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25.9: Other Gas Mixtures

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  • 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,

    \[ K_{ \frac{CO}{CO_2} } = \frac {p^2_{CO_2}} {p^2_{CO} p_{O_2}} \]


    \[ p_{O_2} = \frac {p^2_{CO_2}} {p^2_{CO}} K_{ \frac{CO}{CO_2}} \]

    and hence

    \[ ln \frac{1}{p_{O_2}} = ln K_{ \frac{CO}{CO_2} } +2 ln \frac {p_{CO}} {p_{CO_2}} \]

    \[ = \frac{-\Delta G}{RT} \]

    We see that p_O2 is equivalent to a ratio:

    \[ \frac{p_{CO_2}}{p_{CO}} \]

    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

    \[ \frac{p_{H_2O}}{p_{H_2}} \]

    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.

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