Reaction Progress Cuttoff Criteria

[shensel]

Hello,

Without involving kinetics, what criteria does React use to know when to stop a reaction (i.e. xi=1)? I'm looking to make sure metastable conditions aren't misinterpreted as actual equilibrium. Can I manually react a system indefinitely, or at least until I know that no other reactions will occur?

 

Thanks,

Scott

[Brian Farrell]

Hi Scott,

The reaction progress variable Xi by definition varies from 0 at the start of a reaction path to 1 at the end, but it has meaning only in terms of how you set up the reaction path. If you've set up a simple polythermal path in which temperature varies from 0 to 100 C, for example, then Xi = 0 indicates the initial condition, as always (0 C here), Xi = .5 indicates 50 C, and Xi = 1 is the system heated to 100 C. The program by default takes steps equal to 1/100th of Xi, so here it increases temperature one degree at a time. If the program was instructed to heat water to 200 C, then each step by default would be 2 degrees, and Xi = .5 would correspond to 100 C and Xi = 1 to 200 C. In titration paths (simple reactants), similarly, you specify the amount of a reagent to add over the course of the path, and so adding everything specified corresponds to Xi = 1. Adding half of that corresponds to Xi = .5, and at Xi = 0 you've added nothing.

When interpreting the results of a calculation, it's helpful to plot against the variable that actually controls reaction progress. So in a sliding temperature path, you might set temperature as your x axis variable. In a titration, set the mass reacted (under Reactant properties) of the titrated species. And for a sliding fugacity or activity path, you can find the buffered fugacity or activity value under Reactant properties or under Gas fugacity/Species activity.

Please let me know if that doesn't clear up your question.

Regards,

Brian Farrell
Aqueous Solutions