Hi. It's Mr. Andersen and this is chemistry essentials video 66. It's on Le Chatelier's
Principle, also referred to as equilibrium law. And it's really an elegant principle
that you apply to reversible reactions. And it basically says if there's ever a disturbance
to equilibrium, the reaction will move to counteract that disturbance. And so let's
say for example I'm at an equilibrium state between reactants and products and I all of
a sudden get an influx of reactants, way more reactants. It's going to throw my equilibrium
out. And so what can I do? Well since this is a reversible reaction I can move the reaction
from the left to the right according to Le Chatelier's Principle. And as I do that I
increase the amount of products and I bring it back into balance. That's essentially Le
Chatelier's Principle. And so according to that, in any reversible reaction where we
have reactants and products and it's established in equilibrium, if we ever have a disturbance
and that disturbance can be adding or removing chemicals. It could be a decrease in volume,
an increase in pressure. It could be an increase in temperature, a decrease in temperature
or even dilution, what's going to happen is a new equilibrium is going to be be reestablished
again. And so let me give you an example of that, a graphical example. If we're looking
at the Haber process where we're taking nitrogen and hydrogen gas and covering it into ammonia,
this could be our initial equilibrium, where we have given amounts of concentration of
each of these. But let's say at this point right here, there's an influx of hydrogen.
So we're increasing the amount of hydrogen. So that means we're increasing the reactants
on the left side of the reaction. According to Le Chatelier's Principle the reaction simply
moves to counteract that additional reactant. And so which way is it going to move? It's
going to move to the right. And so what's going to happen to the amount of hydrogen?
It's going to decrease. What's going to happen to the amount of ammonia, it's going to increase.
What's going to happen to the amount of nitrogen? It's going to decrease because it's a reactant
as well. And so a new equilibrium is going to be established. And so if we look at this
at the molecular level, so this is all of our gases. Again they're all mixed in together
but we have varying amounts of that. Let's say we get an increase in the amount of hydrogen
gas. How can we get rid of that hydrogen gas? We can simply combine it with nitrogen, make
more ammonia and then we can reestablish that new equilibrium again. This also applies to
pressure caused by changes in volume as well. And so let's say we're looking at this reversible
reaction where we have dinitrogen tetraoxide which is a clear gas which can move reversibly
to nitrogen dioxide which is going to be a reddish brown gas. And let's say we put a
mixture of these gases at equilibrium inside this chamber right here. And I were to simply
decrease the volume. So if I decrease the volume it's going to be more concentrated
and so we're going to have a darker color. But if you were to watch it for a given amount
of time it will turn back to that color. And so it's important that we understand really
what's going on. And you can't figure it out looking at it this way. Let's look at it at
the molecular level. So originally we had a mixture of those gases that look like that.
And what was it reddish-brownish? Because we had enough of those nitrogen dioxides which
are going to be these ones right here which give it that kind of a color. So what happens
when I squeeze it? What happens when I decrease the volume? I'm increasing the pressure of
those gases. And so according to Le Chatelier's Principle what's it going to do? It's going
to move to counteract that increased pressure. Well how can it do that? Well if you look
here at the reaction, we've got two moles of the nitrogen dioxide on the right side
but only one mole on the left side. And so if we could move this reaction from the right
to the left, we can decrease the number of moles that we have. We can alleviate some
of that pressure and that's really what's going on. And it's going to take a little
bit of time for that to occur, but that reaction is switching from the right back to the left.
This also applies to temperature and energy as well. So let's look at that same reaction.
Again we're taking this colorless dinitrogen tetraoxide and moving it into nitrogen dioxide
which is reddish brown. As we move from the left to the right side that's going to be
an endothermic reaction. What does that mean? It actually takes in heat from the surroundings
as we move from the left to the right side. And so I'm simply going to add that heat down
on the left side and we can now treat it as a reactant. And so what happens if we increase
the temperature? If we increase the temperature, if we increase that heat what it's really
doing is it's essentially adding more reactants. It adding more things on the left side. And
remember when we added more hydrogen in that Haber process what happened? It switched to
the right side. So we had a shift towards the right side. What's that going to do? It's
going to push our reaction to the right. We're going to get a darker color. And so this is
the same concentration of gas but here and here they've heated it up. And so we're seeing
that reaction move to the right side to counteract that increased temperature. Likewise what
happens if we decrease the temperature? It's like decreasing the reactants on the left
side and now it's going to move towards the left side and we're going to have more that
colorless gas. And so Le Chatelier's Principle is wonderful and we can apply it, for example,
to increase yield. And so let's say we're looking at this Haber process where we have
nitrogen gas and hydrogen gas making ammonia. This is an exothermic reaction. And so that
means we could put heat on the right side. And so let's say I want to increase the amount
of ammonia. Well let's look at the concentration. If I want to increase the amount of ammonia,
what would I do? I would simply increase the reactants. If I can increase the concentration
of my reactants it's going to push the reaction to the right side. I'm going to have more
of that ammonia. Let's look at the next one. How could I increase my percent yield by changing
the volume and thereby the pressure? Well if you look on the left side we have 4 moles.
On the right side we have 2 moles. And so if I could squeeze this reaction it's going
to convert towards lower moles. It's going to convert towards that ammonia. I'm going
to have a great percent yield as well. Now let's look at temperature. I mean the temptation
is to say just heat it up and that's going to give us more of the yield. But since this
is an exothermic reaction that means we have more heat on the right side. And so if I increase
the temperature it's actually going to push it back to the left side. And so I would want
to decrease this reaction's temperature in order to get more of that percent yield. In
actuality there's a nice balance because we want it to go quickly and so temperature and
pressure are actually balanced out. And so did you learn which way the reaction is going
to shift based on disturbances be it either concentration or pressure or temperature?
And then finally could you, given a reaction, figure out ways by changing temperature, volume
and concentration to increase percent yield? I hope so. And I hope this was helpful.