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Chemistry: The First Law of Thermodynamics


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  • Type: Video Tutorial
  • Length: 10:56
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 117 MB
  • Posted: 01/28/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Final Exam Test Prep and Review (49 lessons, $64.35)
Chemistry Review (25 lessons, $49.50)
Chemistry: Thermochemistry (12 lessons, $18.81)
Chemistry: An Introduction to Energy (6 lessons, $10.89)

In this lesson, Professor Yee introduces thermodynamics. The First Law of Thermodynamics states that the total energy of the universe is constant. Remember from the lesson on the nature of energy that the universe is defined as the system + surroundings, and thus the First Law of Themodynamics correlates to chemical reactions. Another way to state this law is that the change in energy is equal to heat + work. Next, Professor Yee teaches us that the change in energy and energy itself are state functions. A state function is a property that is determined by the state of condition and not how the current state was reached. He explains this better using the concepts of altitude and distance. The altitude of a city is a state function because it is not dependent upon how you reach the city. The distance you drive to get to the city is not a state function because it will depend upon the route you take driving there.

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 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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An Introduction to Energy
The First Law of Thermodynamics Page [1 of 2]

How do we know that hydrogen and oxygen when we mix them together and apply a spark are going to react to form
water? How do we know if we have a car parked on a hill and we release the brake that it is going to roll downhill and
not uphill? How do we know if we blast a tangerine into a million pieces that if we sit around for a while those pieces
are not going to collect from all over the room and reassemble themselves into a complete tangerine again? That is
the study of thermodynamics. We’re ready to start examining some tenets of thermodynamics.

Scientists have codified certain observations in thermodynamics and called them laws. Now these laws are not real
laws in the sense that you can prove them but they describe reality and we’ve never found any violations of them so
we call them laws.

The first law that I’d like to talk about is called “The First Law of Thermodynamics.” The First Law of Thermodynamics
in words is that the total energy of the universe is constant. We can express that in symbolic language by saying that
DE, the change in internal energy of a system is equal to the heat transferred from the system to the surroundings or
from the surroundings to the system, plus the work done by the system on the surroundings or by the surroundings on
the system. Now these two things might seem incongruous and they might seem like they don’t have anything to do
with each other, but the thing to remember is we define the universe as the sum of the system and surroundings. In
other words the universe is only broken up into two pieces, the system and the surroundings. When we say that heat
is transferred from the system to the surroundings, or work is done by the surroundings on the system, the change in
internal energy has to be consistent with the idea that stuff goes somewhere. The heat goes somewhere from the
system. It goes into the surroundings and if work is done, work has to be done by the surroundings on the system.

The sum of these two quantities is equal to the change in energy, but overall the total energy of the universe is

To understand more fully this idea, we have to define the concept of a state function and what we’re going to see is
that DE, the changes in internal energy, is a state function, and q the heat transferred, and w the work done, these
two things are not state functions individually. So q is not a state function, w is not a state function but DE and E the
internal energy are state functions. Some other things that you can think about that are state functions are pressure,
volume, temperature, and a quantity that we haven’t talked about yet, enthalpy.

Let me first define what a state function is. A state function is a property of a system that is determined by the state or
condition of the system, and not by how the system got to that state. That doesn’t sound like very much. Probably
didn’t even make sense the first go around. But let me give you some examples of things that are state functions and
things that aren’t state functions and maybe it will be a little more clear.

Here’s a graphic of San Francisco, which is at zero feet in altitude because it’s at sea level and Denver, the Mile High
City, which is at 5,280 feet. So this represents an altitude difference between San Francisco and Denver. Altitude is
a state function because it doesn’t depend on how you get to Denver. You can drive however you want to get from
San Francisco to Denver. You can go up and across I-80 and come up through Wyoming and then come down into
Denver and you’re still going to be at 5,280 feet above sea level. Similarly, you could go from San Francisco, drive
down to southern California to Bakersfield across and take I-70 across; you’re still, despite the fact that you traveled a
different route at 5,280 feet. Think about this. The distance traveled on roads is not a state function because, in all
likelihood, the number of miles that you travel is going to be different depending on whether you take I-80 across to

Denver or I-70 across to Denver. Distance traveled is not a state function whereas altitude is a state function.
Let me give you another example. Here’s my system, and instead of energy we’re going to talk about money. The
money here is the energy of my system and I’m going to make changes to the energy of my system. What am I going
to do? Suppose I’m going to divide it into giving money to Joe or Sam. So here’s my system and I’m going to make a
change to my system. Say I give $100 to Sam and $500 to Joe. I’ve made a $600 change in the energy of my
system, in the number of dollars in my bank account. My bank account has gone down by $600 and I’ve given $100
to Sam and $500 to Joe.

Let’s change the problem. The first time we gave $100 to Sam and $500 to Joe. Now let’s give $300 to Sam and
$300 to Joe. We’ve made exactly the same change in our bank account so the bank account balance is a state
function but how much Sam or Joe got, that changed. The value of the amount of money that each of my friends got
changes, even though the balance of my bank account didn’t change. The difference in my bank account was a constant at $600. DE represents my bank account, Joe represents w -- w and q, work and heat, and they’re not state
functions. They can be up 500 and 100 or 300 and 300, but the change in my bank account, the same $600.

If we take a gallon of gasoline, gasoline is octane 18 CAH and we’re going to burn in it in the presence of oxygen, and
we’re going to make carbon dioxide and water, and this is not a balanced reaction. Let’s balance it. So this is now a
balanced reaction. If we perform this reaction it turns out a change in energy occurs. That change in energy only
depends on the fact that we started with 1 mole of octane and 12.5 moles of 2 O to give 8 moles of CO2 and 9 moles
of water. So that’s our DE and that’s constant. But it should be really clear that we can change the relative
distribution of work and heat and here’s the idea. If we take that gallon of gasoline and the 12.5 moles of oxygen and
we throw a match in it, we’re going to make a pretty good bonfire and we’re going to turn the energy associated with
this reaction into pure heat. We’re going to heat up a bunch of air. That’s if the distribution is all q and no w. Instead
we can take that gallon of gasoline and put it in a car and drive the car to the top of the hill. Now we’ve done some
work because we said that raising a weight in a gravitational field, that’s doing work. We’ve also made some heat
because the gas is coming out of the back of the tailpipe of the car; they’re typically hot. The engine gets hot. The
radiator gets hot. The water in the radiator gets hot so we make a lot of q too but the point is we do a little bit of w.

The DE, the change in energy, is exactly the same. If we collected all the gas coming out of the tailpipe we would
collect the same amount of carbon dioxide and water as when we just burned the gallon of gasoline at the bottom of
the hill. We’ve changed the distribution of how we’ve utilized that energy, and the way that the energy can be
distributed is between q and w.

So what have we learned? The total energy of the universe is a constant. That’s the First Law of Thermodynamics.
I’ve also introduced the concept of a state function and said that the internal energy change is a state function, but q
and w, the heat and work are not. We’re not quite ready to answer the question of why a car doesn’t spontaneously
roll uphill or why a tangerine that’s been blasted into a million pieces doesn’t spontaneously reassemble. We’re not
ready to answer that question yet. We’re going to find out that the fact that internal energy change is a state function
is a very powerful tool for learning about our universe.

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