化学反应工程Chapter 2.ppt

PROBLEMSP33~35, 2.1~2.3, 2.7, 2.9, 2.11, 2.14 化 学 反 应 工 程 Testing Kinetic Models Two problems make the search for the correct mechanism of reaction difficult. First, the reaction may proceed by 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. 化 学 反 应 工 程 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 C0 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 [C0] = [C] + [X] 化 学 反 应 工 程 We then also assume that either or that the intermediate is in equilibrium with its reactants; thus, where =0 化 学 反 应 工 程 EXAMPLE 2.1 SEARCH FOR THE REACTION MECHANISM The irreversible reaction (11) A + B = AB (10) has been studied kinetically, and the rate of formation of product has been found to be well correlated by the following rate equation: What reaction mechanism is suggested by this rate expression if the chemistry of the reaction suggests that the intermediate consists of an association of reactant molecules and that a chain reaction does not occur? 化 学 反 应 工 程 SOLUTION If this were an elementary reaction, the rate