Lecture 5

PolyE-304 Polymer Reaction EngineeringDr.M.Bilal Manzoor

Polymer and process Engineering UET, Lahore

Testing Kinetic ModelsTwo problems make the search for the correct mechanism of reaction difficult.

First, the reaction may proceed by more than one mechanism, say free radical and ionic, with relative rates that change with conditions.

Second, more than one mechanism can be consistent with kinetic data. Resolving these problems is difficult and requires an extensive knowledge of the chemistry of the substances involved. Leaving these aside, let us see how to test the correspondence between experiment and a proposed mechanism that involves a sequence of elementary reactions.

In these elementary reactions we hypothesize the existence of either of two types of intermediates.

Type 1. An unseen and unmeasured intermediate X usually present at such small concentration that its rate of change in the mixture can be taken to be zero. Thus, we assume

This is called the steady-state approximation. Mechanism types 1 and 2, above, adopt this type of intermediate, and Example 2.1 shows how to use it.

Type 2. Where a homogeneous catalyst of initial concentration Co is present in two forms, either as free catalyst C or combined in an appreciable extent to form intermediate X, an accounting for the catalyst gives

We then also assume that either

or that the intermediate is in equilibrium with its reactants; thus,

SEARCH FOR THE REACTION MECHANISMExample 2.1

solution

SEARCH FOR A MECHANISM FOR THE ENZYMESUBSTRATEREACTIONExample 2.2Here, a reactant, called the substrate, is converted to product by the action of an enzyme, a high molecular weight (mw > 10 000) protein-like substance. An enzyme is highly specific, catalyzing one particular reaction, or one group of reactions. Thus,enzymeA------ >RMany of these reactions exhibit the following behavior:1. A rate proportional to the concentration of enzyme introduced into the mixture [E,].2. At low reactant concentration the rate is proportional to the reactant concentration, [A].3. At high reactant concentration the rate levels off and becomes independent of reactant concentration. Propose a mechanism to account for this behavior

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