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# Reaction mechanism and rate law

A reaction mechanism is the sequence of elementary steps by which a chemical reaction occurs. Many reaction mechanisms contain one step that is much slower than the others; this step is known as the rate-determining step. If the rate-determining step is the first step in a mechanism, the rate law for the overall reaction can be derived directly from the stoichiometry of the step's balanced equation. Created by Jay.

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• How does one go about determining the elementary steps? how do you know which molecules will form together, before you get to the original equation? is there a certain technique, or is it just trial and error with different molecules?
• Usually, the steps are either given or determined experimentally. One way to go about writing the steps is if you are given the original equation and any intermediates, as you know that they intermediates must be produced and then used. So, to answer your question, it is trial and error in experiments, as far as I am aware. Hope this helps.
• so you use the slow step in the reaction rate equation?
• Yes. Imagine the slowest step as a bottleneck of a ketchup bottle. It hardly matters if the ketchup bottle is full but if the neck of the bottle is too constricted, ketchup will come out slowly.Similarly.the slowest step will determine the rate of formation of products.
Hope this helps. :D
• Doesn't this approximation require adjustments if you were dealing with elementary steps that took much longer than just 1 second? For example, if you had a slow reaction that occurred after 30 minutes and the fast one after 15 minutes, leaving the fast reaction out seems like it won't make for a good approximation. Or do most elementary steps in these mechanisms occur within seconds?
• Think about it this way instead. The slow step is the weakest link in the chain of reaction steps. A fast step will already be complete by the time the slow step finishes one cycle, so eventually every step faster than the slow step is slowed down to the slow step's rate, because the products from the slow step are needed to proceed to a faster step. That means that the overall reaction must proceed at the slow step's rate.
• How would you create a rate law for a mechanism in which the slow step is NOT the initial step?
• In this case, I suppose the overall rate would near the 2nd step rate(the slower one) because of the same reason. The time it takes for step one would be negligble compared to the time it takes for step 2, making our overall rate near the second step's rate.
• In this reaction, can we also consider NO2 the catalyst?
• I see why you are asking this since one NO2 is regenerated, but reactants are not considered to be catalysts. Catalysts are something other than a reactant that are added to a reaction to lower the activation energy and thereby speed up the reaction.
• What if there are 2 fast steps and 1 slow? How do you get the rate from there?
• Why is the rate law R=k[NO2]^2
• what is difference betn parallel, series and normal reactions?
• In a "normal" reaction, reactants give products: A → B.
In a "series" reaction, reactants give a product that then reacts to give another product: A → B → C.
In a "competitive parallel" reaction, a reactant gives two different products: A → B and A → C.