Activation energy for mono, di and triprotic (or basic) acids.?

Question

I reacted HCl, H2SO4 and H3PO4 with zinc and I found the activations energy to be 30.1, 20.3 and 30.9 KJmol-1.
I don't really care if the values are accurate but I have been trying to find an explanation for the trend, something that I can justify numerically... do you think any of the following can have an effect of activation energy?
a. Ka values (for polyprotic Ka = Ka1*Ka2*...)
b. bond strenghts of H-Cl, H-O(-S), H-O(-P)
c. How easily the rest of "non dissociated" molecules of acid dissociate and the rate at which they do
d. ???

I really need help, been stuck on this problem for a couple of days.... does this trend even make sense?
Thanks a lot

personal theories are welcome as well... I couldn't find anythink in the web so I'm guessing its not that easy to answer...

Answer 1

First of all the acid strength of these acids is:

HCl > H2SO4 >> H3PO4

a) This does not even apply - at all!
b) nor does this
c) ah, now we are getting somewhere:

your reaction is:

Zn + 2H+ --> Zn+2 + H2(g)

it's not the rate at which they dissociate, that leads back to Ka, it's how good a H+ source the acid were.

First of all - how did you calculate the Ea? This is inportant, because if your experiment favored a dilute aqueous system, then I would recheck your data, you might just have data points 2 and 3 flip-flopped.

If your soln was conc undiluted acid, with no quenching, I can understand your data, as H2SO4 will slowly, if not at all react with zinc when concentrated, it must be dilute. H3PO4 will always be dilute, usless (and I don't thing so- you were using polyphophoric acid). Conc HCl reacts slow with Zn as well. Both HCl and H2SO4 are however hydroscopic!