1 / 7

Consecutive Elementary Reactions

Consecutive Elementary Reactions. When proposing a reaction sequence, or mechanism, it is important to derive a rate expression that can be tested against experimental data. If we consider the simplest elementary sequence:

Mia_John
Télécharger la présentation

Consecutive Elementary Reactions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Consecutive Elementary Reactions • When proposing a reaction sequence, or mechanism, it is important to derive a rate expression that can be tested against experimental data. If we consider the simplest elementary sequence: • the question is how would reaction products evolve from a system that abides by this mechanism? • The differential equations governing the rate of formation and/or decomposition of the components of the system are: • For A, • For B, • For C,

  2. Integration of these ordinary differential equations gives:

  3. Defining the dynamics of this simplest of reaction sequences is relatively challenging • How would you approach the following catalytic reaction sequence? • The concentration of most reaction intermediates cannot be measured! • The rate constants for each elementary step in the sequence cannot be estimated independently!

  4. Simplifications are available when the decomposition of B (r2) is rapid, relative to the decomposition of A (r1). • In this example, rate comparisons are made on the basis of first-order rate constants i.e. k2 relative to k1 • If r2 is “quick” (k2 = 2 k1) If r2 is “fast” (k2 = 10 k1)

  5. Steady-State Approximation • The steady-state hypothesis (SSH) is an important technique of applied chemical kinetics. • If an intermediate compound in a reaction sequence is very reactive, its concentration reaches a plateau after a short period, called the relaxation time. • The analytical expression of the SSH: the derivative with respect to time of the concentration of reactive intermediates is equal to zero. • If compound B was highly reactive, meaning k1/k20, our rate expressions and their solutions are greatly simplified:

  6. Steady-State Approximation • Steady-State Hypothesis • In a sequence of elementary steps going through reactive intermediates, the rates of the steps in the sequence are equal. • In our consideration of the sequence ABC, the SSH applied to B reduced the rate expressions to: • This suggests that the rate of decomposition of A (-d[A]/dt) equals the rate of formation of C (d[C]/dt) when the intermediate is sufficiently reactive.

  7. Rate Determining Step • Another set of simplifying techniques can be applied when one reaction of a sequence can be identified as rate limiting. • The dynamics of the overall sequence are dominated by the kinetics of this single rate determining step • All other elements of the sequence affect the overall dynamics by supplying the reagents that are needed by the rate limiting reaction. In this ABC example, r1 is much faster than r2, making a rate determining step assignment a useful simplifying assumption.

More Related