Download Carbon dioxide, college study notes - Chemistry of carbon dioxide and more Study notes Green and Environmental Chemistry in PDF only on Docsity!
Carbon Dioxide
Andrew R. Barron
This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License †
Carbon dioxide (CO 2 ) is the most stable oxide of carbon and is formed from the burning of carbon or carbon containing compounds in air or an excess of oxygen, (1). For industrial applications it is usually pre- pared from the decomposition of calcium carbonate (limestone), (2), rather than separation from combustion products.
(1)
1 Phase chemistry of carbon dioxide
Carbon dioxide does not exists as a liquid under normal atmospheric pressure, but solid CO 2 (also known as dry ice) sublimes at -78.5 ◦C (Figure 1). Dry ice (Figure 2) is commonly used as a refrigerant for food or biological sample preservation.
∗Version 1.2: Jan 20, 2010 9:56 am US/Central †http://creativecommons.org/licenses/by/3.0/
Figure 1: The phase diagram for carbon dioxide.
Figure 2: Photograph of a solid block of dry ice.
note: When dry ice is placed in water (especially heated) sublimation is accelerated, and a low-sinking dense cloud of fog (smoke-like) is created. This is used in fog machines, at theaters, concerts, haunted houses, and nightclubs for dramatic eects (Figure 3). Fog from dry ice hovers above the ground unlike other arti cial fog machines (that use partial combustion of oil) where the fog rises like smoke.
Figure 4: The carbon dioxide molecular orbital diagram.
3 Dissolution and reaction with water
Although CO 2 has no dipole moment it is very polar (dielectric constant = 1.60 at 0 ◦C, 50 atm) and consequently dissolves in polar solvents such as water up to a concentration of 0.18% (0.04 M). Most of it (+99%) is present as solvated CO 2 (Figure 5), and only ca. 0.2% is reacted to form carbonic acid, (3), with subsequent equilibria resulting in the formation of bicarbonate (HCO 3 - ) and carbonate (CO 3 2-).
(3)
Figure 5: Typical water solvation of carbon dioxide.
The overall reaction involves a series of equilibria. The rst equilibrium is the formation of carbonic acid, (4). The reaction rates, (5), are on the magnitude of 1 second (i.e., slow), and as a consequence when carbon dioxide is carried in the body an enzyme is present to speed up the reaction.
The 2nd^ equilibrium is as a consequence of rst ionization of carbonic acid to form bicarbonate (HCO 3 - ), (6). In contrast to the rst reaction, (4), this reaction is very fast with a Keq = 1.6 x 10-4^ @ 25 ◦C.
(6)
The 3rd^ equilibrium involves the formation of the carbonate ion(7), and has a Keq = 4.84 x 10-11. Carbonate (CO 3 2-) is a delocalized ligand, which can act as a mono or bidentate or bridging group.
(7)
The formation of carbonic acid is the reason that even in the absence of pollutants (such as SO 2 ) natural rain water is slightly acidic due to dissolved CO 2. The equilibrium associated with carbonic acid is also responsible for the buering of the pH in blood.
4 Reaction chemistry
Photosynthesis in plants reduces CO 2 to organic matter but similar reactions have yet to be developed in non-living systems. Grignards react readily with carbon dioxide to form the carboxylate, which yields the associated car- boxylic acid upon hydrolysis, (8). Similar reactions occur with other organometallic compounds. In addition, CO 2 reacts with alkali metal salts of phenols (phenolates) to yield the hydroxy-carboxylate, (9).
Figure 7: Correlation of Earth average temperature with carbon dioxide and global population.
The Earth's atmosphere has two functions. First, the ozone (O 3 ) in the upper atmosphere screens harmful UV from reaching the surface of the Earth. Second, as solar radiation penetrates the atmosphere a portion of the heat is then retained as a consequence of the CO 2 in the atmosphere. It is this process that modulates the surface temperature and provides a stable environment for life. The failure or alteration of either of these processes can have a dramatic eect on the livability of a planet. Consider the relative position of Venus, Earth, and Mars to the Sun (Figure 8). The closer a planet is to the sun the greater the UV radiation and the greater the heating of the planet; however, the temperature is also greatly modulated by the atmosphere. Venus has an atmosphere comprising 95% CO 2 and has a surface temperature of approximately 450 ◦C. In contrast, while Mars' atmosphere is also 95% CO 2 , it is only 1% as dense as that of Earth's, and thus the surface temperatures range from 40 ◦C during the day (due to radiative heating) to 80 ◦C at night (due to the lack of retained heat because of the thin atmosphere). These should be compared to Earth's atmosphere which is 0.038% CO 2 , which allows for the correct amount of heat to be retained to sustain life. Clearly any signi cant change in the CO 2 content of the atmosphere will change the global temperatures of a planet.
Figure 8: The relative size and position of the planets from the sun.
6 Bibliography
- N. Stern, The Stern Review: The Economics of Climate Change, HM Treasury, London.
- R. B. Gupta and J.-J. Shim, Solubility in Supercritical Carbon Dioxide, CRC Press (2006).
- Carbon Dioxide Capture and Storage: Special Report of the Intergovernmental Panel on Climate Change, Cambridge University Press (2005).
