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 C_A* because we cannot easily measure the concentration of A*, use PSSH to solve for C_A*.

Solving for C_A*

Substituting for C_A* 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 k₂C_A, i.e.,

and the rate law becomes

Apparent first order.

For low concentrations of A, we can neglect k₂C_A 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

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