November 27, 2024, 06:41:03 PM
Forum Rules: Read This Before Posting


Topic: Relationship between kinetics and equilibrium (rate law + Keq)  (Read 8516 times)

0 Members and 1 Guest are viewing this topic.

Offline ch3mistry

  • Very New Member
  • *
  • Posts: 1
  • Mole Snacks: +0/-0
Hey everyone, I've been reviewing kinetics and equilibrium material, and I've noticed something that hopefully someone can clear up.

For an equation like this: aA + bB <--> cC + dD

The rate laws for the forward (f) and reverse (r) reactions would be, respectively:

Rate(f) = kf[A]^n^m, and Rate(r) = kr[C]^x[D]^y, where n, m, x, and y are the exponents which are determined experimentally, not from the stoichiometric coefficients.

The relationship between kinetics and equilibrium in this scenario is Rate(f) = Rate(r), therefore kf/kr, which equals Keq. However I was always taught that the equilibrium expression was:

Keq = [C]^c[D]^d / [A]^a^b, where c, d, a, and b are the stoichiometric coefficients.

My question is if kf/kr = Keq, why are the exponents different? I can't seem to find an explanation for this, but the only thing I can think of is that the above equilibrium expression exponents are actually the same as the experimentally determined rate law exponents, but only for single elementary step reactions are they the same as the stoichiometric coefficients.

If this is true, would that mean for a reaction 2A + B <---> C + D, if the forward rate law was determined to be k or k[A], would the correct Keq equations be something like this?

Keq = [C][D]/, or Keq = [C][D]/[A]

Thanks!

Offline Yggdrasil

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3215
  • Mole Snacks: +485/-21
  • Gender: Male
  • Physical Biochemist
Re: Relationship between kinetics and equilibrium (rate law + Keq)
« Reply #1 on: April 07, 2007, 01:58:26 PM »
It is only correct to say that kf/kr = Keq for an elementary reaction.  For a more complex mechanism, I think it is possible to define Keq in terms of the rate constants but it will be a more complex equation involving more than two rate constants (ie. k1, k-1, k2, k-2, etc.).

Sponsored Links