Carbonate

For other meanings, see Carbonate (disambiguation)

In inorganic chemistry, a carbonate is a salt of carbonic acid.

Chemical properties

The carbonate ion is a polyatomic anion with the empirical formula C O₃²⁻ and a molecular mass of 60.01 daltons; it consists of one central carbon atom surrounded by three identical oxygen atoms in a trigonal planar arrangement. The carbonate ion carries a negative two formal charge and is the conjugate base of the hydrogencarbonate ion, HCO₃⁻, which is the conjugate base of H₂CO₃, carbonic acid.

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

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 undil 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.

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 which is double bonded.

Applications

Soda water (also known as Selzer water) is water with CO₂ dissolved under pressure. The pleasant taste of soda water was discovered by the 18th century chemist Joseph Priestley.

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.

Carbonate-containing salts are industrially and mineralogically ubiquitous. The term "carbonate" is also commonly used to refer to one of these salts or carbonate minerals. Most common is calcite, or calcium carbonate, the chief constituent of limestone. The process of removing carbon dioxide from these salts by heating is called calcination.

The term is also used as a verb, to describe the process of raising carbonate and bicarbonate concentrations in water, see also carbonated water, either by the introduction under pressure of carbon dioxide gas into the water, or by dissoving carbonate or bicarbonate salts into the water.

History

At one time, it was thought that the presence of carbonates in rock was unequivocal evidence for the presence of liquid water. Recent observations of two planetary nebulae reported in the January 17, 2002 issue of the scientific journal Nature indicate that carbonates can form in interplanetary space.

Carbonates were detected in the Gusev Crater on Mars by the Mars Exploration Rover Spirit on January 9, 2004. ^*

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Chemical properties

Applications

History

See also