Learn about Advanced Placement Chemistry, Chemical kinetics 5, in this comprehensive video by bannanaiscool.
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Male Speaker: In the bonding unit in the advanced section, I told you about how the formation of ion or compound is really achieved through a series of steps. Those series of steps are called the reaction mechanism. Okay, so now here is another reaction mechanism or first just take a look at a certain reaction. NO2 plus CO actually makes NO plus CO2 but you know it's not necessarily likely that that collision occurs in the atmosphere to form these two. What can happen is that a different series of reactions could actually be done in order to get this net equation. That's going to be called the reaction mechanism. Now, let's say somebody tells you know what here is your equation and here is the rate laws for, we actually know the rate laws in that equation, equals variables k times concentration of the NO2 and its square. It was experimentally determined. If you were given a reaction mechanism this can tell you a lot about how it would actually works and how it would actually works can tell you a lot about the rate law as well watch. NO2 plus NO2, those two molecules can collide in the atmosphere to form nitrogen trioxide and nitrogen monoxide. The nitrogen trioxide can then react with carbon monoxide to form NO2 and CO2. You know if you add these two equations to get -- you will notice that NO3s were canceled here and NO2 will cancel with NO2 and you will get this plus this makes this plus this and that's the net reaction. So, is this a possible reaction mechanism for this reaction here. Yes, it is so far because we can take all the equations in this case two of them added together to get the net equation, cool now but here is that the second thing that must be obeyed in order for this to be a possible reaction mechanism. This must be the rate law for the rate determining step. Now rate determining step is the slowest step. You know you can only go usually as fast as the slowest step of a reaction. Some slow steps take a long time, but the other one will occur lightening fast and so the reaction time, time that takes for the reaction to occur is going to be based on the slowest step. So, somebody tells you that this first step is the slowest step and the second one is the fast step. The rate determining step is the slowest step. You can write the rate law for a rate determining step by looking at the molecules that are colliding to form the products. You can do that with net equation so all equations so far that we have worked with you can actually write the rate law just by looking at the equation, you have to experimentally determine the order and we know that but here you don't have to do that. All you have to do is look at what we call the molecularity. This is a bimolecular reaction so this one reactant -- unimolecular, if there was three vacant we call termolecular but those are very highly are likely reactions termolecular because three molecules would have to collide with the proper orientation at one time in order to be able to form products. Two molecules more likely to do with one, it's easy to be able to form products and here is the thing. Since this is the slowest step, its rate law is actually just written as NO, the rate equals k times NO2 square. Well, is that the rate determining step, the rate determining step's rate law is the rate law for the overall reaction. Now, the overall reaction I told you it does have that rate law this is the rate law right here for the rate determining step so this is what we call a proper reaction mechanism. One, the mechanism adds together giving an equation to the rate law for the rate determining step which is just determined by looking at the reactants and write again, rate equals k times concentration of NO2 square is the rate law for that reaction so it's a viable reaction mechanism. Now by the way an other thing because NO3 right here was present in that reaction formed here and then present here to react with but in the net reaction isn't found, its called an intermedia