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Reaction Mechanism - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

A reaction mechanism is a series of steps which allows complex reactions to proceed. Explained in terms of collision theory, it is unlikely for more than two particles to collide at the same time with the proper orientation, so reactions involving several reactants are actually composed of several simpler reactions happening in close succession. Often an a substance which is created in one step and used in the next, called an intermediate, is formed.

Alright so when you're dealing with a reaction you might not realize the reaction actually goes through several different things before it gets from the reactants to the products. If you look at this blue reaction it doesn't simply go from ozone to oxygen in one simple step there's actually multiple steps that we're going to actually call reaction mechanisms or the reaction pathway that the reactants go through to actually get to the products. Okay so reaction mechanisms are a series of 2 or more simpler reactions that combine to form the overall reaction and here are some more that you're actually going to need to know before we get started. The overall reaction is not going to be called an overall reaction anymore, we're going to call that the complex reaction okay. And the elementary steps are series of simpler reactions that combine to form that overall complex reaction.

Okay and there're some things that are, that the reaction, here's the reaction down here might be involved in that reaction mechanism but are not shown in this overall complex reaction. But one of them is going to be the intermediate and intermediates are substances produced in one element reaction or step and then consumed in another. And another thing is that catalyst, a catalyst you know that are things that increase the rate of reaction, you put them in the reaction and you also can get them out unchanged, they're not consumed by the reaction, they're left unchanged when dealing with the reaction. Okay so let's look at this complex reaction in blue. It actually does go from ozone to oxygen, it actually goes through these theories of elementary steps that's indicated below. So ozone which comes from the atmosphere also deals with, reacts with chlorine and chlorine comes from the stratosphere, they react together and they actually output some oxygen and some chlorine oxide.

Okay so then also at the same time ozone is also reacting with the uv in the atmosphere and it's going to produce some oxygen gas and some oxygen atom. Now these 2 things are very unstable, this guy is a radical meaning it has an odd number of electrons, it's extremely reactive and this guy is oxygen, oxygen should be a diatomic. It usually is never left by itself. So these guys are very, very unstable, so what's going to happen is, those guys are going to react and they're going to produce more oxygen gas and then chorine. Okay, so since this is being produced and then consumed again, here and here this is our intermediate. We can cross those out, these are not part of our overall reaction it's produced and then consumed. That's our intermediate, we also have an intermediate with oxygen produced and consumed awesome.

Then we have chlorine, chlorine is actually brought into the reaction and then outputted unchanged. This is our catalyst, I'm going to box that off, that is our catalyst. This actually helped the reaction proceed, but it was, when we put it in and we got it out left unchanged, it might have changed within the process but we got it back. That's our catalyst, the catalyst is not, is also not going to be part of the overall complex reaction. So if we were to add this up, I'm going to cross out the catalyst because we're not going to include that either. We get O3+O3 yields O2+O2+O2, 2O3+3O2 is exactly what we originally, our complex will actually state so they should up to get our complex rection.

Okay so what does this all these mean? Like why I'm I even doing this, let's go over here we can actually find the rate of how fast a reaction goes using the rate reaction mechanism. So before we were dealing with, typically when you're finding the rate law you're dealing with empirical data or experimental data. You have in the lab and actually do some trials to actually get that. But if you know a reaction works and how the mechanism breaks down you can actually figure out the rate law just by using the elementary step. Let's talk about that, okay so I broke down, here's my complex reaction down here and I broke it up into its elementary step okay. Now you're going to be given this information whether it's slow, fast or fast and we're going to say this is our rate determining step right here. And what do I mean by rate determining step? Well the rate determining step is our weakest link. It's, the reaction proceeds no faster than the slowest elementary step meaning that we can't go faster than our slowest step. So this is going to be what our rate law is actually going to be dependent upon. So we know our rate law backbone is going to be rate equals k times the concentration of whatever it's involved in.

Since this is our slow step I can just take this reaction and use this reaction, so I can say the rate is a concentration of NO because we know it's a reactant squared. Now when we're dealing with empirical data or data or experimental data I could not do that, I had to figure out what this squared meant but, using empirical data like we did in another video. But this squared can actually come from our rate determining step this actually can tell us what the rate was without having to calculate it. And notice from our complex reaction H2 it doesn't matter, is not part of the rate law. It doesn't actually matter how much hydrogen I had put into my reaction because it's only, the rate is only dependent on how much NO is in the reaction.

And this actually is the illustrations through energy diagram that illustrates that fact, that this is a rate law because notice here's what we put it, but this is the first step nitrogen monoxide is reacting to form our activated complex and our activation energy is extremely high okay. And this is going to be, a lot of energy is going to take a lot of time to get up here and get down to the products. These are a little bit slower, these are faster because our activation energy is smaller okay as our rate mechanism indicates. You're not going to have to calculate this, you're actually going to be given this information but this is just a prove to show you or illustration to show why that is. So not only can you find the rate laws using empirical data, you can actually find rate laws using reaction mechanisms as well by using the coefficient from the for reaction mechanism.