Geochemist's Workbench Support Forum

# Dissolution Kinetics

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Hello GWB,

I am currently trying to run a kinetic model in GWB that emulates talc dissolution. I have a rate equation that does not require a rate constant but an activation energy. This particular rate law is not represented by GWB so I had to create a "Custom Rate Law"...I put in all the script into the command screen and it successfully transfered the information to the talc(kinetic mineral) on the react screen. I give the system initially 6 grams of talc to dissolve according to the rate law...unfortunately, GWB continues asking me for a rate constant which is not required. Could someone provide me some help/solution to this problem.

Thanks,

Drew Syverson

Here is the rate law: kinetic Talc rate_law='surface*(5.0e-9*(activity(''H+'')^2/activity(''Mg2+''))^(1/4)+0.8e-9)*exp(-act_en/(8.314*TK))', surface=9000

The act_en = 45000 J/mol, TK = 298.15 K

Attached is the GWB react system. Talc(Kinetic).rea

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Hello GWB,

I am currently trying to run a kinetic model in GWB that emulates talc dissolution. I have a rate equation that does not require a rate constant but an activation energy. This particular rate law is not represented by GWB so I had to create a "Custom Rate Law"...I put in all the script into the command screen and it successfully transfered the information to the talc(kinetic mineral) on the react screen. I give the system initially 6 grams of talc to dissolve according to the rate law...unfortunately, GWB continues asking me for a rate constant which is not required. Could someone provide me some help/solution to this problem.

Thanks,

Drew Syverson

Here is the rate law: kinetic Talc rate_law='surface*(5.0e-9*(activity(''H+'')^2/activity(''Mg2+''))^(1/4)+0.8e-9)*exp(-act_en/(8.314*TK))', surface=9000

The act_en = 45000 J/mol, TK = 298.15 K

Attached is the GWB react system. Talc(Kinetic).rea

Hi Drew:

Yes, this is a quirk with using the custom rate law functionality - you'll find that if you enter a value for the rate constant, any value, GWB will let you proceed, even though your equation doesn't call for it.

A couple of hints that will also help:

1. Given that you have a temperature dependent rate constant, and are using activation energy in your rate law, select the Preexp / activation energy radio button, and enter a value for both fields.

2. You should incorporate the (1-Q/K) term to reflect tapering of the reaction as you approach saturation, as well as allowing for both the forward and reverse reaction.

3. You have some syntax errors in your rate law. I suggest starting with a simpler form of the rate law (maybe remove inhibiting/promoting species initially) until you get it working correctly. Then add complexity piece by piece.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

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Hi Drew:

Yes, this is a quirk with using the custom rate law functionality - you'll find that if you enter a value for the rate constant, any value, GWB will let you proceed, even though your equation doesn't call for it.

A couple of hints that will also help:

1. Given that you have a temperature dependent rate constant, and are using activation energy in your rate law, select the Preexp / activation energy radio button, and enter a value for both fields.

2. You should incorporate the (1-Q/K) term to reflect tapering of the reaction as you approach saturation, as well as allowing for both the forward and reverse reaction.

3. You have some syntax errors in your rate law. I suggest starting with a simpler form of the rate law (maybe remove inhibiting/promoting species initially) until you get it working correctly. Then add complexity piece by piece.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

Thanks Tom for the quick response...after a little more work and your advice I got the model to work.

Drew

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