Hi. It's Mr. Andersen and this is chemistry essentials video 60. It's on driving non spontaneous
processes. Again those are processes that don't occur without external energy. We also
call them thermal dynamically non favorable. Sometimes we call then endergonic. But if
we were to look at a free energy diagram like that, we're essentially looking at an uphill
reaction where we have a positive delta G. If we shove on it it's just going to return
where it was before. And so how do we lift that? In other words how do we lift that when
the reactants actually have less free energy then the products do? Well there's two ways
that we can do that. First of all we could add energy from the outside. So this is just
a model, but if we add energy from the outside then we can lift those reactants and eventually
have the energy of the products. Where is that energy coming from? It could externally
come from electricity, for example when we're charging a rechargeable battery. Or it might
even come from the sun. And so that's how a solar panel works or how a leaf works, if
we're looking at photosynthesis. Another way we can do it is we can couple it to a spontaneous
process. And so if we were to look at an exergonic or an energy releasing process like this,
where delta G is less than 0, that's going to be a downhill reaction. So you could imagine
if we were to couple those two together, and again this is just a model, then we could
take some of that energy released in that spontaneous reaction and use it in the non
spontaneous reaction. So again, in a non spontaneous process our delta G is a positive value. So
our goal is to make it a negative value. In other words make this spontaneous. We can
do that in one of two ways. We can take energy in from the surroundings. So for example if
we use electricity, like when we're charging a battery, then we can change that delta G
to a negative value. Likewise if we were to take in light for example in photosynthesis,
we can turn that upside down as well. We can also couple it to another reaction or to another
process. So for example in our body we constantly do this using a molecule called ATP. Every
time we convert ATP into ADP we release a little bit of energy and we use that to do
things like think and move and talk. And so let's start with electricity. So if we were
looking at a battery like this, why does a battery eventually run out? It's because our
reactants had more energy than our products do. And so we're losing that energy. Now we're
using that to do work. We're using it and losing it as thermal energy. But eventually
you have a battery that doesn't work. And so how am I going to recharge it? I have to
connect it to electricity. And what that electricity can do is it can actually turn those products
into reactants and then add energy to them. And we can recharge that battery so we can
use it over and over again. Likewise, if we were to look at life on our planet, most of
us are heterotrophs. What does that mean? We take energy in our food. So we take energy
in our food, for example like in glucose, we combine it with oxygen, we make carbon
dioxide and water. But when we do that they're reactants. In other words our food and our
oxygen have a higher amount of free energy than the products that we produce. And so
we would be stuck if it weren't for the autotrophs. If it weren't for plants. What they can do
is turn that backwards. In other words they can take that carbon dioxide and water and
they can add energy to it. Now where are they getting the energy from? They're getting it
from the sun. And so how does photosynthesis work? Well inside a leaf you have chloroplasts.
And inside the chloroplasts you have a membrane like this. And what it's doing is it's taking
in energy from the light. What's it doing with that energy? It's getting its electrons
excited. It's passing those electrons down an electron transport chain. And eventually
those high energy electrons go back into your food. Now who are they making that food for?
For themselves, because they're going to do that same process of respiration. Now where
is a lot of that energy going? A lot of that energy goes to this molecule right here, ATP,
which is used in energy coupling inside our body. And so again we're using external energy
to turn that endergonic reaction upside down. Now that's one of only two ways that we can
do this. We can also energy couple. So what's going on there? We're taking an endergonic
reaction and coupling it to an exergonic reaction. Or we're pairing a non spontaneous with a
spontaneous reaction. So inside our body the molecule that we use over and over and over
again is going to be ATP, or adenosine triphosphate. And on the end it has these three phosphate
molecules. And as this last phosphate pops off, it releases energy with it. And so that's
going to be a spontaneous reaction. It's releasing energy. But then we can break our food down.
And as we do that that releases energy which we can use non spontaneously to create more
ATP. And it is said that you go through your whole body weight in ATP every day. Now what
do we use that ATP to do? We use it to couple to non spontaneous reactions inside body.
So for example how do your nerves work? If we're to look at the lining of a nerve, or
the membrane of a nerve, we have the spontaneous release of the phosphate as we breakdown ATP.
But we're using that for non spontaneous movement of ions inside and outside. If we were to
look at a muscle what we're doing is that spontaneous release of the phosphate is coupled
to a non spontaneous contraction of that muscle fiber. And so again it's not like we're using
electricity. It's not like we're using light. We're storing energy in molecules and then
we're simply coupling that to exergonic reactions. So did you learn to explain how external energy
and energy coupling can be used to drive endergonic reactions? I hope so and I hope that was helpful.