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Thermal chemistry is a sub-topic of thermodynamics.

Â So we need a little introduction to thermodynamics.

Â If you continue on to the next chemistry class,

Â which we've called advanced chemistry, we will study thermodynamics in lot more

Â detail, but let's define thermodynamics here.

Â Thermodynamics is the scientific study of the interconversion of heat and

Â other kinds of energy.

Â So we know that thermochemistry deals with heat.

Â In thermal dynamics we will talk about other types of energy,

Â like Gibbs free energy, for example.

Â But we will focus here on, as I said, on this chapter on just the heat portion.

Â As we study thermodynamics, we need to find something called a state function,

Â or the state of the system.

Â The state of the system is all of the values of all the r,

Â relevant macroscopic properties.

Â So, what temperature is it?

Â That would be part of the state of a system.

Â What is in pressure if it is a gas?

Â 1:13

How much energy is in it?

Â What is its temperature?

Â What is its pressure as I've mentioned, maybe what's its volume?

Â How much do you have volume wise?

Â Now as we talk about the state of the system and we define these things

Â that give you the relevant, relevant macroscopic properties of that system.

Â Some of those are called state functions.

Â These would be the properties that are determined by the state of the system,

Â regardless of how you achieved that condition.

Â 1:49

If I have a an object and I say that it's 6 ounces.

Â Okay, let's say volume amount.

Â Well it's going to be 6 ounces regardless of how you got there.

Â Here in the United States we have Cokes and

Â they come in twelve ounce cans or beverages of various kinds.

Â Come in a twelve ounce can.

Â Now if I had drank half of it, it would be 6 ounces.

Â If it were an empty can and

Â at the factory they only put 6 ounces in, it would be 6 ounces.

Â So it wouldn't matter how you got there.

Â 6 ounces is 6 ounces regardless of how you achieve that.

Â 2:30

So the magnitude of change when you have a state

Â function only depends upon the initial and the final value.

Â We call this path independent.

Â So with our example of our soft drink,

Â did it get 6 ounces because it was full and I drank half?

Â Okay, so final minus initial would be 6 ounces, minus the 12 I started with and

Â I had a change of 6 ounces.

Â Or, did I drink 9 ounces and then spit three back in.

Â All right, that would, that wouldn't matter.

Â It'd be the change overall of 6 ounces.

Â So, for a change in volume, we always will talk about final minus initial.

Â We'll see this often.

Â So, a change is always the final value minus initial value.

Â And it doesn't matter anything else in between, just where you ended and

Â where you started.

Â 3:34

Here's some other examples of state functions.

Â We could talk about energy.

Â It wouldn't matter whether it was total internal energy, kinetic energy,

Â potential energy.

Â Energy is a state function is abbreviated with an E.

Â Capitol E we might use KE for

Â kinetic energy in which case we use a capitol K and a capitol E, and so forth.

Â 4:04

Pressure, that's another one where it is going to

Â be a capital P where we have a change in pressure, we only care what the final and

Â initial, it won't make a difference.

Â 4:14

So we say that state functions are path independent.

Â So these two people climbed up the hill.

Â Some things are path dependent and somethings are path independent.

Â The altitude would be path independent.

Â It's change, where those people are standing, they are at the same at,

Â the altitude.

Â Where they started and

Â where they finished, it doesn't matter what path they took.

Â The change in amplitude would be exactly the same.

Â Another one would be the amount of energy by virtue of the people's position.

Â Where are they now?

Â If they were to roll down the hill,

Â there would be the same amount of potential to do work.

Â Now what is very different is the work it took to get to

Â the top of the mountain, okay?

Â Somebody had to take a very, very long and convoluted path.

Â It took 'em a lot longer to get there.

Â They were up and down and up and down and finally got to the top.

Â That might be very different than somebody who started at the bottom and

Â was able to find a very direct path.

Â Maybe there was a rope attached that they could utilize to pull them up,

Â what have you, it matters how they got there.

Â So work is not a state function.

Â 5:26

Now we made a statement of Law of Conversation of Energy, and

Â this is actually the First Law of Thermodynamics.

Â Energy cannot, can be converted from one form to another, but

Â it cannot be created or destroyed.

Â 5:45

What's not possible, and what was assumed with this statement is if

Â the total amount of the energy in the universe is a constant.

Â It's not possible to know the total amount of energy in the universe.

Â There's way too many complicating factors, down to the atomic level and

Â the electrons moving about having energy associated with them.

Â Molecules flying through the air, having energy associated with that.

Â A car driving down the street having energy associated with that.

Â It's not possible to know the total amount of energy in the universe, but

Â we can determine the change in energy as a process takes place.

Â Now it's not the change in the energy of the universe because that's not changing.

Â But when a process takes place, we could study that system and

Â we can know its change of energy, which is abbreviated delta E.

Â Okay, so the total amount of energy change with any process in the universe is

Â gotta be 0, because the amount of energy in the universe is a constant.

Â But whatever the system is doing,

Â the surroundings has to be doing just the opposite.

Â So if the system is losing, let's say 10 Joules of heat,

Â the surroundings must be gaining 10 Joules of heat.

Â Whatever the one is doing, the other's doing just the opposite.

Â So if it's losing, it's going down.

Â If it's gaining, it's a positive value, but overall the universe is not changing.

Â The other way I like to think of this equation, and

Â more commonly I think of it this way, is that whatever the delta E,

Â the change in energy of the system is, that would have to be

Â equal in magnitude but opposite in sign of the delta E of the surroundings.

Â So I usually think of it this way, so

Â this is been stated exactly the same with this equation here.

Â 7:59

Because those are the only two ways that heat can, or energy can get in or

Â out of a system, if we go back to the first law of thermodynamics,

Â we can restate it with a very simple equation.

Â The delta E, the change in energy of the universe is assumed constant.

Â You are not going to create or destroy energy, it's just being transferred,

Â then the transfer of energy is going to be equal to the heat and the work.

Â So that little equation is one that we need to keep track of.

Â q stands for heat, w stands for

Â work, and you will notice that those two things are lowercase letters.

Â They are not state functions.

Â They are path dependent.

Â It matters how you undergo the change.

Â But overall, E, internal energy, is not a state function.

Â 8:57

Now there's a sign convention associated with q and

Â w that we need to get familiar with.

Â Now, eventually we'll understand the negative and positive of heat, and

Â the negative and positive of work, as we work on calculations, but

Â for now let's just get us familiar with them.

Â If a reaction is exothermic, or a process is giving off heat,

Â okay thermal energy is being transferred out of the system in to the surroundings,

Â the sign for heat is negative.

Â 9:30

Okay? So, exothermic is negative.

Â If on the other hand, a system is gaining thermal energy,

Â it is an endothermic process and it has a positive sign for endothermic processes.

Â 9:43

Now, for work.

Â Work is negative when the work is done by the system on the surroundings.

Â So, if the system is pushing out on the surroundings it's

Â doing work on the surroundings, then that is a negative work.

Â If the work is done by the surroundings on to the systems, so

Â it's pushing in on the system maybe,

Â as a way of thinking of that, squashing in on it, then that is a positive value.

Â So, if you've got a system in which heat is leaving the system and

Â work is being done on by the system on the surroundings.

Â Those are ways that energy is leaving the system.

Â Both of those are negative discussions that I just had.

Â And we would have a total change in energy that's negative.

Â Okay?

Â If, on the other hand, we have a system.

Â And here's the system.

Â And heat is entering the system.

Â And we're doing work on the system by the surroundings, then both of these would be

Â positive, and we would have a positive change in energy of that system.

Â 10:53

Now let's think about work.

Â When a gas expands it does work, so

Â we have a gas that is starting out with this amount of volume, okay.

Â And it is expanding, and when it expands it is pushing out on the surroundings.

Â It is going to change in volume by a certain amount.

Â And that would be represented by final, that's the whole thing, minus the initial,

Â that's over here, and what I have shaded a little bit there is the change in volume.

Â So as a gas expands, it is pushing out on the surroundings.

Â It is doing work on the surroundings.

Â And that would be a negative.

Â When a system does work, [SOUND] pushes out.

Â If the system is doing work on the surroundings,

Â then that is a negative value.

Â 11:47

Now you probably, in terms of work,

Â have seen in some past classes that work is force times distance.

Â If you push an object with a certain amount of force over certain distance,

Â that is one way to calculate work.

Â But when we're talking about gases we're going to define work this way.

Â It is a negative of the pressure times the change in volume.

Â 12:10

Now, the gas expanding, okay, which is what we saw in the previous picture.

Â You can look back at it.

Â If it's expanding, this number is positive.

Â It is getting bigger.

Â Well, pressure is always positive.

Â You can either have no pressure or

Â you can have a positive pressure, and then we change the sign here.

Â And so when, if we're expanding a gas, [SOUND] then work is negative.

Â Now let's see if that makes sense.

Â If the gas is expanding it is doing work on it's surroundings and

Â our sign convention says that is a negative value.

Â 12:52

So you need to know that equation.

Â Now when you use this equation to obtain work,

Â you're going to have units of that are kind of weird.

Â And let me, [SOUND] now this kind of work is called PV work.

Â Whenever a gas is expanding or compressing you can calculate work using this

Â equation, and we will not be using the one you see up here at the top, okay, for

Â problems that we have in this chapter.

Â 13:37

When using this equation, if you were to put, you,

Â if you were to put a pressure and you were to put a volume, a change in volume in,

Â and you obtain work, what would be the units of that work?

Â 13:48

Well, if we use pressure in terms of atmospheres and volume in terms of liters,

Â the unit that we have, we define as liter atmosphere.

Â Now it's reversed of what we see up there, but we never call it an atmosphere liter.

Â The unit is called a liter atmosphere.

Â Now liter atmosphere doesn't look much like a work unit to us.

Â But there is a very easy conversion between leader atmospheres in

Â Joules that you'll need to know and this is it.

Â If there's 101.3 J, that's your typically energy unit, in 1 liter atmosphere.

Â So let's consider this problem.

Â 14:23

We have got a gas that has a volume change from o.5 liters

Â to 2.5 liters and is pushing out on an external pressure of 0 atmospheres,

Â the questions is how much work is done?

Â Well work is equal minus P delta V.

Â 14:47

It's pushing against 0 atmosphere and so there is no work.

Â We can go ahead and plug our numbers in for

Â the change in volume though just to practice.

Â That would be 2.5 liters as a final, minus 0.5 liters, that's the initial and

Â that would give us a work of 0 liter atmospheres, or 0, pick any unit you want.

Â 15:10

So if you're pushing against an external pressure, you're not doing any work.

Â I don't care how much the gas is expanding.

Â So changing a gas's volume doesn't necessarily do work,

Â unless you're pushing out against an external pressure.

Â But let's have you do the work for this gas.

Â It is starting at, and I'm sorry about the break in the line there.

Â It's starting at 0.5 liters, we'll put it up there, all right?

Â And it's expanding to 2.5 liters.

Â It's pushing against an external pressure of 1 atmosphere.

Â Work the problem and select your answer.

Â 15:44

Well, if you picked a negative 200 Joules to one significant figure,

Â that would be the correct answer.

Â Now some of you might have forgot to convert to liter atmospheres, so

Â make sure you do that.

Â When you determine work as a negative, there is the 1 atmosphere, the final,

Â which is 2.5 liters, minus initial of 0.5 liters.

Â It's going to give you 2, or a negative 2 liter atmospheres.

Â But don't forget that you need to convert that liter atmospheres to Joules because

Â that's what's being asked for in this problem.

Â And it's 101.3 Joules per liter atmosphere and

Â that's why to one significant figure you'll have a negative 200 joules.

Â 16:56

a container in here, in which we have this little reaction happening, and

Â gases are being produced as a reaction happens.

Â Okay?

Â Those gases are going to make the container expand.

Â And it's going to push up against that external pressure and we're going to be

Â able to move the volume, not where we see in this picture right now,

Â but we will expand it, and let's erase the thing right there.

Â 17:38

Well did you say they would both be negative?

Â If you said that, you are correct.

Â Now how I know it's a negative q?

Â Because we have the term exothermic, it is giving off heat.

Â Whenever a gas is expanding, okay, we have got a negative work.

Â If you forget that, we know that work is a negative P delta V.

Â If its expanding that's positive, this is always positive,

Â then the we change the sign and we have a negative work.

Â 18:14

From that previous example that we just talked about,

Â an exothermic reaction that is producing gas, can we know the sign of delta E?

Â Is it positive, is it negative, or

Â we can't know unless we know the value of w and q.

Â 18:31

Well, you should have said it's negative, because it's exothermic, that's negative.

Â Because it's expanding, that's negative.

Â And if you take a negative number and

Â add a negative number, this is going to be negative.

Â If both of these are positive, then this will always be positive.

Â The only time you don't know is, is if one is positive and

Â the other is negative, okay?

Â So these are the two for which you would not know the size of the or

Â the sign of E delta E, but for both of 'em being negative or

Â both being positive, you certainly can know.

Â So this is the end of learning objective number 2,

Â which we have looked at, kind of the beginnings of thermodynamics.

Â You will see a lot more about thermodynamics in advanced chemistry.

Â But for now, that's all we really need to examine, the sign convention of heat,

Â the sign convention of work and

Â the idea that those two things are the way we get energy in and out of a system.

Â