Hi. It's Mr. Andersen and this chemistry essentials video 61. It's on kinetic reaction control.
Or more appropriately it's comparing that to thermodynamic reaction control. Lots of
times in a process, a spontaneous process, there's a choice that's being made. And determining
on which path we take we may have different products. And so it's kind of confusing and
I wanted to start with an analogy. Imagine if I asked you to buy a car. But I only gave
you two places that you could go. You could either go to kinetic used cars or you could
go to themodyn new cars. And so you're going to have to make a choice. And depending on
which choice you make you're going to get a different car. If you go to a used car dealership
you might get a car that looks like this. And if you go to a new car dealership they're
going to have a car that look like this. Now there are going to be advantages of both.
In other words what's the advantage of buying a used car? It's going to be super cheap.
It won't be as expensive. But you're going to get a different product. If you have enough
money it's better to get a new car. Because that new car is going to be more reliable.
And so this choice is also being made in spontaneous processes. In other words there's control.
And so it could either be kinetic control. And that kinetic control lots of times is
based on activation energy. It's the cost of that process. And then we also have thermodynamic
control. What's governing that? That's going to be the change in free energy. Or our delta
G. How much difference in free energy is there between the reactants and the products in
this downhill reaction. And so let's go back to the cars for just a moment. So if we start
with that cheap car right here, what's the advantage of that cheap car? It's going to
have a low cost. But it's also going to have a low reliability. If we have that expensive
car, it's going to cost us more money to begin with, but it's going to be more reliable over
time. And so what's governing that choice is really cost. If we don't have the money
to buy a new car then we're simply not going to be able to buy it. But if we do have enough
money it's better to buy that new car because it's going to be more reliable over time.
And so what's really governing this choice? It's going to be that initial cost. Money
is determining the path. Now let's stop talking about cars and then apply this to chemistry
for a moment. And so in a free energy diagram like this we start with our reactants. And
so let's say we have this one pathway where we have reactants and products. And then we
have another pathway like this. What are we getting? We're getting different products
of it. So what's going to govern the difference between these two pathways? Well it's going
to be the activation energy. It's how much energy we have to put in to get that reaction
going, to achieve that transition state. If we don't have enough energy to reach this
transition state then we're going to have to take this lower pathway. Our products are
going to have less stability. They're going to have less energy in them. So what's governing
the other pathway? That's going to be our change in free energy. We would like to take
that pathway if we could. We would like to have more stable products, but we can't unless
we achieve a high enough activation energy. So what's really determining the path. A lot
of the time it's the temperature that we have. And so let me give you an example of that.
Let's say that we have this reaction right here. We have our reactants on the left side
and we have products on the right side. But let's say this product is less stable then
this product. Well if we're looking at this chemical reaction at a really low temperature,
you can see that we're getting really low amounts of that more stable molecule and high
amounts of the less stable molecule. Well what do you think is going to happen if we
increase the temperature? So we're increasing it almost 100 degrees, a little over 100 degrees
celsius. What's going to happen? Well we're able to create more of that more stable molecule.
Why is that? We're taking that thermodynamic pathway. We're taking that new car dealership
pathway. And so in that first process, again, there was kinetic control going on because
we didn't have the energy. We didn't have the money to take that path, but if we do,
it's more advantageous to take the thermodynamic path.And so again, can you explain what a
thermodynamically favored reaction may produce varying amounts of products? Again, it's determined
on which pathway you're taking. If you're controlled by the kinetics, then you're going
to take that lower pathway. And I hope that was helpful.