Submitted by Nancy Williams / Scripps College, Pitzer College, Claremont McKenna College on Tue, 03/03/2009 - 12:23
##### Categories
Prerequisites
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Description

This worksheet gives students practice with deriving and analyzing the rate laws for two step mechanisms. It's a good review of steady-state kinetics, the assumptions one makes in deriving rate laws, and rate determining steps (and how these last affect the rate law). It finishes by connecting these ligand substitution kinetics to Michaelis-Menton kinetics to show that "it's all the same math, we just change the form".

Attachment Size
190 KB
Learning Goals

A student should be able to derive rate laws for two step ligand substitution (or other, similar) mechanisms. I generally revisit this when we get to catalysis and organometallic mechanisms. They should have a clear conceptions of what rate determining steps are, and how they affect the rate law. They should be able to extract rate constants from data by making linear plots of kobs.

Equipment needs

None.

Implementation Notes

See above.

Time Required
at least 50 minutes (I sometimes stretch into a second day)

#### Evaluation

Evaluation Methods

Most recently, I have asked them to derive a rate law from a literature article on a final exam, and they did extremely well, despite it being a fairly complicated example. Once they've practiced this skill a bit, they can master it. All students gave essentially correct answers.

Evaluation Results

Students generally do well on this. I have to lay out the steady-state approximation, how to generate the rate laws for elementary steps, and how to define kobs again (some never learned it in genchem, some have forgotten), but then they are off to the races. I put them in groups of three or four to work on this, and then we wrap up on the board. They have trouble with the idea that it's the realtive magnitude of the k-1 and k2 step that determine RDS, not the relative magnitudes of k1 and k2. They also have trouble with the idea that the rate determining step can change, but they buy in once they can see the math.

Attribution, Non-Commercial, Share Alike CC BY-NC-SA
Joanne Stewart / Hope College

Hi Scott,

I think I get what you're saying about the importance of k-1 in determining the RDS, but I don't understand the point you're making when you say that students "have trouble with the idea that the rate determining step can change."

Adam Johnson / Harvey Mudd College
Does it mean that the rate determining step can change depending on conditions (concentration)?  Or is it more complicated than that?
Nancy Williams / Scripps College, Pitzer College, Claremont McKenna College
Correct, Adam; which step is rate determining can depend on concentrations, temperature, etc. In this case, which step is rate determining is a question of concentrations.
Sheila Smith / University of Michigan- Dearborn

Scott, I'd like to assign this as a homework problem, but the key is attached here instead of in the problem sets.

Nancy Williams / Scripps College, Pitzer College, Claremont McKenna College

Sorry to be slow on this. The key is now in Problem Sets (linked here).

Karen McFarlane Holman / Willamette University

This LO is great!  I adapted is ever so slightly to pertain to the ligand substitution reactions relevant to Pt- and Ru-based anticancer compounds (related to my research) but otherwise the problems were identical.  We did this as an in-class exercise and it was great.  Thanks!