March 28, 2024, 06:32:22 AM
Forum Rules: Read This Before Posting


Topic: Rate Law  (Read 10392 times)

0 Members and 1 Guest are viewing this topic.

Offline kelaklub

  • Regular Member
  • ***
  • Posts: 38
  • Mole Snacks: +1/-3
  • I'm a mole!
Rate Law
« on: May 11, 2006, 11:23:36 AM »
I have uploaded the sample test problem (really bad scan)...

They answer given is A, I think it's typo and should be E. What do you think? Thanks.
« Last Edit: May 11, 2006, 12:54:19 PM by Borek »

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Rate Law
« Reply #1 on: May 11, 2006, 11:38:55 AM »
Looks like typo.

Make it B ;)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline kelaklub

  • Regular Member
  • ***
  • Posts: 38
  • Mole Snacks: +1/-3
  • I'm a mole!
Re: Rate Law
« Reply #2 on: May 11, 2006, 11:51:23 AM »
Hmm, I thougt it was E because I determined y from y from trails 2 and 3, and setting up my equation as...

0.5y = 1

=> y = 0

Is this not right?

Offline kelaklub

  • Regular Member
  • ***
  • Posts: 38
  • Mole Snacks: +1/-3
  • I'm a mole!
Re: Rate Law
« Reply #3 on: May 11, 2006, 11:54:23 AM »
Doooh, I misread the rates from trials 2 and 3, you are right, it is B.

Thanks.

Offline kelaklub

  • Regular Member
  • ***
  • Posts: 38
  • Mole Snacks: +1/-3
  • I'm a mole!
Re: Rate Law
« Reply #4 on: May 11, 2006, 12:06:42 PM »
Oh now that my question is answered, can I remove that scan from the server or leave it on for a day in case a few people want to check out this problem? What would be considered good etiquette?




Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Rate Law
« Reply #5 on: May 11, 2006, 12:53:53 PM »
I have edited your post and added image here, so you can delete it from the outside server. Next time click on Additional options... below the edit field and select image to upload.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Will

  • Organic Dude
  • Chemist
  • Full Member
  • *
  • Posts: 400
  • Mole Snacks: +58/-2
  • Gender: Male
Re: Rate Law
« Reply #6 on: May 11, 2006, 01:16:14 PM »
I've got a similar problem that I wasn't bothered to start a new thread about, but I might as well post it now that this thread has started!

The question is: A reaction is first order with respect to species A and zero order with respect to B. For each mechanism [see pic], explain whether or not it is consistent with the rate equation (rate=k[A]).

My teacher says Mechanism 2 is consistent with the rate equation, although I would've thought that because two steps containing B happen before the step with A in it (which is the rate determining step), that Mechanism 2 isn't consistent with the rate equation (as it would be second order wrt to B?).
Mechanism 3 might be OK because although the first equation involves two species of A (2nd order), A is regenerated in the next equation, so it may only be first order wrt to A, but I am completely unsure about this and I would appreciate a second (or 3rd, 4th etc!) opinion. :)
« Last Edit: May 11, 2006, 01:17:57 PM by will17 »

Offline Donaldson Tan

  • Editor, New Asia Republic
  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3177
  • Mole Snacks: +261/-13
  • Gender: Male
    • New Asia Republic
Re: Rate Law
« Reply #7 on: May 11, 2006, 07:53:52 PM »
Assuming all elementary reactions with A as a reactant as the rate determining step

Consider mechanism 1, the first equation is the rate determining step. Its rate equation is thus rate = k [ A ] [ B ]

Consider mechanism 2, the last equation is the rate determing step. its rate equation is thus rate = k[ A ] [ S ]

Consider mechanism 3, the first equation is the rate determining step. Its rate equation is rate = k[ A ]2

We require the rate equation to be first order with respect to A, so mechanism 3 is rejected.

We require the rate equation to be zero order with respect to B, so mechanism 1 is rejected.

In order to make sure that mechanism 2 fits the stipulated requirement

1. B + Q -> R     r1 = k1[ B ][ Q ]
2. R + B -> S      r2 = k2[ R][ B ]
3. S + A -> Z      r3 = k3[ S ][ A ]

Steps 1 and 2 are very fast reactions. S is quickly formed from B and Q, so the rate of formation of S has no effect on the rate determining step. This means the rate equation is zero order with respect to B. Mechanism 2 is therefore the only mechanism that meets the requirement.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline Will

  • Organic Dude
  • Chemist
  • Full Member
  • *
  • Posts: 400
  • Mole Snacks: +58/-2
  • Gender: Male
Re: Rate Law
« Reply #8 on: May 11, 2006, 08:03:08 PM »
Thanks geodome ;).

The one bit I am still slightly confused about is if S is part of the rate determining step, surely it has to be in the rate equation? If you double the conc. of B then I think you'd quadruple the rate of formation of S, which would increase the overall rate, but the rate is zero order wrt to B ???.

Offline tennis freak

  • Full Member
  • ****
  • Posts: 107
  • Mole Snacks: +10/-9
Re: Rate Law
« Reply #9 on: May 11, 2006, 10:14:33 PM »
hey guys i to have a question that i can't seem to comprehend, help would be greatly appreciated,.
It asks us for the half life and the order of the reaction

here is the data

Time (days)   0     1   2   3   4  5   6   7  10  20
%Reactant    100 79 63 50 40 31 25 20 10   1
Remaining

we know the half life is 3 days but why is it first order?
AP Chemistry Squad Member [004]

Don't be afraid to try something new, remember amateurs built the ark and professionals built the titanic!

Offline JZ_1

  • Regular Member
  • ***
  • Posts: 38
  • Mole Snacks: +2/-7
  • HOW could I still be loosing Scooby Snacks?
Re: Rate Law
« Reply #10 on: May 11, 2006, 10:37:47 PM »
Plot it

Zero order:
    |
[]  |           Slope = - K
    |_______
       Time

First order
      |
ln[] |              Slope = - K
      |_______
           Time

Second order
       |
1/[] |                 Slope = K
       |________
             Time

Whichever graph gives you a straight line is the order of the rxn.

Also you could use the half-life formula
First order
T1/2= .693/k

K is negative for zero and 1st order and positive for second order

Offline tennis freak

  • Full Member
  • ****
  • Posts: 107
  • Mole Snacks: +10/-9
Re: Rate Law
« Reply #11 on: May 11, 2006, 11:39:42 PM »
awsome thanks a bunch, finally somebody who can explain it! :P
AP Chemistry Squad Member [004]

Don't be afraid to try something new, remember amateurs built the ark and professionals built the titanic!

Offline mrdeadman

  • Full Member
  • ****
  • Posts: 262
  • Mole Snacks: +23/-39
  • I got a 5. Hooray!
Re: Rate Law
« Reply #12 on: May 12, 2006, 08:52:49 AM »
awsome thanks a bunch, finally somebody who can explain it! :P
that's how i explained it to you, when you asked that question in class!  :P
AP Chemistry Squad Member [001]

Offline tennis freak

  • Full Member
  • ****
  • Posts: 107
  • Mole Snacks: +10/-9
Re: Rate Law
« Reply #13 on: May 13, 2006, 07:42:33 PM »
you did not nobody knew what they should do, and if you did i don't remember, sorry
AP Chemistry Squad Member [004]

Don't be afraid to try something new, remember amateurs built the ark and professionals built the titanic!

Offline rctrackstar2007

  • Full Member
  • ****
  • Posts: 265
  • Mole Snacks: +18/-40
  • Gender: Male
  • I need scoobie snacks and a 5 on the test!
Re: Rate Law
« Reply #14 on: May 14, 2006, 01:13:11 PM »
you did not nobody knew what they should do, and if you did i don't remember, sorry

ya that's def how we explained it in class lol
AP Chemistry Squad Member [002]

The world is like an atom. The not-quite-as-intelligent people are the nucleus all packed together sharing a common...everything. We, we are the electrons. Granted we're not as smart as these engineers and what-not so we're most likely in the first orbital, but we're the electrons of this giant atom. We all have differing intelligences and ideas and we are separated from the nucleus which makes us better because no one really cares about how a nucleus acts. It's the electrons that make chemistry, except for nuclear chem, of course, which I am a big fan of.

-Your's truly, 2006;
  written to describe the HS chem student apart from the average being

Sponsored Links