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Chemistry: The Nature of Energy

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  • Type: Video Tutorial
  • Length: 13:15
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 141 MB
  • Posted: 01/29/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry Review (25 lessons, $49.50)
Chemistry: Thermochemistry (12 lessons, $18.81)
Chemistry: An Introduction to Energy (6 lessons, $10.89)

Energy is the capacity to do work or transfer heat. Professor Yee introduces Thermochemistry, or the study of energy changes associated with a chemical system. He introduces Kinetic Energy (the energy associated with motion), Potential energy (which is stored energy), and internal energy (which is the sum of kinetic and potential energy). Internal energy is difficult to define in chemistry, as Potential energy is not always evident. However, in chemistry, often the change in energy is most important, and this can be defined as: (final energy - initial energy). Lastly, Professor Yee introduces two different units for measuring energy, Joules and Calories. Joules measure energy as work, with work being the energy that moves an object against a force. Work, in chemistry, is often PV work, or a gas expanding against external pressure. Calories measure heat, or the transfer of energy from one object to another by a change in temperature. 1 cal is equal to exactly 4.184 Joules, and is approximately 1/1000 of a food calorie.

Taught by Professor Yee, this lesson was selected from a broader, comprehensive course, Chemistry. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more."

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

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Thermochemistry
An Introduction to Energy
The Nature of Energy Page [1 of 3]
In this unit we’re going to explore the idea that there’s a lot more to chemical reactions than the rearrangement of
atoms to form new molecules. In fact, associated with most chemical reactions is the change in the amount of energy.
So let’s look at an example.
What you just saw was a hydrogen balloon and the hydrogen reacted with oxygen in the air to form water. An amount
of energy was given off as a result of that reaction. We’ll talk more about exactly how much energy was given off.
The manifestation of the energy given off was that large fireball that you saw and in this room there was a very loud
bang.
So let’s now talk about some of the ideas that we need in order to talk more about chemical reactions and the first
thing that I’d like to do is to find energy. This is perhaps a somewhat unsatisfying definition of energy. Energy is the
capacity to do work or to transfer heat. Here’s the idea. You know sometimes you get up in the morning and you all
energetic. Energy. And what it means is that you have whatever it takes to go out and get the shopping done, get
your car washed, whatever. The idea is that if you have a lot of energy you have the capacity in that case to do work.
Work that scientists talk about has a more specialized definition, but you could go to work and get your work done.
We also as chemists talk about another way that energy is manifest and that is it can transfer from one system to
another via heat. We’ll talk more about that idea later on.
Let’s now divide the energy into two pieces. We’ve already talked about kinetic energy. Kinetic energy is the energy
associated with particles in motion. When we talked about gases, we talked about their average kinetic energy. That’s
a relatively simple idea. A more complicated idea is potential energy, which is stored energy. It isn’t always
absolutely obvious that potential energy is present. For instance, you could have some water in a river and you might
say it doesn’t seem to have a lot of potential energy. But if that water is right at the top of the waterfall, when it goes
over the waterfall it’s now converting potential energy from being very high up into kinetic energy. The water is
starting to move faster and faster and faster as it falls down the waterfall.
Let’s show you an example that links these two ideas, kinetic energy and potential energy. To do that I’m going to
drop a printer. The reason I’m going to drop a printer is because we didn’t have a bowling ball. Here’s a printer.
When it’s sitting on the desk it has no kinetic energy because it’s not moving. In this case it doesn’t have any potential
energy either relative to this point, where now it has potential energy because I’ve raised it up. What we’re going to
see is a conversion of this potential energy into kinetic energy as I drop the printer. Here we go. So we had the
potential energy associated with being high up. We’ll talk more about what systems exhibit potential energy. When
we dropped it, that potential energy got converted into kinetic energy, the energy associated with motion.
Now really, to talk about what we need to talk about here, we need to define both the idea of the system, and the
system is the part of the universe that we’re really interested in. In the case of the printer, the printer was the system.
In the case of the balloon, it was the system. Everything else is called the surroundings. The entire universe consists
of the system and the surroundings together.
What we’re going to be interested specifically as budding chemists is thermochemistry, which involves the energy
changes associated with a chemical system. In this case the system is our chemical reaction or a chemical
experiment and we’re interested in what is going on with that part of the universe.
Let’s define something called the internal energy. The internal energy is the sum of all the contributions of potential
and kinetic energy in the system. The kinetic energy of a sample of gas, we’ve already talked about. But in the case
of a chemical system, what we’re going to be interested in, the potential energy is not necessarily so clear. It’s going
to turn out that there’s potential energy associated with how the atoms are put together, in other words, the bonds.
There could also be potential energy associated with electrons and excited states. You may not have heard about
electrons and excited states yet, but later when you hear about them you might come back and examine this again.
Electrons in excited states represent potential energy.
It turns out that it’s not necessarily convenient to add up all these contributions from potential energy and kinetic
energy in our chemical system. For instance, if we have Avogadro’s number of atoms or molecules, we’re talking
about a huge number of particles. It could be relatively difficult to add everything up. Fortunately, it turns out that
we’re typically only interested in changes in energy and we’re going to define the change in internal energy as the final
internal energy minus the initial internal energy.
Thermochemistry
An Introduction to Energy
The Nature of Energy Page [2 of 3]
It’s also important to talk about the units for energy, and the units that we’re going to see here, there are two. Joules is
more often associated with a concept called work and calories is more associated with a concept called heat. I’ll talk
about those more in a second. And it wasn’t obvious at the time when people were first exploring these things that
work and heat were somehow associated with each other. So what happens is you’ve got two separate systems and
they have to be brought together to make it all work out.
An example of using this idea of units with kinetic energy, you know that the kinetic energy is equal to ½ mv2, m is the
mass of the particle and v is the velocity and we’re going to square that. Now instead of a gas particle, what we talked
about before, let’s imagine a book, a 2-kilogram book, and we’re going to allow it to be moving at 1 meter per second.
That’s the velocity of the book. Say I throw the book across the room and it has a velocity of 1 meter per second, then
the kinetic energy of that book flying across the room is 1 joule. This is the SI unit for energy.
I just mentioned that we have these things called work and heat. It turns out that work and heat are ways that
systems can exchange energy with its surroundings and vice versa. So a system can do work on the surroundings.
The surroundings can do work on the system. The system can put some heat into the surroundings or the
surroundings can put some heat into the system. These are the ways in which the system and the surroundings can
exchange energy.
Let’s define work a little bit more. Work, as a physicist would describe it, is related to moving an object a certain
distance d against a force f. For instance, when I take the printer, it’s sitting on the table. When I raise it up, I’m doing
work. Why am I doing work? The reason I’m doing work is because I’m moving that printer that has a certain mass in
a gravitational field and m x g, that’s a force and if I lift it up a certain distance, that’s d.
In chemistry a typical chemical reaction, the issue of work comes in because of this idea called pv work, that gases
expand against external pressure. In this case it could be the pressure of the atmosphere and that’s doing work. So
whenever you blow up a balloon, what you’re doing is you’re doing work. Another example, I mentioned lifting, and
this is a little more subtle. Separating socks. When you pull socks out of the dryer, very often they’re stuck together
and the reason that they’re stuck together is because of static electricity. In that case, there is a coulomb force that’s
causing the two socks to stick together and when you pull those apart, you’re having to do work. So that’s another
example of work.
How does work relate to the units of energy? Here’s an example, in raising a 1 kilogram mass 1 meter, where gravity
is 9.8 m/sec2 , we’ve got the force here. The force is 1 kilogram times the gravity, g, which is 9.8 m/sec2 and the
distance that we’re raising this 1 kilogram mass is 1 meter and you multiply f times d (1 kg x 9.8 m/sec2 x 1 m) and so
we’ve done 9.8 joules of work when we raised the printer up that high.
Now let’s talk about heat. Heat is a form of energy that gives rise to temperature changes so this is much more
intuitive probably for you. You know when something feels warm. If you go on a cold day and you stand by the
heater it seems to warm you. What’s happening is the heater is transferring energy in the form of heat to you. The
unit of heat is called the calorie, and the calorie is a word that should sound familiar to you because you know that
when you eat food there’s caloric content or calories associated with the food. It turns out that the calorie for food is
1000 times the calorie unit that chemists use. We’ll come back to that in a special topic. The calorie is defined to be
the amount of heat required to heat 1 gram of water from 14.5 C to 15.5 C. It turns out to be a relatively small number.
In fact, it’s actually only 4.184 joules and that’s how it’s going to be defined. In other words we have these two units
for energy. We have to relate them. We’re going to relate them by this identity that one calorie is equal to 4.184 joules
and that’s an exact. Since these are so tiny, what we do is we define both a kilocalorie, or 1000 calories, and a
kilojoule, or 1000 joules, and these turn out to be much more convenient numbers when we’re talking about chemical
reactions.
Let’s review and introduce maybe a little more new material. We have the printer on the desk. Here it is. It has no
kinetic energy, no potential energy. When we raised it up, we did work. It now has a lot of potential energy. When we
dropped it, it converted that potential energy into kinetic energy, and then eventually it came to rest again, and guess
what? The energy that was kinetic energy gets turned into heat and that heat actually warms the pad of paper, and
the desk, and the air around. It might not be obvious that should be the case. Here’s an experiment for you to try
sometime. Get a piece of metal, a pipe or something like that, beat the heck out of it with a hammer. What you’ll find
is that just by pounding on that piece of metal, you can actually heat it up. In other words, the printer hitting the table
Thermochemistry
An Introduction to Energy
The Nature of Energy Page [3 of 3]
actually heats up the table a little bit. It’s a little bit of a leap of faith to believe that all the kinetic energy went into the
table but that’s exactly where it went.

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