# Oregon State University Ecampus Online Chemistry Lecture - Entropy

0 (0 Likes / 0 Dislikes)

Oh hi, we would like to take a look at the gibs real quick.
And that is delta G is = to delta H minus TdeltaS.
What I would like to point out there is how we could experimentally determine values.
Now delta G can be determined from this expression.
Delta G is equal to negative RT natural log of K.
R has been measured experimentally over the years.
And people pretty much agree on a number--8.314 joules per mole kelvin.
You measure the temperature of your system and, this is so sweet, you measure the equilibrium constant.
So if you have a system with a couple of gases you measure products over reactants, this gives it to you.
And we can go ahead and publish delta G values.
Delta H, and this goes way back--this goes way way back to chem 121 type topics,
delta is the heat at constant pressure.
So we do calorimetry.
We go ahead and burn something and measure the heat being given off.
So I will go ahead and write delta H is equal to constant pressure, we call it the heat, Q.
So if we want to know something, like the entropy of a cheeto.
We take a cheeto, we give it a spark to get it started.
We burn it and measure the heat that it gives off.
We call that delta H.
So these two are very simple determined by experimental data.
Now, delta S is a little bit of another story, in particularly for me.
Now what we can do is say 'look if we let the system reach equilibrium then delta G is zero.'
So you have a reaction out on the table and it's delta G is equal to zero.
Now that's rather sweet because what you can do is do an experiment and go ahead and measure delta H
and we can calculate what delta S is.
Because rearranging, add TdeltaS to both sides, divide by the temperature, and delta S, the entropy,
is equal delta H over T.
Now this is extremely difficult to do in real life.
And here's why...we need to maintain equilibrium the whole way.
Remember one of the conditions was that delta G was equal to zero.
If we don't maintain equilibrium, this number is not zero, it's something else.
You might know what it is one moment but you may not know what it is the next because you have a reaction going on.
You might have it ever changing.
So what we like to say--is have a system where the, here it is, processes continually reversible.
So delta H is Q. The heat at constant pressure.
But under this condition what we say is.. it's completely reversible.
Now imagine this, imagine if I took some dust, put inside this metal...actually a milk jug, one gallon milk jug
set it on fire so that it gave off heat and flames, maybe even blew the top off
I can measure the heat, I can say here's the temperature and so here's the heat, if you will,
the heat that was absorbed by the surroundings
I can measure delta S if the system were at equilibrium the whole time.
So what that would mean is I go ahead and light it on fire and say 'look it's expanding a little
but I am not letting the reaction go 100%.' It's something I can reverse and set back down.
It's as though the lid gets blown off a little and I can set it back down.
Lid gets off a little bit more and I can set it back down.
This is a classic problem in engineering when they deal with engines,
cylinders going up and down inside of a chamber and pressures are changing.
They say their cylinder goes up reversibly, like a little, and then back down.
Imagine if you will, there is no air around this, we do it inside of a vaccum.
And then you can imagine maybe a little bit better that you can have reversible steps.
Now that is very difficult to do, often what we do is calculations or simulations.
Because in real life, if I were to go ahead and light this on fire, imagine that I am supposed to stop this
oh my goodness, even smaller than the eye can see, and say that every single step is reversible.
So I have this dust, and it's called lycopodium powder, ground up fern sporn, it has a large surface area.
And I lit a candle in here, and I put some of the dust in here and if I give it a good blast...
squeeze some air in here, the dust will come in contact with the candle and it will go ahead and explode.
So let me go ahead and do this. And imagine, if you will, I am supposed to stop the tape at every fraction of a second
and show, hey it's reversable and I am supposed to take measurements at every one of these little steps.
So it's very very difficult to do.
Got it.