Learn about Advanced Placement Chemistry, Chemical kinetics 2, in this comprehensive video by bannanaiscool.
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Rob Lederer: We are looking for the rate law for this reaction. Somebody is going to ask you what is rate law for this given this experimental data which has concentrations of the reactants or about the products and the initial rate of the reaction or a time zero. How is that concentration affecting the rate? So, we can actually write this rate law in a very rudimentary form right now and then plug what we need to. This is what we you could always write. So write the rate law, right away I go, rate equals k that's ten times the concentration of one of the reactants NH4 positive times the concentration of the NO2 negative. But, here is the thing. You don't know X and Y here which are the powers to which those concentrations are taken. But, the information here is going to help us get those powers. Once we get those, we can actually find K which is a rate constant. Watch how this happens, okay. There is simple way to do it. There is a mathematical way to do, but it is a simple way and the charge will always give you a simple way. Watch this. If you have a trial where these concentrations or these two chemicals are reacting and they give you certain rate. What happens if in trial two, you keep the concentration of the NH4 the same NH4 positive but, you change the concentration of the NO2 negative by doubling it. Well, it looks like the rate has done what, doubled. If the concentration of a certain chemical doubles and the rate doubles, what's the power to which that chemical is taken if this is how a constant which it is. It is the power of one. It's a first order type of reaction for that chemical. So, we know the order is now for the NO2 negative. That's cool, okay. Now, look at this one. When you compare lines of data two and three, trials two and three, look what happens. The concentration of the NH4 positive is doubled here. This stays the same, so that's good. As long as one stays the same, the other changes, we can figure out what's happening. This thing is the same. This doubles when I raise the rate. Now I know that's a kind of cookie way to read scientific notation but did it on purpose to show you. This number times four Thos THoisTthis nubmthos thisiisp;asdgoishaboihasboihsbis this number here. So when the concentration was doubled, the rate went up four times. So what's the order, the power to which this has taken? Yeah, yeah if this doubles and this goes by four and this is constant, this is a two. So, overall we say that this reaction is two plus one is third order overall and look, if you took this one of data right down and plug this number in for this, this number in for this, square and to the first power and you put this in for rate, you can divide these in to this side to find K, the rate constant and that's how you would do that. This is called the differential rate law method or the initial rate method for finding this differential rate law or just called the rate law.