Reduction of carbon element

Carbon element can reduce non-metal oxides and metal oxides
.

From the Gibbs formula △rGm ^Θ = △rHm ^Θ -T△rSm ^Θ , the reaction △rGm ^Θ product versus temperature T is plotted (Figure 13-2), and 3 straight lines are obtained.

The reaction of metal with oxygen, taking manganese as an example

Figure 13-2 carbon as reductant and oxygen to react with the manganese rGm in △ ^[Theta] - Line

It can be seen from Figure 13-2 that the slope of the reaction line for increasing entropy is negative, and the slope of the reaction line for decreasing entropy is positive
.

MnO+C=Mn+CO (e)

Explain that when the temperature T>Ty, the reaction (e) △rGm ^Θ <0, C can reduce MnO and generate CO
.

When the temperature is Tz, the two lines (b) and (d) intersect.
At this time, the △rGm ^{Θ of the} two reactions are equal, and the reactions (b) and (d) are subtracted to obtain the reaction (f)

2MnO+C=2Mn+CO ₂ (f)

Explain that when the temperature T>Tz, the reaction (f) △rGm ^Θ black <0, C can reduce MnO and generate CO ₂
.

If the ΔrGm ^Θ -T line where the metal reacts with oxygen does not intersect the (c) or (b) line, the metal cannot be smelted by the C reduction method
.

Since reaction (c) is an exothermic reaction and a process of increasing entropy, the free energy of the reaction is not only negative, but also gradually decreases as the temperature rises, so the reaction can be combined with some non-spontaneous reactions, The overall reaction proceeded spontaneously
.

SiO ₂ +2C1 ₂ =SiCl ₄ +O ₂

Since SiO _{2 is} more stable than SiCl ₄ , the reaction is not spontaneous, but it is combined with reaction (c), namely

SiO ₂ +2C+2C1 ₂ =SiCl ₄ +2CO

It is equivalent to adding some coke to the SiO ₂ chlorination reaction, and the total reaction progresses spontaneously with △rGm ^Θ <0
.

This approach is reaction coupling in thermodynamics
.