Chemistry Electrochemical and Oxidation-Reduction Principles of Metallurgy
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Topics Covered :

● Electrochemical Principles of Metallurgy
● Extraction of Aluminium by Electrochemical Method
● Extraction of Copper by Electrochemical Method
● Oxidation Reduction

Electrochemical Principles of Metallurgy :

`=>` Principles of thermodyamics are applied to pyrometallurgy. Similar principles are effective in the reductions of metal ions in solution or molten state.

`=>` Here they are reduced by electrolysis or by adding some reducing element.

`=>` In the reduction of a molten metal salt, electrolysis is done. Such methods are based on electrochemical principles which could be understood through the equation,

`DeltaG^(⊖) = - n E^(⊖) F` ..........(42)

here `n` is the number of electrons and `E^(⊖)` is the electrode potential of the redox couple formed in the system.

`=>` More reactive metals have large negative values of the electrode potential. So, their reduction is difficult.

`=>` If the difference of two `E^(⊖)` values corresponds to a positive `E^(⊖)` and consequently negative `ΔG^(⊖)` in equation 42, then the less reactive metal will come out of the solution and the more reactive metal will go to the solution, e.g.,

`Cu^(2+) (aq) + Fe (s) → Cu (s) +Fe^(2+) (aq)` .........(43)

`=>` In simple electrolysis, the `M^(n+)` ions are discharged at negative electrodes (cathodes) and deposited there.

`=>` Precautions are taken considering the reactivity of the metal produced and suitable materials are used as electrodes.

`=>` Sometimes a flux is added for making the molten mass more conducting.

`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
The process of extraction of metals by electrolysis of their fused salts is called electrometallurgy. Here electrons serve as reducing agent.
So next we will be discussing about applications of electrolysis to metallurgy.

Extraction of Aluminium from alumina :

`=>` In the metallurgy of aluminium, purified `Al_2O_3` is mixed with `Na_3AlF_6` or `CaF_2` which lowers the melting point of the mix and brings conductivity.

`=>` The fused matrix is electrolysed.

● Cathode : Steel
● Anode : Graphite

`=>` The graphite anode is useful here for reduction to the metal.

`=>` The overall reaction may be taken as :

`2Al_2O_3 +3C → 4Al +3CO_2` .........(44)

`=>` This process of electrolysis is widely known as `text(Hall-Heroult)` process.

`=>` The electrolysis of the molten mass is carried out in an electrolytic cell using carbon electrodes.

`=>` The oxygen liberated at anode reacts with the carbon of anode producing `CO` and `CO_2`. This way for each kg of aluminium produced, about 0.5 kg of carbon anode is burnt away.

`=>` The electrolytic reactions are :

Cathode : `Al^(3+)` (melt) `+3 e^(-) → Al (l)` .......(45)

Anode : `C(s) + O^(2-) ` (melt) `→ CO (g) + 2e^(-)` ......(46)

`C(s) +2O^(2-) ` (melt) `→ CO_2 (g) +4e^(-)` .........(47)

Copper from Low Grade Ores and Scraps :

`=>`Copper is extracted by `text(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` (equations 42; 48).

`Cu^(2+) (aq) +H_2(g) → Cu (s) +2H^(+) (aq)` ...........(48)

Oxidation Reduction :

`=>` Some extractions are based on oxidation particularly for non-metals.

`=>` A very common example of extraction based on oxidation is the extraction of chlorine from brine (chlorine is abundant in sea water as common salt).

`2Cl^(-) (aq) +2H_2O (l) → 2OH^(-) (aq) +H_2 (g) +Cl_(2) (g)` ........(49)

`=>` The `ΔG^(⊖)` for this reaction is + 422 kJ. When it is converted to `E^(⊖)` (using `ΔG^(⊖) = – nE^(⊖)F`), we get `E^(⊖) = – 2.2 V`.

`=>` Naturally, it will require an external e.m.f. that is greater than `2.2 V`.

`=>` But the electrolysis requires an excess potential to overcome some other hindering reactions.

`=>` Therefore, `Cl_2` is obtained by electrolysis giving out `H_2` and aqueous `NaOH` as byproducts.

`=>` Electrolysis of molten `NaCl` is also carried out. But in that case, `Na` metal is produced and not `NaOH`.

`=>` Extraction of gold and silver involves leaching the metal with `CN^–`. This is also an oxidation reaction (`Ag → Ag^+ or Au → Au^+`). The metal is later recovered by displacement method.

`4Au (s) +8CN^- (aq) +2H_2O (aq) + O_2 (g) → 4 [ Au (CN)_2]^(-) (aq) +4OH^(-) (aq) ` ........(50)

`2[Au(CN)_2]^(-) (aq) +Zn(s) → 2Au (s) +[Zn(CN)_4]^(2-) (aq)` .............(51)

● In this reaction zinc acts as a reducing agent.