How do you create a mechanism from something that's first order?

Question

Topics - Chemical Kinetics

Question from Iodine clock reaction lab.

Answer 1

Hi Lynny C.,

Brief answer: you don't, unless you have a simple one-step reaction (unimolecular).

More detailed: what the first order overall kinetics tell you is that the rate-limiting step in an overall reaction is unimolecular (generally -- there could be an inert material involved that you don't have a clue on). But, unless you have a little more grasp of the possible reactions of your various chemical species, you can't get any further than that, just from the fact that you observed a first-order portion of the whole possible reaction space.

In particular for iodine clock reactions (note that there are about 4 common types, and as a chemist, you could come up with more!), only some of them are first-order in I- . And at that, they would be second order in S₂O₈ - or some other species, if you had more data!

I would suggest you look up "Iodine Clock Reaction" on Wikipedia to get more info.

-- Cheers, --Mr. d.