[nj_bartel]

 http://img215.imageshack.us/my.php?image=rxnva7.jpg

Is step 1 or 2 the rate limiting step?

Step 2 has the higher activation energy and step 1 has the higher absolute energy value.  My book seems to say the highest activation energy determines rate limiting step, while I recall my professor saying that the activation energy that reached the highest energy value would serve as the rate limiting step.  Which is it?

Thanks

[azmanam]

activation energy is a delta E value.  largest delta E is RDS.  Step 2 is RDS

[nj_bartel]

In my notes I have word for word off the board

"Rate Limiting Step (rls): Governed by the transition state (TS) which has the highest absolute energy.
   -Tells us which step dictates the reaction rate"

Is this not contradictory? =/

[azmanam]

That seems to me to be contradictory.

First, energy isn't an absolute value - it's relative.  That's why we always put a generic, ambiguous E on the y axis, instead of actual values.

Here's from my organic text (Advanced Organic Chemistry, Carey & Sundberg) (emphasis added):

Comparison with the form of the expression for the rate of any single reaction step (rate = k_r{A}{B}, where {} is substituted for brackets meaning concentration) reveals that the magnitude of ΔG^╪ will be the factor that determines the magnitude of k_r at any given temperature.  Qualitative features of reaction mechanisms are often described in the context of transition-state theory and illustrated with potential energy diagrams.  The potential energy diagrams for a hypothetical one-step bimolecular reaction and for a two-step reaction are shown in Fig. 4.1 (attached).  The (diagram on the right) depicts a two-step reaction in which an intermediate having a finite lifetime is involved.  Two transition states are then involved.  The higher activation energy of the first transition state implies that the first step would be slower and therefore rate-determining.

Now, in looking at your diagram again, I am immediately reminded of kinetic vs. thermodynamic control, where the starting material might actually be in the center, and your products are the two states on the outsides.  In that case, forming the product on the left would be slow relative to forming the product on the right.  Without labels on your diagram it's hard to tell.  If the reaction proceeds from left to right, step 2 is slow.  If it proceeds from the center outward, forming the product on the right is slow