Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Rate Law for Halogenation of a Hydrocarbon?  (Read 866 times)

0 Members and 1 Guest are viewing this topic.

ruetherford

  • Very New Member
  • *
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 1
Rate Law for Halogenation of a Hydrocarbon?
« on: December 06, 2018, 08:57:11 AM »

Hi, I'm working on this problem for PChem and am getting some rather hairy answers!


Work:

The rate of the reaction should be: [tex]R = \frac{-d[RH]}{dt} = \frac{-d[Br_{2}]}{dt} = \frac{d[RBr]}{dt} = \frac{d[HBr]}{dt}[/tex]


I used the steady-state approximation to solve for: [tex]0 = \frac {d(Br)}{dt} = 2k_{1}Br_{2} - k_{2}BrRH + k_{2}RBr_{2} - k_{4}BrR[/tex] and [tex]\frac{d(R)}{dt}  = 0 = k_{2}*Br*RH - k_{3}*R*Br_{2} - k_{4}*Br*R[/tex]


And eventually got: [tex][R]   = \frac{[ B][RH]k_2 - [Br_2]k_1}{[Br2]k_3}[/tex] and [tex]
  = \frac{[Br2]([R]k_3 + k_1)}{[RH]k_2}[/tex]

However, when I go on to plug these back into the SS equations (to get [R] and [ Br] in terms of [RH] and [Br2] only), I get some pretty nasty-looking stuff and end up with a crazy-looking quadratic. Am I approaching the problem wrong?
« Last Edit: December 06, 2018, 11:51:04 AM by Borek »
Logged

Corribus

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +425/-20
  • Online Online
  • Gender: Male
  • Posts: 2617
  • A lover of spectroscopy and chocolate.
Re: Rate Law for Halogenation of a Hydrocarbon?
« Reply #1 on: December 07, 2018, 09:53:27 AM »

First, maybe it's just a typo in your syntax here but your first steady state approximation expression is wrong (there should be a k3 term). I'm also not following how you got from your steady state expressions to your "and eventually got" expressions. What happened to your k4 terms?
Logged
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

mjc123

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +219/-9
  • Offline Offline
  • Posts: 1532
Re: Rate Law for Halogenation of a Hydrocarbon?
« Reply #2 on: December 09, 2018, 09:59:02 PM »

If you add the equations for d[Br·]/dt and d[R·]/dt, you get that k1[Br2] = k4[R·][Br·].
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.078 seconds with 20 queries.