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Unformatted text preview: V. REACTION MECHANISMS When a reaction occurs the balanced stoichiometric reaction indicates what the reactants and products are and what stoichiometric amounts are involved in the reaction. They do not, however, necessarily indicate what is actually taking place when the reaction occurs. Consider the gas phase reaction H 2(g) + Br 2(g) 2HBr (g) . If the reaction actually occurred with a collision between the H 2 and Br 2 molecules, this reaction would be referred to as an elementary reaction (i.e., a reaction that is actually taking place). The rate of any elementary reaction is well defined. It is equal to the product of the concentrations of the reactant molecules to the powers of their stoichiometric coefficients. Therefore if the above reaction was an elementary reaction the rate of the reaction would be rate = k[H 2 ][Br 2 ] . Experimentally it is found that the above expression does not agree with the experimental rate law so that the above reaction is not an elementary reaction (i.e., this reaction is not how the reaction actually occurs). In such cases it is necessary to construct a mechanism for the reaction. A mechanism is a series of elementary reactions (i.e., reactions that are actually occurring) that describe how the reaction occurs. Normally when the reactions in the series (called the steps) are added together the stoichiometric reaction is obtained. Each step has a well defined rate law discussed above. In order to obtain the experimental rate law one must place conditions (to be discussed below) on the steps in the mechanism. The mechanism can contain chemical species called intermediates that are not present in the stoichiometric reaction. Since the rate law can only contain chemical species that are present in the stoichiometric reaction any intermediates in the rate law must be re-expressed in terms of chemicals present in the stoichiometric reaction. There are three types of reactions that are normally used for the steps in the mechanism. The first type is called a unimolecular reaction and it has the form A k products where a single molecule A reacts to form products. The rate for a unimolecular reaction is rate = k[A] where k is the rate constant. The second type of elementary reaction is called a bimolecular reaction and it has the form A + B k products 1 which involves a collision between the molecules A and B. The rate for a bimolecular reaction is given by rate = k[A][B] where k is the rate constant. Notice that if B is the same molecule as A the rate would be k[A] 2 . The third type of elementary reaction is called a termolecular reaction and has the form A + B + C k products which involves a simultaneous collision between the three molecules A, B and C. This type of step is not used very often because of the probability of a three molecule collision that leads to products is very small. The rate for a termolecular reaction is given by rate = k[A][B][C] where k is the rate constant. where k is the rate constant....
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- Spring '08