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Welcome to this first lecture in statistic molecular thermodynamics.
So many of you may have been drawn into the course based on the introduction
video which involved me doing a demonstration of the thermite reaction.
So I'd like to spend this first video talking a bit about the philosophy of the
course, how it's going to be laid out. And also I, I hate to do a demonstration
without then providing some information about it afterwords.
And I told you that part of the course would involve learning how to do the
calculations that would let you, yourself, determine whether a reaction
like the thermite reaction would take place, and if so would it release a lot
heat, for instance. I think of that as sort of a big picture
thermodynamics calculation. And by big picture, if I had to pick some
sort of analogy, it'd be a big picture like building a house.
So a grand project. And at the end you've got some satisfying
outcome predicting properties of a chemical reaction.
However, the truth is, this course is designed to be fairly fundamental.
So, before you can build a house, you've got to build a hammer.
And so, a fair amount of the course will involve from first principles, building
up the tools that we'll need to understand thermodynamical concepts, and
the to apply those concepts. Nevertheless, in the first few lectures,
we're going to be a little bit light. I'm going to give you a, a sort of a big
picture overview of what happened in the thermite reaction and if you don't follow
it perfectly right now, that's fine, I just want to give you a feel for it.
And by the end of the course, if you were, were to return to that video you
will find, I think, that you understand it perfectly and that'll be a real goal.
So the first two or three videos in the series, are going to say a bit about why
you might want to learn thermodynamics. Explain what sorts of things that one can
do with it. And then afterwards, we'll dive into the
building a hammer part. So, let me return to thermite.
And now I'll show you a balanced chemical equation.
And so I told you that we had a little flowerpot filled with rust and aluminum
powder. And so I have my balanced equation here.
I've got iron oxide, and that is rust, and I've got aluminum as a solid, and if
I were to look at the products involved, there's been a transfer of the oxygen
atoms from the rust, to the aluminum. And that makes alumina, and that's the
source actually from which we extract aluminum on earth, it's a mineral.
Of course, I wrote it from left to right as I know it happened in the pot and as I
described it to you. But you might ask yourself, why wouldn't
one write it the other way? Why not have the alumina on the left side
with solid iron going to iron oxide plus aluminum powder?
And so a key question from a thermodynamic standpoint is, which way
does the reaction go, and why. Moreover, will the metal that's produced
melt? And how will we know?
So these are key concepts. And the energetics of chemical reactions
are just tremendously important in all sorts of chemical processes.
So for example, did you know that sulfuric acid is by mass, the most
produced chemical on Earth? Over 180 million tons of sulfuric acid
are made every year. And the heat that's involved in that
process, has to be accounted for properly in order to build plants that can that
can do the reactions necessary. Polyethylene, a very important polymer,
80 million tons of polyethylene made every year.
An the thermochemistry associated with that is absolutely crucial to plant
design. One can go on and on.
The pharmaceutical industry, food products, petroleum cracking, biochemical
processes. They all require knowledge of which way
the reactions go and how much heat will it take or will be released?
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So focusing again on the reaction as it's written In order to determine the
direction it proceeds, we need to assess a thermal chemical quantity called the
enthalpy. So, enthalpy is a property of a
substance, and we might be able to look up, these are common substances.
Enthalpies will be tabulated for common substances.
So one can look up the enthalpy of rust, of aluminum; and when one does that, one
can determine the change in enthalpy on moving from the left to the right.
In this particular instance, we write from left to right because the enthalpy
change is negative. That means that enthalpy is released,
heat is released. We call that an exothermic reaction.
This in fact is a very exothermic reaction.
The amount of heat that's released is minus 850 kilojoules per mole.
So a joule is the SI unit of energy. And during the course there will be many
opportunities to do unit conversions between units of energy.
But 850 kilojoules would be 850,000 joules.
And so the enthalpy of the reactants is substantially higher than the enthalpy of
the products, and that released enthalpy as the reaction proceeds is available to
do things. So what might that enthalpy do?
Well, it can melt solids. And we can ask, how much enthalpy will it
take to melt the solid? That's actually measured by the enthalpy
of fusion of a substance. So fusion is either transformation from a
liquid to a solid, or the reverse, a solid to a liquid, that would be melting.
And in the case of iron, it take 14 kilo-joules per mole to melt one mole of
iron. it takes 11 kilojoules to melt one mole
of aluminum. If we look in our reaction mixture as
written, it's two and two. So let's call that, two times 14 is 28.
Two times 11 is 22. That adds to a nice round number, 50.
Kilojoules per mole, that leaves us 800 kilojoules per mole of remaining enthaply
that's available to do something, or 800,000 joules.
And what else can it do? Well, it can raise the temperature.
So the next question is, how much does the temperature go up given that amount
of available energy and these substances? To answer that questions, you need to
know another thermochemical quantity. It's called the heat capacity.
So, the heat capacity tells you. How much energy does it take to raise the
temperature of a given substance by one degree?
And so you see that in the units here, the heat capacity for iron, 25 joules per
mole degree Celsius. So, a rise in temperature of one degree
Celsius. For each 25 joules per mole of substance.
And, in the case of alumina, 128 joules per mole will raise it one degree
Celsius. So if I add those together, I've got 153.
Double that, because there's. Actually, we'll take double of that,
because there are 2 moles of iron. So, 50.
Plus 128, 178, why don't we round that and say about 200?
So 200 joules per mole, but there's 800,000 joules per mole still available
to raise the temperature, so that's a pretty simple division, and you'll get
about 4,000 degrees. Now how much is the actual temperature
rise? Turns out it's a bit in excess of 2,500
degrees. And the difference just reflects that we
play it a little loose by assuming all of these values are constants over that
entire temperature range. Turns out aluminum actually begins to
vaporize if you get it hot enough, and that takes energy.
But, that's well above the melting point of iron, which is 1,530 degrees Celsius.
and that's why we observed the iron melting.
Becoming a molten red flowing liquid that fell through the m-, bottom of the flower
pot. And then we saw that pretty golden,
golden red glowing substance lying in the sand as it resolidified.
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So I'll, I'll sort of close this section by again providing sort of a practical
utility for this. it's a beautiful demonstration, and I
hope you were as excited as I was doing it.
But, it turns out, it's been used for a long time, it's, it's the method by which
railroad ties are joined together. When you lay track and you want to then
weld it into one continuous steel track you put, and you see here a nice little
picture. You put a thermite reaction on top that
allows molten iron to fall onto the join between the two ties, leading to
ultimately, a continuous piece of steel. And so that's kind of thermodynamics in
real life. And so I've talked about enthalpy and
I've talked about heat capacity and we'll also start to look ultimately at terms
like entropy and free energy. And that's thermodynamics, these various
quantities that control things like temperature and which direction reactions
go and we'll learn how to apply them in order to make useful predictions and
understand chemical phenomena. So that's the end of, of this particular
video. each video's going to end with the
picture of the train dragging us forward down the thermochemical tracks and the
next, video I want to do will be called Benchmarking thermoliteracy, and then
I'll tell you a little story about why it might be good for lots of people to know
something about thermodynamics. See you then.
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Dr. Christopher J. CramerDistinguished McKnight and University Teaching Professor of Chemistry and Chemical Physics
