Video 1.1 - That Thermite Reaction

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Termodinâmica Estatística: Dinâmica Molecular

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University of Minnesota

Termodinâmica Estatística: Dinâmica Molecular

163 classificações

This introductory physical chemistry course examines the connections between molecular properties and the behavior of macroscopic chemical systems.

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Module 1

This module includes philosophical observations on why it's valuable to have a broadly disseminated appreciation of thermodynamics, as well as some drive-by examples of thermodynamics in action, with the intent being to illustrate up front the practical utility of the science, and to provide students with an idea of precisely what they will indeed be able to do themselves upon completion of the course materials (e.g., predictions of pressure changes, temperature changes, and directions of spontaneous reactions). The other primary goal for this week is to summarize the quantized levels available to atoms and molecules in which energy can be stored. For those who have previously taken a course in elementary quantum mechanics, this will be a review. For others, there will be no requirement to follow precisely how the energy levels are derived--simply learning the final results that derive from quantum mechanics will inform our progress moving forward. Homework problems will provide you the opportunity to demonstrate mastery in the application of the above concepts.

Video 1.1 - That Thermite Reaction9:37

Video 1.2 - Benchmarking Thermoliteracy12:10

Video 1.3 - Quantization of Energy14:50

Video 1.4 - The Hydrogen Chloride Cannon12:18

Conheça os instrutores

Dr. Christopher J. Cramer
Distinguished McKnight and University Teaching Professor of Chemistry and Chemical Physics
Chemistry

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?

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.

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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