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Course: MCAT > Unit 5
Lesson 11: Principles of bioenergeticsGibbs free energy introduction
Explore the concept of Gibbs free energy and its role in spontaneous chemical reactions. Understand how it's calculated and its relationship with enthalpy, temperature, and entropy. Discover how it influences metabolic pathways and the energy release in our bodies. Created by Jasmine Rana.
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- She says multiple times that delta G tells you when a reaction will occur. That is NOT correct. The Delta G for breaking down sugar is always negative, but that just tells you whether it is thermodynamically favorable. Doesn't mean that the reaction kinetically can occur because the activation energy may be way too large to overcome without the help of enzymes. That is why sugars do not spontaneously breakdown without being in our bodies. Therefore the preferred word usage should be - delta G means the reaction is favorable, NOT spontaneous.(29 votes)
- Spontaneous and favourable are interchangeable. Error arises when people associate spontaneous reactions with rapid reaction rates.(7 votes)
- in6:38you talked about entropy... what is it??(2 votes)
- Sal has a number of videos on entropy: https://www.khanacademy.org/science/physics/thermodynamics/v/entropy-intuition(4 votes)
- She wrote -ΔG to mean a negative ΔG value, but if the ΔG value is negative, then -ΔG would be a positive value? It should have been written as ΔG < 0 rather than -ΔG.(5 votes)
- Another way to remember the difference....
delta sign for change, then G, then tip the less than sign over to make an 'L' and then your 0. Think of glowworms or lightning bugs that spontaneously GLO. make sense?
Then for non-spontaneous, just reverse your sign and you are G2G.(4 votes) - I thought it was the CHANGE IN TEMPERATURE in the equation.(1 vote)
- What does G stand for? And what is the delta G measure?(0 votes)
Video transcript
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.