Thermodynamics of Metallurgy

Thermodynamics of Metallurgy

Thermodynamics of Metallurgy

The method employed for extracting a metal from its ore depends on the nature of the metals and that of the ore may be related to the position of the metal in the electrochemical series. In general, metals with E < - 1.5 volt yield compounds which are very difficult to reduce and electricity is usually used for the isolation of such metals. On the other hand, noble metals with E > 0.5 volt form easily reducible compounds. A metal higher up in the electrochemical series should be more difficult to reduce to metallic form. As we move down in the electrochemical series, the reduction becomes more and more easy. E of metals provides some idea regarding the selection of an appropriate method for extracting the metal from its compounds.

Free energy is related to standard cell potential.

G = -nF Ecell

 No. of moles electrons

 Faraday constant 96500 C.

In order that the reduction of oxide, halide or sulphite ore by an element may be place simultaneously at a given temperature and pressure, there is decrease in the free energy of the system (-
G).

The more the negative value of G, the higher is the reducing power of an element. The free energy change (G) is related to the heat change (H) as well as to the product of temperature. [G = H - TS]

For a reaction, 2M (s) = O2 (g)  2M O (s)

G becomes smaller with the increase in temperature. This is because the gaseous reactant oxygen is consumed in the reaction leading to the decrease in randomness or entropy of the system hence G becomes negative. With further increase in temperature, T
S becomes more negative value. Since the term TS high G is less.

Reaction becomes more feasible.



Ellingham diagram showing the change in free energy G with temperature of oxides (based on 1 g mole of dioxygen in each case).

Note:
1. In Ellingham diagram, any, metal or non-metal can act as reducing agent for the oxide of other metal that lies above it in Ellingham diagram.
2. Slope of any line will be continuous until there is any phase transition.

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