Hi. It's Mr. Andersen and this is chemistry essentials video 40. It's on the reaction
path. If I want to move this object here from point A to point B it's going to travel along
a path. And it looks like it's a pretty straight path, from A to B. But if we were to look
at the reaction path, in other words the path from reactants to products, it's not going
to be so straight. In other words it's going to follow along a curved energy profile. In
other words that object has a certain amount of energy at the top. We put energy in. And
we're able to get some of that energy back. And so if we're looking at a chemical reaction,
this could be the reactants on this side. And the products on this side. But we have
to put a certain amount of energy into that system in order for that reaction to occur.
And so let's say we have these two molecules, A bonded to B and C bonded to D. And then
we're going to created these products down here. A now bonded to C and B bonded to D.
We have to put some energy into the system. It's just not going to spontaneously occur.
And so we have to add energy until we reach what's called the activation energy. And so
in an elementary reaction, or in a one step reaction we have to break bonds. And then
we have to form new bonds. And a good way to look at how that works as far as energy
goes is to use what's called an energy profile. This is the amount of energy or potential
energy those chemicals had before the reaction. This is after. And these one's right here
are going to be in what's called a transition state. They're transitioning from reactants
to products. And what we really have to do is start to weaken those bonds. And one way
that we can do that is to change the temperature. And so according to the Arrhenius Equation,
the constant or the rate constant in an equation is based on the temperature. The higher the
temperature, the faster the molecules are moving. And the faster, we're more likely
to hit this activation energy. Or the amount of energy required to reach that transition
state. And so what we can use is the rate constant and the temperature to actually calculate
that activation energy. And so in review, what is activation energy? Well when we have
a reaction, this the reactants and this is the products potential chemical energy. But
the activation energy is going to be the amount of energy that we have to put into that system.
And so you think of it now like reactants and products. Here are our two molecules we
have to begin, reactants. Here are our products that we have at the end. And so that transition
state is going to be weakening of these bonds right here. So we can break those bonds apart.
And then we can form those new bonds in those products. And so Svante Arrhenius studied
this. He's a Swedish chemist. Really brilliant guy. Also came up with the idea of the greenhouse
effect. And panspermia, how life could spread through the universe. And so it's a really
brilliant guy. But what he came up with is, looked at the rate equation. And so this is
that rate equation we've talked about. This is the rate of the reaction. This is the rate
constant. A is going to be the concentration of the reactants. And then m is going to tell
us about the order of that reaction. But if we look at all of these things here, which
one is really going to be affected by temperature? It's k. And so there is a temperature dependence
built into k. And so the Arrhenius Equation looks like this. And it basically describes
the temperature dependence of k, or that rate constant. It's pretty scary. And so if we
make it colorful then maybe it's not quite as scary. If we break through the parts of
this equation, k again stands for the rate constant. A is the frequency factor. And that's
basically built on the amount of energy particles have in their proper orientation. We've got
the activation energy. We've got the gas constant. And finally we've got the temperature. Now
where would you be asked to apply this in a chemistry class? Well what we can do is
we can measure two of these things. In other words if we measure the rate constant and
we measure the temperature in a reaction, then we can calculate activation energy using
the Arrhenius Equation. And so this equation by itself is not necessarily helpful in the
chem lab. But what we can do is we can take the natural log of the whole equation. And
if we do that, the natural log of k is equal to this. Now this looks a lot like that y
intercept form. And so this is natural log stands for y. This is going to be our slope.
This is x and this is b. And so let me show you how you could apply that. So in a lab
this is the temperature on the side. And this is k. This is going to be that rate constant.
So this has to be calculated or measured in an experiment. And so what I can do is I can
graph that data. Now what I'm going to do is graph it, not temperature on the x-axis,
but the inverse of temperature on the x-axis. And I'm not going to graph k on the y-axis.
What I'm going to graph is the natural log of k. But if I do that and I have my data
like this, what I can do is I can calculate the slope of that line. Now that slope of
that line is going to be what's in red here. And so the slope is going to be equal to negative
activation energy divided by the rate constant. And so we can measure this on the slope over
here. We know what the gas constant is. And so what we can figure out is the activation
energy. In other words the amount of energy that we have to put into that system in order
for that reaction to occur. And so did you learn to explain the difference between successful
and unsuccessful reactions in terms of energy and orientation? If you did, then you learned
what I hoped you would. And I hope that was helpful.