Hi It's Mr. Andersen and this is chemistry essentials video 58. It's on spontaneous processes.
Those are processes that occur without external energy. And so I'm going to knock over this
tower right here. And as I do that that is a spontaneous process. Now it's not going
to occur until I add a little bit of energy. You can see that that block right in the middle
on the bottom I have to pull that out of the way. But as I do that, that's going to be
a spontaneous process as it falls down. And that's going to just move in one direction.
It's not magically going to move in the other direction. And so in the other direction that
would be a non spontaneous reaction. It's not going to occur. It's statistically impossible
for that to occur. And so as I talk about spontaneous reactions I'm going to treat the
words enthalpy and internal energy the same. There are slight differences, but we're just
going to treat them as synonyms. And so in processes we can either have spontaneous or
non spontaneous processes. We can know that it's a spontaneous process right away if we
ever see a decrease in enthalpy and an increase in entropy. Now if you don't know what these
two things are, I'll put links to videos about both enthalpy and entropy. But if we ever
see a decrease in that amount of internal energy and an increase in that randomness
or that dispersal of that matter, we know that it's a spontaneous reaction. And any
spontaneous process is going to favor the products or the things that are created in
that process. If it's non spontaneous we can know right away if we see an increase in enthalpy
and a decrease in entropy. We know that that's going to favor the reactants and it's going
to be non spontaneous. And so how do you know if we're favoring the products or the reactants?
We can use something called the equilibrium constant to figure that out. And I have a
little model that shows you that. If we ever see an equilibrium constant greater than 1,
we know that that's going to favor the products and it's going to be spontaneous. If it's
ever less than that, less than 1, then we're going to see a non spontaneous reaction. Now
the thing that's tricky about a spontaneous reaction is that it just doesn't occur magically.
Sometimes it can occur over millions of years. And so you probably know that graphite can
be turned into a diamond but it takes a huge amount of energy and it takes a huge amount
of pressure. And so that's going to be a non spontaneous process. It just doesn't occur
on it's own without external energy. It takes a huge amount of energy. But if that's in
one direction, in the opposite direction we're going to have a spontaneous reaction. But
you know that a diamond just doesn't breakdown into graphite. It takes millions of years
for that to occur. And so spontaneous process is probably a bad term. It's better to use
a thermodynamically favored process. That just is too much of a mouthful. And so if
we're looking at a spontaneous process one of the first clues is to look at the enthalpy,
which is the amount of internal energy. And if we ever have a change in enthalpy that
is negative we know that that is probably going to be a spontaneous reaction. So if
we were to look at a thermite reaction it's exothermic. It's giving off that energy. That
means our reactants have more energy than our products. If we're to look at the rusting
of iron like you do in the hand warmers we have a negative enthalpy change. And so that's
a clue that this is going to be a spontaneous reaction. But sometimes we'll have reactions
that occur on their own. But they instead of giving off energy are actually consuming
that. And so if we were to look at this cold pack right here that means that our delta
H is actually going to be a positive value. So it's going to be an increase. And so we
can't just look at enthalpy itself. We have to add entropy to the equation. So imagine
if I had these two spheres. On the left side we've got gas. On the right side we have a
vacuum. And I were to just open it up, what's going to happen to those molecules? Well they're
going to move from the left to the right side. Is there any change in temperature? No. And
so what we're seeing there is a change in entropy. And so if you're looking at a spontaneous
process you don't want to only look at the change in enthalpy, you want a change in that
entropy as well. And so to figure out if we're favoring products or reactants we can use
a term called the equilibrium constant which really works with reversible equations. But
this will work right here. So if we imagine we have reactants and products, the equilibrium
constant is simply the concentration of products divided by the concentration of reactants.
And so let's look at this first scenario. And this is a PHET simulation. I'm going to
put some molecules over here on the left side. And so they're going to spread out. And then
they're going to be acted on by gravity. They're going to bounce back and forth. I started
with a hundred of those molecules on the left side. They're randomly dancing back and forth.
And so as they move to the right side we're changing color. And so we can represent those
as products. So if we look at how many I have on the left side and the right side, on the
left side I have 51. On the right side I have 49. If I were to divide my products by my
reactants, 50 over 50, we would say that my K value is right around 1. And so we're going
to be at equilibrium. In other words we're not favoring either reactants or the products.
Now if we look at this in the next simulation right here, I'm going to change it a little
bit. I'm going to put the ones on the left side, give them a little bit more gravitational
potential energy. And let's start that going. And so again those molecules are bouncing
around. You can see that the energy on the reactant side is actually higher than the
energy on the product side. And so that's pushing those molecules over to the right
side. Or it's favoring those products. If we speed it up and let it run for awhile you
can see now that I have 76 molecules on the right, 24 on the left. So if I were to put
75 over about 25 my K value is going to be around 3. And so that means it's going to
be great than 1. So we're favoring products in this case. And so that's going to be a
spontaneous reaction in this case. Or a spontaneous process pushing it to the right. What have
we done? We've seen a decrease in the enthalpy or decrease in the energy. And if you look
at it carefully you can actually see an increase in entropy as well. Let's go in the opposite
direction. Now I've given less energy on the left side. If we let this run for a little
bit it's hard for them to get up to the right side. It's hard for them to become products.
And so let me speed this up a little bit and we'll watch it change over time. You actually
see less entropy in this model right here. Less change because it takes more energy to
move to the right side. If we were to figure out the K value now our products are at 20.
Our reactants are going to be at 80. So if I were to figure out my K value it's around
0.25. And so it's going to be less than 1. So if my K value is less than 1 that tells
me this is probably a non spontaneous reaction. And so in a spontaneous reaction you know
it's spontaneous right away if you see these two things. If you see a decrease in enthalpy
or a decrease in that internal energy and you see an increase in entropy. It is non
spontaneous if we see the opposite of that. In other words if we see an increase in enthalpy,
in other words if we're looking at an endothermic reaction and we see a decrease in entropy.
So if we were to kind of organize that a little bit, in the upper left what we're seeing here
is an increase in entropy and a decrease in enthalpy. So this right here would be an exothermic
reaction where it's becoming more random over time. Know right away that that's going to
be spontaneous. If we look in the lower right, what are we seeing here? We're seeing an increase
in enthalpy. This is an endothermic reaction. And we're seeing a decrease in entropy. We
know right away this is not spontaneous. Now what about these other two? Well you're going
to have to watch the next video on Gibbs Free Energy to figure out how we do that. And so
did you learn to predict whether a process is spontaneous using delta H and delta S?
I hope so. And I hope that was helpful.