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.