Welcome to Week 6 of statistical molecular thermodynamics.
[SOUND]. In this first video of the week, I want
to begin discussing entropy. So, it turns out this nice studio I'm
sitting in is about about maybe 200 feet from the banks of the Mississippi River.
And there's a bridge over the Mississippi not far from here.
And I'd like you to imagine an experiment maybe many of you have seen.
What if I were carrying a chunk of metallic potassium, standing in the
middle of the bridge, and I tossed it into the Mississippi River, well below
me, fortunately. Well, many of you probably have not
actually seen it with potassium, you've seen it with sodium.
Toss a chunk of sodium in water, it skitters around for a while, it makes
hydrogen gas, the hydrogen gas ignites, there's a little explosion, it's all
pretty dramatic. It's a lot more dramatic with potassium
it turns out. This is one of those, don't try that at
home experiments. But I guess what I want to emphasize is,
I toss that potassium K into the river, K plus H2O.
There's a big explosion and I make potassium hydroxide and a half a mole of
hydrogen gas, assuming I don't ignite that hydrogen gas and end up making water
with the oxygen in the atmosphere. Let me ask you to consider a different
process. What if I take a flask full of potassium
hydroxide in water, just like that little section of the river, and I bubble
hydrogen gas through it. What happens?
Not really much, right? We, it, the direction that it goes, we
know which direction it goes. But why?
Now, some might think oh, well, you know, it goes from potassium plus water to KOH
plus one half H2 because that's the exothermic direction.
Heat is obviously released. Maybe it's exothermicity.
Well, let me tell you about a process or two where there is no exothermicity, so
enthalpy change would be zero. And yet the process we would all agree
from sort of common sense and observation is spontaneous.
So let me take one example. Here I have two vessels separated by a
stopcock, and at the moment that stopcock is closed.
On the left side is an ideal gas and on the right side is a vacuum.
That vessel is completely evac-, evacuated.
I open the stopcock and immediately, so this is a little bit of a thought
experiement because in practice it would take a second, two seconds maybe.
But imagine that I've got a big enough hole in my stopcock that immediately
[SOUND] all the gas is equally distributed across both vessels.