Elements of
Chemical Reaction Engineering
6th Edition



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Essentials of
Chemical Reaction Engineering
Second Edition

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Chapter 9: Reaction Mechanisms, Pathways, Bioreactions and Bioreactors

Active Intermediates

 

Problem Statement

The rate law for the reaction

is found from experiment to be

Suggest a mechanism consistent with the rate law.

Solution

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution:


Solution

Two A molecules collide and energy is transferred from one A molecule to the other molecule making it highly reactive.

This activated molecule (A*) can do one of two things. It (A*) can collide with another molecule to become deactivated (A).

or (2) the activated molecule, A* can decompose to form B and C

For reactions with active intermediates, the reaction coordinated discussed in Chapter 3 now has trough in it and the active intermediate, A*, sit in this trough

Rate Laws

Relative Rates

Net Rates: Rate of Formation of Product

Need to find an expression for CA* because we cannot easily measure the concentration of A*, use PSSH to solve for CA*.

Solving for CA*

Substituting for CA* in Equation (4) the rate of formation of B is

Relative rates overall

For high concentrations of A, we can neglect k3 with regard to k2CA, i.e.,

and the rate law becomes

Apparent first order.

For low concentrations of A, we can neglect k2CA with regard to k3, i.e.,

and the rate law becomes

Apparent second order

Dividing by k3 and letting and k = k1 we have the rate law we were asked to derive



Back to Chapter 9