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Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: AhBeng on April 26, 2012, 09:36:57 AM

Title: Kinetics of an (overall) negative 1st order reaction
Post by: AhBeng on April 26, 2012, 09:36:57 AM

Full Question : Qn8
http://www.rsc.org/images/2000_Rd%20_1_Questions_tcm18-190754.pdf

Answers (not full working though)
http://www.rsc.org/images/2000_Rd_1_Answers_tcm18-190755.pdf

Thing is, I'm unable to obtain the value of k as 0.112 (the given answer).

My method :
Since order of reaction wrt to I3- is 1, and since
rate = c [I3–]^1 where c = k [H3AsO3]^1 [H+]^-1 [I–]^-2,
accordingly using c = ln2/t(1/2) (for any one of the 4 Runs) and substituting both the value of c, as well as the molarities of the other species (for that particular Run) into the equation
c = k [H3AsO3]^1 [H+]^-1 [I–]^-2
we should be able to obtain the value of k, which based on the 4 Runs, are approximately 0.12 to 0.13 (to 2 sf), but the given answer is 0.11 (to 2 sf).

Any assistance would be much appreciated.

Title: Re: Kinetics of an (overall) negative 1st order reaction
Post by: Kemi on April 27, 2012, 11:20:50 AM

I got 0.121 (a mean of the four ks, all cs calculated from linear fits). Maybe they mixed up two digits in the answer? Anyway, I wouldn't worry too much about such a small difference, because you can estimate the rate constant in different ways.

Thanks for sharing this question (and, indirectly, this site: http://www.rsc.org/Education/events-and-competitions/Olympiad/olympiad-questions-answers.asp)!

Title: Re: Kinetics of an (overall) negative 1st order reaction
Post by: AhBeng on April 27, 2012, 11:38:49 AM

All right, thanks for replying  :)