[sodium.dioxid]

If we are measuring the rate of product formation, even the steps after the rate limiting step are built into what we measured (even if they are fast). Why is zero credit given to the steps after the rate limiting step?

[ramboacid]

I think what you are trying to get at though is why, if all steps take some amount of time to take place, then why aren't they included in the total time? The reason is that we make the assumption that each of the steps after the rate determining step are already limited by the lack of reactants that are produced from the RDS, and since they take place as soon as the intermediate is formed, they have a negligible effect on the overall rate and can therefore be ignored. This assumption also fits with experimental evidence.

Yes, it may sound like cheating, but do you really want to calculate find out how long each step takes, if most of the steps take an insignificant amount of time to proceed?

[sodium.dioxid]

Yes, it may sound like cheating, but do you really want to calculate find out how long each step takes, if most of the steps take an insignificant amount of time to proceed?

Then the fast steps must be really fast. Otherwise, we would be crediting the the rate limiting step with the speed contributed by the steps that follow.

I guess it is a matter of HOW fast the steps that follow are. If they are not too far apart speed wise, then that application would be erroneous.

[Jorriss]

Yes, it may sound like cheating, but do you really want to calculate find out how long each step takes, if most of the steps take an insignificant amount of time to proceed?

Then the fast steps must be really fast. Otherwise, we would be crediting the the rate limiting step with the speed contributed by the steps that follow.

Yes, it's an approximation. It doesn't always work. It's not the only approximation. There is the rate-determining step approx, steady state approximation, etc, etc.

[fledarmus]

And if you are really concerned about the exact kinetics of a reaction mechanism, there are ways to test it and determine the rate of individual steps. When I was in grad school, coupling of multiple mass spectrometers was just starting to be a big research area. You could fire a stream of molecule A in the gas phase, combine it with a stream of molecule B, and send it through a series of mass spectrometers spaced at different distances along the flight path. You could then identify different intermediates along the pathway, where the distance was directly related to the reaction time. This let you calculate the rate of each step along the reaction pathway by the appearance and disappearance of various intermediates with time. It was very elegant work for its era.

But in most cases, the rate of one step is at least a couple of orders of magnitude slower than the other steps, and there is a less than 1% error in assuming that the other steps don't contribute at all to the time required by the reaction.

[sodium.dioxid]

But in most cases, the rate of one step is at least a couple of orders of magnitude slower than the other steps, and there is a less than 1% error in assuming that the other steps don't contribute at all to the time required by the reaction.

Good stuff. I guess I was just underestimating how fast the other steps are.