Hi. It's Mr. Andersen and this chemistry essentials video 65. It's on the equilibrium constant.
A chemical weapon that was used in World War I and killed a number of people was phosgene.
Phosgene is created through a reversible reaction where you take carbon monoxide and chlorine
gas. You mix them together and you get this phosgene. And this is a World War II poster
warning people that this chemical might be coming. It gives off a smell kind of like
hay. And since it's colorless that was an important way for you to know that this chemical
might be coming. And so in any kind of a chemical reaction like this it's reversible, the reactants
are converted into products and the products are converted back into reactants. But it's
important that we know do we have more reactants or do we have more products? And so we use
our K value, our equilibrium constant to figure that out. In this chemical reaction at standard
temperature and pressure we're going to have a really large K. What does that mean? This
reaction is going to move towards the right. That means that we're going to have way more
phosgene then we're going to have any of these reactants. And so in a reversible reaction,
reactants are converted into products and back again. And over time it eventually reaches
an equilibrium. Now what is K? K is simply a way to measure when we reach equilibrium,
do we have way more reactants or do we have way more products that are being produced.
And so I've got this model right here that shows us how reversible reactions work. So
there are going to be a bunch of molecules over here. We'll call those reactants and
they'll be green when they're on this side of the chamber and they'll be red when they
move over to the right side. This is a PHET simulation. Over to the right I've created
a K-ometer which is going to measure our K value. So K is going to be the number of products
divided by the reactants. In this case we only have reactants on the left side and so
we have a very large number here, 100, and how many products do we have? Zero. And so
what's our K value going to be? It's simply going to be zero. So let me start this and
see what happens to our K value. And so molecules are moving now from the reactant to the product
side, from the product to the reactant side, but we basically have the same level on both
sides. And so our K value is 1. Now as I tip it towards the right we're getting more products
and so that is going to become a larger number up here and so our value goes up to 3. We
have about three times as many products. Likewise when we pull it in the other direction now
we have way more reactants. And since our reactants, in this case value, are going to
be around 75, products around 25, our K value is going to be around one-third. Now this
model over simplifies a little bit because we only have products and reactants in this
simulation. In a chemical reaction you're going to have lots of products and lots of
reactants. And so if we've got a reversible reaction like this where this represents our
reactants and this represents our products, this is the general equation for K. But if
we were to figure it out for a more complex kind of an equation this is the general way
that we write out K. Where K represents the concentration of one of our products, so that's
going to be C, raised to the power of the number of moles that we have of that chemical.
Likewise the concentration of D raced to the power and that power is going to be moles
of this product as well. Likewise we're going to be put the reactants on the bottom. It's
simply the concentration raised to the moles. What am I talking about? Let me show you an
example of that. If this is our equation right here this is going to be our equilibrium constant.
And so what do we have? Well here's going to be our products on the top. So it's going
to be the concentration of water, right here, raised to the 2 power, because we have two
moles out in front of that. Likewise we've got nitrogen gas raised to the 1 power because
we only have 1 mole of that. We're then going to put our reactants in the bottom. Likewise
both of these are raised to the second power because each of those have two moles. Now
if I were to give you that reversible phosgene reaction, could you right the equilibrium
constant for that? What's our K value going to be equal to? It's simply going to equal
this. Well each of these are raised to the 1 power because we have only one mole of each
of these. So this would be our K value. Now we can calculate K in the lab. And so at standard
temperature and pressure our phosgene is going to be this. This is going to be that value
I showed you at the beginning, 4.57 times 10 to the 9th. What does that mean? We're
going to have way more of this product then we do the reactant at that given point. In
other words you can write this number over 1 on the bottom and it's going to show you
how many of them are going to be the phosgene. Now if we were to change the temperature we
can actually change that K value. If I change that temperature to 200 degrees celsius, so
if I change it and make it a really high temperature, you can see that our K value is going to be
much lower. What does that mean? It's going to be shifted back towards the left and we're
going to have way more of those reactants than we do products. Likewise if we were to
take this equation right here, where we've got nitrogen and oxygen and we're converting
that into nitrogen oxide, what would that look like, our K value? Our K value is going
to be simply this. It's going to be raised to the second power because we have to moles
of this gas. What is our K value for this equation? It's going to be a really, really
small value. What does that mean? That we're going to have way, way more reactants concentration
wise than products. And so it's going to be shifted way over here towards the left side.
Now what if I were to increase temperature at this point? Well, let's say we were to
have a strike of lightning. That's actually going to raise our K value and it's going
to push it over towards the right side. Now we know that in the atmosphere we have nitrogen
gas and we have oxygen gas. But if we have a K value that was really really large at
standard temperature and pressure, all of this would quickly be converted into nitrogen
oxide. But we know that doesn't occur. It only occurs when you have a huge amount of
energy, a huge amount of lightning. And so again in summary, if our K value is ever great
than 1, that equation is going to move from the reactant to the product side. We're going
to have way more products then we do reactants. If it's ever less than 1, then we're going
to have the reverse be true. We're going to have way more reactants then we do products.
And so did you learn to use the values of K to predict which chemicals we're going to
have when it reaches equilibrium? I hope so. And I hope that was helpful.