Which mechanism and which step would best fit the data?

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Discussion Overview

The discussion revolves around identifying which proposed mechanism and which step should be considered the rate-determining step based on provided data. Participants explore the conceptual aspects of comparing mechanisms and their corresponding steps in a chemical reaction context.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion about how to compare the proposed mechanisms and steps, indicating a lack of instruction on this topic.
  • One participant questions whether relevant data was provided in earlier parts of the assignment.
  • Another participant suggests that the data might be referenced in previous questions, prompting further exploration of the data's implications.
  • There are inquiries about the effects of varying concentrations of reactants on the reaction rate, specifically regarding [I-], [H2O2], and [H+].
  • One participant proposes a rate law based on the concentrations of [H2O2] and [I-], but expresses uncertainty about its implications for the rate-determining step.
  • Another participant suggests that the absence of [H+] in the rate law might indicate its role as an intermediate, as intermediates are typically canceled out in the overall reaction mechanism.
  • A suggestion is made to work out the rate law for different cases to further analyze the situation.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the best mechanism or rate-determining step. Multiple competing views and uncertainties regarding the interpretation of the data and its implications for the mechanisms remain evident.

Contextual Notes

Participants express uncertainty about the data's relevance and the assumptions underlying their comparisons. There are also unresolved mathematical steps related to deriving the rate law from the provided data.

Who May Find This Useful

Students preparing for chemistry tests, particularly those focused on reaction mechanisms and rate laws, may find this discussion relevant.

JessicaHelena
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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.
 

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Wasn't there any data given? Perhaps in earlier questions?
 
Oh, I believe it might be this data that they're referring to...
 

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Could anyone help me with this? I do have a test on this tomorrow, and I'd really like some help...
 
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?
 
@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...
 
So if the rate law doesn't involve [H+], does that tell you anything?
 
@mjc123 So [H^+] would be the intermediate? (Since intermediates are canceled between the steps)
 
Try working out what the rate law would be in cases A-D.
 

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