[Jonnyb504]

Ok so Im almost done my kinetics lab and at the end we had to determine how the [HCOOH] influences the rate of reaction. Naturally we doubled the amount and what not. My data shows that it slowed down the reaction rate by almost 8x. The question I am being asked though is from my data tell wether it is HCOOH or HCOO- that is actually reacting with Br2? I have checked around and I think I am correct in saying that it reacts with HCOO- but MY question is WHY?

[AWK]

Br2+HCOOH=2HBr+CO₂

[Jonnyb504]

Ok so are you saying that it is reacting with the HCOOH and not the HCOO-? Because we where told that since HCOOH is a weak acid that it partially disociates into

HCOOH<--->H⁺+ HCOO^-

And they ask us is the Br₂ reacting with the HCOOH or the HCOO^-

[Dan]

The reaction between bromine and the formate ion is much faster than that with formic acid. Possibly because the formation of the hypobromite ester intermediate, which would form more readily with the formate ion, is rate limiting - this is speculation on my part though.
There are various mechanisms at work, it's complex - especially since the pH rises as the reaction proceeds (you are making HBr, a far stronger acid than formic acid), the ratio of formate to formic acid drops, making alternative mechanisms more significant.

I don't have full access to the following papers at the moment, so I haven't read them, but the abstracts may be useful to you:

http://www3.interscience.wiley.com/cgi-bin/abstract/109570723/ABSTRACT
http://www3.interscience.wiley.com/cgi-bin/abstract/109570683/ABSTRACT