For liquids, the fluid volume change with reaction is negligible when no phase changes are taking place. Consequently, we can take
υ = υ0
Then
and so forth for CC and CD.
Consequently, using any one of the rate laws in Chapter 3, we can now find –rA = f(X) for liquid-phase reactions. However, for gas-phase reactions the volumetric flow rate most often changes during the course of the reaction because of a change in the total number of moles or in temperature or pressure. Hence, one cannot always use Equation (4-13) to express concentration as a function of conversion for gas-phase reactions.
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