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We're going to be studying chemical kinetics in our first unit.

Â In this unit we are going to be interested in how fast is chemical reaction is taking place

Â is it really rapid and it happens in a blink of an eye? Is it very slow and would take a thousand years?

Â We are embarking on how you would monitor that, how do you know the rate in which reaction takes place.

Â Well it's easy to monitor it for a car. You can measure how far it travels in a certain amount of time.

Â So we could say that a car travels at 200 miles per hour here in the United States,

Â Or we could say this is a very slow vehicle that doesn't travel that fast by measuring how far it travels over a certain amount of time.

Â Our first learning objective is to describe this concept of rate of reaction.

Â This concept of chemical kinetics and rate is defined by the change in concentration as a function of time.

Â We usually monitor that concentration in units of molarity and the time might be a second interval,

Â and that's what we will usually use in this lesson. It could be in minutes, hour or years for that matter.

Â You can either watch a reactant and see that its concentration is decreasing as time goes by,

Â or you can monitor the product and see how its concentration is increasing as time goes by.

Â Let's look at this schematic of a chemical reaction.

Â The reaction we're going to look at is A+B->C

Â and we see the colors represented, A as blue B as red and C as purple.

Â So I want you to look at the reactant A and what is happening to it as time goes by.

Â Now we see that we have a schematic at 15 minutes, 30 minutes and 45 minutes going by

Â as this reaction proceeds. Well, it is certainly decreasing as time goes by.

Â We're using it up. Now we want to be able to monitor that and define the rate of reaction by monitoring that.

Â So let's think about the changing of concentration. Will the change of concentration be positive or negative?

Â Now remember a change which is represented with delta is always final minus initial.

Â So if you think about how much you have at the end versus how much you have at the begining, will this be positive or negative?

Â Well if you said it would be negative then you would be correct.

Â Now as we continue to think about this reaction A+B->C the concentration is decreasing,

Â the change in concentration is negative, and when we talk about the rate of a reaction

Â we do not say that a reaction is going at a negative rate of speed.

Â So we have to take into account that there is a minus sign with that.

Â This portion that we just looked at is negative.

Â You just said that in the previous slide, and this portion has to be positive.

Â The rate of the reaction is always defined as a positive speed we place this minus sign in there

Â A negative times a negative is a positive, and this way the rate of the reaction will always be positive.

Â Now you can't travel in a car at a negative rate of speed.

Â You can go 100 miles per hour, 15 miles per hour, or 47 kilometers per hour,

Â but you can't go a negative 47 kilometers per hour.

Â You might go backwards, but you still go backwards at a positive rate of speed.

Â So we have to place that minus sign in there for monitoring reactants to obtain a rate of the reaction.

Â Now let's look at C. C is our product. What is happening to it as time goes by.

Â Well, it is increasing. We're getting more and more of it.

Â So we're monitoring the rate of reaction in terms of this C,

Â We are not going to have to have that negative sign in there.

Â We can just do change in concentration of C with respect to time.

Â No negative sign is needed.

Â Ok, now we have two different reactions. The one we've been looking at, A+B->C,

Â and D+E->F. Now, I want you do look at these images and tell me which one of those reactions is faster.

Â 4:38

Well, if you said that number 1 was faster then you would be correct.

Â What we need to do is to watch a reactant or a product as time goes by in those two reactions

Â and we will see that we are using up the reactant A, which was blue, or the reactant D, which was yellow,

Â we're using up reactant A more rapidly than D is dissapearing, therefore A has a faster rate of speed.

Â Alright, let's look at this graph.

Â We're monitoring a very basic reaction and A is turning into B.

Â A is represented by the black, so it is decreasing as time goes by.

Â B is represented by the red, and it is increasing as time goes by.

Â My question for you is this. Is rate of reaction a constant during the course of the reaction?

Â Is it running at the exact same rate of speed?

Â Is the change of concentration of the same time interval the same?

Â Well it is not. If it were it would be a nice linear graph.

Â But it is dropping off.

Â If we look at this time interval from five to ten seconds we see that we are increasing B by this amount.

Â This is the same time interval. It is a give second time interval from here to here.

Â but its change in concentration is much smaller

Â So we have got a smaller rate of speed as time goes by.

Â 6:20

So we have to do calculations over a certain rate and we have to define what that time interval is.

Â And it's going to be different depending upon which time interval we choose time as the reaction takes place.

Â When you're comparing reactions you would want to be monitoring it during the same time interval

Â to say this one is faster or this one is slower.

Â Now if we have that time intervale smaller and smaller

Â eventually it gets so small that it's a point in time

Â and it would be and instantaneous time.

Â An instantaneous rate could be obtained at that.

Â Now if we think about this in terms of a car traveling.

Â If you're going down the road for a couple hours and you measure the distance that you cover in those two hours

Â let's say that you went 140 miles in that two hours of time.

Â You could say that over those two hours that you traveled at 70 miles per hour.

Â Well, here in the United States that is a typical speed for interstate travel

Â and that's not going too fast.

Â But let's say you're traveling down the interstate and an officer clocks you going 100 miles per hour in that instance.

Â Now that's an instantaneous rate, and you can't argue to officer that over the two hours

Â you were traveling you were only averaged 70 miles per hour therefore you shouldn't get a ticket

Â he knows that at that instant you were going too fast.

Â 8:57

Now here we have a reaction which involves gases.

Â And you can monitor the change in pressure as time goes by.

Â We have a slope of the tangent line here to determine the instantaneous pressure change as time goes by.

Â So rates can be defined as change in pressure as well as change in concentration.

Â This is using PV=nRT

Â n/V is molarity, so if I take my v over here and my R and T over there

Â and these are constants for the reaction, let's say our temperature is constant as time goes by,

Â and this portion right here, mols per volume would be synonymous for molarity,

Â we can see that there is a relationship between molarity and pressure.

Â Therefore, you can be monitoring pressure as time goes by

Â and not have to do it in terms of concentration every time.

Â So, this the end of our first learning objective

Â how you define a reaction in terms of change of concentration of either product or reactant.

Â