Voiceover: Okay so up until now we've spent a lot
0:03of time talking about ideal gases but it turns out
0:06that no gases are actually ideal.
0:09And although the ideal gas law gives us a pretty accurate
0:12idea of what's going on, there are slight deviations
0:14when we're talking about a real gas.
0:17And so what I want to do is I want to review the conditions
0:20of an ideal gas and we'll see where
0:22these deviations for real gases occur.
0:25And first we said that an ideal gas is one that has
0:28molecules that don't occupy any actual space.
0:32So no actual space, and a real gas's particles
0:36do occupy space.
0:38It's a microscopic amount of space but it's real.
0:41And a Dutch chemist named J.D. Van der Waals
0:44actually studied this deviation.
0:46So Van Der Waals, and he also found that
0:51unlike ideal gases, real gas particles do
0:54exert a small amount of force on each other
0:58and these forces were actually named after him.
1:00We call them Van Der Waals forces now.
1:02So, Van Der Waals found that the degree to which
1:05a real gas deviates from an ideal gas
1:09has a lot to do with a magnitude of the
1:12inner molecular forces and the size of the particles.
1:15Let's draw a system and let's fill it with some particles.
1:21What do you think is happening if all of the particles
1:24in this container are slightly attracted to each other?
1:28Well in a small way that gas is going to be
1:31imploding or caving in on itself.
1:34So caving in on itself.
1:35Because all of these particles are a little bit attracted
1:39to each other, and what do you think this is affecting?
1:41Well it's going to affect the pressure because now the
1:44gas isn't pushing out on the sides of the container
1:47as much as we thought it was, so for real gas pressure
1:51we need to be able to correct for the
1:53inner molecular forces.
1:54But that's kind of tricky because we've got to think
1:56about what the inner molecular forces are dependent on.
1:59And first, I'd say that some of these forces is dependent
2:03on the concentration of the particles
2:05because more particles means more attraction.
2:09So the concentration of anything is just the amount of
2:12that thing per the space that it occupies and the amount
2:15of our particles or the moles in the space that they occupy
2:19is the volume so the concentration is the moles per volume.
2:24And, N divided by V, our moles per volume would be
2:28alright if we're talking about all of these particles being
2:31attracted to one particle but they're all actually being
2:35attracted to each other as well.
2:37So all of these particles are also attracted to each other
2:42and what that means is that force actually increases
2:44exponentially as particle concentration increases
2:48and so we need to make it N divided by V squared.
2:51Now this is kind of a little bit of a crazy thought
2:54but the thing that characterizes the inner molecular forces
2:57between particles is actually different for different particles.
3:01And so we need to account for the different characteristics
3:04of different gases and the atoms that are in their particles.
3:08So each gas has a different intrinsic factor
3:11that we need to consider.
3:12We call that A, and we can think of A as the attraction
3:16coefficient but it's usually a value that's given to us
3:19in a table because it's unique
3:21for the gases that we're dealing with.
3:24So putting these thoughts together, we find that the
3:26P-Ideal, the pressure ideal is actually equal
3:30to the pressure observed plus this consideration
3:34of the attraction coefficient times the concentration
3:37of these particles squared.
3:39Now it's important to make note that the forces
3:42may attract each other, these particles forces
3:45may attract each other and cause the system
3:48to kind of implode but they might
3:49also very well repel each other.
3:52And so if they repel each other the value of A
3:55would be negative so it can be positive or negative
3:58depending on whether the particles are attracted
4:00or repulsed from each other and this makes pretty logical
4:04sense if you think about because the pressure
4:06we observe is an understatement of the ideal pressure
4:11if the system is slightly imploding so we would
4:15need to add this correction.
4:17And it's an exaggeration of an ideal gas if the particles
4:21are repelling from each other and pressing out
4:24on the sides even more and in that case
4:27we would need to subtract the correction.
4:30So Van der Waals taught us to correct for the inner
4:33molecular forces when we're dealing with a real gas.
4:37Now the other deviation was related to the size
4:39of the particles in the free space that they take up
4:42which affects the value for our ideal volume.
4:47So each gas has particles that are little different size
4:50again dependent on the atoms or the molecules
4:52that are in that gas.
4:54Hydrogen particles for example are quite small but
4:57if we're talking about methane particles well those
5:00are quite a bit bigger.
5:01And so each gas has an empirical volume factor
5:04to consider for and we call this factor B.
5:07And I usually think of B as my bigness coefficient.
5:11B for bigness, so we consider the size of the molecules
5:15and we also have to factor in
5:16how many particles, molecules or atoms there are.
5:21And that's just our moles or N.
5:25So we multiply the moles by our bigness coefficient
5:28and we can write this as an equation if we say that
5:31the volume ideal, the ideal volume is the volume
5:36of the container minus the space occupied
5:40by the particles which would be N times B.
5:44And as another quick note, unlike A, B will always
5:48be positive because the molecules will always
5:50take up some amount of space.
5:53So starting with the ideal gas law which is PV is equal
5:57to NRT, let's insert our corrected pressure for a real
6:02gas and our corrected volume for a real gas
6:05into the equation.
6:06So starting with our corrected pressure we know
6:08that the ideal pressure is equal to the pressure
6:11observed plus our attraction coefficient times our
6:16concentration, our molar concentration squared.
6:20So P observed plus A times N over V squared.
6:25And then we can insert our volume, so the ideal volume
6:28is actually equal to the volume of the container
6:32minus the number of particles times
6:35the bigness coefficient or B.
6:38So VC minus NB and the corrected product
6:42of the real pressure and the real volume now
6:46is equal to NRT.
6:48So by corrected for the inner molecular forces and the
6:51volume that's taken up by the particles now we have
6:54an equation that applies to real gases
6:57which we call the Van der Waals equation.
7:00And it looks pretty terrifying and there are tons
7:03of values in here but it's really now big deal
7:06because the only difference between this
7:08and the ideal gas equation is just
7:10the corrected pressure and the corrected volume.