Which mechanism and which step would best fit the data?

[JessicaHelena]

Homework Statement

Two mechanisms are proposed (as seen in the screenshot). Which mechanism and which step as the rate determining step would best fit the data?

Homework Equations

I think this is more of a conceptual problem ...

The Attempt at a Solution

I apologise for not strictly following this format, but I am really lost, because the teacher didn't really teach us much about comparing the best-fit mechanisms and steps. How do I compare between the two? Both look equally good to me.

 

[Borek]

Wasn't there any data given? Perhaps in earlier questions?

 

[JessicaHelena]

Oh, I believe it might be this data that they're referring to...

 

[JessicaHelena]

Could anyone help me with this? I do have a test on this tomorrow, and I'd really like some help...

 

[mjc123]

Compare I and II. What is the effect on the rate of doubling [I^-] while keeping everything else constant? So how does the rate depend on [I^-]?
Compare II and III. What is the effect of doubling [H₂O₂]?
Compare III and IV. Here you double both [H₂O₂] and [H⁺]. But you have already determined the effect of doubling [H₂O₂]. So what is the effect of doubling [H⁺]?
Now you know the dependence of the rate on the 3 concentrations, what does that tell you about the rate-determining step?

 

[JessicaHelena]

@mjc123
So from your questions, I can get the rate law, which would be something like
Rate = k*[H_2O_2]*[I^-], and we can plug in any row's data to find k.

However, I'm not sure what this tells me about the rate-determining step...

 

[mjc123]

So if the rate law doesn't involve [H⁺], does that tell you anything?

 

[JessicaHelena]

@mjc123 So [H^+] would be the intermediate? (Since intermediates are canceled between the steps)

 

[mjc123]

Try working out what the rate law would be in cases A-D.