Tag: production of metals

Questions Related to production of metals

Identity the principle behind the concentration of bauxite ore.

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. Magnetic property of impurities in white bauxite

  2. Low specific gravity of $Al _{2}O _{3}$

  3. Amphoteric nature of $Al _{2}O _{3}$

  4. High melting point of $Al _{2}O _{3}$

Show answer
Correct Option: C
Explanation:

$Al _2O _3 + 2NaOH + 3H _2O \rightarrow 2Na[Al(OH) _4]$
$Al _2O _3 + 6HCl \rightarrow 2AlCl _3 + 3H _2O$

Hence, in both the ways, Al can be concentrated, which owes to its amphoteric nature. 

Use the relationship $\Delta \,G^{\circ}\,=\,- nFE^{\circ} _{cell}$ to estimate the minimum voltage required to electrolyse $Al _2O _3$ in the Hall-Heroult process.

$\Delta G^{\circ} _{f}(Al _2O _3)\,=\,-1520\,kJ\,mol^{-1}$

$\Delta G^{\circ} _{f}(CO _2)\,=\,-394\,kJ\,mol^{-1}$

The oxidation of the graphite anode to $CO _2$ permits the electrolysis to occur at a lower voltage than if the electrolysis reactions were :

$Al _2O _3\,\rightarrow\,2Al\,+\,3O _2$.

What is the approximate value of low voltage?

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. 2 V

  2. 3 V

  3. 4V

  4. 5V

Show answer
Correct Option: A
Explanation:

Net reaction in Hall-Heroult process is

$3C\,+\,2Al _2O _3\,\rightarrow\,4Al\,+\,3CO _2$

Or $4Al^{3+}\,+\,12e^{-}\,\rightarrow \,4Al,$

Number of electrons (n) = 12

$\Delta\,G^{\circ }\,=\,3 \Delta\, G^{\circ } _f (CO _2)\,-\,2 \Delta\,G^{\circ } _f(Al _2O _3)$

$=-3\,\times\,394\,-\,2(-1520)$

$= 1858\ kJ$


$\Delta G^{\circ}\,=\, -nFE^{\circ} _{cell}$

$-E^{\circ} _{cell}\,=\, \displaystyle \frac {\Delta G^{\circ}}{nF}\, =\, \displaystyle\frac {1858\,\times\, 1000}{12\, \times\, 96500}$

$= 1.60\ V$

Thus, Hall-Heroult process takes place at lower voltage.

In electrtolysis of $Al _2O _3$ by Hall-Heroult process:

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. Cryolite $Na _3[AIF _6]$ lowers the melting point of $Al _2O _3$ and increases its electrical conductivity.

  2. $Al$ is obtained at cathode and probably $CO _2$ at anode

  3. both (a) and (b) are correct

  4. none of the above is correct

Show answer
Correct Option: C
Explanation:

The electrolysis of alumina by Hall and Heroult's process is carried by using a fused mixture of alumina and cryolite  along with minor quantities of aluminum fluoride and fluorspar. The addition of cryolite and fluorspar increases the electrical conductivity of alumina and  lowers the fusion temperature. Thus, the option A is correct. Also at anode, alumina reacts with fluorine to give oxygen. The liberated oxygen reacts with carbon to form $CO$ and $CO _2$. These gases are liberated at anode.

Which one of the following statements is incorrect?

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. In the aluminothermite process, aluminium acts as reducing agent.

  2. Lead is extracted from its chief ore by both carbon reduction and self-reduction.

  3. Zinc is extracted from its chief ore by carbon reduction.

  4. Extraction of gold involves the leaching of ore with cyanide solution followed by reduction with zinc.

  5. In Hall-Heroult process, the electrolyte used is a molten mixture of alumina, sodium hydroxide and cryolite.

Show answer
Correct Option: E
Explanation:
(1) $Cr _{2}O _{3}+2Al\rightarrow Al _2O _3+2Cr^{3+}$
(2) $2PbS+2PbO\rightarrow 2PbO+2SO _{2}$
$PbS+2PbO\rightarrow 3Pb+SO _{2}$ (self reduction)
$PbO+C\rightarrow Pb+CO$ (carbon reduction)
(3) $ZnO+C\rightarrow Zn+CO$ (carbon reduction)
(4) $4Au(S)+8CN(aq)+2H _{2}O(aq)+O _{2}(g)\rightarrow 4[Au(CN) _{2}]^{-}(aq)+4OH^{-}(aq)$ - leaching
$2[Au(CN] _{2}]^{-}(aq)+Zn(S)\rightarrow 2Au(S)+[Zn(CN) _{4}]^{2-}(aq)$ - reduction
(5) It is molten mixture of alumina and cryolite

Which of the following is not a method of refining of metals?

chemistry production of metals refining of metals basics of metallurgy basic metallurgical processes

  1. Electrolysis

  2. Smelting

  3. Poling

  4. Liquation

Show answer
Correct Option: B
Explanation:

Smelting is process in which metal is extracted from its ore by heating and melting. It is not a method of refining metal. Methods of refining metals are electrolysis poling, liquation, zone refining, vapour phase refining, distillation.

Which one of the following is the true in electrolytic refining?

chemistry production of metals refining of metals basics of metallurgy basic metallurgical processes

  1. Impure metal is made cathode.

  2. Impure metals is made anode.

  3. Impure metals is made cathode and pure metal as anode.

  4. Both electrodes must be of pure metal.

Show answer
Correct Option: B
Explanation:
  • In Electrolytic refining, the impure metal is made to act as an anode. A strip of the same metal in pure form is used as a cathode. 
  • They are put in a suitable electrolytic bath containing a soluble salt of the same metal. 
  • The more basic metal remains in the solution and the less basic ones go to the anode mud.
  • Zinc, copper is refined by this method.

Four metals and their methods of refinement are given
(i) $Ni,\ Cu,\ Zr,\  Ga$
(ii) Electrolysis, van Arkel process, zone refining, Mond's process
Choose the right method for each:

chemistry production of metals refining of metals basics of metallurgy basic metallurgical processes

  1. $Ni:$ Electrolysis, $Cu:$ van Arkel process, $Zr:$ Zone refining, $Ga:$ Mond's process

  2. $Ni:$ Mond's process, $Cu:$ Electrolysis, $Zn:$ van Arkel process, $Ga:$ Zone refining

  3. $Ni:$ Mond's process, $Cu:$ van Arkel process, $Zr:$ Zone refining, $Ga:$ Electrolysis

  4. $Ni:$ Electrolysis, $Cu:$ Zone refining, $Zr:$ van Arkel process, $Ga:$ Mond's process

Show answer
Correct Option: B
Explanation:

$Ni$ is refined by Mond's process.

$Cu$ is refined by Electrolysis.
$Zn$ is refined by Van Arkel process.
$Ga$ is refined by Zone refining.

Mark the correct statements:

(i) mercury can be refined by the process of distillation.

(ii) In poling, the molten impure metal is stirred with green poles of wood.

(iii) In electrolyte refining of metals, impure metal is made as cathode and a thin strip of pure metal is made as anode.

chemistry production of metals refining of metals basics of metallurgy basic metallurgical processes

  1. (i) and (ii)

  2. (i) and (iii)

  3. (ii) and (iii)

  4. (i), (ii) and (iii)

Show answer
Correct Option: A
Explanation:

Low boiling point metals like Mercury are purified with distillation.

Hence,  statement (i) is correct.
 
The heat of  copper makes the pole emit wood gas that reduces the cuprous oxide to copper.
In poling, the molten impure metal is stirred with green poles of wood.
Hence,option (ii) is correct.

In electrolyte refining of metals, impure metal is made as Anode  and a thin strip of pure metal is made as cathode.
Hence, option (iii) is incorrect.

When stones are separated from rice, we throw away the solid component.

chemistry production of metals refining of metals basics of metallurgy basic metallurgical processes

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

When stones are separated from rice, we throw away the solid component. Stones are non useful whereas rice are useful. So we through away non useful solid and then we can have rice for cooking.

During extraction of metals, electrolytic refining is used to obtain pure metals. Material which will be used as anode, for refining of silver metal by this process, is:

chemistry production of metals refining of metals basics of metallurgy basic metallurgical processes

  1. zinc

  2. impure silver

  3. pure silver

  4. iron

Show answer
Correct Option: B
Explanation:

Electrolysis is the dissolving of metal from one electrode of an electrolytic cell to another electrode in pure form. It gives the highest purity metal product. The impure silver is cast into a slab which is the anode, while pure silver sheet metal is the cathode.