In our discussion of gases we talked about which elements in
the periodic table existed as gases at room temperature.
The elements which are gases include, hydrogen, helium,
oxygen, nitrogen, fluorine, chlorine and all the noble gases. We
took a few moments to look at several of these gaseous elements.
I'll expect you to know the standard state phase of every element
in the periodic table. Now you might be thinking, holy cow, are
you nuts, but remember 2 of the elements are liquids and 10 of
the elements are gases. So it is not so bad. Also I'll expect you
to know the color of each of the gaseous elements.
Hydrogen is the most abundant element in the universe.
Hydrogen is a colorless, odorless, tasteless gas. If boils at
-252. degrees C and freezes at -259.1 degrees C. Water is a good
source of hydrogen, as is natural gas. The formula for hydrogen
is, H2.
Helium was the 1st noble gas to be discovered. It was
identified on the sun before it was found on the earth. It
discovery was made by a French astronomer Pierre Jules Casar
Janssen in 1868. Helium is colorless, odorless, tasteless gas at
room temperature. It boils as -268.6 degrees C. It is the only
element that cannot be converted to a solid by cooling alone.
(Note: the pressure must be increased to 26 atmospheres before
helium solidifies at -272 degrees C.) The formula for helium is
the same as its symbol, He.
Oxygen, O2, is the most abundant element in
the earths crust. O2 was discovered in the late 18th
century by the English chemist Joseph Priestly. A colorless gas,
at room temperature it condenses to a blue liquid at -183 degrees
C and freezes to a pale blue solid at -218 degrees C. Obtained by
the liquification of air.
Although nitrogen is the most abundant element in the
atmosphere it is not particularly abundant on earth because there
are few compounds containing nitrogen which are solids. Nitrogen
is a colorless, odorless, tasteless gas. It boils at -196 degrees
C and freezes at -210 degrees C. It is obtained from
liquification of air. We saw liquid nitrogen in class when I
poured it on the balloon containing carbon dioxide and helium.
Fluorine is a pale yellow gas with a boiling point of
188 degrees C. It freezes at 218 degrees C. Fluorine
is an extremely reactive gas and very difficult to work with in
its a elemental state. I cannot show you a sample of fluorine
beause of its reactivity. In fact, it is so reactive it even
forms compounds with noble gas elements like xenon. We will talk
about this interesting behavior later. Fluorine is commonly found
in many minerals including fluorspar, cryolite, and fluorapatite.
Argon was identified by the physicist Lord Rayleigh
and Sir William Ramsay. Their individual experiments identified
the colorless, odorless, tasteless gas in 1894. It was the first
nobel gas isolated on earth.
Later in 1898 Ramsay and an assistant Morris Travers isolated
neon, from a sample of impure oxygen. They were also able to show
that air contained two other element which they identified
Krypton and Xenon.
Radon is obtained as a disintegration product of
radium. It is a radioactive gas. Radon-222 decays by alpha
particle emission to a variety of solid radioisotopes. Two of
these isotopes are polorium-218 and polorium-214 are also alpha
emitted. As solids these remain in the lung. Radon, as a gas, is
inhaled and then exhaled.
Chlorine is a pale greenishyellow gas, with a
boiling point of 101 degrees C and a melting point of
34 degrees C. Chlorine is the most important of the
halogens.
Under appropriate conditions substances which are normally
liquids or solids can exist in the gas phase. In such cases the
gaseous phase of the substance is called vapor.
After looking at several of the gaseous elements using the
Periodic Table Videodisc we explored the behavior of gases by
pouring liquid nitrogen over a balloon filled with carbon dioxide
gas. Specifically we were interested in what happened to the
volume of the gas in the balloon as we lower the temperature. As
I poured the liquid nitrogen over the balloon we observed the
balloon getting smaller, until it got completely flat as though
no gas was present at all. When I picked up the balloon and
started shaking it we could hear the sound of a solid inside the
balloonn. As I held the balloon it began to return to its
original shape. As I continued to shake the balloon the sound due
to the solid inside diminished as the balloon got larger. Finally
when we could not hear the solid rattling around inside the
balloon it had returned to it original size.
This experiment demonstrated the relationship of temperature
and volume for any gas. As the temperature was lowered the volume
of the gas also decreased. Here is a QuickTime
Movie (this is a large file so if you are using a modem wait
and view this file on campus) of what happens when a balloon
containing carbon dioxide gas is placed into liquid nitrogen.
Here are a few slides from the
movie. There is a direct relationship between the temperature and
the volume of a gas. Here is a QuickTime
Movie of what happens when a balloon containing helium is
placed in liquid nitrogen.
Had we recorded some data we could plot the temperature
versus volume for the gas. I have obtained some data from a
different experiment where I was able to carefully measure both
the temperature and the volume of a gas. Here is the animation showing the plot of
temperature versus volume for a gas. (ATTENTION: This is an early
animation of mine and you will need to click the mouse to advance
through the animation. So if the animation stops and appears to
be doing nothing, just click the mouse once. Be sure the mouse
cursor is in the animation window when you click it.)
To view the animation of this experiment your browser
must have the Shockwave
plug-in from MacroMedia. If you do not have the
plug-in installed on your computer get it before trying
to run the animation.
In the next experiment with a gas we explored how the volume
of a gas depended on pressure. Here is a movie
of the experiment like we did in class. We used an apparatus
which consisted of a syringe containing air connected to a
pressure transducer to measure the pressure. The pressure
transducer measured the pressure in pounds per square inch (lb in-2).
In class we began the experiment with 30 mL of air and the
pressure reading was 14.7 lb in-2. I then began to
reduce the volume of the air in the syringe by pushing on the
syringe plunger. We continued to collect data (pressure and
volume readings). The data we collected is summarized in the
table below;
Volume (mL)
|
Pressure(lb in-2))
|
30
|
14.7
|
25
|
17
|
20
|
21
|
15
|
26.5
|
10
|
34
|
8
|
40
|
Overall we can see as the volume decreases the
pressure increases. This is an inverse relationship. Here is an animation showing the plot of pressure
versus volume for a gas. The data plotted in this animation is
different from the data collected in class, but the relationship
is the same. The units on pressure are not lb in-2 but
atm . An atm is an atmosphere and the relationship between lb in-2
and atm is
14.7 lb in-2 = 1 atm
(ATTENTION: This is an early animation of mine
and you will need to click the mouse to advance through the
animation. So if the animation stops and appears to be doing
nothing, just click the mouse once. Be sure the mouse cursor is
in the animation window when you click it.)
To view the animation of this experiment
your browser must have the Shockwave
plug-in from MacroMedia. If you do not have the
plug-in installed on your computer get it before trying
to run the animation.
We'll learn about plotting this data in the
in-class (lab) problem set during the week of September 7, 1999.
What is pressure?
Pressure is a measure of the force of an object, or
collection of particles, on a given area. Another way of saying
the same thing, is pressure is the force exerted on an object
divided by the area over which the force is distributed.
Mathematically, we would write this relationship in the following
way;
![](../../1225/Lecture/Chapter13/Gas1.h1.gif)
We need to make a distinction between pressure and force.
Something familiar to all of us is going down to the gas station
to pump up a bicycle tire. Tires require between 40 psi (pounds
per square inch) to 130 psi. We know adding more air to the tire
increases the pressure. The number of psi increase. The units
(psi) describe the weight exerted by the gas divided by the area
over which the weight is distributed. We also understand that P
is proportional to the number of moles of gas in a container.
The difference between force and pressure can be described in
terms of a person walking on a frozen lake. Up right, the
person's weight (mass x gravity) is distributed over a
small area (area of shoes) so the pressure exerted by the person
is high. If the ice is thin it may break as a result of the
person standing on it. By lying down flat on the ice the
individual's weight is distributed over a greater area and the
pressure decreases.
Atmospheric pressure is a term that should also be familiar
to you. The local weather report includes a map of the US with
indicated regions of high and low pressure. This suggests that
pressure is not constant. If we read the newspaper or watch a
weather report the pressure is stated and it changes each day.
You also know that at high altitudes (show an airplane)
the pressure is lower than on the ground.
We can measure the pressure
exerted by the atmosphere by filling a hollow glass tube with
mercury, (a tube longer than 76. cm) and while plugging the
bottom of the tube inverting it into a pan containing mercury.
(Note: the mercury barometer was first developed by Evangelista
Torricelli in 1643) Such a device is called a barometer
(
). The atmospheric pressure is obtained by measuring
the distance between the surface of the mercury in the reservoir
and the top of the mercury in the tube. Well you might ask, why
didn't the mercury run completely out of the tube when it was
inverted into the pan? The answer is the pressure exerted by the
atmosphere supports the column of mercury. Another way to
describe this is the weight of mercury in a column 760 mm high is
equal to the weight of the air above the surface of the mercury
in the pan.
We can demonstrate the presents of atmospheric pressure by
the following simple experiment (
QuickTime
movie, 1.6 M file)(this version is displayed in a
larger frame size/2.4
M file). If a soda can is partially filled with water and the
water heated to boiling, nearly all of the air is swept from
inside the can. If the top of the can is quickly sealed and the
water in the vapor phase rapidly cooled a large difference in
pressure is obtained. This can be accomplished by inverting the
can and immersing it in a container of water at room temperature.
We will discuss aspects of this experiment later in the lecture.
Area of the pop can is 2(pi)r(h + r) = 2(3.14)(1.125)(4.75+1.125)
= 32 in2. Pressure is
![](../../1225/Lecture/Chapter13/Gas1.h2.gif)
so the weight of the atmosphere on the can is 14.7(32)
= 470 lbs. Inside the can the weight of any gas is close to zero
since there is very little gas in the can. Normally when a can of
soda is open the pressure inside and outside the can are the
same, and nothing happens to the can walls. But if we creat a
large difference in pressure inside and outside the can we see
what happens.