Hi. It's Mr. Andersen and this chemistry essentials video 63. It's on the reaction quotient. One
of my favorite games when I was a little kid was hide and seek. But lot's of times at the
end you would have one person who's still hidden and the person who's searching for
them can't find them. And so we would always play a game hotter or colder. As they get
closer you would say you were getting warmer, you're getting hotter. And as you're moving
away you're getting colder. And that would be an easy way for them to find that last
person. And so in a reversible reaction we have something similar. And so we identify
K as the equilibrium constant which is going to be a ratio of products to reactants in
a reversible reaction. But that only occurs when we reach equilibrium. When we find what
we're looking for. But in science we have a reaction quotient, Q, which is going to
be the same thing. It's calculated the same way, but it's where we are before we actually
reach equilibrium. So it allows us to see how far we are away. It's like playing hotter
or colder. And so in a reversible reaction the reactants become products, products become
reactants and over time they eventually reach equilibrium. And when we reach equilibrium
we can calculate the equilibrium constant which is simply K equal to the concentration
of products divided by the concentration of the reactants. But along that pathway we could
also calculate Q which is going to be the concentration of products to reactants at
any given time. And over time Q is going to approach and eventually become K, once we
reach equilibrium. And so a model kind of works like this. Imagine if we have a free
energy diagram and this ball is going to roll back and forth. But let's say it eventually
stops right here. At the bottom is going to be K. And where it's at now would be Q. So
we know that it's going to move towards the left. If we let it roll a little bit more
it eventually pauses. Now we have a new Q. Now we know that it's going to move right
and eventually it's going to reach K or that equilibrium. And so if we use an energy diagram
like this, K is going to be right here. And Q is going to be any way along that path.
And so here I've got a model which is essentially a reversible reaction model. On the left side
we've got a bunch of molecules. We'll call them A. And as they bounce over to the right
side they become B. And so what I'm doing is simply keeping track of the number of reactants
and products. Reactants are going to be green and products are going to be red. And I'm
keeping track of that every 50 seconds. And you can see that it varies at the beginning.
At the beginning you can see here that we had 100 reactants and 0 products. But right
now as we get 350 seconds in it's starting to stabilize. And so I could gather data in
a model kind of like that. Now I'm going to take this data and I'm going to graph it.
And we're going to have a graph that looks like this. So at the beginning we have all
reactants and zero products. But you can see over time it's eventually reaching this equilibrium.
And so let's say that at the end we reach equilibrium. And again this is simply a model.
But how would be calculate that equilibrium constant? It's simply going to be the concentration
of products divided by the concentration of reactants. And so if I were to calculate that
our products is 65 and our reactants is 35. And so let's say K is 1.9. And so at the end
when it reaches equilibrium we'll say that our K value is going to be 1.9. Well, what's
Q? Q is going to be values along before we reach equilibrium. And so what's going to
be our Q at that first point? It's going to be 0. How did I calculate that? I simply took
the number of products which is 0 divided by the reactants which is 100 and that's going
to give me a Q value of zero. Now if I go to the next one, so if I go to 50 how am I
going to figure that out? It's going to be my products which are 0.42 divided by 0.58.
And so that's going to be 0.72. And now let's watch what happens to our Q value as we move
closer and closer to equilibrium. You can see it stabilizes here. But I simply don't
have enough products. It's lower than that. Now I over shoot it. And my Q value goes a
little bit too high. Then it eventually approaches it. And it eventually becomes K. Q & K are
exactly the same thing. And so this is a model but it shows you what's going on in a chemical
reaction. K is what we have at the end and Q is what we have along the way. And so if
we were to summarize a little bit in a reversible reaction, if my Q value is less than my K
value, the reaction has to move to the right. What does that mean, move to the right? I
have to convert more of my reactants into products. What happens if my Q value is greater
than my K value? Then I have to move to the left. And what do I have when my Q value equals
my K value? I'm at equilibrium. Or they're going to be the same ratio. Now it's sometimes
hard to remember this. And so if you put it on a number line it helps you quite a bit.
If I put my Q and my K values on a number line like this, so K is where I want to be
and Q is where I am, which way am I going to have to move along this number line? I'm
going to have to move from the right to the left. So I'm going to have to move left. I'm
going to have convert more of my products into reactants so I can eventually reach K.
Now this only works in a homogeneous kind of a state. In other words if we were to look
liquids and solids or gases to solids or gases to liquids, this reaction quotient is not
going to work. Now if we apply it to an actual chemical reaction, I've got this reversible
reaction up here. 2 moles of these makes 1mole of this and 2 moles of this, I can use what's
called an ICE table to look at the initial concentrations and then my final concentration
is at equilibrium. Why do they call it an ICE table? Well you can see here that I have
my initial concentrations, my equilibrium at the end and then I have my change or I
have my I C E. And this is going to be measured in molarity. And so if we were to calculate
what my Q value is right here, here's my Q value right here, well you can see that since
I have no products at this point my Q is going to be equal to 0. Now how do I figure out
my K value? If I'm given my concentrations at equilibrium I could calculate K. Now I'm
going to show you how to do that in the next video on equilibrium. But let's just say it
is 650. And again, watch the next video on equilibrium if you don't know how to do that.
So which way is this reaction going to move? It is going to move to the right? Is it going
to move towards the left? Well you can see if I put them on a number line, if I put K
on a number line and Q on a number line I have to move to the right, which should make
total sense. I have to convert these reactants to products if I'm ever going to approach
equilibrium. So it's going to move towards the right. And so if I were to show you what
those values are going to be, so how did I get these values? If I take this number subtract
that, I'm going to get this value right here. You should start to see a pattern in that
change row. And so if I look at it, all of these values are going to be the same molarity
change. Well why is that the same molarity change? It's because we have the same moles
up here. If we look at this one it's 0.019. Why is that? There's only going to be 1 mole
right there. And so stoichiometry is incredibly important. We have to make sure that that
change matches the stoichiometry of our equation. And so did you learn to determine the effect
of chemical reaction manipulation on Q and K values? I hope so. And I hope that was helpful.