Hi. It's Mr. Andersen and this is chemistry essentials video 71. It's on free energy and
the equilibrium constant. Over the last 20 videos we have been talking about two things.
Thermodynamics and the importance of delta G, where we can think of delta G as the amount
of free energy available in the products and the reactants and the reaction occurring as
we kind of move from reactants to products. And then we started talking about equilibrium.
And the importance of K, which is a measure of the concentration of products to reactants.
As K is equal so are the reactants and products. As it increases we shift towards the right.
As it decreases we shift towards the left. But it's important that you understand that
we can interchange these two things, delta G and K. And that they are inherently tied
together. And so in any reaction we can look at it using thermodynamic reasoning. And the
importance of changes in free energy or changes in delta G. And also looking at it as an equilibrium
reactions where we've got K, this equilibrium constant, which tells us are we shifting more
towards the right or towards the left. And that there is a algebraic connecting between
these two. Where delta G equals the negative rt, so that's going to be the gas constant
because times the absolute temperature times the natural log of K. And so we can interact
between the two. And if we figure out our delta G, which I mean you could use the appendix
in the back of most chemistry books to figure it out, that tells us a lot about what's going
to happen to K. And so let's say K is equal to 1. We have an equal amount of reactants
and products. Well if we plug that into this reaction that means our delta G or a change
in delta G is going to be equal to 0. What happens if our K value is greater than 1?
In other words it's shifted toward the right? We'd now have a delta G that's going to be
a negative value. So we think of this as an exothermic reaction. It's spontaneous. It's
moving toward the right. And then if we look at a K value much less than 1, then we're
going to get a delta G of a positive value. So that's an endothermic reaction. It's consuming
energy. It's non-spontaneous. And so it's really tying together these two big thoughts
that we've been talking about for a long time. Now generally in AP chemistry we've only talked
about exothermic or endothermic. But you should also understand there is energy involved in
this. And so we could think of this as an exergonic or giving off energy reaction or
an endergonic reaction. Now what is that important? Well in biological systems it's important
to look at the amount of total energy that's either given off or being consumed. Not necessarily
the heat that's given off or consumed. And so if we look at this interaction right here
between delta G and K, and let's look at a few reversible reactions. So in this one,
the breakdown of water into its different ions, we would call a delta G value, if we
measure that, of 79.9 kiloJoules. Now let me show you what the K value is. It's going
to be a really really small value. What do we know about a positive delta G? That means
it's an uphill reaction, non-spontaneous reaction. What do we know about a really small K value?
That means it's going to be shifted more towards the left, or shifted more towards the reactants
in this case. If we look at another reaction, in this case we've got a delta G that's a
negative value. What's that tell us about its K value? That's going to be a positive
value. So this is a spontaneous reaction, or a downhill reaction. And in this case it
shifted way towards the right. So you should be able to predict, if I give you a delta
G, what's going to be my K value? Is it going to be positive or negative? That would be
the first question. And is it going to be a really small or a really large value? So
since our delta G is a positive value we know that our K value is going to be really really
small, just using this equation up here. And so you could plug in a gas constant and our
absolute temperature and that's going to be easily calculated. And so if we were to summarize
it again, how are these two tied together? Delta G is negative, then we've got a K equal
to a value greater than 1. That's going to favor the products. Or it's going to move
more towards the right. If it's equal to 0, K equals 1. And then if delta G is going to
be a positive value our K value is going to be much less than 1, or it's going to favor
the reactants. And so why is this important? Well if we're looking at biology, if we're
looking at the chemistry of life, it really tells us how we can couple reactions together.
And so if we look at this famous reaction up here, what we've got is glucose combining
with oxygen and we're making carbon dioxide and water. And so what is this? This is going
to be cellular respiration. It's taking place in all the cells of your body right now. If
we were to look it up in the appendices we'd find our delta G value is going to be negative
2880 kiloJoules. So we've got a delta G value that's a negative value. What does that mean?
It's going to shift towards the right. So what would our K value be? It's going to be
greater than 1. So that's going to favor our products. It's going to move towards the right.
This is a spontaneous reaction if we can get it going. And we use enzymes inside the cells
to do that. Now what are we using that energy to do? Well we've got another reaction here.
We've got adenosine diphosphate. And if we add a phosphate to it we make ATP. What do
you use ATP to do? It's used in all of the cells of your body. It's essentially cellular
coinage that your cells use to do work. I mean that's how you think. That's how you
move. That's how you kind of do all of the chemical reactions inside your body. And so
what's going on inside our cells? More appropriately inside the mitochondria of our cells is that
we're coupling this exergonic reaction, or this reaction that gives off energy with an
endergonic reaction, one that consumes it. So you could think of these as gears. As we're
converting that sugar into carbon dioxide and water we're using the energy that's released
in that exergonic reaction and we're tying it to an endergonic reaction. And so did you
learn to relate delta G to K? In other words, what happens as we decrease our delta G, as
we shift it more towards the right, what happens to our K? And vice versa? I hope so. And not
only do I hope that was helpful, this is the last video in this series and so I hope the
whole AP chemistry series has been helpful. I hope you have been along with me. And thanks
for watching.