Hi. It's Mr. Andersen and welcome
to my podcast on Gibbs Free Energy. Whenever I get to Gibbs free energy in the year I can
see my kids eyes gloss over and they're confused. And even Willard Gibbs talks about this in
this quote. He said the idea of entropy and the second law of thermodynamics will seem
far fetched and may repel beginners as obscure and difficult for comprehension. So what does
that mean? I'm going to try and tackle something that's been a problem for years. I'm going
to try to make Gibbs free energy more understandable. So that you really know what it is and understand
the power of it. Especially in biology. And so I think the first place to start is the
name itself. The idea of free kind of gets in the way. And so what I'd like to go back
to is old name, which it used to be called available energy. And so let's thing of delta
G as available energy. Or energy that we can actually use to do work. The other thing that
I think makes it hard is the equation itself is confusing. We've got delta G. We've got
enthalpy. We've got entropy. We've got words that you're maybe not understanding. And so
what I'm going to try to do is pare this down to the easiest explanation that I can. And
then we can get into more of the specifics a little bit later. And so the best place
to start when you're trying to understand available energy or free energy is to look
at spontaneous reactions. Spontaneous reactions are reactions that once you give them a little
bit of a push, they'll actually go on their own. These reactions also will tend to release
energy. And they give energy to their surroundings. Quintessential example in life is cellular
respiration. But let's not get there yet. So the three reactions that I'm going to talk
about here are, number one a ball rolling down a slide. Number two, diffusion. And then
number three, a cherry bomb that's exploding. And so we're going to apply Gibbs free energy
to each of these. So let's start with a ball at the top of a slide. And the ball at the
top of the slide is going to roll down to the bottom. And so the first thing that I
want to give you is what's called enthalpy or total energy of a system. And so if we
say this is a system. What happens to the total energy of the system as the ball rolls
down the slide? Well if you know anything about potential energy, the amount of potential
energy that we have up here is going to be greater than the amount of potential energy
that we have down here. And so the total energy of a system, we'll call that H or enthalpy.
The total energy of the system has gone from a big H to a little H. In other words the
total energy of the system has decreased when the ball rolls down. Now if I had to push
it up again, then we would add energy to the system. But this is a spontaneous reaction.
And the enthalpy or the total energy is decreasing. Now in biology we don't move around because
of potential energy or mechanical potential energy. In biology our energy or our potential
energy is actually in these bonds. In other words there's a huge amount of potential energy
in this bond between the carbon and the hydrogen. And so this right here is glucose. And if
we can release some of that energy, we can do it to do work. And so again, what is H?
H stands for the total energy of the system and it looks like in a spontaneous reaction
that's actually going to get smaller. Or decrease. Okay. Next one. Let's talk about diffusion.
So in diffusion imagine it right here that we've got a bunch of molecules in this container
and they're bouncing around. And I remove this wall. So if I remove that wall, what's
going to happen to the molecules? The molecules are going to spread out to fill that area.
We call that diffusion. Now entropy is, we use the symbol S for that, entropy is a measure
or the disorder of a system. Or sometimes we call that the randomness of the system.
And so let's compare this. Right here we've got a bunch of molecules on this side. And
then we've got a lot of space over here. So what happens to the disorder of that system
as I do diffusion? Well it's becoming more disordered. In other words the entropy is
increasing. What's an example of that? Let's say I go into your room and your room's a
mess. It looks like this. If I say clean up your room, then you could go like that. And
so what happens through diffusion or in the spontaneous reaction, well let's say remove
this wall. What's going to happen now? We're going to get even more disorder. So we're
going to even get a bigger S value. And so in this spontaneous reaction it looks like
the S value is increasing. Okay. Last one is that cherry bomb. So let's say we have
a cherry bomb. We put it on the desk. Does it explode? No. And one of the reasons it
doesn't is the temperature is really low. And so let's say I add a bonfire to the situation.
So I increase the temperature. Does that make it a more spontaneous reaction or is the reaction
more likely to happen or less? Well it's more likely to happen if I increase the temperature.
More likely to get an explosion. Okay. So those three things, total energy or enthalpy,
entropy or S, and temperature can effect spontaneous reactions. And so now let's apply that to
Gibbs free energy. And so before we actually get to the equation, let's do a little algebra
here. So let's say I wrote this equation. X = Y - AB. Okay. So let's say we had this
equation right here and I were to decrease this value, the Y value. What would that do
to the X value? It would decrease it, right? Let's say I were to increase the A value and
increase the B value, what would that do to the X value? Well since we're subtracting
right here that would decrease it as well or make it go even farther down. And so let's
go back and summarize those three spontaneous reactions. In the first one the ball rolled
down. So what happened to our H value? Well our enthalpy of the system decreased. What
should that do to our Gibbs free energy or our available energy? It should decrease that
value. What happened here when we increased the entropy of the system or increased the
delta S? Well if we increase the delta S that should also decrease the Gibbs free energy.
So now we have two things decreasing that. And what happened here? Well if we increase
the temperature that made it more spontaneous. So if we increase that, we also decrease the
Gibbs free energy. So what's the moral of the story? Moral of the story is that if the
delta G ever decreases or if it's ever less than zero, that's a spontaneous reaction.
Likewise, if the delta G is greater than zero, let's turn to the next slide, that's going
to not be a non-spontaneous reaction. Okay. So in summary. If the delta G is less than
zero that's a spontaneous reaction or we call that an exergonic reaction or an energy releasing
reaction. If it's greater than zero that's an endergonic reaction. And then finally if
nothing happens to the available free energy then it's just at equilibrium. The quintessential
example of a spontaneous reaction in life is going to be cellular respiration. Photosynthesis
is an example of an endergonic reaction. And so let's talk about each of those. So let's
say we're doing cellular respiration. So what I said earlier is that glucose has a certain
amount of energy within the bonds. And so let's do cellular respiration. So I take in
sugar, my body is going to combine that with oxygen. We're going to convert that to carbon
dioxide and water. So let's animate and see what happens. Wow. So a lot of stuff happens.
So let's go back. So first of all let's look at the entropy of the system. What happened?
Did it become more or less ordered? It seems like it became more random for sure. What
happened to the enthalpy of the system? Well we're releasing energy. So we have energy
out here. So we have released energy. That means that the total energy that was contained
within those bonds has actually decreased. And so you can look at the value right down
here. The delta G of cellular respiration is -686 kcal/mol. That means if you had a
bout a third of a pound of glucose or sugar, you were to break that down in your body,
you would release about 686 calories, big C, of energy. So we call that an exergonic
reaction. Or it's releasing energy. Now we can actually plot that on a energy diagram.
So glucose itself had a certain amount of free energy to start with. But in this cellular
respiration, we end up with less energy. So this is a spontaneous or a exergonic reaction.
Now it doesn't just explode into flames, the sugar that's sitting on your counter or the
sugar that's inside your body. And so in order to get that to work, you have to put a certain
amount of energy into the system. And that amount of energy we call activation energy.
And so if you look back to the first three examples I gave you, the ball just didn't
roll down the slide on its own. You had to push it. And the cherry bomb just didn't explode
on its own. You had to add a little bit of energy to that. We call that activation energy.
But there's a net loss in energy. And so we call that an exergonic reaction. Let's think
about photosynthesis. What do you need for photosynthesis? Really only three things.
Carbon dioxide, water and sunlight. Oh, four things. Because you also need plants. And
so in photosynthesis what happens, well we store that energy in glucose. And so if we
look at our delta G value it's a positive or we call that a endergonic reaction. If
we were to actually draw the energy diagram of that, we'd have a lower amount of energy
to start with. A greater amount of energy at the end. And so we're storing energy. And
then it has more energy at the end in the form of that glucose. Now where do we get
the activation energy? In photosynthesis we get that from the sun. And so those two reactions,
releasing energy and storing energy allow life to exist. But our day to day life doesn't
use glucose. Doesn't use, we do use glucose, but our second to second kind of a functioning
doesn't use glucose. We actually use something called ATP. And so ATP or adenosine triphosphate
is the energy coinage of our cells. It's what you're using right now. We can store it as
energy and then we can kind of cash it in. So let me show you what happens. ATP is broken
down into ADP and it releases a certain amount of energy. If we look at that the breakdown
of ATP to ADP or adenosine diphosphate, it only has two phosphates right here, plus a
phosphate group, is an exergonic or the delta G value is going to be negative. If we go
back and convert that ADP back into ATP, now you can see right here our delta G value is
actually positive. Or if we break it, we release energy and then we can store that again when
we make ATP again. And so right now when you move your finger like this, what's actually
powering that muscle is ATP releasing energy. Allows that muscle to contract and then we
can do that over and over and over again. And so you use respiration to get energy.
But we actually store that in ATP over time. So what is the secret of life? And how does
all of this work? Well energy comes from the sun. So we have a certain amount of energy
up here. And we store that energy through photosynthesis in sugars. And so the release
of energy by the sun, the delta G is going to be less than zero. It's an exergonic or
an energy releasing reaction. But photosynthesis, photosynthesis is going to have a delta G
value that is greater than zero. In other words it requires energy and it pairs that
to the energy coming from the sun. Next we make bread out of that. And then through cellular
respiration, respiration, we are going to release energy. So the delta G value now is
less than zero. We use that to store ATP. Now that ATP is used to breakdown into ADP.
And so that's going to be a delta G value of less than zero and that's going to release
energy. But to convert that ADP back into ATP is going to be a delta G value greater
than zero. And so available energy and free energy is super important. It's converting
energy in sun eventually into energy that we can use. And eventually that energy ends
as heat. So the energy is converted that whole way through but it just becomes more and more
disordered as we get to the end. And so that's free energy. And I hope that's helpful.
