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(i) Name the most common ore of the metal aluminium from which the metal is extracted. Write the chemical formula of the ore.
(ii) Name the process by which impure ore of aluminium gets purified by using concentrated solution of an alkali.
(iii) Write the equation for the formation of aluminium at cathode during the electrolysis of alumina.

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Last updated date: 22nd Mar 2024
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MVSAT 2024
Answer
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Hint: It is an oxide of aluminium. Aluminium is extracted by electrolysis of this ore, dissolved in molten Cryolite or in $A{{l}_{2}}{{O}_{2}}.{{H}_{2}}ON{{a}_{2}}Al{{F}_{6}}$ .
It is a method of Concentration of Ores, in which gangue particles are removed.
In electrolysis different reactions occur at the respective electrodes, try answering the question using this fact.

Complete step by step solution:
- The minerals from which metals can be extracted conveniently and profitably are called ores.
- We know that Aluminium (Al) is the most abundant metal in the earth crust (8.3% by mass).

(i)The most common ore of the metal aluminium from which the metal is extracted is Bauxite ore chemical formula is $A{{l}_{2}}{{O}_{2}}.{{H}_{2}}O$.

(ii) Here we will see the process by which impure ore of aluminium gets purified by using concentrated solution of an alkali is known as Baeyer’s Process.
-Now we know that Baeyer’s process is leaching of alumina from Bauxite ore. Let’s see the impurities involved in bauxite are : silica ,iron oxides ,titanium oxides .
-In this Alumina dissolved into 45 - 50% NaOH solution at 473 – 523 K and 35 -36 bar pressure. Chemical equations involved are:

     \[A{{l}_{2}}{{O}_{3}}(s)+2NaOH(aq)+3{{H}_{2}}O(l)\to 2Na\left[ Al{{\left( OH \right)}_{4}} \right]\left( aq \right)\]

Here we see that for proper formation of ppt, the solution is seeded with freshly prepared sample of hydrated $A{{l}_{2}}{{O}_{3}}$

     \[2Na\left[ Al{{\left( OH \right)}_{4}} \right]\left( aq \right)+C{{O}_{2}}\left( g \right)\to A{{l}_{2}}{{O}_{3}}.x{{H}_{2}}O\left( s \right)+2NaHC{{O}_{3}}\left( aq \right)\]

In this step sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated up to 1470K to give back pure alumina

     \[A{{l}_{2}}{{O}_{3}}.x{{H}_{2}}O\left( s \right)\underrightarrow{\Delta }A{{l}_{2}}{{O}_{3}}\left( s \right)+x{{H}_{2}}O\left( g \right)\]

(iii) Here we can see how Al is extracted. The pure alumina is mixed with cryolite along with calcium fluoride and aluminium fluoride to lower the melting point of the mix and increase the conductivity.

- Steel tanks are lined with carbon that acts as cathode and graphite anodes are used in the electrolysis process. Molten alumina decomposes and the following reaction takes place.

     \[A{{l}_{2}}{{O}_{3}}\left( molten \right)\to 2A{{l}^{3+}}+3{{O}^{2-}}\]

At cathode:

     \[A{{l}^{3+}}\left( l \right)+3{{e}^{-}}\to Al\left( l \right)\]

At anode:

\[\begin{align}
  & C\left( s \right)+{{O}^{2-}}\left( l \right)\to CO\left( g \right)+2{{e}^{-}} \\
 & C\left( s \right)+2{{O}^{2-}}\left( l \right)\to C{{O}_{2}}+4{{e}^{-}} \\
\end{align}\]

This process is called the Hall – Heroult process.
Therefore, we can conclude that Bauxite ore $A{{l}_{2}}{{O}_{2}}.{{H}_{2}}O$ is the most common ore of
Aluminium and by Baeyer’s Process aluminium gets purified by using concentrated solution of an alkali. The reaction at cathode during the electrolysis of alumina is \[A{{l}^{3+}}\left( l \right)+3{{e}^{-}}\to Al\left( l \right)\].

Note: - While writing equations keep in mind that at cathode Reduction takes place and at anode Oxidation, you can easily remember it as LOAN – left oxidation anode negative.
- There are many ores from which we can extract aluminium but, in the question, we have asked about the most common ore.
-Cathode and anode are separate electrodes, also at anode oxidation occurs while reduction occurs at cathode. Keep in mind this fact to avoid mistakes in writing Cathode reactions.

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