Tag: chemistry

Questions Related to chemistry

Which of the following statements is correct?
(i) Copper is extracted by hydrometallurgy from low-grade ores.
(ii) In the electrolytic reduction of alumina, graphite is acting as an anode and steel vessel with the lining of carbon acts as an anode.
(iii) In Hall-Heroult process, aluminium is obtained at anode and CO and $CO _2$ are produced at the cathode.

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

  1. (i) only

  2. (i) and (ii) only

  3. (ii) and (iii) only

  4. All of these

Show answer
Correct Option: B
Explanation:
$(i)$ Copper is extracted by hydrometallurgy from low-grade ores. It is leached out using acid or bacteria. The solution containing $Cu^{2+}$ is treated with scrap iron or $H _2$.
$Cu^{2+} (aq)+H _2 (g)\longrightarrow Cu (s)+2H^+ (aq)$
$(ii)$ In the electrolytic reduction of alumina. The graphite act as anode and steel with a lining of carbon acts as an anode.
$(iii)$ The process of extracting Aluminium from Alumina $(Al _2O _3)$ by electrolysis is called Hall-Heroult process. In this process Aluminium is produced at cathode which is obtained by outlet, oxygen is released at an anode. It reacts with carbon of anode producing $CO$ and $CO _2$.

Cryolite and fluorspar are mixed with $Al _2O _3$ during electrolysis for extraction of aluminium to:

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

  1. increase the mass of the reaction mixture

  2. get other products at anode like fluorine

  3. lower the melting point and increase the conductivity of the electrolyte

  4. reduce aluminium oxide by cryolite

Show answer
Correct Option: C
Explanation:

In the metallurgy of Aluminium, purified $Al _2O _3$ is mixed with $Na _3AlF _6$ or $CaF _2$ which lowers the melting point of mix and brings conductivity.

In the metallurgy of aluminium:

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  1. $Al^{3+} $ is oxdised to $Al _{(s)}$

  2. graphite anode is oxidised to carbon monoxide and carbon dioxide

  3. oxidation state of oxygen changes in the reaction at anode

  4. oxidation state of oxygen changes in the overall reaction involved in the process

Show answer
Correct Option: B
Explanation:
The oxidation state of $'O'$ does not change i neither the reaction occurring at the anode or the overall reaction as shown below.
At anode:-$C+{ O }^{ 2- }\longrightarrow CO+2{ e }^{ - };C+2{ O }^{ 2- }\longrightarrow C{ O } _{ 2 }+4{ e }^{ - }$
The released electrons used at cathode and here Graphite  is oxidized to $'CO'$ and $C{ O } _{ 2 }$

Purification of aluminium by electrolytic refining is called:

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

  1. Serpeck's process

  2. Hoope's process

  3. Hall's process

  4. Baeyer's process

Show answer
Correct Option: B
Explanation:

Purification of aluminium by electrolytic refining is called Hoope's process. Serpeck, Hall's and Baeyer process are chemical process.

In the extraction of aluminium,
Process X: applied for red bauxite to remove iron oxide (chief impurity)
Process Y: applied for white bauxite to remove Z (chief impurity). 

Then, Process X and impurity Z are:

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

  1. X $=$ Hall - Heroults process & Y$=$ $SiO _{2}$

  2. X $=$ Bayers process and Z $=$ $SiO _{2}$

  3. X $=$ Serpecks process and Y $=$ iron oxide

  4. X $=$ Baeyers process and Y $=$ iron oxide

Show answer
Correct Option: B
Explanation:

Bayer's process is used for beneficiation of red bauxite.
Serpeck's process is used for beneficiation of white bauxite. 
Reaction to remove impurity from white bauxite is:
$SiO _2+2C\rightarrow Si+2CO$ .

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

The following question is relevant to the extraction of Aluminium:
Along with cryolite and alumina, another substance is added to the electrolyte mixture. the substance is ___________.

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  1. Fluorspar

  2. Mica

  3. NaCl

  4. None of these

Show answer
Correct Option: A
Explanation:

Fluorspar is the substance act as a solvent and it increases conductivity. (Pure alumina melts at about $2000^oC$ and is a bad conductor of electricity. If fused cryolite and fluorspar is added, the mixture melts at $900^oC$ and  $Al _2O _3$ becomes a good conductor of electricity.)