[rleung]

Hi,

While reviewing some of my old stuff from chem lab, I came across a question that doesn't quite make sense.  Somehow, I was able to get the right answer, but I can't seem to come up with the same answer now.  Here's the question:

The following reaction is 1st order in A and B, and before the reactions are started, [A] is the same for all 3 reaction mixtures.  The value of k (rate constant) is unknown prior to experiment.

A+B => C

The following absorbances at 600 nm of the product C are recorded for these three mixtures.

time (sec)    Rxn Mix 1         Rxn Mix 2        Rxn Mix 3
0                0                     0                   0
60              0.08                0.05               0.11
120            0.120              0.075             0.165
180            0.140              0.088             0.193

Which one of the following statements is consistent with this set of data?

a) rxn 3 is run at a lower temperature for rxn 2
(*this one is dead wrong since rxn 3 shows the greatest [C] from absorbance data, so if anything, rxn 3 has a higher rate, which implies it is run at a higher temperature than rxn 2)

b) at t=90, for rxn 3 is larger than for rxn 2
(*this can't be true since rxn 3 seems to produce the greatest amount of product [C], so it must have LESS reactants (namely, ), than rxn 2, NOT more)

c) rxn 2 was mixed before the other mixtures
(*there's no way for you to see this from the data given unless you are given the starting concentrations of all 's, which presumably control the rate of this first-order reaction since I assume the rate law to be rate=k )

d) if at t=0 is the same in all mixtures, k of rxn 2 is smallest
(*this is just plain wrong since concentrations don't affect the k value itself)

e) rxn 2 contains the largest [C] at 160-s
(*this is the right answer, but I do not understand why.  Although we are not given [C] at 160-sec, from the data, it's clear that rxn 3 produces the greatest amount of [C] compared to the other two at any given time up to and including t=180-sec, so I am confused as to why this choice is right)

Oh my gosh, I just realized how much I typed.  Sorry to make this such a long post.  Any input would be appreciated.  Thanks

Ryan

[rleung]

So any thoughts on this one?

[Albert]

e) rxn 2 contains the largest [C] at 160-s
(*this is the right answer, but I do not understand why.  Although we are not given [C] at 160-sec, from the data, it's clear that rxn 3 produces the greatest amount of [C] compared to the other two at any given time up to and including t=180-sec, so I am confused as to why this choice is right)

Are you sure this is the right answer?

[rleung]

Well, I know I marked it and didn't have points subtracted.  Could be a misgrading.  Does another answer choice look better?

[sdekivit]

well the absorbances are form [C] and if the law of Lambert-Beer is valid here then the statement e is wrong, since A ~ c

Recall: A = e * c * l

[rleung]

Does another answer choice look better?  Everything else looks bad. The only one where it "could" be right is c) rxn 2 was mixed before the other mixtures, but that is being presumptuous since you can't really infer that from the data (plus, that would be a bad experiment).

Ryan

[Albert]

Does another answer choice look better?  Everything else looks bad. The only one where it "could" be right is c) rxn 2 was mixed before the other mixtures, but that is being presumptuous since you can't really infer that from the data (plus, that would be a bad experiment).

Ryan

In my opinion, C is wrong because all reactions begin at the same time.

time (sec)    Rxn Mix 1         Rxn Mix 2        Rxn Mix 3
0                0                     0                   0
60              0.08                0.05               0.11
120            0.120              0.075             0.165
180             0.140              0.088             0.193

As you pointed out, A could be correct, but, unfortunately, the problem never mentions temperature.

So, to sum up, I would choose B. Since A is constant throughout the experiment, the only difference should be the initial concentration of the other reagent (namely, B).
But this is just my opinion...

[sdekivit]

well look at the question carefully: the reaction is first order in A and B, thus is both dependant on A and B:

s = k * [A] *

[A] is the same for all mixtures at the beginning of the reaction (given)

Thus: The only explanation for the higher absorbances is that prior to the experiment mixture 3 has the largest , then mixture 1 and mixture 2 has the least

--> so b is the correct answer, at t = 90 mix 3 has a larger than mix 2.

So i agree with albert

[rleung]

Ahh, I see.  I ruled out B originally b/o I assumed that if you have more [C] (product), you would have less (reactant).