Mechanisms+and+the+Rate-Determining+Step

Mechanisms are a part of Chemistry known as [|kinetics]. Although most reactions happen because of a collision between two reactants that supply enough activation energy to cause the reaction, some are not so straightforward. A general reaction could be A+Bà C+D where A and B react to form C and D. However, a reaction that does not follow a simple progression like this is made up of a **reaction mechanism,** or a series of straightforward reactions. Each reaction in the mechanism is known as an **elementary step**. An overall reaction can be **unimolecular** if one molecule falls apart into its compenents, like the combustion or rearrangement of one molecule: Aà B+C An overall reaction can be **bimolecular** if two molecular reactants collide when oriented correctly (two molecules might collide but not react because the two molecules are not oriented correctly): A+Bà C+D Overall reactions with three molecules are very rare and are referred to as **termolecular**: 2A+Bà E+F One example of a reaction mechanism for the overall reaction 2A+Bà E+F may be: A+Bà C C+Aà D Dà E+F By adding up each of the elementary steps and cancelling C and D, the overall reaction of 2A+Bà E+F is found. C and D are known as the **intermediates** because they are formed and utilized in the reaction, but are not present in the equation for the overall reaction.
 * Reaction Mechanisms **

[|Catalysts] can also be part of a reaction mechanism. The chemical substance known as a catalyst increases or decreases the rate of a chemical reaction. Unlike other reagents, catalysts are not consumed by the reaction. Rather, catalysts are used in one elementary step and reformed in another. Therefore, the catalyst does not appear in the overall reaction's equation.

While an intermediate is formed and then used up, a catalyst is used up and then reformed. This can be seen in the following example where the overall reaction is A+Bà C+D: catalyst + A à intermediate + C intermediate + B à D + catalyst

Every reaction mechanism has an elementary step that is slower than the rest. This slower step is known as the **rate-determining step** because it determines and limits how quickly the overall reaction can occur. Because of this, the rate law of the overall reaction is the same as the rate law of the slow, or rate-determining, step. In an elementary step, the rate law is the reactant(s) raised to its coefficient(s). For example: Aà B+C Rate=k[A]
 * Rate-Determining Step **

and

A+2Bà products //Tip: Remember that coefficients become exponents in the rate law.// For a mechanism with a slow initial step, the rate law is simply the rate law of the slow step. If was the given equation and the following mechanism had been proposed:
 * Mechanisms with a Slow Initial Step **

The rate of the overall reaction would be because this is the rate law of the slow step. //Tip: Remember that in finding the rate law, only overall reactants and products are used, not intermediates.//

The mechanism of a chemical reaction with a fast initial step can be verified by using. If was the given chemical equation with a rate law of. And if the given mechanism was
 * Mechanisms with a Fast Initial Step **

the rate law would be. But this rate law does not agree with the given rate law because NOCl2 is an intermediate. So, the intermediate must be replaced with reactants that are part of the overall reaction. The first step is to find the forward reaction (//k//1) and the reverse reaction (//k//-1) and set them equal to each other.

Next, solve for the reverse reaction.



Then, substitute this into :



Finally, put this rate and the slow rate together



The mechanism is correct for this reaction because its rate is the same as the experimentally found one. 

**Practice**
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