Carbonate: Wikis

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Ball-and-stick model of the carbonate ion, CO 2−3

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, CO 2−3. The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group O=C(O-)2.

The term is also used as a verb, to describe carbonation, the process of, raising the concentrations of carbonate and bicarbonate ions in water to produce carbonated water and other carbonated beverages — either by the addition carbon dioxide gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

In geology and mineralogy, the term "carbonate" can refer both to carbonate minerals and carbonate rock (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion, CO 2−3. Carbonate minerals are extremely varied and ubiquitous in chemically-precipitated sedimentary rock. The most common are calcite or calcium carbonate, CaCO3, the chief constituent of limestone (as well as the main component of mollusc shells and coral skeletons); dolomite, a calcium-magnesium carbonate CaMg(CO3)2; and siderite, or iron (II) carbonate, FeCO3, an important iron ore. Sodium carbonate ("soda" or "natron") and potassium carbonate ("potash") have been used since antiquity for cleaning and preservation, as well as for the manufacture of glass. Carbonates are widely used in industry, e.g. in iron smelting, as a raw material for Portland cement and lime manufacture, in the composition of ceramic glazes, and more.

Contents

Structure and bonding

The carbonate ion is the simplest oxocarbon anion. It consists of one carbon atom surrounded by three identical oxygen atoms, in a trigonal planar arrangement, with D3h molecular symmetry. It has a molecular mass of 60.01 daltons and carries a negative two formal charge. It is the conjugate base of the hydrogen carbonate (bicarbonate) ion, HCO3, which is the conjugate base of H2CO3, carbonic acid.

The Lewis structure of the carbonate ion has two (long) single bonds to negative oxygen atoms, and one short double bond to a neutral oxygen Simple, localised Lewis structure of the carbonate ion

This structure is incompatible with the observed symmetry of the ion, which implies that the three bonds are equally long and that the three oxygen atoms are equivalent. As in the case of the isoelectronic nitrate ion, the symmetry can be achieved by a resonance between three structures:

Resonance structures of the carbonate ion

This resonance can be summarized by a model with fractional bonds and delocalized charges:

Delocalisation and partial charges on the carbonate ion Space-filling model of the carbonate ion

Chemical properties

Metal carbonates generally decompose on heating, liberating carbon dioxide and leaving behind an oxide of the metal. This process is called calcination, after calx, the Latin name of quicklime or calcium oxide, CaO, which is obtained by roasting limestone in a lime kiln.

A carbonate salt forms when a positively charged ion, M+, attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound:

2 M+ + CO32− → M2CO3
M2+ + CO32− → MCO3
2 M3+ + 3 CO32− → M2(CO3)3

Most carbonate salts are insoluble in water at standard temperature and pressure, with solubility constants of less than 1×10−8. Exceptions include sodium, potassium and ammonium carbonates, as well as many uranium carbonates.

In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a dynamic equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion is prevalent. In more acid conditions, aqueous carbon dioxide, CO2(aq), is the main form, which, with water, H2O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate is basic, sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid.

Carbonated water is formed by dissolving CO2 in water under pressure. When the partial pressure of CO2 is reduced, for example when a can of soda is opened, the equilibrium for each of the forms of carbonate (carbonate, bicarbonate, carbon dioxide, and carbonic acid) shifts until the concentration of CO2 in the solution is equal to the solubility of CO2 at that temperature and pressure. In living systems an enzyme, carbonic anhydrase, speeds the interconversion of CO2 and carbonic acid.

Acid-base chemistry

The carbonate ion (CO32−) is a moderately strong base. It is a conjugate base of the weakly acidic bicarbonate (IUPAC name hydrogen carbonate HCO3), itself a moderately strong conjugate base of the still weakly acidic carbonic acid. As such in aqueous solution, the carbonate ion seeks to reclaim hydrogen ions (protons).

To test for the presence of the carbonate anion in a salt, the addition of dilute mineral acid (e.g. hydrochloric acid) will yield carbon dioxide gas.

Organic carbonates

In organic chemistry a carbonate can also refer to a functional 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 formula ROCOOR′, or RR′CO3. Important organocarbonates include dimethyl carbonate, the cyclic compounds ethylene carbonate and propylene carbonate, and the toxic triphosgene.

Biological significance

It works as a buffer in the blood as follows: when pH is too low, the concentration of hydrogen ions is too high, so you exhale CO2. This will cause the equation to shift left, essentially decreasing the concentration of H+ ions, causing a more basic pH.

When pH is too high, the concentration of hydrogen ions in the blood is too low, so the kidneys excrete bicarbonate (HCO3). This causes the equation to shift right, essentially increasing the concentration of hydrogen ions, causing a more acidic pH.

Carbonate salts

  • Carbonate overview:
H2CO3 He
Li2CO3 BeCO3 B C N O F Ne
Na2CO3 MgCO3 Al Si P S Cl Ar
K2CO3 CaCO3 Sc Ti V Cr MnCO3 FeCO3 CoCO3 NiCO3 CuCO3 ZnCO3 Ga Ge As Se Br Kr
Rb2CO3 SrCO3 Y Zr Nb Mo Tc Ru Rh Pd Ag2CO3 CdCO3 In Sn Sb Te CI Xe
Cs2CO3 BaCO3 Hf Ta W Re Os Ir Pt Au Hg Tl2CO3 PbCO3 Bi Po At Rn
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo
La2(CO3)3 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Presence outside Earth

It is generally thought that the presence of carbonates in rock is strong evidence for the presence of liquid water. Recent observations of the Planetary nebula NGC 6302 shows evidence for carbonates in space,[1] where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.

Significant carbonate deposits have not been found on Mars via remote sensing or in situ missions, even though Martian meteorites contain small amounts. Groundwater may have existed at both Gusev[2] and Meridiani Planum.[3]

References

  1. ^ Kemper, F., Molster, F.J., Jager, C. and Waters, L.B.F.M. (2002) The mineral composition and spatial distribution of the dust ejecta of NGC 6302. Astronomy & Astrophysics 394, 679-690.
  2. ^ Squyres et al., (2007) doi 10.1126/science.1139045
  3. ^ Squyres et al., (2006) doi 10.1029/2006JE002771

See also

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Simple English

A carbonate is a chemical compound that is made of carbon and oxygen. When added to an acid, a carbonate will produce carbon dioxide, water and a chemical salt. It is found naturally in the earth.

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