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Rate constants for redox reactions

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[admin note: this post represents recovered forum content after a critical hard drive failure - it is not in it's original form]



Dec 6 2007, 11:45 AM

Does React require that rate constants for redox reactions be entered in units of molal/sec? In React v 6.0, there is text next to the input field for the rate constant that says "molal/sec", which is leading to my confusion. It seems like this would limit the utility of the kinetic redox function since the units of a rate constant must be adjustable in order to properly cancel terms in a rate law equation.


For instance, if I am interested in modeling the oxidation of FeSO4(aq), and the available literature data [Huffman & Davidson, 1956] states that the oxidation reaction is second order in Fe2+ and first order in O2, yielding a rate law of R=k*[Fe2+]2*[O2], then it seems that the appropriate units for the rate constant would be L*mol-1*atm-1 in order to yield a rate law whose units are mol*L-1*sec-1. This is the units that the [H&D, 56] rate constants are reported in.


So does the kinetic redox function actually care what units the rate constant is entered in, or does it assume that that the user has done his/her homework and entered in numerical values for the reaction order(s) and rate constant to yield a rate law with the correct output units?





Dec 6 2007, 02:26 PM

Hi Joel:


In version 6, React requires that promoting and inhibiting species in the built-in kinetic rate law for redox are represented using activities (ie. unitless). If you wish to represent a rate law using concentrations instead, you would need to factor the species' activity coefficients into the rate constant (assuming that they differ significantly from unity). An alternative approach would be to cast the rate law using React's extensive custom rate law capabilities (see p. 45 in the Reaction Modeling Guide).


Version 7 makes things even easier. In version 7, GWB's built-in rate law is implemented as follows...





... meaning that you can now cast inhibiting and promoting species as activities or concentrations:






Rate constant units are automatically matched to the inhibiting or promoting species type that you select, and are displayed on the interface.


You can always check that your rate law has been implemented correctly by looking at the kinetic reaction rate and species activities/concentrations in your React output file.


I hope that helps,


Tom Meuzelaar

RockWare, Inc.

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