Carbon dioxide


Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at normally-encountered concentrations it is odorless. As the source of carbon in the carbon cycle, atmospheric is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater.
It is a trace gas in Earth's atmosphere at 428 parts per million, or about 0.043% having risen from pre-industrial levels of 280 ppm or about 0.028%. Burning fossil fuels is the main cause of these increased concentrations, which are the primary cause of climate change.
Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian was regulated by organisms and geological features. Plants, algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis, which produces oxygen as a waste product. In turn, oxygen is consumed and is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration. is released from organic materials when they decay or combust, such as in forest fires. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate, which causes ocean acidification as atmospheric levels increase.
Carbon dioxide is 53% more dense than dry air, but is long lived and thoroughly mixes in the atmosphere. About half of excess emissions to the atmosphere are absorbed by land and ocean carbon sinks. These sinks can become saturated and are volatile, as decay and wildfires result in the being released back into the atmosphere., or the carbon it holds, is eventually sequestered in rocks and organic deposits like coal, petroleum and natural gas.
Nearly all produced by humans goes into the atmosphere. Less than 1% of produced annually is put to commercial use, mostly in the fertilizer industry and in the oil and gas industry for enhanced oil recovery. Other commercial applications include food and beverage production, metal fabrication, cooling, fire suppression and stimulating plant growth in greenhouses.

Chemical and physical properties

Structure, bonding and molecular vibrations

The symmetry of a carbon dioxide molecule is linear and centrosymmetric at its equilibrium geometry. The length of the carbon–oxygen bond in carbon dioxide is 116.3 pm, noticeably shorter than the roughly 140 pm length of a typical single C–O bond, and shorter than most other C–O multiply bonded functional groups such as carbonyls. Since it is centrosymmetric, the molecule has no electric dipole moment.
As a linear triatomic molecule, has four vibrational modes as shown in the diagram. In the symmetric and the antisymmetric stretching modes, the atoms move along the axis of the molecule. There are two bending modes, which are degenerate, meaning that they have the same frequency and same energy, because of the symmetry of the molecule. When a molecule touches a surface or touches another molecule, the two bending modes can differ in frequency because the interaction is different for the two modes. Some of the vibrational modes are observed in the infrared spectrum: the antisymmetric stretching mode at wavenumber 2349 cm−1 and the degenerate pair of bending modes at 667 cm−1. The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected in Raman spectroscopy at 1388 cm−1, with a Fermi resonance doublet at 1285 cm−1.
In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a Coulomb explosion imaging experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations based on an ab initio potential energy surface of the molecule, is that none of the molecules in the gas phase are ever exactly linear. This counter-intuitive result is trivially due to the fact that the nuclear motion volume element vanishes for linear geometries. This is so for all molecules except diatomic molecules.

In aqueous solution

Carbon dioxide is soluble in water, in which it reversibly forms , which is a weak acid, because its ionization in water is incomplete.
The hydration equilibrium constant of carbonic acid is, at 25 °C:
Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as molecules, not affecting the pH.
The relative concentrations of,, and the deprotonated forms and depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water, the bicarbonate form predominates becoming the most prevalent at the pH of seawater. In very alkaline water, the predominant form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.
Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate ion :
This is the true first acid dissociation constant, defined as
where the denominator includes only covalently bound and does not include hydrated. The much smaller and often-quoted value near 4.16 × 10−7 is an apparent value calculated on the assumption that all dissolved is present as carbonic acid, so that
Since most of the dissolved remains as molecules, Ka1 has a much larger denominator and a much smaller value than the true Ka1.
The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the carbonate ion :
In organisms, carbonic acid production is catalysed by the enzyme known as carbonic anhydrase.
In addition to altering its acidity, the presence of carbon dioxide in water also affects its electrical properties. File:Millipore co2.svg|thumb|400px|Electrical conductivity of carbondioxide saturated desalinated water when heated from 20 to 98 °C. The shadowed regions indicate the error bars associated with the measurements. A comparison with the temperature dependence of vented desalinated water can be found . When carbon dioxide dissolves in desalinated water, the electrical conductivity increases significantly from below 1 μS/cm to nearly 30 μS/cm. When heated, the water begins to gradually lose the conductivity induced by the presence of , especially noticeable as temperatures exceed 30 °C.
The temperature dependence of the electrical conductivity of fully deionized water without saturation is comparably low in relation to these data.

Chemical reactions

is a potent electrophile having an electrophilic reactivity that is comparable to benzaldehyde or strongly electrophilic α,β-unsaturated carbonyl compounds. However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with are thermodynamically less favored and are often found to be highly reversible. The reversible reaction of carbon dioxide with amines to make carbamates is used in scrubbers and has been suggested as a possible starting point for carbon capture and storage by amine gas treating.
Only very strong nucleophiles, like the carbanions provided by Grignard reagents and organolithium compounds react with to give carboxylates:
In metal carbon dioxide complexes, serves as a ligand, which can facilitate the conversion of to other chemicals.
The reduction of to CO is ordinarily a difficult and slow reaction:
The redox potential for this reaction near pH 7 is about −0.53 V versus the standard hydrogen electrode. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.
Photoautotrophs use the energy contained in sunlight to photosynthesize simple sugars from absorbed from the air and water:

Physical properties

Carbon dioxide is colorless. At low concentrations, the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor. At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.53 times that of air.
Carbon dioxide has no liquid state at pressures below 0.51795 MPa. At a pressure of 1 atm, the gas deposits directly to a solid at temperatures below 194.6855 K and the solid sublimes directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called dry ice.
Liquid carbon dioxide forms only at pressures above 0.51795 MPa ; the triple point of carbon dioxide is 216.592 K at 0.51795 MPa . The critical point is 304.128 K at 7.3773 MPa. Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid. This form of glass, called carbonia, is produced by supercooling heated at extreme pressures in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon dioxide and germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released.
At temperatures and pressures above the critical point, carbon dioxide behaves as a supercritical fluid known as supercritical carbon dioxide.
Table of thermal and physical properties of saturated liquid carbon dioxide:
Temperature
Density
Specific heat
Kinematic viscosity
Thermal conductivity
Thermal diffusivity
Prandtl Number
−501156.341.841.19 × 10−70.08554.02 × 10−82.96
−401117.771.881.18 × 10−70.10114.81 × 10−82.46
−301076.761.971.17 × 10−70.11165.27 × 10−82.22
−201032.392.051.15 × 10−70.11515.45 × 10−82.12
−10983.382.181.13 × 10−70.10995.13 × 10−82.2
0926.992.471.08 × 10−70.10454.58 × 10−82.38
10860.033.141.01 × 10−70.09713.61 × 10−82.8
20772.5759.10 × 10−80.08722.22 × 10−84.1
30597.8136.48.00 × 10−80.07030.279 × 10−828.7

Table of thermal and physical properties of carbon dioxide at atmospheric pressure:
Temperature
Density
Specific heat
Dynamic viscosity
Kinematic viscosity
Thermal conductivity
Thermal diffusivity
Prandtl Number
2202.47330.7831.11 × 10−54.49 × 10−60.0108055.92 × 10−60.818
2502.16570.8041.26 × 10−55.81 × 10−60.0128847.40 × 10−60.793
3001.79730.8711.50 × 10−58.32 × 10−60.0165721.06 × 10−50.77
3501.53620.91.72 × 10−51.12 × 10−50.020471.48 × 10−50.755
4001.34240.9421.93 × 10−51.44 × 10−50.024611.95 × 10−50.738
4501.19180.982.13 × 10−51.79 × 10−50.028972.48 × 10−50.721
5001.07321.0132.33 × 10−52.17 × 10−50.033523.08 × 10−50.702
5500.97391.0472.51 × 10−52.57 × 10−50.038213.75 × 10−50.685
6000.89381.0762.68 × 10−53.00 × 10−50.043114.48 × 10−50.668
6500.81431.12.88 × 10−53.54 × 10−50.04454.97 × 10−50.712
7000.75641.133.05 × 10−54.03 × 10−50.04815.63 × 10−50.717
7500.70571.153.21 × 10−54.55 × 10−50.05176.37 × 10−50.714
8000.66141.173.37 × 10−55.10 × 10−50.05517.12 × 10−50.716