- [Voiceover] So we have some words here
that relate the different reactions
and whether they absorb or release
different types of energy.
So the first word here, exothermic.
Exothermic the root of the word is therm
which relates to heat and these word indeed
means a reaction that releases heat.
Releases, it releases heat.
And one way to think about it if you're thinking about
constant pressures or change in enthalpy
it can be viewed as your,
how much heat you absorb or release.
So a negative change in enthalpy
means that you're releasing heat.
One way to think of, if you view enthalpy as heat content
you have less heat content after the reaction than before
it was meant you release heat.
Which means you're change in enthalpy is going to be
less than zero so these all mean the same thing.
Well, this is true.
You're releasing heat.
This is a same thing as releasing heat if you talk about
constant pressure.
Constant pressure which is a reasonable
assumption if you're doing something in a beaker
that's open to the air or if you're thinking
about a lot of different biological systems.
Now based on that logic what do you think this word means,
endothermic.
Well endothermic, therm
same root and now your prefix is endo
so this is a process that absorbs heat.
Absorbs heat.
Or if you're thinking of a constant pressure,
you can say your enthalpy after the reaction
is gonna be higher than the enthalpy before the reaction.
So your delta H is going to be
greater than zero.
All right, fair enough.
Now let's look at these two characters over here.
Exergonic and endergonic
so exergonic the root here is ergon
and you might not be as familiar with that
as you are with therm but you might have heard
the word ergonomic.
Say, hey that's a nice ergonomic desk.
That means it's a desk that's good to do work at
or it's a nice ergonomic chair.
An ergon does indeed come from the Greek for work.
And so exergonic is a reaction that releases work energy
or at least that's what the word implies.
Releases, let me do that in the same color.
This is something that is going to
release work
energy.
And endergonic, same logic,
well that's gonna be something based on just
the way the word is setup that absorbs work energy
or uses work
energy.
Now one of our variables
or properties that we can use to think about energy
that can be used for work is Gibbs free energy
and the formula for Gibbs free energy,
if we're thinking about constant pressure and temperature,
so let me write that down.
So if we're talking about constant pressure
and temperature then the formula
for Gibbs free energy or you can even view this
as a definition of Gibbs free energy.
The change in Gibbs free energy,
let me do this in another color.
The change in Gibbs free energy is equal to
our change in enthalpy
minus, use in the different color.
Minus our temperature
times our change in entropy
and if this looks completely foreign to you,
I encourage you to watch the video
on Gibbs free energy but the reason why
this is related to energy for work
is okay, look I have my,
whether I'm absorbing or I'm releasing heat
and I'm subtracting out entropy which is kind of the energy
that is going to the disorder of the universe
and what's left over is the energy that I can do for work.
That's one way to think about it.
So you can see that this relates work energy
to change in enthalpy right over here.
So exergonic, something that releases
work energy could say that has less work energy
after the reaction than before it,
your delta G is going to be less than zero.
So let me write that down.
So here our delta G is going to be less than zero
and these things, these are reactions that release
work energy, we've seen it
in the video on Gibss free energy.
We consider this to be spontaneous.
Spontaneous.
These are going to move forward.
So these over here, the ones that absorb work energy,
well they're gonna have more work energy
in the system than before is one way to think about it.
So your delta G
is going to be greater than zero
and we say these are not spontaneous.
So these are not spontaneous.
Now that we have the definitions out of the way
and we have a way to relate these variables,
let's look at these different scenarios of things
that are exothermic and exergonic
or exothermic and endergonic and see why
they make an intuitive sense.
So in this first reaction it's exothermic, our delta H
is less than zero.
That means it has less enthalpy after the reaction
than before which means it released heat
and so you can see here, this heat is being released.
And where did that energy come from?
Well when it bonds
in this new configurations on a net basis
the electrons are able to go to lower energy states
and release that energy.
And heat, if you're thinking about in a microscopic scale
it's something that's raising your temperature
at least locally which it means just about transferring
kinetic energy to these microscopic molecules.
Remember when you're talking about heat or temperature
you're thinking about these macro variables
but on a microscopic variable,
you're talking about kinetic energies and potential energies
and things like that.
So what's happening is these electrons
or when they get into a new configuration
and they're going to release energy
and that can be transferred to the individual molecules.
So you see here, we've released energy
and we also have an increase in entropy.
We have more entropy after the reaction
than before the reaction.
We have more objects right over here,
there's more states in which they could actually be in
and they're actually moving faster.
So this one, we see if you just apply,
if you apply the formula over here
this is gonna be less than zero.
This over here, delta S is going to be greater than zero.
The temperature, that's gonna be absolute temperature
in terms of Kelvin so it's always going to be positive
and so this whole term is going to be positive
so you're gonna have a negative, minus a positive
it's going to be negative.
So our delta G
is going to be less than zero and we see
that this is spontaneous.
This is going to move forward and it makes sense,
it releases energy, the electrons like it.
It creates a more disordered state.
Another way to think about it is think about
trying to do the reaction the other way,
you're gonna have to get some energy for those electrons
to get into a higher energy state when they form
these new bonds your gonna have to get
these four constituents together in the exact right way.
That seems a lot less likely to happen
than going in from the left to the right.
Now let's think about something that absorbs heat
and this one's a little bit counter intuitive.
It absorbs heat but it's still going to be spontaneous.
It's still going to be exergonic.
It's still going to happen.
So delta H is greater than zero
so it absorbs heat to happen.
So I have these two molecules
with these different constituents, they are about to collide
and we're saying that the temperature is high.
If the temperature is low, this might not be spontaneous
but if the temperature is high enough it will be.
So the temperature high in a microscopic basis,
you're saying, okay these things just have them,
really high kinetic energy,
they're going to ram into each other really fast
and they're gonna ram into each other so fast
that they can form all these other constituents.
So you have the net entropy,
you have the net entropy has increased.
Even though over here our electrons
are in a higher energy state to form this configuration
so it had to absorb heat,
so it had to absorb heat energy.
So we could say heat but heat on a microscopic level,
we're just talking about kind of kinetic energy
of these molecules.
So it have to absorb it but where did that energy come from?
Well, it came from the kinetic energy of the molecules.
They might had a certain kinetic energy before
but then some of that gets lost
so when they all get banged up
into their different configurations.
If you're saying, well I still don't get this.
Think about trying to do this reaction the other way.
Try to get these four constituents in the right time,
all together, even though if they're happening,
if they're put together at the right way
their electrons could configure in a way to release energy
but this is super high temperature.
This is a really, really chaotic system.
It's not gonna go from right to left,
it's gonna go from left to right.
When it's really chaotic, things are banging
each other really fast, you're more likely to go
on a direction of higher entropy.
So now let's look at,
and so this is spontaneous even though it absorbs heat.
If you're not draining the heat away locally,
your temperature at least around these molecules
it will go down.
But as source we're assuming constant temperature
for this so you can assume that in a macro level
that temperature dissipates and gets absorbed
outside of the system somehow.
Now, let's look at this configuration.
It's exothermic so delta H is less than zero,
less enthalpy after the reaction than before
so it's releasing heat but it's not spontaneous.
And it's not spontaneous because it's reducing
the entropy in the world.
It's reducing the entropy in the world
and the entropy matters because our temperature is high.
One way to look at this equation is entropy doesn't matter
when temperature is low.
Temperature is really scaling your entropy
but when temperature is high, entropy start to take over.
This variable starts to matter a lot.
So over here, because entropy is negative,
this isn't gonna, this thing's not gonna actually happen.
So if these things were coming together very slowly,
their electrons could configure in just the right way
so that they can get to a lower energy state
and release energy.
But they're buzzing pass each other so fast
that they're not gonna have a chance to do that.
If you think about it the other way, this reaction
is much more likely to happen.
If you have a bunch of this diatomic molecules
running around, they're gonna bump into each other so fast
that they're gonna knock the constituents off of
these diatomic molecules or at least the way it's depicted,
it kinda looks like a diatomic molecule.
And they might absorb some of that kinetic energy
in doing it, in order to go from right to left
but that's more likely to happen.
So from left to right not spontaneous
because entropy really matters at this high temperature.
Then finally, and this one's pretty intuitive
something that needs heat, something that needs heat energy
and has a reduction in entropy
that's definitely not going to be spontaneous.
So this is greater than zero, this is less than zero
but then you're subtracting it so this whole thing
is greater than zero, this delta G
is going to be greater than zero.
Delta, let me do that in a green color.
This delta G is going to be greater than zero
and it makes sense that you have these two molecules
that you have to get together in just the right way.
They need heat in order to proceed with this reaction
to kind of excite, to excite the electrons
to higher energy state to get into this,
I guess you could say less stable bond.
Why would they do that?
The reaction is much more likely to go in this way
or if you had a bunch of these molecules
they're all knocking into each other,
they get into a more stable configuration
and there's more entropy when they split up
than when they actually stay together.
So delta G greater than zero.
This is endergonic and endothermic
and of course, this one was delta G greater than zero.
Even though this would release energy
that the things that's so chaotic they're not gonna have
a chance to do that and you're much more likely to go
in the direction of maximizing entropy
and so this one also is not spontaneous.