EK 30 min exam Chem 2015 Reaction Rates

[studentdoctor611]

---

Members do not see this ad.

Hey guys,

This is from EK 30 min exam 1 Passage 2 in Chem.

The passage question is: Why can Equation 1 be used to measure the rate of Reaction 1?

I understand that the rate of the reaction is based off of the slow step, considering that we can look at this problem as a 2-step reaction. But, I can't quite understand how to know that Reaction 1 is the slow step. Is it because there are more moles in reaction 1 that are reacting and it takes longer for a greater amount to react?

Thanks!

 

[aldol16]

I understand that the rate of the reaction is based off of the slow step, considering that we can look at this problem as a 2-step reaction. But, I can't quite understand how to know that Reaction 1 is the slow step. Is it because there are more moles in reaction 1 that are reacting and it takes longer for a greater amount to react?

I don't think that this question hinges on knowing which step is the slow step at all. It hinges only on the fact that Reaction 2 cannot proceed at all until reaction 1 proceeds to generate I3, which is required for reaction 2. It has everything to do with the mathematical expression of rates:

rate of reaction 1 = d[I3]/dt

d[I3]/dt = 1/2*d[S2O3] by looking at the stoichiometry of reaction 2

By substitution, rate of reaction 1 = 1/2*d[S2O3]/dt.

This equation works because the I3 formed by reaction 1 is directly consumed by reaction 2 and doesn't do anything else. Perhaps this would be easier if you posted the question and answer choices.

 

[sazerac]

We need to know the answer choices. This question could go in any number of directions, some of which have nothing to do with rates.