[itsgood819]

step 1- H2O2 + I + H+  H+ + OI- + H2O
step 2- H+ + OI- + 2I- + H+  I3- + H2O
step 3- I3- + 2 S2O3 2-  3I- + S4O6 2-

I got 2H+ + H2O2 + 2S2O3 2- --> 2H2O + S4O6 2-as my overall reaction, but I'm not sure if that's right.

Also, would the Iodine be a catalyst? In the lab, we used KI and we changed the concentration of it to measure the rate, but since it doesn't show up in the overall reaction, would the Iodine even show up in the overall rate law? We also changed the concentrations of Na2S2O3 and H2O2, and I think those are part of the overall rate law.

Thanks so much!

[Sophia7X]

I did a similar lab some time ago. 
I checked your final equation, I think it looks good.

The iodide is a catalyst because you see it as a reactant, but it doesn't participate in the final equation. So it would not be a part of the overall rate law; catalysts and intermediates are excluded from rate law expressions.

[itsgood819]

Thank you! 

One more question though- which one would be the rate determining step? I have a little trouble with this. Would I have to look for the intermediate?

[vladi307]

Check this. I think that will be useful 

[itsgood819]

Okay, just to make sure, are these rate laws right for the elementary steps?

step 1- R= k₁[H2O2][I-][H+]
step 2- R= k₂[H+]²[OI-][I-]²
step 3- R= k[I₃-][S2O32-]²

and I think I've figured it out- if the rate law matches the overall rate law that I've found before, then it's the rate determining step right?

[Vidya]

yes you are right

rate determining step is the one which is there in your rate law.