Understanding Gibbs Free Energy: The Key to Exergonic and Endergonic Reactions

inside our body we have many different types of exonic and endergonic reactions now we know to basically calculate to

determine whether reaction is actually exonic or endergonic we have to calculate what the Gibs free energy is

of that reaction because ultimately it's the giftsfree energy value that tells us whether reaction is spontaneous or

non-spontaneous now the mathematical equation that allows us to actually calculate the magnitude of Gibbs free

energy is this equation here so the Gibbs free energy of that particular reaction under those conditions is equal

to the Gibs free energy under standard conditions when the concentration of products and reactants is equal to one

molar plus this entire quantity so 2.33 * r t log Q where R is simply the gas constant t is the temperature in Kelvin

and Q is the reaction quotient and that tells us the ratio of the concentration of products to the

reactants now what exactly is the meaning of Delta G well Delta G tells us the amount of free energy that is

produced or used up when a chemical reaction takes place under certain conditions so if a chemical reaction

takes place and it releases is Gibs free energy that reaction will have a negative Delta G value and such a

reaction is set to be spontaneous exergonic and so this reaction releases useful energy that can be used to power

other processes and reactions inside our cells and inside our body now if the Delta G is positive what that means is

that particular reaction will be endergonic nonspontaneous and a positive Delta g means we have the

input energy for that reaction to actually take place and in fact inside our body we can use exergonic reactions

to produce energy and then that energy can be used to carry out Ender Gonic reactions now what about when the Delta

G is zero well when the Delta G is zero that means our reaction has achieved equilibrium and this Q value will become

a K the equilibrium constant now what about the Delta G with this degree symbol what's the meaning of

this quantity well this describes the free energy value of the reaction under specific conditions called standard

State conditions and this describes conditions when the concentration of the reactants and products is equal to one

molar now to see what we mean by that let's take a look at the following graph so this energy graph basically contains

the Y AIS that's the Gibs free energy and the xaxis is the reaction progress and in this particular example I used

this chemical reaction so formic acid Associates into the conjugate base and produces the H+ ion now let's suppose

the concentration of this is one molar and the concentration of these two products is also one molar so when this

is the case we see that when one mole of formic acid at a concentration of one mole or transforms into one mole of

conjugate base and one mole of H+ ion which are also at a concentration of one molar then we see the Delta G between

the products and the reactants is given by 21 21.3 K or 21,300 jewles of energy

now because the energy the free energy of the products is higher than the free energy of the reactants that means this

reaction is endergonic and so we need to input 21.3 K of energy to actually drive this

reaction in the forward Direction so this reaction as described here under standing conditions is endergonic it's

nonspontaneous it's reactant favor and this is in accordance with the fact that formic acid is a weak acid and will not

dissociate to a very large extent now just because the Delta G degree symbol the Delta G understand

State conditions is positive for these conditions does not mean that Delta G will be positive under some other

conditions in fact by changing the Q value by changing the concentrations of the reactants and products we can

ultimately transform this endergonic reaction into an exergonic reaction and this is a very important concept because

it is continually used inside our body our body changes the concentrations of endergonic reactions to basically

transform them into exergonic reactions now if we look at the following equation this equation tells us exactly that so

what the equation tells us is if this quantity is positive then this doesn't necessarily have to be positive if this

is positive but this entire term is more negative than this is positive as a result of this Q value then a positive

quantity plus a negative value that is greater than this magnitude will give us a Delta G that is negative and it's this

Delta G It's The Sign of this Delta G not this one that ultimately dictates whether reaction is actually product

favored or reactant favored and to see what we mean by that let's carry out the the the following

calculation uh so in this particular case we know that Delta G standard State condition is equal to

21.3 K now the question is what exactly should the Q value be for this Delta G to actually be negative and for our

reaction to be spontaneous product favorite and the way that we're going to solve this problem is by basically using

some type of negative value for this Delta G so let's suppose Delta G is any negative value for let's so let's

suppose it's -5 K so this quantity is5 k and this quantity is positive 21.3 K now because the gas constant R is given to

us in Jewels so 8.314 Jews per Kelvin time mole let's transform these quantities into Jewels

so this quantity is equivalent to 21,300 Jews and that's a positive value while this quantity we said just a

moment ago we're going to use 5 KJ or equivalently - 5,000 Jew now the goal is to ultimately calculate what the Q value

has to be what the ratio of the concentrations of the products to the reactants has to be for the reaction to

actually be product favored for this to be -5000 a negative value now notice we call uh we could have also used -5 Jews

or -1 million Jews basically any Nega value here works because any negative value means this reaction will be

spontaneous under that Q situation under that uh concentration of products and reactants so if we take this equation

now and solve for log of Q we get that log of Q is equal to So This this term is brought to this side so it's this

minus this on the top and then we divide by this quantity so 2.3 303 then the gas constant is 8.314 Jew per mole time

Kelvin and the temperature we're going to assume is let's say 25° C so equivalently 298 Kelvin now if we solve

for Q we get that Q is equal to 10 to the power of this ratio now this divided by this gives us -

4.61 and so if we carry out this calculation raise 10 the^ of - 4.61 we get about

2.45 * 10 to -5 now what this means is when the Q value when this quantity when the ratio

of the concentration of the products and the reactants is equal to uh 0.000000

245 the reaction this reaction here will actually be X ergonic it will be spontaneous and it will be product

favored so what we basically show is even though this reaction is endergonic under standard conditions when the

concentrations of these are equal if we change the concentrations around we can actually

transform that endergonic nonspontaneous reaction into an exergonic spontaneous reaction as can be seen in the following

example where we used the Delta G on negative quantity so ultimately it's the Delta G that determines whether our

reaction under those concentration conditions and that temperature value is actually spontaneous or not this doesn't

necessarily have to be negative or positive it doesn't matter what This Is It ultimately matters that this is a

negative value to actually dictates spontaneity and this concept is important because in our body our body

can change the concentrations and therefore transform these endergonic reactions into exergonic spontaneous

reactions as we'll see when we discuss processes such as the crep cycle glycolysis and so forth