Reaction Mechanism

a reaction mechanism is a step by step sequence of elementary steps, by which an overall chemical change occurs. Now, when we use the term elementary step, an elementary step is just one step in a series of reactions that show the progress of a reaction at the molecular level. Now here we're gonna have a reaction mechanism overview. So we're gonna stay here step one and step to represent our two elementary steps. So this would be elementary step one. An elementary step two. In elementary step one, we have cl gas plus O three gas, giving us cielo gas plus O two, gas. In elementary step two, we have cielo gas plus cielo gas, gives me cl gas plus C L +02 gas. Now, both elementary steps together. Well, that represents our reaction mechanism. So these two elementary steps together, our reaction mechanism. And we can say here that our overall reaction can be created by canceling out what we call our catalysts and reaction intermediates. So if we take a look here, we're going to say that a catalyst is a compound shown as a reactant in the first step, and then as a product in the final step. So here the first step is just elementary step. One final step is elementary step two. Now, if this mechanism of another mechanism we see has let's say four elementary steps, then the first step would still be the initial step and it will be the very last one. Step four, that would be our final step. Okay, but in this case, since there's only two, this is the first step. This is the second step. Now our reaction intermediate. This is a compound that first appears as a product in one elementary step, and then later as a reactant in another elementary step. Now these are canceled out and by counseling them out. What's left behind comes down to form our overall reaction. So if we looked at the overall reaction, this would come down here. This came down here, this came down here and then this came down over here. So keep this in mind. When you're looking at a reaction mechanism, a reaction mechanism is just all of the elementary steps together. If we were to cancel out our catalysts and our reaction on our reaction intermediates, that should give you the overall reaction for the equation. Okay, so just keep this in mind these key terms when it comes to any type of reaction mechanism and elementary step.

consider the following reaction mechanism for the formation of nitrobenzene. Alright, so identify the reaction intermediate or intermediates and catalysts are catalysts. So, here, if we take a look all of this together represents our reaction mechanism. Each individual reaction represents our elementary step, right? So here this would be elementary step one. This would be elementary step to an elementary step three. Alright, so remember, a reaction intermediate appears first as a product, then later as a reactant. So, if we take a look here, let's try to find them. Let's see. So here, H two, N 03 appears as a product, and then later we see it appear as a reactant. So that would definitely be a reaction intermediate. But is that the only 1? We would say no, that's not the only one. Who else does that? Here's another one. C six, H six N 02 plus here. C 686 and 02 plus here, that is also a reaction intermediate. It appears first as a product and later as a reactant. Do we see any others? Yes, we see this. H. S. So four minus appears as a product and then later as a reactant. Now, do we have any catalysts? If we look, we do remember catalyst will appear in the very first step as a reactor. Then it will appear in the very last elementary step as a product. So, here, this would be our catalyst and that'd be our only catalyst involved in this entire reaction mechanism. So, just keep in mind a reaction intermediate appears first as a product and then later as a reactant. A catalyst appears as one of the first reactant, and then later in the very last elementary step. It appears as one of the products, so as long as you can remember that, who knows, the difference between a reaction intermediate and a catalyst?

Now, Molecular charity is associated with the number of moles for reactant molecules within an elementary step. Here we take a look, we have one mole of carbon dioxide gas reacting with one mole of water liquid to produce one mole of carbonic acid. If we were to take a look here, we'd say that we have a total of two moles of reactivates. Now the number of moles of reactant gives us different names for the molecular charity. If we had only one mole of a reactant, then the molecular charity would be called uni. Molecular. If there are two moles of reactant like we have in this example, then that would be called by molecular. Now, I know when we're naming Covid and compounds in the past, when we say to we'd say die like carbon dioxide, but for Molecular charity we don't use the prefix di we use the prefix by so two moles of reactant Sequels by molecular. If we had three moles of reactant total, then that would be ter molecular. When we talked about numerical prefixes in the past three would be try but there's no such thing as try. Molecular. So again, when it's three moles of reactant, its term molecular Okay, so when we talk about molecular charity, just count the total number of reactive molecules and you'll be able to determine the molecular charity of any elementary step

the elementary reaction of two moles of nitrogen dioxide reacting with one mole of flooring gas to produce two moles of N. 02 F gas is an example of a blank type of reaction. All right, so here we're talking about molecular charity. So, we need to determine the total number of reactant moles involved. So here we have two moles of nitrogen dioxide and one mole of flooring gas Together that totals three moles of reactant. So unique molecular means we only have one mole of of reactant by molecular would mean to Tetra would mean four. So, the answer is either B or deep. So it's either try molecular or term molecular. Remember from our earlier videos when it comes to molecular charity, three moles of reactant is referred to as term molecular. Try molecular does not exist. So here the answer would be option deep.

now we're going to stay here in a reaction mechanism. One of the elementary steps is classified as a slow step. And when we say slow step, we're going to say that is the rate determining step. That limits the overall rate of a chemical reaction. Because remember you're only as fast as your slowest step. Now, we're going to say for a slow step, we're going to say that the coefficients of the reactant are equal to the reaction orders of the rate law. Remember we talked about calculating rate laws beforehand by using a chart and a list of values. But now we can look at the reaction mechanism, locate our slow step, use its coefficients, and that can give us the reaction orders. Right? So this is just another way of connecting rate law to our chemical reactions. In this case are chemical reactions are elementary steps that comprise a reaction mechanism.

consider the phone? Elementary steps. What is the rate law of the reaction mechanism? So, if we take a look here, we have a fast step, a slow step and a fast step. So step one is locate the slow step of the reaction mechanism. So here is our slow step. The second elementary step. As long as the reactant are not intermediates, their coefficients will equal the reaction orders of the rate law. So let's let's just focus for right now, on the slow step, we look at the reactant of the slow step and it's one mole of N 02 and one mole of N 205. So for right now we'd say rate equals K N 02 to the one because the coefficient is one and then N 205 Also to the one because the coefficient is one. But if we were to look closer, we'd see that N 02. We have an issue because N 02 is indeed a reactant reaction intermediate because N 02 1st appears as a product and then later as a reactant. Now this leads us to step two. Therefore reacting is an intermediate canceled out with the product intermediate. Alright, so Rno two that we needed from the slow step Gets canceled out by this N here. So that leads us to an issue because if it gets canceled there, that means it gets canceled in our rate law. So now it's no longer there. So this leads us to step three. Now, If step two happens, use the elementary step possessing the product intermediate. Alright, so here, elementary step one. This is our product intermediate. That canceled out with my intermediate from the slope step. What we're gonna do now is gonna take over its reaction. We're going to say for that elementary step, the coefficients, the coefficient coefficients of the reactant still equal the reaction orders. And for that elementary stuff, the coefficients of the products equal the inverse of the reaction orders. So let's see what that means. We come back up here, The N 02 that we needed is now gone. So here we're gonna use the first elementary step we're going to take over. It's reacting its product. We have one mole of N 205 and we have one mole of N 03. If we were to look we see that we have one mole of N here and another mole of N 205 here. This never got canceled out. So we can still use it in total, that's two moles of N 205. So if we come down here Kay, N 205, we have a total of two moles. So reaction orders too. Now the product that we're gonna use Is N 03 and there's one mole of it. So we come down here 1003. Remember we just said that for the elementary step that we're using, the coefficients of the product equals the inverse. So we wouldn't put one year, We put -1 the inverse. So this year would represent our rate law based on the reaction mechanism given to us above. Again, you can see that it gets a bit tricky if one of the reactions that we need from the slow step happens to be an intermediate, because then we have to look at the next elementary step that was involved and we have to incorporate its reactant and its products to give us our overall rate law. Now, this doesn't happen all the time. So in the time that it does just remember these steps that we took in order to find your final rate law.