Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Other gas mixtures

The oxygen required to cause oxidation in the gas phase need not to come from oxygen gas. Consider the following reaction:

2CO (g) + O2 (g) = 2CO2 (g)

For this reaction,

$${K_{{{CO} \over {C{O_2}}}}} = {{p_{C{O_2}}^2} \over {p_{CO}^2.{p_{{O_2}}}}}$$

or $${p_{{O_2}}} = {{p_{C{O_2}}^2} \over {p_{C{O_{}}}^2.{K_{{{CO} \over {C{O_2}}}}}}}$$

and hence

$$\displaylines{ \ln {1 \over {{p_{{O_2}}}}} = \ln {K_{{{CO} \over {C{O_2}}}}} + 2\ln {{p_{C{O_{}}}^{}} \over {p_{C{O_2}}^{}}} \cr = {{ - \Delta G} \over {RT}} \cr} $$

We see that pO2 is equivalent to a ratio: $${{p_{C{O_2}}^{}} \over {p_{CO}^{}}}$$ .

Another nomographic scale may be added to the diagram, with a new origin, C where the CO/CO2 line crosses the y-axis.

Similarly for the reaction 2H2 + O2 = 2H2O; pO2 is equivalent to $${{p_{{H_2}O}^{}} \over {p_{{H_2}}^{}}}$$ Adding a further nomographic scale to the diagram, we see that the equilibrium pressure ratios of CO and CO2 or H2 and H2O for a given oxidation of metal, or reduction of an oxide, can be deduced at a given temperature from the diagram.