Video Transcript
In this video, we will learn about
the properties of the components of air, how to separate these components, and the
reasons for doing this, in other words, the uses of different air fractions. Let’s start by briefly looking at
the composition of air.
Our atmosphere is a mostly
homogeneous mixture of gases, some particulate matter, and water vapor. Dry air, which does not contain
water vapor or particulate matter, is composed of approximately 78 percent nitrogen
gas by volume. In other words, about 78 percent of
the volume of the atmosphere is made of nitrogen. 21 percent is made of oxygen, and
the small fraction that remains is made of other gases. These other gases include argon,
carbon dioxide, the noble gases neon, helium, krypton, and xenon, as well as gases
like methane, nitrogen dioxide, which is a pollutant, and ozone, O3. So, this is the composition of dry
air.
But remember though that air does
in fact contain water vapor. There is more gaseous water or
water vapor in the air at sea level and less further away from the surface of the
earth. Water vapor is approximately a
quarter of a percent by mass of the whole atmosphere. Now, let’s compare the main
components of air according to the physical properties.
Let’s look at nitrogen, oxygen,
carbon dioxide, and water vapor. Nitrogen gas with a triple bond is
the least reactive of these four air fractions and has the highest average bond
energy, 942 kilojoules per mole. This tells us that it requires a
lot of energy to break this triple bond. The next highest average bond
energy is in carbon dioxide. A value of 799 kilojoules per mole
is for one of the carbon oxygen double bonds. This high value tells us that
carbon dioxide is also fairly unreactive.
Oxygen gas, however, with a double
bond is fairly reactive. It has a much lower average bond
energy than one of the double bonds in carbon dioxide. And water is the most reactive of
the four fractions. Its average bond energy is for one
of the hydrogen oxygen single bonds. Only 459 kilojoules per mole are
needed to break this bond. The density of these air fractions
are unique to each fraction.
Density is usually measured in
grams per centimeter cubed or kilograms per meters cubed. And the values for nitrogen,
oxygen, and carbon dioxide are given at standard temperature and pressure, which are
zero degrees Celsius and one bar. We can see that nitrogen has the
lowest density then oxygen with a slightly higher density then carbon dioxide being
the most dense of the three fractions.
Remember that water exists as a gas
above its boiling point of 100 degrees Celsius. However, water can exist as a vapor
below 100 degrees Celsius and even at zero degrees Celsius. But this depends on the
conditions. Just above zero degrees Celsius,
water is generally found as a liquid and its density is about one gram per
centimeter cubed. There is a definite trend in the
boiling points on this table for these fractions.
Liquid nitrogen boils into a gas at
the lowest temperature of negative 196 degrees Celsius. Oxygen is converted into a gas from
a liquid at negative 183 degrees Celsius, while carbon dioxide goes straight from a
solid to a gas at minus or negative 79 degrees Celsius. In other words, carbon dioxide
undergoes sublimation. So, technically, for carbon
dioxide, we should not refer to it as boiling point but perhaps sublimation
point.
These physical properties or
physical characteristics are unique to each fraction, in other words, unique to each
chemical substance. They help us identify what
substance is present. For example, if you had a gas whose
boiling point was negative 196 degrees Celsius, you could identify that gas as being
nitrogen. The property of boiling point is
very useful. It is what allows us to separate
the components or fractions of air. Let’s investigate that a bit
further.
To separate the fractions of air,
the air is first filtered to remove solid particulates and dust, in other words, to
purify the air. Air is then compressed under high
pressure. A cooling process then begins. As the air gets cooler. It reaches a temperature of about
six degrees Celsius. At this temperature, water vapor is
mostly all condensed and filtered off. Further cooling to negative 79
degrees Celsius solidifies gaseous carbon dioxide to solid carbon dioxide. This is then removed.
The fractions that remain are
nitrogen, oxygen, and a small amount of argon. When the cooling process reaches
negative 183 degrees Celsius, oxygen gas liquefies. Then, at negative 186 degrees
Celsius, argon liquefies. And at negative 196 degrees
Celsius, nitrogen liquefies. The liquid oxygen, argon, and
nitrogen is then further cooled to about negative 200 degrees Celsius. And this liquid mixture is fed into
a special plant for fractional distillation.
Fractional distillation is a
suitable method to separate different liquids from a mixture of liquids. It is a physical separation because
no chemical bonds are broken. Substances are merely separated by
virtue of the different boiling points.
The separation of nitrogen, argon,
and oxygen happens in a fractionating column. The mixture enters the column at
negative 200 degrees Celsius at the bottom. The column is cooler at the top and
warmer at the bottom. As the liquid enters the bottom of
the column, it warms up slowly. When it reaches a temperature of
negative 196 degrees Celsius, which is nitrogen’s boiling point, the liquid nitrogen
is converted to a gas, which then bubbles up the column and is collected at the
top.
The mixture is further warmed until
negative 186 degrees Celsius, which is argon’s boiling point. Argon is converted from a liquid to
a gas and begins to bubble up the column. It is usually collected at its own
special outlet point. However, many fractionating column
diagrams don’t show a special outlet point for argon, so we’ll just simplify it and
leave it off for now.
So, we know nitrogen gas boils off
first and is collected at the top of the column. We know that argon gas boils off
second and is collected at its own special outlet point. What remains in the column is
liquid oxygen. It is warmed just a degree more to
about negative 185 degrees Celsius to ensure that all the argon has left the
mixture. And then, a valve at the bottom of
the fractionating column is opened, and the liquid oxygen is allowed to run off and
is collected.
Now, we know about the fractions in
air, their unique properties, and how to separate them. We need to now ask ourselves, why
are the fractions of air separated? In other words, what use do we have
for these different fractions? Nitrogen gas, we have seen, is
inert and unreactive. For this reason, it is perfect to
use as the atmosphere in food packaging and storage, for example, in ground coffee
tins and in potato chip bags. It is the lack of oxygen gas and
water vapor in these bags that prolongs the freshness of the food and helps prevent
oxidation.
Nitrogen is one of the reactants in
the Haber process, which is the industrial preparation of ammonia gas. Ammonia gas is a starting material
for many other reactions. Liquid nitrogen, because it is so
cold, is used to freeze dry foods and cryogenically freeze biological tissue
samples. In the lab, nitrogen gas is used as
an inert atmosphere over chemical reaction mixtures.
Oxygen gas is used in oxyacetylene
torches in welding and metal cutting, as well as in medicine, for example, in
breathing equipment. Carbon dioxide gas also has many
uses. Some of them include use in fire
extinguishers, to carbonate or make soft drinks fizzy, and in its solid form as dry
ice, which is used to keep food cool. You may have seen this, for
example, in mobile ice cream carts.
Argon is often used as the
atmosphere in light bulbs as it is inert and unreactive, even when the filament gets
white-hot. Argon lasers are used in eye
surgery. And argon, like nitrogen, is used
as an inert atmosphere in chemical reactions. It’s time to practice a
problem.
The components of air can be
separated in a fractionating column, as shown. (a) Fraction A is collected at the
top of the column. Which range of temperatures must be
maintained at the top of the column? (A) Negative 195 to negative 190
degrees Celsius, (B) negative 190 to negative 185 degrees Celsius, (C) negative 185
to negative 180 degrees Celsius, (D) negative 205 to negative 200 degrees Celsius,
or (E) negative 200 to negative 195 degrees Celsius.
The question asks about separating
the components of air. To separate the components of air,
air must first be filtered to remove dust and particulates. It is then compressed under high
pressure and cooled in stages. In the first stage, air is cooled
to about six degrees Celsius, at which point almost all the water has condensed and
this is removed by a filter. The air is now dry and water
free. It is further cooled to negative 79
degrees Celsius, at which point carbon dioxide gas solidifies to solid carbon
dioxide. The solid carbon dioxide is then
easily removed.
The last stage in cooling is to
cool the remaining components, which are nitrogen and oxygen, to negative 200
degrees Celsius, at which point they are both in the liquid form. In reality, there is a tiny
fraction of argon also present in the mixture, but this fraction is less than one
percent. And here, we will assume that the
main fractions are nitrogen and oxygen.
The mixture of liquid nitrogen and
liquid oxygen are then fed into a fractionating column, where they are separated by
a process called fractional distillation. This process is similar to the
process for separating the components of crude oil. The mixture is fed into the bottom
of the column, where it is warmer. At the top of the column, it is
cooler. The negative 200 degrees Celsius
liquid mixture of nitrogen and oxygen is then warmed slowly.
When the temperature reaches
negative 196 degrees Celsius, which is nitrogen’s boiling point, the nitrogen boils
off and leaves the top of the column as fraction A. The temperature at the top of the
column is maintained just above nitrogen’s boiling point, or just slightly warmer
than nitrogen’s boiling point. It cannot be much warmer than
this. Otherwise, oxygen might also boil
off. Oxygen’s boiling point is negative
183 degrees Celsius. And so, the temperature range at
the top of the column must be cooler than oxygen’s boiling point. So, the range of temperatures that
must be maintained at the top of the column is negative 195 to negative 190 degrees
Celsius.
Question (b) fraction B is
collected at the bottom of the column. Which range of temperatures must be
maintained at the bottom of the column? The answer options are the same as
for part (a).
When all the nitrogen gas has left
as fraction A at the top of the column, what is left is liquid oxygen. This is fraction B. We saw that oxygen’s boiling point
is negative 183 degrees Celsius. And to prevent the oxygen from
boiling off, the temperature at the bottom of the column is maintained between
negative 190 and negative 185 degrees Celsius, which is cooler than oxygen’s boiling
point.
Ensuring that the oxygen remains as
a liquid, a valve or tap is then opened at the bottom of the fractionating column,
and the oxygen in liquid form is easily collected. So, which range of temperatures
must be maintained at the bottom of the column? The answer is negative 190 to
negative 185 degrees Celsius, which is just cooler than oxygen’s boiling point.
Question (c) identify the major
component of fraction A. (A) gaseous argon, (B) liquid
nitrogen, (C) liquid oxygen, (D) gaseous nitrogen, or (E) gaseous oxygen.
We have seen that the major
component of fraction A is gaseous nitrogen because nitrogen boiled off when the
temperature in the column reached about negative 196 degrees Celsius. So, the major component of fraction
A is gaseous nitrogen.
Question (d) identify the major
component of fraction B. And the answer options are the same
as for part (c).
We have seen that the major
component of fraction B is liquid oxygen. Because often nitrogen comes off as
a gas, the only component left is oxygen. It’s kept at a temperature cooler
than its boiling point, in other words, cooler than negative 183 degrees Celsius,
and easily drained out the bottom of the column by opening a tap or a valve. So, the major component of fraction
B is liquid oxygen.
Let’s summarize what we’ve learned
about the fractional distillation of air. We learned that the main components
of air are nitrogen and oxygen gas. A very small percentage of air is
composed of carbon dioxide, argon, and noble gases as well as water vapor. We saw that the fractions each have
unique physical properties and that boiling point, in particular, is useful in order
to separate these fractions. We learned about the process of
fractional distillation used to separate nitrogen and oxygen. And lastly, we looked at the
applications or uses of the different air fractions.
