knauss

Thanks for jumping in, Brian. As you note, "affinity" is defined as -2.303RTlogQ/K. When the reaction modeling guide called the variable "logQoverK" affinity, I wrote that first rate expression assuming that it was literally affinity. I was being incredibly literal! When it occurred to me that the variable was just what it says, logQ/K, putting that in the A&H form for the rate law gets you effectively exp(lnx) = x, ie, it gives the thermo term as (1-Q/K), just like the first form of the rate law that I used. Duh! As for the extra parens, I tested that to be sure it wasn't an issue. It is, however, a very good point that you raise. Cheers.

Never mind. OK, I see my problem. I was being literal. The table in the manual defined the variable logQoverK as "affinity", but it really is log(Q/K). If I use the expression (1-exp(2.303*logQoverK)), you get the same result as using (1-Q/K) for this thermo-driver term in the rate law. Cheers.

Not an extensive user of GWB and first attempt at a simple custom rate law. Have a quick question about syntax for using an affinity term. Test case is a simple dissolution of albite into a pH ~4 HCl solution. The react input attached called "2-mech.rea" uses the expression (1-Q/K) for this "affinity" term. Seems to run OK. The second input called "2-mech-affinity.rea" tries to use the helper function "logQoverK", which is defined as being "affinity" in the modeling guide. In this case I want to use the Aagaard & Helgeson form for the affinity term: (1-e^(-A/RT)), where A is the affinity. My syntax for this is clearly wrong, because the output shows the albite reaction rate for step #0 as being "-1.#IO". It appears to be running, but is just stuck at the first step. What would be the correct form for this? I'm guessing it's something quite simple that I've missed, being a newbie using custom rate laws in GWB. Thanks. 2-mech.rea 2-mech-affinity.rea