Acarbonate is asalt ofcarbonic acid, (H2CO3),[2] characterized by the presence of thecarbonate ion, apolyatomic ion with the formulaCO2−3. The word "carbonate" may also refer to acarbonate ester, an organic compound containing thecarbonate groupO=C(−O−)2.
The term is also used as a verb, to describecarbonation: the process of raising the concentrations of carbonate andbicarbonate ions in water to producecarbonated water and other carbonated beverages – either by the addition ofcarbon dioxide gas under pressure or by dissolving carbonate or bicarbonate salts into the water.
Ingeology andmineralogy, the term "carbonate" can refer both tocarbonate minerals andcarbonate rock (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion,CO2−3. Carbonate minerals are extremely varied and ubiquitous in chemically precipitatedsedimentary rock. The most common arecalcite orcalcium carbonate,CaCO3, the chief constituent oflimestone (as well as the main component ofmollusc shells andcoral skeletons);dolomite, a calcium-magnesium carbonateCaMg(CO3)2; andsiderite, oriron(II) carbonate,FeCO3, an importantiron ore.Sodium carbonate ("soda" or "natron"),Na2CO3, andpotassium carbonate ("potash"),K2CO3, have been used since antiquity for cleaning and preservation, as well as for the manufacture ofglass. Carbonates are widely used in industry, such as in iron smelting, as a raw material forPortland cement andlime manufacture, in the composition ofceramic glazes, and more. New applications of alkali metal carbonates include: thermal energy storage,[3][4] catalysis[5] and electrolyte both in fuel cell technology[6] as well as in electrosynthesis ofH2O2 in aqueous media.[7]
TheLewis structure of the carbonate ion has two (long) single bonds to negative oxygen atoms, and one short double bond to a neutral oxygen atom.
This structure is incompatible with the observed symmetry of the ion, which implies that the three bonds are the same length and that the three oxygen atoms are equivalent. As in the case of theisoelectronicnitrate ion, the symmetry can be achieved by aresonance among three structures:
This resonance can be summarized by a model with fractional bonds anddelocalized charges:
Stalactites and stalagmites are carbonate minerals.
Metal carbonates generally decompose on heating, liberating carbon dioxide leaving behind an oxide of the metal.[2] This process is calledcalcination, aftercalx, the Latin name of quicklime orcalcium oxide, CaO, which is obtained by roasting limestone in alime kiln:
CaCO3 → CaO + CO2
As illustrated by its affinity forCa2+, carbonate is a ligand for many metal cations.Transition metal carbonate and bicarbonate complexes feature metal ions covalently bonded to carbonate in a variety of bonding modes.
Lithium,sodium,potassium,rubidium,caesium, andammonium carbonates are water-soluble salts, but carbonates of 2+ and 3+ ions are often poorly soluble in water. Of the insoluble metal carbonates,CaCO3 is important because, in the form ofscale, it accumulates in and impedes flow through pipes.Hard water is rich in this material, giving rise to the need for infrastructuralwater softening.
Acidification of carbonates generally liberatescarbon dioxide:
CaCO3 + 2 HCl → CaCl2 + CO2 + H2O
Thus, scale can be removed with acid.
In solution the equilibrium between carbonate, bicarbonate, carbon dioxide and carbonic acid is sensitive to pH, temperature, and pressure. Although di- and trivalent carbonates have low solubility, bicarbonate salts are far more soluble. This difference is related to the disparatelattice energies of solids composed of mono- vs dianions, as well as mono- vs dications.
Inaqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid participate in adynamic equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, thebicarbonate ion is prevalent. In more acid conditions, aqueouscarbon dioxide,CO2(aq), is the main form, which, with water,H2O, is in equilibrium with carbonic acid – the equilibrium lies strongly towards carbon dioxide. Thussodium carbonate is basic,sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid.
Inorganic chemistry a carbonate can also refer to afunctional group within a larger molecule that contains a carbon atom bound to three oxygen atoms, one of which is double bonded. These compounds are also known as organocarbonates or carbonate esters, and have the general formulaR−O−C(=O)−O−R′, orRR′CO3. Important organocarbonates includedimethyl carbonate, the cyclic compoundsethylene carbonate andpropylene carbonate, and the phosgene replacement,triphosgene.
Three reversible reactions control thepH balance of blood and act as abuffer to stabilise it in the range 7.37–7.43:[9][10]
H+ + HCO−3 ⇌ H2CO3
H2CO3 ⇌ CO2(aq) + H2O
CO2(aq) ⇌ CO2(g)
ExhaledCO2(g) depletesCO2(aq), which in turn consumesH2CO3, causing the equilibrium of the first reaction to try to restore the level of carbonic acid by reacting bicarbonate with a hydrogen ion, an example ofLe Châtelier's principle. The result is to make the blood more alkaline (raise pH). By the same principle, when the pH is too high, the kidneys excrete bicarbonate (HCO−3) into urine as urea via theurea cycle (or Krebs–Henseleit ornithine cycle). By removing the bicarbonate, moreH+ is generated from carbonic acid (H2CO3), which comes fromCO2(g) produced bycellular respiration.[11]
Crucially, a similar buffer operates in the oceans. It is a major factor in climate change and the long-term carbon cycle, due to the large number of marine organisms (especially coral) which are made of calcium carbonate. Increased solubility of carbonate through increased temperatures results in lower production of marinecalcite and increased concentration of atmospheric carbon dioxide. This, in turn, increases Earth temperature. The amount ofCO2−3 available is on a geological scale and substantial quantities may eventually be redissolved into the sea and released to the atmosphere, increasingCO2 levels even more.[12]
It is generally thought that the presence of carbonates inrock is strong evidence for the presence of liquid water. Recent observations of theplanetary nebulaNGC 6302 show evidence for carbonates in space,[13] where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.
^Silverthorn, Dee Unglaub (2016).Human physiology. An integrated approach (Seventh, Global ed.). Harlow, England: Pearson. pp. 607–608,666–673.ISBN978-1-292-09493-9.
^IPCC (2019)."Summary for Policymakers"(PDF).IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. pp. 3–35.
^Kemper, F., Molster, F.J., Jager, C. and Waters, L.B.F.M. (2001) The mineral composition and spatial distribution of the dust ejecta of NGC 6302.Astronomy & Astrophysics394, 679–690.
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