Hello, I am trying to calculate the rate of reaction of a reactant in both liquid and vapor phase. Any guidance will be highly appreciated.
reaction rate in perfect gases (in vapor phase) is usually proportional to the product of volume concentration in some powers. For example, for a reaction
A+1/2 B -> C
d[C]/dt=K* [A]*[B]^0.5, where K is the velocity constant which is a function of the temperature T, and [C], [A ]and [B] are volume concentrations (no reverse reaction is included).
The rate of the reaction in liquid phase is usually not directly proportional to the product of volume concentrations. It can be expressed as a product of molalities along with the appropriate activity coefficients of the reagents in the solution.
Thus, the rate of the same reaction in a solution will be expressed as
d[C]/dt=K* [A]*[B]^0.5*fa*fb^0.5, where fa and fb are the corresponding activity coefficients of A and B which are, in turn, the functions of the concentrations.
The latter are generally defined through the chemical potentials of the reagents mu as
mu=mu (s)+RTln [A]*fa,
with the basis f->1 as [A]->0, i.e the activity coefficient goes to 1 in the infinitely diluted solution. Note that other choices for activity coefficients basis could be used, and thus their numerical values depend on that basis.
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