Carbonates are a large group of minerals that are widely distributed. The minerals of the carbonate class include salts of carbonic acid, most often these are salts of calcium, magnesium, sodium, copper. In total, about 100 minerals are known in this class. Some of them are very widespread in nature, such as calcite and dolomite.

Structurally, all carbonates belong to the same basic type - anions 2- are isolated radicals in the form of flat triangles.

Most carbonates are anhydrous simple compounds, mainly Ca, Mg and Fe with a 2- complex anion. Less common are complex carbonates containing additional anions (OH) - , F - and Cl - . Among the most common anhydrous carbonates, trigonal and orthorhombic carbonates are distinguished. Carbonates usually have a light color: white, pink, gray, etc., with the exception of copper carbonates, which have a green or blue color. The hardness of carbonates is about 3-4.5; the density is low, except for the Zn, Pb, and Ba carbonates.

An important diagnostic feature is the effect on carbonates of acids (HCl and HNO 3), from which they boil to some extent with the release of carbon dioxide. By origin, carbonates are sedimentary (biochemical or chemical sediments) or sedimentary-metamorphic minerals; surface, characteristic of the oxidation zone, and sometimes low-temperature hydrothermal carbonates are also distinguished.

The main minerals are carbonates

		Syngony	Hardness
Calcite	Calcite CaCO3
	Rhodochrosite MnCO3
	Magnesite MgCO3
	Siderite PeCO3
	Smithsonite ZnCO3
Dolomite	Dolomite CaMg(CO3)2
Aragonite	Aragonite CaCO3
	Witherite VaCO3
	Strontianite SrCO3
	Cerussite PbCO3
Malachite	Malachite Cu2(CO3)(OH)2
	Azurite Cu3(CO3)2(OH)2
rare earth carbonates	Bastnasite Ce(C03)R
	Parisite Ca (Ce, La) 2 × 3 F 2
	Sodium Na 2 CO 3 10H 2 O
	Nahcolite NaHCO3
Nyerereita	Nierreite Na2Ca(CO3)2

Many of the widespread carbonates, especially calcite, magnesite, siderite, and dolomite, have similar crystal morphology, close physical properties, occur in the same aggregates, and often have a variable chemical composition. Therefore, it is difficult, and sometimes impossible, to distinguish them by external signs, hardness, cleavage. A simple technique for diagnosing carbonates by the nature of their reaction with hydrochloric acid has long been used. For this, a drop of dilute (1:10) acid is applied to the carbonate grain. Calcite reacts actively, and a drop of solution boils from the released CO2 bubbles, dolomite reacts weakly, only in powder, and magnesite - when heated.

More reliable results are obtained by the following laboratory studies: accurate determination of their refractive indices; carrying out microchemical reactions on polished rock plates with reagents that color different minerals in different colors; thermal analysis (determination of the decomposition temperature of a mineral, each carbonate has its own temperature); x-ray studies.

carbonate deposits

The most common carbonate is calcite. Transparent calcite is called Icelandic spar, opaque calcareous spar. Calcite forms rocks such as limestone and chalk. The overwhelming amount of calcite was formed due to its biogenic accumulation. At the same time, calcite of hydrothermal origin is also known. In soils, calcite accumulates as a result of the reaction of calcium released during weathering with carbon dioxide in the soil air; soils of arid regions are especially often rich in calcite. Calcite and dolomite form marble. Siderite is a typical mineral of swamp ores; its endogenous origin is rarely noted. Malachite is a beautiful ornamental stone; like the mineral azurite Cu3(CO3)2(OH)2, which is close to it in composition and properties, it is formed on the Earth's surface as a result of the oxidation of copper sulfides.

The use of carbonates

Calcium, magnesium, barium carbonates, etc. are used in construction, in the chemical industry, optics, etc. Soda (Na2CO3 and NaHCO3) is widely used in technology, industry and everyday life: in the production of glass, soap, paper, as a detergent, in gas station of fire extinguishers, in confectionery business. Acid carbonates play an important physiological role, being buffer substances that regulate the constancy of the blood reaction.

Carbonates, which make up about 1.7% of the mass of the earth's crust, are sedimentary or hydrothermal minerals. From a chemical point of view, these are salts of carbonic acid - H2CO3, the general formula is ACO 3 - where A is Ca, Mg, Fe, etc.

Carbonates have ionic crystal lattices; characterized by low density, glassy luster, light color (except for copper carbonates), hardness 3-5, reaction with dilute HCl.

General properties - crystallize in rhombic and trigonal systems (good crystalline forms and cleavage along a rhombus); low hardness 3–4, predominantly light color, reaction with acids (HCl and HNO3 ) with the release of carbon dioxide.

The most common are: calcite CaCO 3, magnesite Mg CO 3, dolomite CaMg (CO 3) 2, siderite Fe CO 3.

Carbonates with a hydroxyl group (OH):

Malachite Cu 2 CO 3 (OH) 2 - green color and reaction with HC l ,

Azurite Cu 3 (CO 3) 2 (OH) 2 - blue, transparent in crystals.

The genesis of carbonates is diverse - sedimentary (chemical and biogenic), hydrothermal, metamorphic.

When heated, acidic carbonates turn into normal carbonates:

With strong heating, insoluble carbonates decompose into oxides and carbon dioxide:

Carbonates react with acids stronger than carbonic (almost all known acids, including organic ones) with the release of carbon dioxide, these reactions are qualitative reactions for the presence of carbonates in solution:

Of the normal carbonates, only alkali metal, ammonium and thallium salts are soluble in water. Due to hydrolysis, their solutions show an alkaline reaction. The normal carbonates of calcium, barium, strontium and lead are sparingly soluble. All acid carbonates are highly soluble in water; acid carbonates of strong alkalis also have a slightly alkaline reaction.

These are rock-forming minerals of sedimentary rocks (limestone, dolomites, etc.) and metamorphic minerals - marble, skarns.

Carbonates are widely used in ferrous metallurgy as a flux and as a raw material for the production of refractories and lime. They are used in construction, optics, metallurgy, as fertilizers. Malachite is used as an ornamental stone. Large accumulations of magnesite and siderite are a source of iron and magnesium.

Bicarbonates of sodium, calcium and magnesium are found in dissolved form in mineral waters, and also, in small concentrations, in all natural waters, except for atmospheric precipitation and glaciers. Calcium and magnesium bicarbonates cause the so-called temporary hardness of water. With strong heating of water (above 60 ° C), calcium and magnesium bicarbonates decompose into carbon dioxide and slightly soluble carbonates, which precipitate on heating elements, the bottom and walls of dishes, the inner surfaces of tanks, boilers, pipes, valves, etc. , forming a scum.

Normal carbonates are widely distributed in nature, for example: CaCO 3 calcite, CaMg (CO 3) 2 dolomite, MgCO 3 magnesite, FeCO 3 siderite, BaCO 3 witherite, BaCa (CO 3) 2 barite calcite, etc. There are also minerals that are basic carbonates , for example, malachite CuCO 3 Cu (OH) 2.

Calcite, CaCO 3 . Name from Greek. "calc" - burnt lime. Synonym- limestone. The name was proposed by Haidinger in 1845 and, like the name of the chemical element, comes from lat. calx (genus calcis) - lime.

Sedimentary organogenic, hydrothermal. Crystals in the form of rhombohedrons. Perfect cleavage along the rhombohedron. Boils under the action of dilute HCl in the cold. Varieties: transparent, colorless - Icelandic spar, rhombic white - aragonite. The strata of sedimentary rocks consist mainly of calcite: chalk, limestone, marble. Calcite also consists of calcareous tuff - travertine.

In its pure form, calcite is white or colorless, transparent (Icelandic spar) or translucent, depending on the degree of perfection of the crystal structure. Impurities color it in different colors. Ni colors green; cobalt, manganese calcite - pink. Finely dispersed pyrite stains bluish and greenish. Calcite with an admixture of iron - yellowish, brownish, red-brown; with an admixture of chlorite - green. Carbonaceous matter often imparts an uneven black color to calcite. Known crystals with numerous inclusions of bituminous substances, they have a yellow or brown color.

The line is white, the density is 2.6-2.8, the fracture is stepped, the hardness on the Mohs scale is 3, the cleavage is perfect along the main rhombohedron, the luster is glassy to pearly. Effervescent on contact with dilute hydrochloric acid (HCl). A variety of twins of intergrowth and germination according to numerous laws, as well as deformation twins, is characteristic. Transparent crystals have light birefringence, which is especially well observed through cleavage surfaces in rhombohedral punches or thick plates.

Ferrous metallurgy consumes millions of tons of limestone as a flux. In addition, limestone is fired for lime in the construction industry. Icelandic spar is used in optics to make polarizers.

magnesite, MgCO 3 . Named after the Greek province of Magnesia. Synonym: magnesian spar. The shape of the crystals is rhombohedral with perfect cleavage along the rhombohedron. In most cases, it occurs in the form of granular aggregates of snow-white color with a conchoidal fracture (“amorphous” magnesite) and in gray elongated grains. Hydrothermal.

The composition is close to theoretical. Of the impurities, Fe is of the greatest importance; less Mn, Ca. Crystals are rare. Usually dense aggregates of various grain sizes up to porcelain-like. Porcelain magnesite often contains admixtures of opal and magnesium silicates. Fragile. The hardness of 4-4.5 in porcelain-like is up to 7 (due to the finely dispersed admixture of opal). Color white, grey, rarely yellowish Occurs in hydrothermal deposits or as a weathering product of ultramafic rocks.

With dilute acids, magnesite reacts without effervescence, which is different from similar calcite. Reaction with HCl only in powder when heated.

Magnesite is used for the production of refractories and binders, in the chemical industry. It is used for the production of refractory bricks. It is also an ore of magnesium and its salts.

An important raw material for the production of refractory bricks and dressing powders. The use of dolomitic limestone improves the quality of sinter, pellets and reduces the viscosity of blast-furnace slags. Deposits: Satka (Russia), Veych (Austria), Liao Tong and Shen-King (North East China), Quebec (Canada).

Malachite, CuCO 3 × Cu(OH) 2 . Name from Greek. "Malakhe" - mallow (meaning the green color of mallow leaves).

Malachite (from the Greek poplar and mallow) is a mineral, the main copper carbonate (copper(II) dihydroxocarbonate). The composition of the mineral is almost exactly expressed by the formula CuCO 3 ·Cu(OH) 2 , but a more accurate modern spelling of the crystal chemical formula of malachite is Cu 2 (CO 3) (OH) 2 . An obsolete synonym is carbonic copper green.

The syngony is monoclinic. Twins by (100). Hardness 3.5-4.0; density 3.7-4.1 g / cm³. Color green in different shades; the luster is different, depending on the composition: glassy in crystals or silky in fine-fibrous aggregates and pieces.

The habitus of crystals is prismatic, lamellar, acicular. Crystals tend to split with the formation of spherocrystals, fine-fibered spherulites, spheroidolite dendrites.

When heated in a flask, it releases water, carbon dioxide and turns black:

The solubility of malachite in acids with the release of carbon dioxide, as well as in ammonia, which turns into a beautiful blue color, is characteristic.

Since ancient times, a method for obtaining free copper from malachite has been known. Under conditions of incomplete combustion of coal, in which carbon monoxide is formed, the following reaction occurs:

Azurite, 2CuCO 3 × Cu(OH) 2 . The name is derived from the Persian Lazvard, meaning blue. The mineral is blue in color with a vitreous luster, brittle. Tv. 3.5-4. The color of the line is bluish-blue, the cleavage is perfect, the fracture is conchoidal.

One of the most common secondary minerals containing copper. An indicator and search feature of copper ores, azurite is itself a copper ore, although less valuable than malachite.

It is formed in the near-surface oxidation zones of most copper sulfide deposits; it is found in secondary copper ores together with malachite. It is unstable under weathering conditions and is easily replaced by malachite. Often in the rock there are banded adhesions of azurite and malachite, which are sometimes cut and polished - this variety is called azuro-malachite.

Sintered, earthy, concentrically shelly. Boils under the action of dilute HCl. Used as decorative ornamental stones, ores for copper.

Siderite, FeCO 3 . Name from Greek. word for iron. A synonym is iron spar. The mineral is of sedimentary origin, brown in color, soluble in mineral acids. When oxidized, it turns into brown iron ore. An important ore for the production of iron, as it contains up to 48% iron and no sulfur and phosphorus. The aggregates are granular, earthy, dense, sometimes in spherical concretions.

The color of the line is white, the luster is glassy, ​​translucent, the hardness is 3.5 - 4.5, the cleavage is perfect, the density is 3.96 g / cm³.

Origin: Hydrothermal - occurs in polymetallic deposits as a vein mineral. Easily weathers to limonite. Usually in granular yellowish-white, brownish masses. Reacts with cold HC1, the drop of which turns green. Color: Yellowish brown, brown, gray, yellowish gray, greenish gray.

Siderite contains up to 48.3% Fe and is used as iron ore. Deposits: Bakalskoe (Southern Urals), Kerchskoe (Ukraine).

Rhodochrosite, MnCO 3 . Name from Greek. "radon" - rose and "chros" - color. Synonym: manganese. Usually in the form of granular aggregates of pink, raspberry color, the line is white. Reacts with cold HCl.

There are isomorphic series MnCO 3 - CaCO 3 and MnCO 3 - FeCO 3 . Manganese is partially replaced by magnesium and zinc. Iron-bearing varieties: ponite and ferrorhodochrosite. Syngony is trigonal. Thick tabular, prismatic, rhombohedral, scalenohedral crystals. Twins at (0112) are rare. Cleavage perfect according to (1011). Aggregates: granular, dense, columnar, spherical, shelly, crusts. Color: pink, red, yellowish gray, brown. Glass luster. Hardness 3.5-4. Specific gravity 3.7.

Hydrothermal mineral of medium- and low-temperature deposits of lead, zinc, silver and copper, in association with siderite, fluorite, barite, alabandine, etc. It occurs in high-temperature deposits with rhodonite, garnet, brynite, tephroite and in pegmatites with lithiophyllite.

In sedimentary manganese deposits, it associates with marcasite, calcite, opal, etc. In this case, it has industrial value. In the weathering crust of manganese and iron-manganese deposits. In metamorphosed primary sedimentary manganese deposits.

Used as manganese ore. Deposits: Chiaturskoe (Georgia), Polunochnoe (Northern Ural), Obrochishche (Varna, Bulgaria).

An aqueous solution of carbon dioxide has the properties of a weak acid: it colors (very weakly) litmus red. Based on this property, it can be concluded that carbon dioxide in solution is partially in the form of carbonic acid (H 2 CO 3), which in turn partially dissociates into ions:

CO 2 + H 2 O ↔ H 2 CO 3,

H 2 CO 3 ↔ 2H + + CO 3 2-.

Carbonic acid can react with one or two equivalents of a strong base, forming primary, or acidic carbonates (hydrocarbonates) and secondary, or neutral (normal) carbonates:

H 2 CO 3 + MON → MHCO 3 + H 2 O;

H 2 CO 3 + 2MOH → M 2 CO 3 + 2H 2 O.

Carbonic acid, as a dibasic acid, dissociates in two stages:

H 2 CO 3 ↔ H + + HCO 3 -

HCO 3 - ↔ H + + CO 3 2-.

Salts, carbonic acid, carbonates; hydrolytically cleaved in aqueous solution. Equilibria are established in their solutions:

M 2 CO 3 + H 2 O ↔ MOH + MHCO 3

MHCO 3 + H 2 O ↔ MOH + H 2 CO 3

Therefore, carbonates show an alkaline reaction, and this is true not only for secondary or "neutral", but also for primary or "acid" carbonates (hydrocarbonates). Only in relation to such indicators, for which, as for phenolphthalein, the alkali → acid color transition occurs when the solution is still weakly basic, primary carbonates (hydrocarbonates) react to cold (0 ° C and slightly higher) as "acids".

The hydrolytic cleavage of (secondary) sodium carbonate is, according to Ausrbach, at 18 °C in 0.1 N. solution of 3.5% in 0.01 N. − 12.4%. At 0.1 n. in a solution of sodium carbonate, the concentration of hydroxide ions is, therefore, at 18 ° C 3.5-10 -3 mol / l. In a solution of sodium bicarbonate, it is at the same temperature 1.5 10 -6 mol / l.

Primary carbonates (hydrocarbonates) of alkali, alkaline earth and some other divalent metals are known. All of them are easily soluble in water. An exception is sodium hydrocarbonate, on the low solubility of which the Solvay method of obtaining soda is based. When solutions of bicarbonates are boiled, they are converted into normal carbonates with elimination of CO 2 .

Secondary or normal carbonates are formed mainly by mono- and divalent metals. Normal carbonates, with the exception of alkali metal carbonates, are sparingly soluble in water.

In addition to alkali metal carbonates, ammonium carbonate is also readily soluble. Quite easily soluble and monovalent thallium carbonate.

All carbonates are decomposed by non-volatile acids. Very weak acids (such as boric and silicic, and, accordingly, their anhydrides) decompose carbonates only when calcined.

Alkali metal carbonates can be melted without decomposition. Other carbonates decompose when heated, splitting off CO 2: M 2 CO 3 \u003d M 2 O + CO 2.

This decomposition is facilitated by the removal of the formed CO 2 (pressure reduction) or the elimination of the oxide M 2 O from the mixture. The latter can be achieved by adding a heat-resistant acid or its anhydride, such as SiO 2 , which forms a salt with a basic oxide. The decomposition of carbonates upon calcination with anhydrides of very weak, but heat-resistant acids, such as boric and silicic acids, is based on this property.

carboxylic acids compounds that contain a carboxyl group are called:

Carboxylic acids are distinguished:

• monobasic carboxylic acids;
• dibasic (dicarboxylic) acids (2 groups UNSD).

Depending on the structure, carboxylic acids are distinguished:

• aliphatic;
• alicyclic;
• aromatic.

Examples of carboxylic acids.

Obtaining carboxylic acids.

1. Oxidation of primary alcohols with potassium permanganate and potassium dichromate:

2. Hydrolysis of halogenated hydrocarbons containing 3 halogen atoms at one carbon atom:

3. Obtaining carboxylic acids from cyanides:

When heated, the nitrile hydrolyzes to form ammonium acetate:

When acidified, acid precipitates:

4. Use of Grignard reagents:

5. Hydrolysis of esters:

6. Hydrolysis of acid anhydrides:

7. Specific methods for obtaining carboxylic acids:

Formic acid is obtained by heating carbon monoxide (II) with powdered sodium hydroxide under pressure:

Acetic acid is obtained by catalytic oxidation of butane with atmospheric oxygen:

Benzoic acid is obtained by oxidation of monosubstituted homologues with a solution of potassium permanganate:

Cannicaro's reaction. Benzaldehyde is treated with 40-60% sodium hydroxide solution at room temperature.

Chemical properties of carboxylic acids.

In an aqueous solution, carboxylic acids dissociate:

The equilibrium is shifted strongly to the left, because carboxylic acids are weak.

Substituents affect acidity through an inductive effect. Such substituents pull the electron density towards themselves and a negative inductive effect (-I) arises on them. Pulling the electron density leads to an increase in the acidity of the acid. Electron donor substituents create a positive inductive charge.

1. Formation of salts. Reaction with basic oxides, salts of weak acids and active metals:

Carboxylic acids are weak, because mineral acids displace them from the corresponding salts:

2. Formation of functional derivatives of carboxylic acids:

3. Esters when an acid is heated with alcohol in the presence of sulfuric acid - an esterification reaction:

4. Formation of amides, nitriles:

3. The properties of acids are determined by the presence of a hydrocarbon radical. If the reaction proceeds in the presence of red phosphorus, it forms the following product:

4. Addition reaction.

8. Decarboxylation. The reaction is carried out by fusing an alkali with an alkali metal salt of a carboxylic acid:

9. Dibasic acid easily splits off CO 2 when heated:

Additional materials on the topic: Carboxylic acids.

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