Hi. It's Mr. Andersen and this is chemistry essentials video 38. It's on elementary reactions.
In other words one step in an overall reaction mechanism. And so in this demonstration right
here what I'm doing is combining A and B. So A is going to be yellow. B is going to
be this purple grey molecule up here. And so you can see if I launch A at B, there's
no reaction that takes place. And the reason there's no reaction is that there's not enough
energy. In other words as these molecules are colliding there's not enough energy contained
within the molecules to actually have that reaction go. So let me pull it back a little
bit more and angle the shot. And I'm going to angle the shot at molecule A and B. And
we'll find that again no reaction takes place. And so we have to have enough energy in a
collision. And also it has to be oriented correctly. So now I'm going to pull it way
back. Hit it right on. And you can see that A is reacting now with B to make C. And then
this is proceeding over and over and over again like that. And so in an elementary reaction,
remember we have to be looking specifically at the collision of those molecules. And so
an elementary reaction is based on collisions. Again collisions could just be made of one
molecule. And you may want to think about that for a second. How do we have a collision
just with one molecule. If we do, that's a unimolecular elementary reaction. And it's
going to simple be A turning into products. A great example of this would be radioactive
decay. When for example we have carbon 14 just decaying. And as it does that that would
be a unimolecular. It's just that one molecule with itself. This would be the rate law for
that. So it would just be a first-order reaction. If we look at 2 molecules combining together
then we get a bimolecular elementary reaction. So A for example combining with A to make
products would be bimolecular. This is the rate law for that. So you can see that this
is going to be a second-order reaction. Or if we have A combining with B, like we did
at that animation at the beginning, the rate law would then just be k times the concentration
of each of those reactants. But both of these together are second-order reactions. Now could
you imagine 3 molecules coming together at the same point and the same time? If they
do we call that a termolecular elementary reaction. It's incredibly rare. But when it
does occur then it would be a third-order reaction. Again collisions are going to create
reactions between molecules. And so they have to have enough energy. If they have enough
energy we can start to stress those bonds and form new ones. And they have to be oriented
correctly. In other words they have to hit right on for us to have a collision. And so
this is something I've talked about a lot but really haven't explained it in detail.
If we're looking at an equation like this, the stoichiometry, in other words we're measuring
how much we have on one-side. And that could be in moles or grams and predicting how much
we should get on the other side is totally predictable. You can just look at the equation.
Make sure it's a balanced equation. And so there's huge predictability of that. But how
fast it occurs, or that rate law is not predictable. It's measurable. We have to measure it for
each reaction. And you might have always been thinking why is that? Well the reason why
is that every reaction as we write it doesn't show the whole picture. In other words what
we're really showing is the reaction mechanism. So it's all the steps that we have in that
reaction. In other words we're forming intermediates that are not even found in that overall reaction.
And since it's much more complex than we might see, then we can't predict it. We have to
measure it. But if we can break it down to these elementary reactions or elementary steps
and we know exactly how many molecules are hitting one another, then it becomes totally
predictable. And so let's look at a unimolecular elementary reaction. So this is simply A breaking
down into products. It would be a first order reaction. And a great example would be radioactive
decay. So it's simply carbon 14 decaying over time. And so watch during this animation.
Look at how many of those reactions we have. And at the beginning I just have a few carbon
14. So there's just a few reactions. And then again it slows down over time. Now let's add
way more carbon 14. You can see that we were going to have a way quicker reaction at the
beginning because we have more of those molecules. Or if we even have more, look at all the starbursts
that are showing that radioactive decay. And so again these are just molecules by themselves.
And so this one step would be a unimolecular elementary reaction. As we move towards a
bimolecular, these are going to be two molecules hitting each other. So let's say A hits A.
And in this animation I'm not going to show you the actual reaction occurring. But what
I want you to do is kind of count the number of collisions if you can in your head. So
let's watch with 5 molecules. There's one, two, three, four. Now as I increase it look
how many collisions we're having. Way more collisions. And then as we increase it to
20 molecules, way more collisions. So you can see why a second-order reaction occurs
the way it does. Because we have so many more molecules or so many potential collisions
that we could have. Now we could also have different reactants. A hitting B. And let's
say it starts to form C over time. Well we're going to do the same thing here. Now we've
got 5 of each of the molecules. So we can see the collisions. But again not enough energy
to start breaking them up. Now let's break it to 10 and 10. Now we're starting to see
reactions occurring. And now let's do 20 of each. And you can see we have tons of reactions
going because now we have more collisions. More energy at that point. That would be bimolecular
second-order. And then finally we can have termolecular elementary steps or elementary
reactions. So let's say A plus B plus C forms a new product. So you could imagine what are
the odds of that ever happening? It's incredibly rare. They all have to come together at the
same time. Have enough energy. And then be oriented correctly and make a product. It
does occur and when it does it would be a third-order reaction. In other words it's
the concentrations of A times B times C. And so did you learn that elementary reactions
are simply steps in an overall reaction? And if we know what that elementary step is and
we know how many molecules are colliding then we can make predictions about the rate law.
And I hope that was helpful.