Consecutive Elementary Reactions

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# Consecutive Elementary Reactions - PowerPoint PPT Presentation

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:

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Presentation Transcript
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,
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!
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)
• 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: