20/11/2016
Carbon dioxide
What is carbon dioxide and how is it discovered?
Joseph Black, a Scottish chemist and physician,
first identified carbon dioxide in the 1750s. At room
temperatures (20-25 o C), carbon dioxide is an
odourless, colourless gas, which is faintly acidic
and non-flammable.
Carbon dioxide is a molecule with the molecular
formula CO2 . The linear molecule consists of a
carbon atom that is doubly bonded to two oxygen
atoms, O=C=O.
Although carbon dioxide mainly consists in the
gaseous form, it also has a solid and a liquid form.
It can only be solid when temperatures are below
-78 o C. Liquid carbon dioxide mainly exists when
carbon dioxide is dissolved in water. Carbon dioxide
is only water-soluble, when pressure is maintained.
After pressure drops the CO2 gas will try to escape
to air. This event is characterised by the CO2
bubbles forming into water.
CO 2-molecule
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Properties of carbon dioxide
There are several physical and chemical properties,
which belong to carbon dioxide.
Here we will sum them up in a table.
Property Value
Molecular weight 44.01
Specific gravity 1.53 at 21 o C
Critical density 468 kg/m 3
Concentration in air 370,3 * 107 ppm
Stability High
Liquid Pressure < 415.8 kPa
Solid Temperature < -78 o C
Henry constant for solubility 298.15 mol/ kg * bar
Water solubility 0.9 vol/vol at 20 o C
Where on earth do we find carbon dioxide?
Carbon dioxide can be found mainly in air, but also
in water as a part of the carbon cycle. We can show
you how the carbon cycle works, by means of an
explanation and a schematic representation. -->
Move to the Carbon Cycle .
Applications of carbon dioxide by humans
Humans use carbon dioxide in many different ways.
The most familiar example is its use in soft drinks
and beer, to make them fizzy. Carbon dioxide
released by baking powder or yeast makes cake
batter rise.
Some fire extinguishers use carbon dioxide
because it is denser than air. Carbon dioxide can
blanket a fire, because of its heaviness. It prevents
oxygen from getting to the fire and as a result, the
burning material is deprived of the oxygen it needs
to continue burning.
Carbon dioxide is also used in a technology called
supercritical fluid extraction that is used to
decaffeinate coffee. The solid form of carbon
dioxide, commonly known as Dry Ice, is used in
theatres to create stage fogs and make things like
"magic potions" bubble.
The part carbon dioxide plays in environmental
processes
Carbon dioxide is one of the most abundant gasses
in the atmosphere. Carbon dioxide plays an
important part in vital plant and animal process,
such as photosynthesis and respiration. These
processes will be briefly explained here.
Green plants convert carbon dioxide and water into
food compounds, such as glucose, and oxygen. This
process is called photosynthesis.
The reaction of photosynthesis is as follows:
6 CO2 + 6 H 2 O --> C 6 H 12O 6 + 6 O 2
Plants and animals, in turn, convert the food
compounds by combining it with oxygen to release
energy for growth and other life activities. This is
the respiration process, the reverse of
photosynthesis.
The respiration reaction is as follows:
C6 H 12O 6 + 6 O 2 --> 6 CO 2 + 6 H 2O
Photosynthesis and respiration play an important
role in the carbon cycle and are at equilibrium with
one another.
Photosynthesis dominates during the warmer part of
the year and respiration dominates during the
colder part of the year. However, both processes
occur the entire year. Overall, then, carbon dioxide
in the atmosphere decreases during the growing
season and increases during the rest of the year.
Because the seasons in the northern and southern
hemispheres are opposite, carbon dioxide in the
atmosphere is increasing in the north while
decreasing in the south, and vice versa. The cycle
is more clearly present in the northern hemisphere;
because it has relatively more land mass and
terrestrial vegetation. Oceans dominate the
southern hemisphere.
Influence of carbon dioxide on alkalinity
Carbon dioxide can change the pH of water. This is
how it works:
Carbon dioxide dissolves slightly in water to form a
weak acid called carbonic acid, H 2 CO 3, according
to the following reaction:
CO2 + H 2 O --> H 2 CO3
After that, carbonic acid reacts slightly and
reversibly in water to form a hydronium cation, H3 O
+, and the bicarbonate ion, HCO3 -, according to the
following reaction:
H2 CO 3 + H 2O --> HCO3 - + H 3O +
This chemical behaviour explains why water, which
normally has a neutral pH of 7 has an acidic pH of
approximately 5.5 when it has been exposed to air.
Carbon dioxide emissions by humans
Due to human activities, the amount of CO2
released into the atmosphere has been rising
extensively during the last 150 years. As a result, it
has exceeded the amount sequestered in biomass,
the oceans, and other sinks.
There has been a climb in carbon dioxide
concentrations in the atmosphere of about 280 ppm
in 1850 to 364 ppm in 1998, mainly due to human
activities during and after the industrial revolution,
which began in 1850.
Humans have been increasing the amount of carbon
dioxide in air by burning of fossil fuels, by producing
cement and by carrying out land clearing and forest
combustion. About 22% of the current atmospheric
CO 2 concentrations exist due to these human
activities, considered that there is no change in
natural amounts of carbon dioxide. We will take a
closer look at these effects in the next paragraph.
Environmental problems - the greenhouse effect
The troposphere is the lower part of the
atmosphere, of about 10-15 kilometres thick. Within
the troposphere there are gasses called
greenhouse gasses. When sunlight reaches the
earth, some of it is converted to heat. Greenhouse
gasses absorb some of the heat and trap it near the
earth's surface, so that the earth is warmed up.
This process, commonly known as the greenhouse
effect, has been discovered many years ago and
was later confirmed by means of laboratory
experiments and atmospheric measurements.
Life as we know it exists only because of this
natural greenhouse effect, because this process
regulates the earth's temperature. When the
greenhouse effect would not exist, the whole earth
would be covered in ice.
The amount of heat trapped in the troposphere
determines the temperature on earth. The amount
of heat in the troposphere depends on
concentrations of atmospheric greenhouse gasses
and the amount of time these gasses remain in the
atmosphere. The most important greenhouse
gasses are carbon dioxide, CFC's (Chlor-Fluoro-
Carbons), nitrogen oxides and methane.
Since the industrial revolution in 1850 began, human
processes have been causing emissions of
greenhouse gasses, such as CFC's and carbon
dioxide. This has caused an environmental problem:
the amounts of greenhouse gasses grew so
extensively, that the earth's climate is changing
because the temperatures are rising. This unnatural
addition to the greenhouse effect is known as global
warming. It is suspected that global warming may
cause increases in storm activity, Melting of ice
caps on the poles, which will cause flooding of the
inhabited continents, and other environmental
problems.
Together with hydrogen , carbon dioxide is the main
greenhouse gas. However, hydrogen is not emitted
during industrial processes. Humans do not
contribute to the hydrogen amount in the air, this is
only changing naturally during the hydrological
cycle, and as a result it is not a cause of global
warming.
Increasing carbon dioxide emissions cause about
50-60% of the global warming. Carbon dioxide
emissions have risen from 280 ppm in 1850 to 364
ppm in the 1990s.
In the previous paragraph various human activities
that contribute to the emission of carbon dioxide
gas have been mentioned. Of these activities fossil
fuel combustion for energy generation causes about
70-75% of the carbon dioxide emissions, being the
main source of carbon dioxide emissions. The
remaining 20-25% of the emissions are caused by
land clearing and burning and by emission from
motor vehicle exhausts.
Most carbon dioxide emissions derive from
industrial processes in developed countries, such
as in the United States and in Europe. However,
carbon dioxide emissions from developing countries
are rising. In this century, carbon dioxide emissions
are expected to double and they are expected to
continue to rise and cause problems after that.
Carbon dioxide remains in the troposphere about
fifty up to two hundred years.
The first person who predicted that emissions of
carbon dioxide from the burning of fossil fuels and
other burning processes would cause global
warming was Svante Arrhenius, who published the
paper "On the influence of carbonic acid in the air
upon the temperature of the ground" in 1896.
In the beginning of the 1930 it was confirmed that
atmospheric carbon dioxide was actually
increasing. In the late 1950s when highly accurate
measurement techniques were developed, even
more confirmation was found. By the 1990s, the
global warming theory was widely accepted,
although not by everyone. Whether global warming
is truly caused by increasing carbon dioxide in the
atmosphere, is still debated.
Rising carbon dioxide concentrations in air in the
past decades
The Kyoto treaty
World leaders gathered in Kyoto, Japan, in
December 1997 to consider a world treaty
restricting emissions of greenhouse gases, mainly
of carbon dioxide, that are thought to cause global
warming. Unfortunately, while the Kyoto treaties
have worked for a while America is now trying to
evade them.
Carbon dioxide and health
Carbon dioxide is essential for internal respiration
in a human body. Internal respiration is a process,
by which oxygen is transported to body tissues and
carbon dioxide is carried away from them.
Carbon dioxide is a guardian of the pH of the blood,
which is essential for survival.
The buffer system in which carbon dioxide plays an
important role is called the carbonate buffer. It is
made up of bicarbonate ions and dissolved carbon
dioxide, with carbonic acid. The carbonic acid can
neutralize hydroxide ions, which would increase the
pH of the blood when added. The bicarbonate ion
can neutralize hydrogen ions, which would cause a
decrease in the pH of the blood when added. Both
increasing and decreasing pH is life threatening.
Apart from being an essential buffer in the human
system, carbon dioxide is also known to cause
health effects when the concentrations exceed a
certain limit.
The primary health dangers of carbon dioxide are:
- Asphyxiation . Caused by the release of carbon
dioxide in a confined or unventilated area. This can
lower the concentration of oxygen to a level that is
immediately dangerous for human health.
- Frostbite . Solid carbon dioxide is always below
-78 oC at regular atmospheric pressure, regardless
of the air temperature. Handling this material for
more than a second or two without proper
protection can cause serious blisters, and other
unwanted effects. Carbon dioxide gas released from
a steel cylinder, such as a fire extinguisher, causes
similar effects.
- Kidney damage or coma . This is caused by a
disturbance in chemical equilibrium of the carbonate
buffer. When carbon dioxide concentrations
increase or decrease, causing the equilibrium to be
disturbed, a life threatening situation may occur.
