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We're going to be studying chemical kinetics in our first unit.
In this unit we are going to be interested in how fast is chemical reaction is taking place
is it really rapid and it happens in a blink of an eye? Is it very slow and would take a thousand years?
We are embarking on how you would monitor that, how do you know the rate in which reaction takes place.
Well it's easy to monitor it for a car. You can measure how far it travels in a certain amount of time.
So we could say that a car travels at 200 miles per hour here in the United States,
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.
Our first learning objective is to describe this concept of rate of reaction.
This concept of chemical kinetics and rate is defined by the change in concentration as a function of time.
We usually monitor that concentration in units of molarity and the time might be a second interval,
and that's what we will usually use in this lesson. It could be in minutes, hour or years for that matter.
You can either watch a reactant and see that its concentration is decreasing as time goes by,
or you can monitor the product and see how its concentration is increasing as time goes by.
Let's look at this schematic of a chemical reaction.
The reaction we're going to look at is A+B->C
and we see the colors represented, A as blue B as red and C as purple.
So I want you to look at the reactant A and what is happening to it as time goes by.
Now we see that we have a schematic at 15 minutes, 30 minutes and 45 minutes going by
as this reaction proceeds. Well, it is certainly decreasing as time goes by.
We're using it up. Now we want to be able to monitor that and define the rate of reaction by monitoring that.
So let's think about the changing of concentration. Will the change of concentration be positive or negative?
Now remember a change which is represented with delta is always final minus initial.
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?
Well if you said it would be negative then you would be correct.
Now as we continue to think about this reaction A+B->C the concentration is decreasing,
the change in concentration is negative, and when we talk about the rate of a reaction
we do not say that a reaction is going at a negative rate of speed.
So we have to take into account that there is a minus sign with that.
This portion that we just looked at is negative.
You just said that in the previous slide, and this portion has to be positive.
The rate of the reaction is always defined as a positive speed we place this minus sign in there
A negative times a negative is a positive, and this way the rate of the reaction will always be positive.
Now you can't travel in a car at a negative rate of speed.
You can go 100 miles per hour, 15 miles per hour, or 47 kilometers per hour,
but you can't go a negative 47 kilometers per hour.
You might go backwards, but you still go backwards at a positive rate of speed.
So we have to place that minus sign in there for monitoring reactants to obtain a rate of the reaction.
Now let's look at C. C is our product. What is happening to it as time goes by.
Well, it is increasing. We're getting more and more of it.
So we're monitoring the rate of reaction in terms of this C,
We are not going to have to have that negative sign in there.
We can just do change in concentration of C with respect to time.
No negative sign is needed.
Ok, now we have two different reactions. The one we've been looking at, A+B->C,
and D+E->F. Now, I want you do look at these images and tell me which one of those reactions is faster.
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Well, if you said that number 1 was faster then you would be correct.
What we need to do is to watch a reactant or a product as time goes by in those two reactions
and we will see that we are using up the reactant A, which was blue, or the reactant D, which was yellow,
we're using up reactant A more rapidly than D is dissapearing, therefore A has a faster rate of speed.
Alright, let's look at this graph.
We're monitoring a very basic reaction and A is turning into B.
A is represented by the black, so it is decreasing as time goes by.
B is represented by the red, and it is increasing as time goes by.
My question for you is this. Is rate of reaction a constant during the course of the reaction?
Is it running at the exact same rate of speed?
Is the change of concentration of the same time interval the same?
Well it is not. If it were it would be a nice linear graph.
But it is dropping off.
If we look at this time interval from five to ten seconds we see that we are increasing B by this amount.
This is the same time interval. It is a give second time interval from here to here.
but its change in concentration is much smaller
So we have got a smaller rate of speed as time goes by.
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So we have to do calculations over a certain rate and we have to define what that time interval is.
And it's going to be different depending upon which time interval we choose time as the reaction takes place.
When you're comparing reactions you would want to be monitoring it during the same time interval
to say this one is faster or this one is slower.
Now if we have that time intervale smaller and smaller
eventually it gets so small that it's a point in time
and it would be and instantaneous time.
An instantaneous rate could be obtained at that.
Now if we think about this in terms of a car traveling.
If you're going down the road for a couple hours and you measure the distance that you cover in those two hours
let's say that you went 140 miles in that two hours of time.
You could say that over those two hours that you traveled at 70 miles per hour.
Well, here in the United States that is a typical speed for interstate travel
and that's not going too fast.
But let's say you're traveling down the interstate and an officer clocks you going 100 miles per hour in that instance.
Now that's an instantaneous rate, and you can't argue to officer that over the two hours
you were traveling you were only averaged 70 miles per hour therefore you shouldn't get a ticket
he knows that at that instant you were going too fast.
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Now here we have a reaction which involves gases.
And you can monitor the change in pressure as time goes by.
We have a slope of the tangent line here to determine the instantaneous pressure change as time goes by.
So rates can be defined as change in pressure as well as change in concentration.
This is using PV=nRT
n/V is molarity, so if I take my v over here and my R and T over there
and these are constants for the reaction, let's say our temperature is constant as time goes by,
and this portion right here, mols per volume would be synonymous for molarity,
we can see that there is a relationship between molarity and pressure.
Therefore, you can be monitoring pressure as time goes by
and not have to do it in terms of concentration every time.
So, this the end of our first learning objective
how you define a reaction in terms of change of concentration of either product or reactant.
Dr. Allison SoultLecturer
Dr. Kim WoodrumSr. Lecturer
