Course Reader Final Practice "2016 Final" Q4 part B

Moderators: Chem_Mod, Chem_Admin

Posts: 18
Joined: Wed Sep 21, 2016 2:58 pm

Course Reader Final Practice "2016 Final" Q4 part B

Postby ERIKTORRESDisc3C » Wed Mar 15, 2017 2:30 pm



So I was able to do part A for this problem but when it came to part B I did not know exactly what to do to find the rate limiting step and I am dear need of help on this. I don't know if I am overthinking this or not but I need clarification on what steps to take.

Q4 B: "Which student(s) mechanism is valid according to the experimentally determined rate law? Be sure to note which step must be the rate-limiting step for the mechanism(s) to give the correct rate law."

Posts: 17895
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 406 times

Re: Course Reader Final Practice "2016 Final" Q4 part B

Postby Chem_Mod » Thu Mar 16, 2017 12:06 pm

Look at the cumulative reactants for all three mechanisms, ignoring intermediates.

Student A:
ClO, O2; ClO

Student B:
2ClO; 2O2

Student C:
2ClO; O2

So both student A and student C have reaction mechanisms with 2 ClO and 1 O2, which matches the experimentally determined rate law. For either of them to be true, the second step for both mechanisms must be the slow step. If the first step for either mechanism was the slow step, then the rate law we'd expect for A is rate = k[ClO][O2], and B is rate = k[ClO]^2.

Chiao-Wei Cheng 3e
Posts: 16
Joined: Wed Sep 21, 2016 2:55 pm

Re: Course Reader Final Practice "2016 Final" Q4 part B

Postby Chiao-Wei Cheng 3e » Sat Mar 18, 2017 1:51 pm

After reading this post, I'm even more confused. In my course reader it said that both Student B and Student C has the correct mechanism, both with the second step as the rate-limiting step, NOT student A and Student C.

Posts: 13
Joined: Fri Jun 17, 2016 11:28 am

Re: Course Reader Final Practice "2016 Final" Q4 part B

Postby deantuazon2G » Sat Mar 18, 2017 4:48 pm

Also, can you clarify how to determine which is the slow step in addition to clarifying what the actual answer is.

Aileen Nguyen 3N
Posts: 26
Joined: Wed Sep 21, 2016 2:56 pm
Been upvoted: 1 time

Re: Course Reader Final Practice "2016 Final" Q4 part B

Postby Aileen Nguyen 3N » Sat Mar 18, 2017 10:25 pm

You can see that A does not work because the intermediates do not cancel out

the cumulative reactants for B should be 2CLO and O2 since one of the O2's are cancelled out

So the two valid ones are B&C

Student B:
2ClO; O2

Student C:
2ClO; O2

you can tell that the second steps are the rate limititing since ignoring the intermediates you write the late laws and the second ones match the overall rate law since the rate limiting step defines the rate law
Last edited by Aileen Nguyen 3N on Sat Mar 18, 2017 10:34 pm, edited 1 time in total.

Return to “Method of Initial Rates (To Determine n and k)”

Who is online

Users browsing this forum: No registered users and 1 guest