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I like to think of elementary steps as straight forward steps so what you see is what you get. While the overall reaction may contain intermediates that aren’t included, the elementary step contains the exact reactants and products and so we can derive the rate law for the elementary step right from the chemical equation.
An elementary step is one part of the reaction mechanism. Each overall reaction is broken up into the individual reactions that occur, and each of these reactions is an elementary step. The rate law of the slow step will match the experimentally determined rate law.
An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state. There are no intermediates and only one transition state. It can be unimolecular, bimolecular, or termolecular. With regards to rate laws, a unimolecular reaction is often first order reactions. A bimolecular reaction is a second-order reaction. Termolecular reactions are rare but they are third-order reactions because all three particles have to collide at the same place and same time.
Janet is right! Elementary reactions are the only kind where you can derive the rate law directly from the chemical reaction and it’s stoichiometric coefficients. Normally, reactions are composed of many elementary steps, so we cannot get the rate law just from looking at the overall chemical reaction
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