Our bodies are constantly active.
Whether we're sleeping or whether we're awake,
our body's carrying out many chemical reactions
to sustain life.
Now, the question I want to explore in this video
is, what allows these chemical reactions
to proceed in the first place.
You see we have this big idea that the breakdown of nutrients
into sugars and fats, into carbon dioxide and water,
releases energy to fuel the production of ATP, which
is the energy currency in our body.
Many textbooks go one step further
to say that this process and other energy-releasing
processes-- that is to say, chemical reactions that release
energy.
Textbooks say that these types of reactions
have something called a negative delta G value,
or a negative Gibbs-free energy.
In this video, we're going to talk
about what the change in Gibbs free energy,
or delta G as it's most commonly known is,
and what the sign of this numerical value
tells us about the reaction.
Now, in order to understand delta G,
we need to be talking about a specific chemical reaction,
because delta G is quantity that's
defined for a given reaction or a sum of reactions.
So for the purposes of simplicity,
let's say that we have some hypothetical reaction where
A is turning into a product B. Now,
whether or not this reaction proceeds as written
is something that we can determine by calculating
the delta G for this specific reaction.
So just to phrase this again, the delta
G, or change in Gibbs-free energy,
reaction tells us very simply whether or not
a reaction will occur.
Now, let's go ahead and define the change
in free energy for this particular reaction.
Now as is implied by this delta sign, we're measuring a change.
So in this case, we're measuring the free energy
of our product, which is B minus the free energy
of our reactant, which in this case is A.
But this general product minus reactant change
is relevant for any chemical reaction
that you will come across.
Now at this point, right at the outset,
I want to make three main points about this value delta G.
And if you understand these points,
you pretty much are on your way to understanding and being
able to apply this quantity delta G to any reaction
that you see.
Now, the first point I want to make has to do with units.
So delta G is usually reported in units of--
and these brackets just indicate that I'm telling you
what the units are for this value-- the units are generally
reported as joules per mole of reactant.
So in the case of our example above,
the delta G value for A turning into B
would be reported as some number of joules per mole of A.
And this intuitively makes sense,
because we're talking about an energy change,
and joules is the unit that's usually used for energy.
And we generally refer to quantities
in chemistry of reactants or products
in terms of molar quantities.
Now, the second point I want to make
is that the change in Gibbs-free energy
is only concerned with the products
and the reactants of a reaction not the pathway of the reaction
itself.
It's what chemists call a "state function."
And this is a really important property of delta G
that we take advantage of, especially in biochemistry,
because it allows us to add the delta G
value from multiple reactions that are taking place
in an overall metabolic pathway.
So to return to our example above,
we had A turning into a product B.
But what if the product B turned into another product C?
If we wanted to calculate the overall Gibbs-free energy
for A going to C, we could instead
calculate the individual delta G for each step
of the reaction that is A going to the product B,
and B going to the product C. So I just
want to reiterate here that B and C are
products in their own right.
They're not transition states.
But what we're seeing here is that in some cases
we may not be able to measure the change in Gibbs-free energy
going from A to C directly.
So instead, we can add together the individual change
in Gibbs-free energy for each step,
because remember Gibbs-free energy is a state function.
And if we do that, we ultimately get the change
in Gibbs-free energy for the overall reaction
of A going to C.
Now one fun way that I kind of remember the state function
like quality of delta G, as well as some other variables
in chemistry, is that my chemistry professor
used to tell us that life is not a state function.
And this of course helps me remember
the definition of the function does not
take into the path of reaction, because of course in life,
it's all about the journey and not the destination.
But in chemistry, sometimes it's the opposite.
Now, the third point that I want to make
is that delta G unlike temperature, for example, which
can be readily measured in a lab for a particular situation,
delta G is something that can be calculated but not measured.
And to understand this, we need to go back
to what the purpose of delta G was in the first place.
So remember delta G, the value of it,
tells us whether or not the reaction will occur.
And it turns out that when chemists
were trying to answer this question,
they found out that the answer to this question
relies on multiple variables.
There's not just one thing that determines whether or not
a reaction will occur.
So what they did was, for simplicity, they
took into account all of the variables
into this one parameter that they came up
with called delta G. And the way they
did this was by creating an equation.
So they said, the change in Gibbs-free energy
is equal to the change in enthalpy,
or heat content, of a particular reaction minus the temperature
of the reaction times the change in entropy,
or broadly speaking randomness, between products and reactants
in a particular reaction.
Therefore, as I mentioned before,
we can go ahead and calculate one single value that
takes into account all of the variables that
affect the extent and degree to which a reaction will occur.
And it turns out that we can actually
measure the change in enthalpy, the temperature, and the change
in entropy for a reaction, so that works out quite well.
Now, at this point, you probably have a question of OK,
I see that I have an equation to calculate
delta G for a reaction, but what does
this value that kind of pops out of this equation
tell me about a reaction?
So let's go ahead and go back to our hypothetical reaction
of A going to B.
Let's draw a diagram that will help
us understand this reaction better.
So I'm going to go ahead and draw a y-axis and an x-axis.
On the y-axis will be the quantity free energy
in units of joules, let's say.
And on the x-axis will be the quantity of a reaction
coordinate.
And this is kind of an abstract parameter that simply
is a way for us to kind of monitor
the progress of a reaction over time.
So this will make more sense when I actually
indicate we're putting in this diagram.
So let's say that our reactants A have
a much higher free energy than the products of our reaction,
which is B in this case.
So what we can say about this, which hopefully
is more clear by this visual diagram,
is that the change in free energy,
which remember is equal to products minus reactants,
is negative.
Or we say it's less than 0.
On the other hand, let's say that we started off
with reactant A that had a much lower free energy
than the product B. Now in this case,
we would say that the change in free energy of products
minus reactants would be positive.
Now, the key takeaway here is that for any chemical reaction
that has a negative delta G value,
we say that the reaction proceeds spontaneously.
That is, it proceeds without an input of energy.
So I'm just going to write spontaneous there.
On the other hand, when a delta G value is positive,
that is when the conversion of reactants to products requires
a gain of energy, we say that it's a non-spontaneous reaction
and cannot proceed unless there is an input of energy.
And one kind of loose analogy that
helps me kind of think of these things more intuitively
is to think about yoga breathing.
So imagine that you're taking a deep, deep breath in, and all
of this breath that you have inside of your body
makes you feel kind of unstable and wanting to burst.
So I kind of think of that as starting off
at a high free energy state.
So let's say we're starting off with A.
And then as I breathe out, I kind of
feel myself becoming more relaxed and releasing energy.
And that brings me to B, which has a lower free energy.
And that of course, breathing out, is a spontaneous process.