of a metal by Mond’s process and by van Arkel Method.

Show Answer

Answer

Two basic requirements are

(i) Metal should form volatile compound with available reagent.

(ii) The volatile compound should be easily decomposable so that it can be recovered easily.

(a) Mond’s process includes conversion of $Ni$ to $Ni(CO)_4$ and then decomposition of $Ni(CO)_4$ to $Ni$.

$$ \begin{gathered} Ni+4 CO \longrightarrow Ni(CO)_4 \\ Ni(CO)_4 {\xrightarrow{450-470 K}} Ni+4 CO \end{gathered} $$

(b) van Arkel method includes conversion of $Zr$ to volatile $ZrI_4$ and then decomposition of $ZrI_4$ to $Zr$ and $I_2$.

$$ Zr+2 I_2 \stackrel{870 K}{\longrightarrow} ZrI_4 \xrightarrow{2075 K} Zr+2 I_2 $$

32. Although carbon and hydrogen are better reducing agents but they are not used to reduce metallic oxides at high temperatures. Why?

Show Answer

Answer

Because at high temperatures, carbon and hydrogen react with metals to form carbides and hydrides respectively. So, carbon and hydrogen are not better reducing agents to reduce metallic oxide.

33. How do we separate two sulphide ores by froth floatation method? Explain with an example.

Show Answer

Answer

Two sulphide ores can be separated by adjusting the proportion of oil and water or by using depressants, e.g., if an ore contains $ZnS$ and PbS both, a depressant $NaCN$ is added during froth floatation. It forms a complex with $ZnS$ and prevents it from coming into froth. PbS comes into the froth and thus separated.

34. The purest form of iron is prepared by oxidising impurities from cast iron in a reverberatory furnace. Which iron ore is used to line the furnace? Explain by giving reaction.

Show Answer

Answer

(a) Haematite $(Fe_2 O_3)$ ore is used to line the furnace.

$$ \underset{\substack{\text { Haematite } \\ \text { lining }}}{\text { (b) } Fe_2 O_3}+\underset{\substack{ \text{impurites}\\ \text { (present in cast iron) }}}{3 C} \rightarrow \underset{\substack{\text { Wrought } \\ \text { iron }}}{2 Fe}+3 CO $$

35. The mixture of compounds $A$ and $B$ is passed through a column of $\mathbf{A l} \mathbf O_3$ by using alcohol as eluant. Compound $A$ is eluted in preference to compound $B$. Which of the compounds $A$ or $B$, is more readily adsorbed on the column?

Show Answer

Answer

Compounds which are adsorbed to the more extent comes out later while the another one which absorbed to the less extent comes out readily. Since, compound ’ $A$ ’ comes out before compound ’ $B$ ‘, the compound ’ $B$ ’ is more readily adsorbed on the column.

36. Why is sulphide ore of copper heated in a furnace after mixing with silica?

Show Answer

Answer

Silica (flux) being an acidic flux removes the impurities of iron oxide the basic impurity, by reacting with it. Thus, iron silicate (slag) is formed.

$$ \underset{\text { Gangue }}{FeO}+\underset{\text { Flux }}{SiO_2} \longrightarrow \underset{\text { Slag }}{FeSiO_3} $$

37. Why are sulphide ores converted to oxide before reduction?

Show Answer

Answer

This is because oxides are reduced to metals easily while sulphides are not. So, sulphide ores are converted to oxide before reduction.

38. Which method is used for refining $Zr$ and Ti? Explain with equation.

Show Answer

Answer

$Zr$ and $Ti$ are refined by van Arkel method. It involves 2 steps

(a) Formation of iodide e.g., zirconium

$$ Zr+2 I_2 \longrightarrow ZrI_4 $$

(b) Decomposition of iodide

$$ ZrI_4 \xrightarrow{1800 K} \underset{\text { Pure }}{Zr}+2 I_2 $$

39. What should be the considerations during the extraction of metals by electrochemical method?

Show Answer

Answer

During extraction of metals by electrochemical method the following two points must be considered

(i) Reactivity of metals If the metals are quite reactive and are expected to react with water then the metals should be extracted by the electrolysis of their purified molten ore rather than their aqueous solution.

(ii) Suitability of electrodes The electrods selected should not react with the product of electrolysis. If at all they react, then the electrodes must be made up of a material which is quite cheap since their periodic replacement should not increase the cost of the process.

40. What is the role of flux in metallurgical processes?

Show Answer

Answer

Role of flux in metallurgical process

(i) Flux is used to remove the gangue by combining with it. Thus, slag formation takes place.

(ii) It makes the molten mass more conducting.

41. How are metals used as semiconductors refined? What is the principle of the method used like germanium, silicon etc?

Show Answer

Answer

Metals of high grade like germanium, silicon etc., used as semiconductors are refined by zone refining method.

Principle This method is based upon the principle that impurities are more soluble in molten state of metal than in solid state.

42. Write down the reactions taking place in blast furnace related to the metallurgy of iron in the temperature range 500-800 K.

Show Answer

Answer

Chemical reactions occurring in blast furnace related to the metallurgy of iron in the temperature range $500-800 K$ are

(i) $3 Fe_2 O_3+CO \longrightarrow 2 Fe_3 O_4+CO_2$

(ii) $Fe_3 O_4+4 CO \longrightarrow 3 Fe+4 CO_2$

(iii) $Fe_2 O_3+CO \longrightarrow 2 FeO+CO_2$

43. Give two requirements for vapour phase refining.

Show Answer

Answer

Two requirements for vapour phase refining are

(i) The metal should form a volatile compound with an available reagent.

(ii) The volatile compound should be easily recovered by decomposition.

44. Write the chemical reactions involved in the extraction of gold by cyanide process. Also give the role of zinc in the extraction.

Show Answer

Answer

(i) $4 Au(s)+8 CN^{-}(aq)+2 H_2 O(aq)+O_2(g) \longrightarrow 4[Au(CN)_2]^{-}(aq)+4 OH^{-}(aq)$

(ii) $2[Au(CN)_2]^{-}(aq)+Zn(s) \longrightarrow 2 Au(s)+[Zn(CN)_4]^{2-}(aq)$

Zinc acts as a reducing agent in this reaction.

Matching The Columns

45. Match the items of Column I with items of Column II and assign the correct code.

Column I Column II
A. Pendulum 1. Chrome steel
B. Malachite 2. Nickel steel
C. Calamine 3. $Na_3 AlF_6$
D. Cryolite 4. $CuCO_3 \cdot Cu(OH)_2$
5. $ZnCO_3$

Codes

A B C D
(a) 1 2 3 4
(b) 2 4 5 3
(c) 2 3 4 5
(d) 4 5 3 2
Show Answer

Answer

(b) A. $\rightarrow$ (2)

B. $\rightarrow(4)$

C. $\rightarrow(5)$

D. $\rightarrow(3)$

A. Pendulum is made up of nickel steel.

B. Molecular formula of malachite is $Cu CO_3 \cdot Cu(OH)_2$.

C. Molecular formula of calamine is $ZnCO_3$.

D. Molecular formula of cryolite is $Na_3 AlF_6$.

46. Match the items of Column I with the items of Column II and assign the correct code.

Column I Column II
A. Coloured bands 1. Zone refining
B. Impure metal to volatile complex 2. Fractional distillation
C. Purification of Ge and Si 3. Mond’s process
D. Purification of mercury 4. Chromatography
5. Liquation

Codes

A B C D
(a) 1 2 4 5
(b) 4 3 1 2
(c) 3 4 2 1
(d) 5 4 3 2
Show Answer

Answer

(b) A. $\rightarrow$ (4)

B. $\rightarrow$ (3)

C. $\rightarrow(1)$

D. $\rightarrow(2)$

A. Coloured bands are observed in chromatography.

B. Impure metal is converted to volatile complex by using Mond’s process.

C. Purification of $Ge$ ans $Si$ are purified by zone refining method.

D. Purification of mercury is done by fractional distillation.

47. Match items of Column I with the items of Column II and assign the correct code.

Column I Column II
A. Cyanide process 1. Ultrapure Ge
B. Froth floatation process 2. Dressing of ZnS
C. Electrolytic reduction 3. Extraction of Al
D. Zone refining 4. Extraction of Au
5. Purification of $Ni$

Codes

A B C D
(a) 4 2 3 1
(b) 2 3 1 5
(c) 1 2 3 4
(d) 3 4 5 1
Show Answer

Answer

(a) A. $\rightarrow$ (4) $\quad$

B. $\rightarrow$ (2) $\quad$

C. $\rightarrow$ (3) $\quad$

D. $\rightarrow$ (1)

A. Cyanide process is used for extraction of Au through formation of anionic complex $.[AuCN)_2]^{-}$.

B. Froth floatation process is used for dressing of ZnS.

C. Electrolytic reduction method is used for extraction of aluminium. Graphite electrode is used for this purpose.

D. Zone-refining is used for purification of Ge.

48. Match the items of Column I with the items of Column II and assign the correct code.

Column I Column II
A. Sapphire 1. $Al_2 O_3$
B. Sphalerite 2. $NaCN$
C. Depressant 3. $Co$
D. Corundum 4. $ZnS$
5. $Fe_2 O_3$

Codes

A B C D
(a) 3 4 2 1
(b) 5 4 3 2
(c) 2 3 4 5
(d) 1 2 3 4
Show Answer

Answer

(a) A. $\rightarrow$ (3)

B. $\rightarrow(4)$

C. $\rightarrow(2)$

D. $\rightarrow(1)$

A. Sapphire is a gemstone which contain Co.

B. Molecular formula of sphalerite is $ZnS$. C. $NaCN$ is used as a depressant in froth floatation method.

D. Molecular formula of corundum is $Al_2 O_3$.

49. Match the items of Column I with items of Column II and assign the correct code.

Column I Column II
A. Blisterred $Cu$ 1. Aluminium
B. Blast furnace 2. $2 Cu_2 O+Cu_2 S \longrightarrow 6 Cu+SO_2$
C. Reverberatory furnace 3. Iron
D. Hall-Heroult process 4. $FeO+SiO_2 \longrightarrow FeSiO_3$
5. $2 Cu_2 S+3 O_2 \longrightarrow 2 Cu_2 O+2 SO_2$

Codes

A B C D
(a) 2 3 4 1
(b) 1 2 3 5
(c) 5 4 3 2
(d) 4 5 3 2
Show Answer

Answer

(a) A. $\rightarrow$ (2)

B. $\rightarrow(3)$

C. $\rightarrow(4)$

D. $\rightarrow(1)$

A. Blisterred $Cu$ can be prepared by means of following chemical reaction

$$ 2 Cu_2 O+Cu_2 S \longrightarrow 6 Cu+SO_2 $$

B. Iron is extracted by using blast furnace.

C. In reverberatory furnace formation of slag occurs as

$$ FeO+SiO_2 \longrightarrow \underset{Slag}{FeSiO_3} $$

D. Hall-Heroult process is used for extraction of aluminium.

Assertion and Reason

In the following questions a statement of Assertion (A) followed by a statement of Reason ( $R$ ) is given. Choose the correct answer out of the following choices.

(a) Both assertion and reason are true and reason is the correct explanation of assertion.

(b) Both assertion and reason are true but reason is not the correct explanation of assertion.

(c) Assertion is true but reason is false.

(d) Assertion is false but reason is true.

(e) Assertion and reason both are wrong.

50. Assertion (A) Nickel can be purified by Mond’s process. Reason (R) $Ni(CO)_4$ is a volatile compound which decomposes at $460 K$ to give pure $Ni$.

Show Answer

Answer

(a) Both assertion and reason are true and reason is the correct explanation of assertion. Nickel can be purified by Mond’s process in which formation of a volatile compound $Ni(CO)_4$ takes place which further decomposes to $Ni$ at $460 K$.

51. Assertion (A) Zirconium can be purified by van Arkel method.

Reason (R) $ZrI_4$ is volatile and decomposes at $1800 K$.

Show Answer

Answer

(a) Both assertion and reason are true and reason is the correct explanation of assertion. Zirconium can be purified by van Arkel method which include formation of volatile $ZrI_4$ which decomposes at $1800 K$ to $Zr$.

52. Assertion (A) Sulphide ores are concentrated by froth flotation method. Reason (R) Cresols stabilise the froth in froth floatation method.

Show Answer

Answer

(b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Sulphide ores are concentrated by froth floatation method. Sulphide ore particles are preferentially wetted by oil, become lighter and rise to the surface along with the froth while gangue particles are preferentially wetted by water, become heavier and thus settle down at the bottom of the tank and cresols stabilise the froth in froth floatation method.

Formation of froth is main reason for extraction of metal. Metal ore comes out along with froth.

53. Assertion (A) Zone refining method is very useful for producing semiconductors.

Reason (R) Semiconductors are of high purity.

Show Answer

Answer

(b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Zone refining method is very useful for producing semiconductors of high purity as in this process pure metal crystallises while impurities pass on into adjacent molten zone when impure metal rod is heated.

54. Assertion (A) Hydrometallurgy involves dissolving the ore in a suitable reagent followed by precipitation by a more electropositive metal.

Reason (R) Copper is extracted by hydrometallurgy.

Show Answer

Answer

(b) Assertion and reason both are correct but reason is not the correct explanation of assertion.

Hydrometallurgy involves dissolving the ore in suitable reagent followed by precipitation with the help of more electropositive metal in which pure metal is get replaced by more electropositive metal.

Long Answer Type Questions

55. Explain the following

(a) $CO_2$ is a better reducing agent below $710 K$ whereas $CO$ is a better reducing agent above $710 K$.

(b) Generally sulphide ores are converted into oxides before reduction.

(c) Silica is added to the sulphide ore of copper in the reverberatory furnace.

(d) Carbon and hydrogen are not used as reducing agents at high temperatures.

(e) Vapour phase refining method is used for the purification of Ti.

Show Answer

Thinking Process

This problem is based on concept of Ellingham diagrams and purification of metal.

Answer

(a) As shown in Ellingham diagram which relates Gibbs free energy and temperature at below $710 K$.

$\Delta G_{(C, CO_2)}<\Delta G_{(C, CO)}$ So, $CO_2$ is a better reducing agent than $CO$ while above $710 K$ it becomes a very good reducing agent.

(b) Generally, sulphide ores are converted to oxides before reduction as reduction of oxides can easily be done using $C$ or $CO$ depending upon metal ore and temperature.

(c) Silica is a flux added to the sulphide ore of copper in the reverberatory furnace leading to the formation of slag

$$ FeO+SiO_2 \rightarrow \underset{Slag}{FeSiO_3} $$

(d) Carbon and hydrogen are not used as reducing agents at high temperature. At high temperature carbon and hydrogen readily form their carbides and hydrides respectively.

(e) Vapour phase refining method is used for the purification of $Ti$ as

$$ Ti+2 I_2 {\xrightarrow{523 K}} TiI_4 {\xrightarrow{1700 K}} Ti+2 I_2 $$



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