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Understanding Reaction Mechanisms: Molecularity, Rate Determining Steps, and Catalysts

This piece explains the concept of reaction mechanisms in chemistry, defining the molecularity and elucidating simple versus complex reactions. It details the production of NO2 from NO and O2, highlighting the role of reaction intermediates and the slowest step, known as the rate determining step. The discussion extends to chain reactions, their specific mechanisms, and the effects of catalysts in lowering activation energy, including the behavior of free radicals. Additionally, it touches upon the importance of enzymes as biological catalysts that facilitate reactions in living organisms.

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Understanding Reaction Mechanisms: Molecularity, Rate Determining Steps, and Catalysts

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  1. Reaction Mechanisms • A reaction mechanism is step or series of steps that make up a reaction. • Molecularity refers to the number of reactant molecules (ions, atoms or molecules) involved in an elementary step or making up an activated complex. • Simple reaction will have one step where complex reactions will consist of a number of steps. • Consider the production of NO2 from NO and O2. 2 NO + O2 2 NO2 This reaction consists of elementary steps. 1) 2 NO  N2O2 (fast, molecularity 2) 2) N2O2 + O2 2 NO2 (slow, molecularity 2) 2 NO + O2 2 NO2 • N2O2 is a reaction intermediate which is short lived and difficult to isolate.

  2. Often it is difficult to determine the actual mechanism of an reaction. In this case it must be determined through experimentally determined relationships. • The rate determining step is the slowest step in the reaction mechanism. • The rate determining step will therefore have the highest activation energy.

  3. Chain Reactions • These reaction have mechanisms which involve a step or steps where a reaction intermediate is continually being regenerated. • Many consist of chain initiation, chain propagation and chain termination. • Chain termination occurs when the reactants are used up and the intermediates combine with themselves

  4. Catalyzed Reactions • Catalysts can provide a lower activation energy by producing an intermediate. • The intermediate that is formed in many cases is a species with an unpaired electron and is known as a radical. • Radicals are highly reactive due to the unpaired electron. • This is what occurs when CFCs are released into the atmosphere and aid in the depletion of the ozone. • Enzymes are biological catalysts which are usually large protein molecules with a distinct 3-D shape which provides an active site where substrates (i.e. the reactants) can interact with each other.

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