| Carbonic Acid |
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Carbonic acid is the only inorganic Carbon Acid , and has the Formula H2CO3. It is also a name sometimes given to Solution s of Carbon Dioxide in Water , which contain small amounts of H2CO3. The salts of carbonic acids are called '''bicarbonates''' (or '''hydrogencarbonates''') and '''carbonates'''. Carbon dioxide dissolved in water is in Equilibrium with carbonic acid: ::CO2 + H2O H2CO3 The Equilibrium Constant at 25°C is 1.70×10−3: hence, the majority of the carbon dioxide is not converted into carbonic acid and stays as CO2 molecules. In the absence of a Catalyst , the equilibrium is reached quite slowly. The Rate Constant s are 0.039 s−1 for the forward reaction (CO2 + H2O → H2CO3) and 23 s−1 for the reverse reaction (H2CO3 → CO2 + H2O). The equilibrium between carbon dioxide and carbonic acid is very important for controlling the acidity of body fluids, and almost all living organisms have an Enzyme , Carbonic Anhydrase , which catalyzes the conversion between the two compounds, increasing the reaction rate by a factor of nearly a billion. ACIDITY OF CARBONIC ACID Carbonic acid has two acidic hydrogens and so two Dissociation Constants : ::H2CO3 HCO3− + H+ :::''K''a1 = 2.5×10−4 mol/L; p''K''a1 = 3.60. ::HCO3− CO32− + H+ :::''K''a2 = 5.61×10−11 mol/L; p''K''a2 = 10.25. Care must be taken when quoting and using the first dissociation constant of carbonic acid. The value quoted above is correct for the H2CO3 molecule, and shows that it is a stronger acid than oxygen substituent. However, carbonic acid only ever exists in solution in equilibrium with carbon dioxide, and so the concentration of H2CO3 is much lower than the concentration of CO2, reducing the measured acidity. The equation may be rewritten as follows (''c.f.'' Sulfurous Acid ): ::CO2 + H2O HCO3− + H+ :::''K''a = 4.30×10−7 mol/L; p''K''a = 6.36. This figure is often quoted as the dissociation constant of carbonic acid, although this is ambiguous: it might better be referred to as the acidity constant of carbon dioxide, as it is particularly useful for calculating the PH of CO2 solutions. INSTABILITY OF CARBONIC ACID It has long been recognized that it is impossible to obtain pure hydrogen bicarbonate at room temperatures (about 20 oC or about 70 oF). However, in 1991 scientists at NASA's Goddard Space Flight Center (USA) succeded in making the first pure H2CO3 samples. They did so by exposing a frozen mixture of water and carbon dioxide to high-energy radiation, and then warming to remove the excess water. The carbonic acid that remained was characterized by infrared spectroscopy. The fact that the carbonic acid was prepared by irradiating a solid H2O + CO2 mixture has given rise to suggestions that H2CO3 might be found in outer space, where frozen ices of H2O and CO2 are common, as are cosmic rays and ultraviolet light, to help them react. It has since been shown, by theoretical calculations, that the presence of even a single molecule of water causes carbonic acid to revert to carbon dioxide and water fairly quickly. Pure carbonic acid is predicted to be stable in the gas phase, in the absence of water, with a calculated Half-life of 180,000 Years . There is a Hypothetical acid Orthocarbonic Acid which is even more hydrated, being C(OH)4. CARBONIC ACID AND RAIN WATER A solution of carbon dioxide in water in equilibrium with the atmosphere (0.033% CO2) has a pH of 5.6. Rain water is normally not quite saturated in CO2, and has a pH of around 6 in the absence of atmospheric pollutants. This effect is separate from the phenomenon of Acid Rain , where industrial pollutants such as Sulfur Dioxide dissolve in rain water and lower its pH drastically. However, the acidity of rain water has important Geological consequences for Carbonate Rocks such as Chalk and Limestone . An equilibrium is established between the Calcium Carbonate of the rock and Calcium Bicarbonate in solution: ::CaCO3 + CO2 + H2O Ca(HCO3)2 This can erode underground caverns around fault lines which water runs down. As the calcium-rich water evaporates, the calcium carbonate precipitates, often as Stalactite s and Stalagmite s. Water drawn from chalk Aquifer s contains dissolved calcium carbonate, and is described as "hard" . SEE ALSO REFERENCES
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