Hi. It's Mr. Andersen and this is chemistry essentials video 41. It's on multistep reactions.
Imagine I'm going to build a toy out of the parts that are sitting right here. So maybe
I take a base. And then I carefully attach a wheel to it. Can you figure out what I'm
making? Add a few more wheels. A few more parts. Maybe a window. And pretty soon I have
a toy train. But I didn't just build that in one step. I had to build it step after
step after step. And so chemical reactions occur along a same path. We have the reactants.
We have the products. But it doesn't just magically appear. We have to go through step
after step. And we have all these intermediates along the way. And so when you're looking
at an overall reaction, remember it's not the whole story. The whole story is each of
those elementary steps added together. Now each of those elementary steps are based on
the collisions between particles. And so that collision theory is going to tell us a lot
about the speed of which that reaction is occurring. And so adding all of those up,
we get what's called the reaction mechanism. Or the overall steps of the reaction. And
this can only be postulated. In other words when you're looking at an overall reaction,
we can make predictions about the stoichiometry. In other words how much mass or atoms or moles
we're going to have at the beginning and at the end. But if we're looking at the reaction
mechanism, we can't do that. In other words we can't predict what's going to happen. We
can only measure it. And so the rate law can be measured experimentally. But each of those
steps have to be figured out. And that's what chemistry does. So if we're to look at this
overall reaction right here, I could make immediate predictions about the stoichiometry
of this reaction. In other words I know I have 1 mole of this on the left. I should
get 1 mole of this on the right. I could figure out the mass on the left. And the mass on
the right. But when I look at that overall reaction I know nothing about the rate law.
It can be measured, but I can't make predictions about it. And the reason why is this is based
on a reaction mechanism. In other words this is not just one step. This is two steps. Or
scientists think that this is two steps. We have this first elementary step. And then
this second elementary step. And you can see there are things showing up here that don't
even occur in that overall reaction. And so if we're looking at the reaction mechanism,
it can only be postulated. Chemists can put forward an idea of what they think is going
on. But we actually can't make predictions. However, if we break it down into its elementary
reactions and we know that's what's going on, then we can make predictions about it.
And so if we know this is the first elementary step, then we can take a look at that and
we can figure out things like the rate law. If we know this that first elementary step,
we know that these two things are colliding together. We know that this is a bimolecular
reaction. We could even model what's going on here. And so we could make predictions
about the rate law. The rate law is simply going to be k, or the rate constant. And this
is going to be a second-order reaction. Because this is the concentration of each of these
two reactants. And so let me give you an example of this. This is one of my favorite chemists.
This is Melvin Calvin. He got a Nobel Prize for this work. And what we was trying to do
is unlock what leaves do. In other words leaves are able to take in carbon dioxide and make
sugar. But it's not just one step. It's a series of steps that are going on. And until
he figured it out, it was really a black box. We didn't know what was going on. And so what
he did is he would stop the reactions at different points. He would figure out the intermediates
of that. And then he came up with what we call now the Calvin Cycle named in his honor.
And so remember, reactions don't occur just in one step. There's a series of elementary
steps within that. We can make predictions about that. But once we get to the level of
the elementary reaction we could figure out what's going on. And I hope that was helpful.