(i) The extraction of metals from their oxides can be carried out by using different reducing agents.
(ii) Consider the following reaction
\(\frac{2}{\mathrm{y}} \mathrm{M}_{\mathrm{x}} \mathrm{O}_{\mathrm{y}(\mathrm{s})} \rightarrow \frac{2 \mathrm{x}}{\mathrm{y}} \mathrm{M}_{(s)}+\mathrm{O}_{ 2(\mathrm{~g})}\)            (1)
(iii) The above reduction may be carried out with carbon. In this case the reducing agent carbon may be oxidized to either CO or CO₂
\(\mathrm{C}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2(\mathrm{~g})}  \)                  (2)
\(2 \mathrm{C}+\mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{(\mathrm{g})} \)                (3)
(iv) If CO is used as a reducing agent
\(2 \mathrm{CO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{2(\mathrm{~g})}\)           (4)
(v) A suitable reducing agent is selected based on the thermodynamics considerations.
(vi) We know that for a spontaneous reaction, the change in free energy (\(\triangle\)G) should be negative.
(vii) Therefore, thermodynamically, the reduction of metal oxide with a given reducing agent can occur if the free energy change for the coupled reaction is negative.
(viii) Hence, the reducing agent is selected in such a way that it provides a large negative \(\triangle\)G value for the coupled reaction.

In this method, the metal is treated with a suitable reagent which can form a volatile compound with the metal.
Then the volatile compound is decomposed to give the pure metal.

1. This process is called as Hall-Heroult process.
Cathode: In this method, electrolysis is carried out in an iron tank lined with carbon which acts as the cathode.
Anode: The carbon blocks immersed in the electrolyte acts as a anode.
Eletrolyte: A 20% solution of alumina, obtained from the bauxite ore is mixed with molten Cryolite and is taken in the electrolysis chamber.
2. About 10% calcium chloride is also added to the solution.
3. Here Calcium chloride helps to lower the melting point of the mixture.
Temperature: The fused mixture is maintained at a temperature of above 1270 K.
4. The chemical reactions involved in this process as follows
(a) Ionisaiton of alumina: \({ A }l_{ 2 }{ O }_{ 3 }\longrightarrow { 2Al }^{ 3+ }+{ 3O }^{ 2- }\)
(b) Reaction at cathode: \(2{ Al }^{ 3+ }_{(melt)}+{ 6e }^{ - }\longrightarrow { Al }_{ (l) }\)
(c) Reaction at anode: \(6{ O }^{2-}_{(melt)}\longrightarrow { 3O }_{ 2 }+{ 12e }^{ - }\)
5. Since carbon acts as anode the following reaction also takes place
(a) \({ C }_{ (s) }+{ O }^{ 2- }_{(melt)}\longrightarrow CO+{ 2e }^{ - }\)
(b) \({ C }_{ (s) }+{ 2O }^{ 2- }_{(melt)}\longrightarrow { CO }_{ 2 }+{ 4e }^{ - }\)
6. Due to the above two reactions, anodes are slowly consumed during the electrolysis.
7. The pure aluminium is formed at the cathode. The net electrolysis reaction can be written as
\({ 4Al }^{ 3+ }_{(melt)}+{ 6O }^{ 2- }_{(melt)}+{ 3C }_{ (s) }\longrightarrow { 4Al }_{ (l) }+{ 3CO }_{ 2(g) }\)

Ellingham diagram for the formation of ZnO and CO₂ intersects around 1200 K. Above this temperature ZnO lies above Carbon which indicates Carbon is the better reducing agent CO is more effective reducing agent only below 983 K.

In this extraction, a basic flux, quick lime (CaO) is used, since the silica gangue present in the ore is acidic in nature. The quick lime combines with it to form calcium silicate (slag).
CaO_(s) + Sio_2(s) ⟶ CaSio_3(s)
Flux       Gangue    Slag

(i) Concentration of the ore
(ii) Extraction of crude metal
(iii) Refining of crude metal

1. Reduction of oxides of active metals such as sodium, potassium etc., by carbon is thermodynamically not feasible.
2. Such metals are extracted from their ores by using electrochemical methods.
3. In this technique, the metal salts are taken in a fused form or in solution form.
4. The metal ion present can be reduced by some suitable reducing agent or by electrolysis.
5. Gibbs free energy for the electrolysis process is given by
Here,
 \(\Delta\)G^o = -nFE^o
 \(\Delta\)G^o - Standard Gibb's free energy change
n - number of electrons involved,
F - Faraday,
E^o - Standard electrode potential
6. If E^o is positive then the \(\Delta\)G^o is negative and the reduction is spontaneous
7. Hence a redox reaction is planned in such a way that the e.m.f of the net redox reaction is positive.
8. When a more reactive metal is added to less reactive metal salt solution, the more reactive metal will go into the solution.
Example:
\({ Cu }_{ (s) }+2{ Ag }^{ + }_{ (aq) }\longrightarrow { Cu }^{ 2+ }_{ (aq) }+2{ Ag }_{ (s) }\)
\({ Cu }^{ 2+ }_{ (aq) }+{ Zn }_{ (s) }\longrightarrow { Cu }_{ \left( s \right) } +{ Zn }^{ 2+ }_{ (aq) }\)

(i) Gangue: The ores are associated with nonmetallic impurities, rocky materials and siliceous matter which are collectively known as gangue.
Eg: SiO₂ is the gangue present in the iron ore (Fe₂O₃)
(ii) Slag: In the smelting process, a flux combines with Silica gangue forming slag.
CaO_(s) + SiO_2(s) → CaSiO_3(s)
Flux     + gangue → Slag

(i) Ellingham diagram is constructed based only on thermodynamic considerations. It gives information about the thermodynamic feasibility of a reaction. It does not tell anything about the rate of the reaction. More over, it does not give any idea about the possibility of other reactions that might be taking place.
(ii) The interpretation of \(\triangle\)G is based on the assumption that the reactants are in equilibrium with the product which is not always true.

a) i) Ellingham diagram for the formation of Al₂O₃ and MgO intersects around 1500^oC. Above this temp Mg lies above the Aluminium line. Hence only above 1500^oC Aluminium might be excepted to reduce magnesia.
ii) Ellingham diagram for the formation of MgO lies below the formation of Al₂O₃. Hence MgO is more stable than Al₂O₃. Hence Magnesium could reduce Alumina.
1. Below 983K, formation of CO line lies below many of the metal oxide formation in Ellingham diagram, hence CO is more effective reducing agent than Carbon.
2. But above this temperature Carbon lies below other metal oxides.
b) Around 1200K Carbon lies below the formation of Fe₂O₃. Hence it is possible to reduce Fe₂O₃ by coke at 1200K.

Zone refining :
1. Zone refining method is based on the principles of fractional crystallisation.
2. When an impure metal is melted and allowed to solidify, the impurities will prefer to be in the molten region. In this process the impure metal is taken in the form of a rod.
3. One end of the rod is heated using a mobile induction heater which results in melting of the metal on that portion of the rod.
4. When the heater is slowly moved to the other end the pure metal crystallises while the impurities will move on to the adjacent molten zone.
5. As the heater moves further away, the molten zone containing impurities also moves along with it.
6. The process is repeated several times by moving the heater in the same direction again and again to get pure metal.
7. This process is carried out in an inert gas atmosphere to prevent the oxidation of metals.
8. Elements such as germanium (Ge), silicon (Si) and galium (Ga) that are used as semiconductor are refined using this process.