define this portion of my expression, delta H of the system minus T delta S

of the system as delta G, so I can use this to calculate delta G of the system

and what I see is that if Delta G of the system is less than 0

then I will have a spontaneous process because this corresponds to Delta S

of the universe

being greater than 0. If I make constant temperature

I see that I get delta G equals delta H minus T Delta S

because I see the difference in products and reactants for the Gibbs free energy

for Delta H and for Delta S, but my T value remains constant.

So, what is free energy? We talk about Gibbs free energy.

Well, it's the energy actually available or free

to do work. When we talk about the enthalpy of a reaction we know that energy is

given off for energies gained, but it doesn't mean that all that energy is usable for doing

work, only part of it is because some of it goes to the dispersal of

energy so what we want to worry about is what is the Gibbs free energy

that's the amount we actually have available to do work.

If there's no subscript as there aren't here we can assume that we're always

talking about the system.

So, delta H equals delta H of the system. The the same goes for delta

G and for Delta S. These are all in terms of the system.

So what does Delta G tell us?

Well, if delta G is less than 0 we have a reaction that is spontaneous in the forward direction.

If its greater than 0 it's spontaneous in the reverse direction or non spontaneous

in the forward direction.

If we have a delta G value equal to 0 then we know we have a system at equilibrium.

because it's neither spontaneous are non spontaneous in either direction.

So let's look at what happens when we have various values of Delta S and Delta H.

We could have both of these values as less than 0 or greater than 0,

so what we want to look at is the different combinations to determine the

relative value of delta G.

When will the process be spontaneous and when will it be non-spontaneous.

Let's first look at delta H is less than 0 and Delta S is a greater than 0.

so if delta H is less than zero then what I see is that my delta H value is negative

and my Delta S value is positive because it's great in 0

but the term, the second term, will be negative

so delta G will always be less than 0 in that case.

If I look at

Delta H greater than 0 and Delta S less than zero

I can't say delta H is greater than zero so it's positive

Delta S is less than 0, so it's negative,

but when I take the negative of a negative I get a positive value

and in that case I see that delta G is going to be positive.

Now we're going to look at an example to figure out what happens

when Delta S is less than 0 and Delta H is less than 0.

We have our equation delta G

equals Delta H minus T delta S. If Delta H is negative

and Delta S is negative which statement is true?

We know the sign of Delta G will depend on the temperature for Delta H and

delta S both less than zero.

The question becomes when will it be positive and when will it be

negative? We want to look at the difference in temperatures. When we're at a

high temperature

versus when we're at a low temperature. What I can do is

say I know I have a minus sign in front and this term,

delta S is negative, therefore minus T times a negative delta S will be greater

than 0 or have a positive value.

As T gets higher, a higher temperature, the T Delta S term becomes

more positive. It is increasing