Page 108 - Chemical equilibria Volume 4

84 Chemical Equilibria
3.3.2.8. Extension of the metal–oxide Ellingham diagrams to redox by
gaseous mixtures of oxidizing/reducing agents
The Ellingham diagram of the oxides also involves information
pertaining to the equilibria of reduction of oxides by CO/CO 2 mixtures and
H 2/H 2O mixtures.
We shall illustrate that information by examining the reduction of liquid
zinc oxide at 700°C by the CO/CO 2 mixture.

For that, we use the data relative to the equilibria:

2CO + O 2 = 2CO 2 [3R.22]

2Zn liq + O 2 = 2ZnO [3R.23]

The reaction under study is given by the balance equation:
[3R.24]
CO + ZnO = CO 2+Zn
We can see that, between the three balance reactions, we have the
relation:

[3R.22] – [3R.23]=2[3R.24] [3.58]
At equilibrium, the affinity of reaction [3R.24] can be written in the form:

⎛ P ⎞
A = A 0 − Rln⎜ T CO 2 ⎟ = 0 [3.59]
24 24 ⎜ P ⎟
⎝ CO ⎠ equ

In view of relation [3.58], the standard affinity is given by:

1
0
A 0 =− Δ 0 (Δ g =− g − Δ g 0 ) [3.60]
24 24 22 23
2
By taking account of equations [3.59] and [3.60], we obtain:

⎛ P ⎞
Δ g − 4.6R log⎜ T CO 2 ⎟ = Δ g 0 [3.61]
0
22 ⎜ P ⎟ 23
⎝ CO ⎠ equ