Carbonate

Carbonate ion model, CO₃²⁻.

The carbonates are the salts of carbonic acid or esters with the group R-O-C(=O)-O-R'. The salts have in common the anion CO₃^2- and are derived from carbonic acid H₂CO₃. Depending on the pH (the acidity of the solution) they are in chemical equilibrium with bicarbonate and carbon dioxide.

Most carbonates, apart from the alkali metal carbonates, are sparingly soluble in water. Due to this characteristic they are important in geochemistry and are part of many minerals and rocks.

The most abundant carbonate is calcium carbonate (CaCO₃), which is found in different mineral forms (calcite, aragonite), forming sedimentary rocks (limestone, marl) or metamorphic (marble) and it is often the natural cement of some sandstones.

Replacing part of the calcium with magnesium, the dolomite CaMg(CO₃)₂ is obtained, named after the French geologist Déodat Gratet de Dolomieu.

Many carbonates are unstable at high temperatures and lose carbon dioxide as they transform into oxides.

Description

Carbonates in nature are formed from carbonic acid in aqueous solution derived from the dissolution of carbon dioxide in water and in which a small fraction forms carbonic acid according to the reaction:

CO₂ + H₂O ↔ H₂CO₃

This reaction is very slow and therefore part of the CO₂ dissolved in water is not in the form of carbonic acid. Carbonic acid is in equilibrium with the bicarbonate anion and the carbonate anion according to the following equations:

H₂CO₃ + H₂O ↔ HCO₃^- + H₃O⁺ (K₁ = 2 10⁻⁴)

and

HCO₃^- + H₂O ↔ CO₃^2- (K₂ = 4.8 10⁻¹¹)

For many years it was thought that in the carbonate ion only two oxygens carried the negative charge. Crystallographic studies of calcite by W.L. Bragg X-rays revealed that all three C-O bonds were identical. This observation required a new structure and it was not until the development of resonance theory that in 1931 a new structure was proposed in which each oxygen shared 2/3 of the the negative charge and in which the bonds were partial, that is, halfway between simple and double:

In this way, the carbonate ion has a planar geometry with a C-O bond length of 1.29 Å, between the single bond (1.34 Å) and the double bond (1.20 Å). Each C–O bond is made up of a single σ bond and a 2/3 fraction of a π bond. The carbonate ion is one of the simplest of the oxocarbon ions.

Applications

Carbonates are applied in many fields. Minerals such as calcite and dolomite are important in the manufacture of cement and construction stone. Often their use is linked to the specific compound, such as obtaining quicklime (CaO) from calcium carbonate. Other carbonate minerals constitute metal ores: barium whiterite; strontium strontianite; iron siderite; manganese rhodochrosite; zinc smithsonite, and lead cerussite.

One of the most industrially important carbonates is sodium carbonate. In nature it is found in some saline lagoons, for example in Egypt, although the vast majority is obtained from lime in the Solvay process. It is an intermediate in obtaining products as diverse as soap, sodium percarbonate used as a bleach, glass, soda (NaOH), etc.

Wood ash is also largely made up of carbonates. These have even given the name to an element, potassium (K) from the English "pot ash = ash under the boiler" since it was usually obtained from this source.

Due to their character as salts of a weak acid, carbonates are also used as cheap bases. In this application, carbon dioxide is released and the salt formed by the metal that was attached to the carbonate and the rest of the neutralized acid remain in solution:

MCO₃ + 2HX → H₂O + CO₂ + MX₂

Where M is a metal, such as calcium or magnesium, and X is the anion of an acid. In this way, calcium carbonate in water would release carbon dioxide and calcium hydroxide, giving rise to basic solutions according to the following chemical balances:

(1) CaCO₃ + H₂O → HCO₃^- + Ca(OH)⁺

(2) HCO₃^- + H₂O+ Ca(OH)⁺ → H₂CO₃ + Ca(OH)₂ and carbonic acid in water decomposes releasing carbon dioxide according to:

(3) H₂CO₃ → H₂O + CO₂

This reaction is used in the detection of carbonates due to its characteristic effervescence in which water and carbon dioxide are released. There are various methods of quantifying the carbonates present in a sample, by quantifying CO ₂ detached. Some include the passage of the CO₂ released by an aqueous solution of barium hydroxide, and the quantification of the precipitated barium carbonate, while others such as "Bernard" calcimetry consist of in the volumetric detection of carbon dioxide.

Added of pseudohexagonal spruce of about 4 cm. The aragonite is a calcium carbonate mineral with orthrombic crystal structure.

Calcium carbonate is part of the formulation of toothpastes.

Minerals

There are about 80 carbonate minerals, most of which are rare. Even so, while nitrates and borates are scarce minerals in nature, the simpler carbonates are quite abundant. They are part of the chemical composition of numerous rocks, being the essential component of limestone, dolomites and crystalline marbles. They are also ores of numerous metals. The most important minerals of the carbonate group are calcite, aragonite (CaCO₃), and dolomite (MgCa(CO₃)₂). Other important ones are smithsonite (ZnCO₃), rhodochrosite (MnCO₃) and magnesite (MgCO₃). These are grouped into sub-groups called the calcite group (trigonal system), the aragonite group (orthorhombic system) and the dolomite (trigonal) groups and monoclinic carbonates such as malachite (Cu₂CO₃(OH)₂) and azurite (Cu₃(CO₃) ₂(OH)₂), with the presence of hydroxide ions in its structure.

Carbonates usually appear in the form of minerals such as calcite in which calcium can be replaced by Mn²⁺, Fe²⁺ or Mg ^{ions >2+}, in isomorphic series that extend from (I) siderite (FeCO₃) to magnesite (MgCO₃) passing through ankerite (FeCa(CO₃)₂) and dolomite (MgCa(CO₃)₂); (II) from magnesite and siderite to rhodochrosite (MnCO₃) and (III) from dolomite and ankerite to kutnahorite.^,

Carbonates tend to have a medium hardness with typical values of 3 - 5 on the Mohs scale.

Carbonated salts

In the following periodic table, the carbonated salts are arranged according to the corresponding elements, indicating their molecular weight (g/mol). In addition to the carbonates indicated in the following periodic table, there are also others such as ammonium carbonate (NH₄)₂CO₃ and ammonium bicarbonate (NH₄)HCO₃.

Chemistry

The chemistry of the carbonate and the chemical balances of its derivative ions play a fundamental role in the formation of the calcareous landscapes or karsts as that of the Enchanted City of Cuenca, Spain

As discussed above, carbonates react with acids releasing water and carbon dioxide. In the case of calcium carbonate:

CaCO3 + 2HCl → CaCl2 + H2O + CO2

Carbon salts are formed in aqueous media in which there are dissolved metal cations and in which a stream of carbon dioxide is infused. This process is schematized by the following reactions:

1) CO₂ (g) + H₂O → H₂CO₃ (acu.)

2) H₂CO₃↔ H⁺ + HCO₃^-

3) HCO₃^- ↔ H⁺ + CO₃^2-

In turn, metal cations with positive charges, M⁺, M²⁺ or M³⁺ will react with the carbonate anion to give the following rooms:

4) 2M⁺ + CO₃^2- → M₂CO₃

5) M²⁺ + CO₃^2- → OLC₃

6) 2M³⁺ + 3CO₃^2- → M₂(CO₃ )₃

Reaction (5) is the case of the formation of calcareous or karst deposits in the form of stalactites, stalagmites or incrustations in areas rich in limestone and carbonate rocks:

Ca²⁺(acu.) + CO₃^2- → CaCO₃ (s)

Thermal stability

Carbonates decompose thermally, releasing carbon dioxide and giving rise to the corresponding metal oxide according to the following reaction:

M₂CO₃ → M₂O + CO₂ (alkali cations: Li ⁺, Na⁺, K⁺, Rb⁺, Cs⁺)

MCO₃ → MO + CO₂ (alkaline earth cations: Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ra²⁺)

The thermal stability of carbonates grows with the size of the cations that compose them. In this way, for a given period the stability grows with the group (alkaline earth > alkaline) and for a given group it increases as we descend: MgCO₃ (400°C), CaCO₃ (900°C), SrCO₃ (1290°C), BaCO₃ (1360 °C). The calcite decomposition process is called calcination, derived from the process of obtaining quicklime from calcium carbonate:

CaCO3 → CaO + CO2 (g)

Organic carbonates

Organic carbonates are esters of carbonic acid. They can be formed from phosgene (Cl₂C=O) and the corresponding alcohol.

Especially polycarbonates based on dialcohols such as 2,2-bis(4-hydroxyphenyl)-propane (HOC₆H₄-C(CH₃ )₂-C₆H₄OH)) have gained importance as highly transparent materials in the manufacture of CDs or as a substitute for of the glass.