AP Chemistry Energy, Heat, and Work

Hello, and welcome back to Educator.com.0000

Welcome back to AP Chemistry.0002

We're going to start, today, on our unit discussing thermochemistry, and there are going to be about three or four lessons discussing thermochemistry--chemistry related to heat.0004

Today, we're going to talk about energy, heat, and work--some of the basic, fundamental concepts--and eventually, we will get more into the chemistry of it.0015

Let's just jump in and get started.0023

The first thing that we want to definitely make sure you know is that temperature and heat are not the same thing.0026

So, temperature and heat are not the same thing; this is probably the single biggest misconception.0032

OK, so temperature is a measure of the average kinetic energy of the particles in the sample that you are taking the temperature of.0048

Heat is the thing that actually flows when there is a temperature differential.0078

In other words, heat is the thing that flows from a hot object to something that is colder.0085

Or, it doesn't necessarily need to be an object; any time there is...let's say you have some hot thing, or something that is at a higher temperature; it doesn't have to be hot; there just has to be a difference in temperature.0089

So, if something is 30 degrees Celsius, and let's say the air surrounding it is 10 degrees Celsius, heat is that energy, if you will, that flows from the higher-temperature object into the lower-temperature object.0099

Heat is the energy that actually flows; what temperature does is actually measure the random motions of the molecules.0113

There is a difference between the two; they are related, but they are not the same thing.0121

Heat is the thing that flows from hotter to colder--we'll say objects (objects could also just be the air)--due to the temperature differential.0126

Differential, as you know, just means the difference.0162

Heat is a form of energy; it is a very disordered form of energy.0166

OK, so heat--profoundly important; thermochemistry is going to be all about the heat.0179

Work is another form of energy.0187

Clearly, energy can come in different ways; you can have light energy; it could be heat energy; work is another form of energy; all of these things are energy.0199

They are represented by the same unit, and the idea, essentially, of major areas of science, is: How do we convert one form of energy to another form of energy?0208

That is really what is going on.0217

Energy...work is another form of energy, and we define it as Work=Fd, which is force times distance.0221

So, the best way to think about that is: if I apply a certain force to an object, and if I move it a distance d, well, the work is going to be the force that I apply, times the distance that I actually move it.0232

Now, notice: work, the way we discuss it in science, is not the same sort of way of everyday usage of the word "work."0243

I can stand and I can push against a wall, and I'll definitely start sweating, and I'll definitely start getting tired, eventually.0252

But, the wall isn't moving; I'm applying a force, but the wall isn't moving.0258

Force is a positive value, but distance is 0; the wall isn't moving.0261

Scientifically, there is no work actually being done; yes, I am doing work, in the sense that I am actually expending energy, but no actual work is being done by the force.0267

So, there is a difference: when we say "work," in chemistry, in physics, in engineering, we're talking about a force that causes something to move.0277

OK, so let's look at some units.0287

Force equals mass times acceleration; that is Newton's second law.0290

So, if you want to give a certain body of a certain mass a given acceleration, you have to apply a certain amount of force to it, and the relation between these three variables is F=ma.0297

Well, mass is in kilograms; that is the standard unit of mass.0306

Acceleration is in meters per square second.0313

Well, this unit right here--the kilogram-meter per square second--this is called a newton.0317

A newton is a unit of force, symbolized with an N.0323

Now, we just said that work equals a force times a distance; well, force, we just said, is a newton; and distance is in meters; so now, we have this unit of a newton-meter.0327

This unit, which is a (well, let me go ahead and actually write out everything so you see it)...0346

We said that a newton is a kilogram-meter per square second, and then we multiply it by a meter, which is there; so this is the force; this is the distance; and we end up with kilogram-meter squared per second squared, and this is called a Joule.0355

This is our standard unit of energy, the Joule.0377

If you want, you can think of it as kilogram-meter squared per second squared, or you can think of it as kilogram-meter per square second, times meter--a newton-meter.0381

So, a newton-meter is a Joule, or if you want to break down the newton and sort of put some things together, you can get kilogram-meter squared per second squared.0390

It depends on the problem you are working on; sometimes you will just deal with newtons; sometimes, you will need to break it up.0399

As you go on in science, you will realize that you will need to sort of break up more of these things, in terms of units, to deal with certain constants.0404

Constants--depending on the problem, you might have to use different constants, which are the same constant, but in different units, they have different values.0412

OK, so the biggest piece of advice I can give is: watch your units.0421

If we're talking about a mass, make sure that kilogram matches with gram; or, if something is in two different units, make sure that kilograms, meters, seconds, Joules, newtons...that everything matches; otherwise, your values will be wrong.0426

OK, so let's talk about the two ways to actually transfer energy.0442

The two ways to transfer energy--to transfer energy from one object to another, from one thing to another, from one environment to another, from the outside to an inside, to transfer energy: we have two ways of doing it.0448

We can transfer it as heat or work, or both.0465

That is it; so, in some sense, this is really simple.0473

We're going to have some sort of a system, and we want to either give energy to it or take energy away from it; well, there are only a couple of ways we can do that.0477

We can either put heat into it or take heat out of it, or we can do work on it, or it can do work on us.0485

So, let's be a little more specific here; we'll draw a picture, and we're going to start talking about systems and surroundings.0491

OK, so I have a system; a system is defined as the thing that we are interested in.0500

I know that that is kind of a vague definition--"thing that we are interested in"--it will make more sense as we do some problems and as we talk about it more.0513

Well, for chemistry, most systems are going to be the reaction: the reaction, and maybe the vessel that is containing it...a solution of hydrochloric acid plus sodium hydroxide--that is your system...the beaker plus the water; that is the system.0525

The surroundings--that is everything else.0540

That is everything else; so, very, very intuitive--nothing strange is going on here.0545

We said that we can transfer energy in two ways: we can do it via heat, or we can do it via work.0550

This is our system; this is (I'm sorry, I should put the system in here)...I'll call this sys, and out here is the surroundings.0557

So now, this is the boundary, and it's always going to be something; there is going to be some sort of a boundary that separates your system from your surroundings.0575

Now, we're going to transfer heat in and out; we're going to transfer work in and out.0583

We have to pick a point of view.0588

In chemistry, we always look at the system's point of view; it's not always the case; there might be other circumstances, maybe in engineering, where you need to look at the surroundings' point of view; but in chemistry, we always look at it from the system's point of view.0591

From the system's point of view, when heat is added to a system (OK, so we say heat in), that heat is positive.0602

When heat is given off of a system, that heat is negative; in other words, the system is losing heat--it's going from a certain value of heat, it's losing heat, and its final heat is actually less than that.0617

So, energy and heat is flowing out; so heat is negative.0631

That is the sign convention; if heat is put into the system, that means more energy; so it has a certain amount of energy; we put more heat in; it goes up; that is positive.0635

We are looking at it from the system's point of view; notice, it would be the reverse if it were the surroundings' point of view.0643

The other way to transfer energy is work.0648

Now, the system could do work on the surroundings; the surroundings could do work on the system.0651

We will be a little more specific about what that means, but the same sign convention applies.0657

If I do work on the system--work on the system--that is positive work.0662

That means I have put energy into the system via the work that I have done to it.0676

If the system does work on the surroundings, that is going to be negative work.0680

This is called work done by the system.0687

So, in this case, these prepositions are very, very important; so, when you read your questions, makes sure you read them carefully, because it will say "work done on the system" or "work done by the system."0693

It won't tell you whether it is positive or negative; you have to decide.0701

So, work done by the system--that means the system used its energy to do something to the surroundings, and in the process of doing it, it transferred its energy to the surroundings.0706

Now, energy is depleted.0715

Work is a little more abstract than heat; with heat, you have a pretty good intuitive sense--you know when something gets colder or gets hotter.0718

Work is a little different, but again, we will do some problems, and we'll get used to it.0725

So, two ways to transfer energy: heat and work; work into the system is positive; heat into the system is positive; heat out of the system is negative; work out of the system is negative.0730

Again, from the system's point of view--we are always looking at it from the system's point of view.0744

Whenever a process actually gives off heat (the system gives off heat), we call it exothermic.0749

It makes sense, right?--exo, outside.0755

And then, heat in is called endothermic--an endothermic process; that means the system is taking in heat.0758

These are very, very important words, and we will use them in the context of our problems.0767

OK, so let's look at just a couple of quick examples.0773

If I take methane gas, and if I combust it (burn it in oxygen), I produce two moles of CO₂, 1 mole of CH₄; I need 2 moles of oxygen; I produce 2 moles of CO₂ and I produce 2 moles of water, and I produce heat.0776

As you know, any time you burn something, it gives off heat; you feel the heat radiation; that is giving off energy as heat.0797

This is an exothermic process.0804

Burning, combustion, is an exothermic process.0810

If I take nitrogen gas, and if I react it with oxygen gas, in order to produce a couple of moles of nitrogen monoxide gas, as it turns out, I have to add heat to the system.0813

So, what happens: if I have a vessel, and if I actually ran this reaction, what would happen is: the vessel itself would get cold--I would feel the vessel get cold--and the reason is, now the reaction--in order for it to work--needs energy put into it.0826

So, it takes heat away from the glass (let's say it's in a glass container); it sucks heat out of the glass container, and because the heat from the surroundings is not actually going into the glass fast enough, when I touch the glass, I feel cold.0843

So, it isn't that the glass got cold; it is that the heat was pulled out of it, bringing it to a lower temperature.0856

That is what is going on.0863

This is an endothermic process.0864

The system evolves heat and gives it up.0870

This system pulls in heat--needs the energy in order to perform its functions (in this case, the systems are the reactions).0873

OK, now let's go ahead and put some mathematics to this.0883

We said that there are two ways to transfer energy: heat and work; well, the change in energy of a system is going to be the heat that goes in or out of the system, plus the work that goes in or out of the system.0890

Again, heat and work are just two different types of energy.0909

So, if a system starts in one state of energy, and it goes to another state of energy, a couple of things have happened.0912

It has either lost or gained heat, lost or gained work, or some combination of both.0920

This is our fundamental equation; this is actually a statement of the first law of thermodynamics--that energy can neither be created nor destroyed; that any energy change is going to take one form or the other: heat, work, or both.0926

But, energy is conserved; that is what this equation is saying.0942

OK, so these heat and work thermodynamic quantities--they have two parts.0947

They have a magnitude, and they have a sign.0957

Well, again, we dealt with the sign issue here; with thermodynamics, oftentimes we have to stop and think about the physics of what is going on, in order to decide what the sign is going to be.0963

Often, we know what the magnitude is, but we have to make sure to keep the sign straight; otherwise, the mathematics will not work out.0974

All right, now let's fiddle with this equation; let's talk a little bit more specifically about this thing called work.0982

OK, so a common type of work in chemistry (in fact, the most common in general) is work done by a gas.0991

So, it's either going to be...well, work done by a gas: notice, here is that "by" again; that is expansion; and when I draw the picture in a minute, it will show you what I mean.1015

Or, work done to a gas, which we call compression.1030

Let's say I have this container, and it has a moveable piston that can move up and down; this is a chamber that has the volume.1044

So, I can push this piston down and squeeze this volume--make it smaller--or I can pull the piston up and make the volume bigger.1056

In this particular case, if there is a gas in here, and it expands, that means the gas is actually pushing against the external pressure of the atmosphere.1064

It is doing a certain amount of work; if I actually push the piston down, and the volume gets smaller, this pressure is higher than this pressure; I am actually pushing this down--I am doing work on the system here.1073

Let's take a look at what this looks like mathematically.1088

We just said that the change in energy is equal to q + w, the heat that goes in and out of a system plus the work that is done on or by the system.1091

OK, q: well, we said that work is equal to force times distance; great--simple substitution.1101

We'll leave q alone, and now, here is what I'm going to do: I know that there is a relationship between force and pressure and area; as it turns out, if I take the force of something, and if I divide it by the area (remember, back when we talked about gases, we said that pressure is equal to the force per unit area, so...) I have this F here; if I divide by A, in order to retain this, I have to multiply by A, so I basically have multiplied this F/d by A/A, right?1111

I did this in order to manipulate this F a little bit.1144

Now, I have F/A, and I have distance times area.1147

Well, this thing right here--I see it in the profile, but really, it is just a cylinder.1153

Let's see what we get here: we have q; well, force over area is pressure, and distance times area is...well, distance is...let's say that this force here is a pressure that is being applied to this piston.1160

The area is, of course, the area of this circular part; so there is a certain pressure being applied; well, the area times the distance...the distance is the distance that this piston actually goes down or up, right?1186

So, this is the distance; well, area times the distance, as far as the circular cylinder is concerned, is just volume.1204

You end up with: Change in energy equals q; P is constant, so the only thing that is changing here is, from this to this, whether we either press down or pull up.1215

Pressure...PΔV; this is pressure times the change in volume.1233

This is what we mean by pressure, volume, and work.1238

So, when we push down on a gas, we are actually doing work on the gas; we are doing work to the system.1242

When the gas expands and pushes the piston up, then the gas is actually pushing against an external pressure, and it is actually doing work on the surroundings.1251

This final equation...this is often how we will do it: we're going to be dealing with some gas in a container, and this is going to be the expression that we use: The change in energy of the system is going to be the heat that flows in or out of the system, plus the change in volume, based on a certain amount of pressure.1266

This pressure is always going to be the external pressure.1291

It is an external pressure that pushes down, and squeezes it, and makes the volume smaller; or, it's going to be the pressure against which the gas pushes in order to make it bigger.1294

In either case, it is a pressure that is pushing against the boundary here.1306

So, it is always an external pressure--that is what this P is, not the pressure of the gas.1311

OK, so now let's talk about some signs.1318

All right, this is going to be important; so now, we're going to actually talk about this term right here, and we're going to relate...so we know that work and pressure times volume are equivalent in terms of magnitude whenever we are doing something like this.1322

So now, we want to talk about what sign convention we are going to use.1339

All right, so let's just write "Work = PΔV"; so, work equals pressure times the change in volume.1345

If I start at a certain volume, and if the gas expands, that means the gas inside is pushing against an external pressure; well, the final volume is bigger than the initial volume.1351

Final volume, minus initial volume, is going to be bigger than 0.1369

Well, in this particular case, it is bigger than 0; the pressure is bigger than 0; here, this work is going to be positive; this number is going to be positive.1375

Pressure times the change in volume, for an expansion, is positive; but, because it is expanding, it is the system that is doing work on the surroundings; therefore, the work is negative, because again, we said we are looking from the system's point of view.1383

If the system is pushing against this piston and expanding, it is doing work on the surroundings.1398

In other words, work is leaving the system.1404

Therefore, work is negative; so, by putting this negative sign, we also account for the other direction.1406

If the outside pressure is pushing down on the piston, and making the volume smaller, that means the final volume is smaller than the initial volume.1413

Therefore, ΔV, which by definition is final minus initial, is less than 0.1425

Well, if ΔV is negative, and P is positive; this negative sign--negative times negative--makes the work positive, which works with the fact that now the surroundings are doing work on the system.1430

The work is positive, because the system is now taking in energy in the form of work.1444

It is positive from the system's point of view.1451

The energy of the system is increasing.1453

So, this is the actual relationship: work is negative pressure times the change in volume (change in volume always being final volume minus initial volume--which is the definition of Δ--final minus initial).1456

OK, so with this little bit of a background, let's just jump into the problems, and I think we can start to make sense of some of this.1473

Let's do Example 1: So, we want to calculate the change in energy for an endothermic process in which 29.6 kilojoules of heat flows (and notice: we said endothermic), and where 13.7 kilojoules of energy is done on the system.1482

OK, so we want to calculate: we say that, for an endothermic process, 20.6 kilojoules of heat is flowing.1540

Endothermic means heat is coming into the system; that means heat is positive.1545

"Where 13.7 kilojoules of energy is done on the system": on the system means work is flowing into the system--that means work is also positive.1551

So, we write our equation: change in energy equals q + w; q is heat; w is work.1559

We said an endothermic process, so it's positive, so it's going to be positive 20.6 kilojoules; so notice, the problem will give you a magnitude; it won't necessarily tell you what direction it is going.1569

If it said exothermic, then I would have to use -20.6.1582

If this said "by the system," I would have to use -13.7.1585

But here, it's endothermic, so heat is positive, and then the work is also positive 13.7, because it is done on the system.1589

Kilojoules...this is a simple arithmetic problem.1597

So, we end up with 34.3 kilojoules of energy.1600

I put heat into it in the amount of 20.6 kilojoules; I did work on the system by 13.7 kilojoules; the total amount of energy that I imparted to the system is a positive 34.3 kilojoules of energy.1608

Oh, by the way, I should let you know: oftentimes, another unit of energy is something called the calorie; it's an older unit of energy, and it is defined as the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius.1626

The calorie is a unit of heat; it's a unit of energy, because that is what heat is--it's just energy.1640

So, as a conversion factor, one calorie equals 4.184 Joules.1646

We are probably not going to run into calorie too much; we may or may not; we'll find out; but just so you know...1653

You have heard of the word "calorie"; you hear about it all the time; that is what it is: 1 calorie is that many Joules--it's a unit of heat; it's a unit of energy.1658

OK, so Example 2: What is the work associated with the expansion of a gas from 0.750 liters to 1.760 liters at constant external pressure of 13 atmospheres?1667

The gas is in a container; there is a pressure of 13 atmospheres on the outside; and again, pressure is always going to be external.1724

On the outside, 13 atmospheres is pressing on it.1732

But, the gas is actually expanding from the inside, and it goes from .750 liters to 1.760 liters.1736

In other words, it is pushing out against the surroundings.1746

It is doing work on the surroundings, which means that energy is flowing out of the system and into the surroundings, which means that it is actually going to be negative.1750

Well, work equals negative pressure, times change in volume, equals negative 13 atmospheres; change in volume--we have 1.760 liters, minus 0.750 liters, and when I do this, I end up with -13.13 (I don't know if you can read that) liter-atmosphere.1761

OK, so notice: atmosphere, liter; this is not exactly a unit of energy that you have seen, although it is a unit of energy; it just happens to be different units, and I will show you what this is.1809

The conversion factor here is this: 1 liter-atmosphere is equal to 101.3 Joules of energy.1822

I'll show you in a minute where this actually comes from.1833

But again, it just goes back to the point: units are very, very important; just because you end up with some unit that seems strange doesn't necessarily mean that it is strange.1835

So, all of the math that we have done is correct; work is equal to pressure times change in volume; pressure is in atmospheres; volume is in liters; this is actually a unit of energy--it's a unit of work.1844

It is actually equivalent (by a conversion factor) to a Joule.1858

So, don't let this throw you off.1863

Now, let's go ahead and do the conversion.1865

When we multiply this out, we end up with 1330.4 Joules, or (can't have these stray lines floating around--let's erase this) 1.33 kilojoules.1867

There we go.1887

OK, so let me see what I have here.1890

OK, so really quickly...you know what, let's skip the unit conversion part; if you can, you are welcome to look it up, but the liter-atmosphere--one liter-atmosphere is equal to 101.3 Joules, so that conversion will always be there for you.1898

Let's just keep going on with these examples.1918

All right, another problem here: Now, a balloon is inflated from 4.00x10⁶ liters to 4.50x10⁶ liters by the addition of 1.4x10⁸ Joules of heat energy.1922

So, I add some heat to the system, and I add 1.4x10⁸ Joules of heat.1972

Now, if the balloon expands against a constant pressure of 0.95 atmospheres, what is the ΔE for the process?1981

OK, so let's just draw a quick picture here: we have this balloon--obviously, we are talking about a really, really large balloon; we pump in heat--pumping in heat energy--so heat is going to be positive.2019

This balloon is going to expand, which means that it is going to push out against the .95 atmospheres; so now, the balloon is actually doing work on the surroundings.2034

Work is going to be negative.2043

Now, let's work it out.2049

ΔE is equal to q plus w; we have worked out our sign convention--heat is going in; work is going out, because it is expansion.2052

We also know that work is equal to -PΔV, so let's go ahead and calculate our work.2062

The work is equal to -0.95 atmospheres (that is the external pressure; and again, pressure, volume, work; PΔV; that P is the external pressure against which you are pushing, or that is pushing on you, or pushing on the system), times 4.50x10⁶, minus 4.00x10⁶.2076

We end up with (when we do this mathematics) -475,000 liter-atmospheres; and then, when we multiply by 101.3 (oops, that is not kilojoules--the conversion factor is 101.3 Joules per liter-atmosphere), we end up with a total of -4.8x10⁷ Joules of work.2103

Well, we just (oh, no, we definitely don't want that...OK) said that ΔE equals q + w; we calculated w--that is -4.8x10⁷ Joules--so now ΔE equals q plus that.2141

Well, q, positive--it is 1.4x10⁸ Joules (you know what, I am always confused about whether I should write it out or do the letters; for years it has been like this--OK, I'll just do the letter), minus 4.8x10⁷ J, so our change in energy is 9.2x10⁷ J.2163

Notice what has happened here: energy flowed in as heat.2196

Work ended up expanding the gas; the gas ended up pushing against the external atmosphere; so energy came in as heat; energy left as work.2202

The total energy change for this process was 9.2x10⁷ Joules; this is a positive value, so there is now more energy in the system that stayed as heat than went out as work.2214

That is all this says--the fundamental equation; this is the first law of thermodynamics.2231

The change in energy is equal to the heat that goes in and out of a system, plus the work that goes in and out of a system.2235

Energy itself can be transformed, but it cannot be created or destroyed; it always has to be accounted for.2241

OK, thank you for joining us here at Educator.com.2247

We'll see you next time for some more thermochemistry; goodbye.2250