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lecture+6sf - Reaction Mechanism A reaction mechanism is a...

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Reaction Mechanism A reaction mechanism is a sequence of elementary steps which describe the process by which reactants change to products in a chemistry reaction. An elementary step is a reaction which directly describes collisions on a molecular level. Most overall reactions are not elementary. Consider the reaction for the combustion of propane C 3 H 8 + 5O 2 → 3CO 2 + 4H 2 O Why is this reaction not an elementary reaction? The reaction shows 1 molecule of C 3 H 8 reacting with 5 molecules of O 2 . This is the correct overall stoichiometry, but could not be a description of molecular collisions. Such a description would involve 6 molecules (1 C 3 H 8 and 5 O 2 ) colliding at once. This is highly unlikely. In a mixture of gases, the most likely number of molecules to collide at once is two. Such a collision is bimolecular. Rarely, but still possible, three molecules collide at once. This is called termolecular. It is also possible for one molecule to come apart. This is called unimolecular. In order for a mechanism to be correct: (1) The steps must add up to the overall reaction (2) Each step must be elementary. This means that it could involve no more than three (usually two) molecules colliding. (3) It must be consistent with the experimental rate law. We have said before that the rate law cannot be predicted from the overall reaction. However, we can infer the rate law for elementary reactions, since these are directly describing molecular collisions. Consider a bimolecular elementary reaction: A + B → products In this bimolecular collision, the collision frequency, and thus the rate, is proportional to both concentrations. Doubling the A concentration doubles the collision frequency of A with B; doubling the B concentration doubles the collision frequency of A with B; doubling both A and B concentrations quadruples the collision frequency of A with B. This is mathematically expressed as: Rate = k[A][B] for a bimolecular collision of A with B In a similar way, we can infer the rate law for other elementary reactions.
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Elementary Reaction Molecularity Rate Law A → product Unimolecular Rate = k[A] A + B → product Bimolecular Rate = k[A][B] A + A → product Bimolecular Rate = k[A] 2 2A + B → product Termolecular Rate = k[A] 2 [B] A + B + C → product Termolecular Rate = k[A][B][C] The molecularity of a reaction is the number of molecules colliding. For elementary processes, the molecularity equals the order. A unimolecular process is first order, a bimolecular process is second order, and a termolecular process is third order. Let’s apply these principles to several examples. 2NO 2 (g) + F 2 (g) → 2NO 2 F(g) Could this reaction be elementary? Conceivably yes, since there are three molecules involved.
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lecture+6sf - Reaction Mechanism A reaction mechanism is a...

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