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 oC),
carbon dioxide is an odorless, colorless 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 oC. 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 characterized by
the CO2 bubbles forming into water.
CO2-molecule
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
Molecular weight
Specific gravity
Critical density
Concentration in air
Stability
Liquid
Solid
Henry constant for solubility
Water solubility
Value
44.01
1.53 at 21 oC
468 kg/m3
370,3 * 107 ppm
High
Pressure < 415.8 kPa
Temperature < -78 oC
298.15 mol/ kg * bar
0.9 vol/vol at 20 oC
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 H2O --> C6H12O6 + 6 O2
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:
C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O
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,
H2CO3, according to the following reaction:
CO2 + H2O --> H2CO3
After that, carbonic acid reacts slightly and reversibly in water to form a
hydronium cation, H3O+, and the bicarbonate ion, HCO3-, according to the
following reaction:
H2CO3 + H2O --> HCO3- + H3O+
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 CO2 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 2025% 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.
Resources:
http://www.oism.org/pproject/s33p36.htm
http://cdiac.ornl.gov/pns/faq.html
http://www.ilpi.com/msds/ref/carbondioxide.html
Living in the Environment, a book by G. Tyler Miller