[cliverlong]

Hi

In the text book I am using there are lots of suggestions that are particular rate equation suggests a particular mechanism via a rate determining step

For example the reaction given (Edexcel June 2006) was

A + 3B -> AB₃

and via data we determine

rate = k[A] [ B ]²

Now the suggestion is:

A + B -> AB (rate determine step)

AB + B -> AB₂ (rds)

AB₂ + B -> AB3  (fast)

OR

B + B -> B₂ (rate determine step)

A + B₂ -> AB₂ (rds)

AB₂ + B -> AB3  (fast)



These deductions seem complete magic (hocus pocus) to me - I just don't understand how these are done.

Can anyone help me understand how to get from rate equations to mechanisms?


Thanks

Clive

[cliverlong]

Any suggestions on this?

Even the simplest example?

Should I ask this question on a different forum? (physical chem perhaps?)

I was asked about this today and I had to reply "I haven't a clue" - much to the amusement of all.

Clive

[Astrokel]

My view is the species involving in the rate determining step must collide together in a single step and there shouldn't be two rds in a give mechanism.

Try this, Propenyl ethanoate + bromine with concentrated ethanoic acid as solvent.

The rate law = [CH3CO2CH2CHCH2][Br2]^2

Propose the rate determining step.

[cliverlong]

My view is the species involving in the rate determining step must collide together in a single step and there shouldn't be two rds in a give mechanism.

Try this, Propenyl ethanoate + bromine with concentrated ethanoic acid as solvent.

The rate law = [CH3CO2CH2CHCH2][Br2]^2

Propose the rate determining step.

If I apply your reasoning (if I am on your wavelength) this would suggest 3 molecules (1 ester, 2 of bromine) need to collide to form an intermediate complex that then rearranges bonds and falls apart to the product.

However, I am sure I have read or been told that the simultaneous collision of three or more molecules is so unlikely as to be almost impossible as a configuration for a reaction. So that 3-molecule interpretation of the rate equation seems unlikely.

Clive

[cliverlong]

I can handle the simplest examples

1) if rate equation is k[X] and there are other reactant, Y and Z say, then I can interpret that as [X] splits up (free radical or ionic) . The product is then very reactive and binds quickly th other reactants

2) If rate equation is k[A][B ] then I would suggest the reaction is rate determined by the collision of a molecule of A with a molecule of B to form an "activated" complex that then splits to form products.

3) Any higher order rate equation seems tricky, at least (see my first posting) to interpret.

Clive