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Natural calcium compounds (chalk, marble, limestone, gypsum) and their simplest processing products (lime) have been known to people since ancient times. In 1808, the English chemist Humphry Davy electrolyzed wet slaked lime (calcium hydroxide) with mercury cathode and received calcium amalgam (calcium-mercury alloy). From this alloy, having driven away mercury, Davy obtained pure calcium.
He also proposed the name of a new chemical element, from the Latin "calx" denoting the name of limestone, chalk and other soft stones.
Being in nature and getting:
Calcium is the fifth most abundant element in the earth's crust (more than 3%), forms many rocks, many of which are based on calcium carbonate. Some of these rocks are of organic origin (shell rock), showing the important role of calcium in wildlife. Natural calcium is a mixture of 6 isotopes with mass numbers from 40 to 48, with 40 Ca accounting for 97% of the total. Other calcium isotopes have also been obtained by nuclear reactions, for example, radioactive 45 Ca.
To obtain a simple substance of calcium, electrolysis of melts of its salts or aluminothermy is used:
4CaO + 2Al \u003d Ca (AlO 2) 2 + 3Ca
Physical properties:
A silver-gray metal with a face-centered cubic lattice, much harder than the alkali metals. Melting point 842°C, boiling point 1484°C, density 1.55 g/cm 3 . At high pressures and temperatures around 20 K, it transforms into the state of a superconductor.
Chemical properties:
Calcium is not as active as the alkali metals, yet it must be stored under a layer of mineral oil or in tightly sealed metal drums. Already at ordinary temperature, it reacts with oxygen and nitrogen in the air, as well as with water vapor. When heated, it burns in air with a red-orange flame, forming oxide with an admixture of nitrides. Like magnesium, calcium continues to burn in an atmosphere of carbon dioxide. When heated, it reacts with other non-metals, forming compounds that are not always obvious in composition, for example:
Ca + 6B = CaB 6 or Ca + P => Ca 3 P 2 (also CaP or CaP 5)
In all its compounds, calcium has an oxidation state of +2.
The most important connections:
Calcium oxide CaO- ("quicklime") a white substance, an alkaline oxide, reacts vigorously with water ("extinguished") turning into hydroxide. Obtained by thermal decomposition of calcium carbonate.
Calcium hydroxide Ca(OH) 2- ("slaked lime") white powder, slightly soluble in water (0.16g/100g), strong alkali. A solution ("lime water") is used to detect carbon dioxide.
Calcium carbonate CaCO 3- the basis of most natural calcium minerals (chalk, marble, limestone, shell rock, calcite, Icelandic spar). In its pure form, the substance is white or colorless. crystals, When heated (900-1000 C) decomposes, forming calcium oxide. Not p-rim, reacts with acids, is able to dissolve in water saturated with carbon dioxide, turning into bicarbonate: CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2. The reverse process leads to the formation of calcium carbonate deposits, in particular formations such as stalactites and stalagmites.
It occurs in nature also in the composition of dolomite CaCO 3 *MgCO 3
Calcium sulfate CaSO 4- a white substance, in nature CaSO 4 * 2H 2 O ("gypsum", "selenite"). The latter, when heated carefully (180 C), turns into CaSO 4 * 0.5H 2 O ("burnt gypsum", "alabaster") - a white powder, when mixed with water, again forming CaSO 4 * 2H 2 O in the form of a solid, strong enough material. Slightly soluble in water, in excess of sulfuric acid it can dissolve, forming hydrosulfate.
Calcium phosphate Ca 3 (PO 4) 2- ("phosphorite"), insoluble, under the influence of strong acids passes into more soluble calcium hydro- and dihydrogen phosphates. Feedstock for the production of phosphorus, phosphoric acid, phosphate fertilizers. Calcium phosphates are also part of apatites, natural compounds with the approximate formula Ca 5 3 Y, where Y = F, Cl, or OH, respectively, fluorine, chlorine, or hydroxyapatite. Along with phosphorite, apatites are part of the bone skeleton of many living organisms, incl. and a person.
Calcium fluoride CaF 2 - (natural:"fluorite", "fluorspar"), insoluble in white. Natural minerals have a variety of colors due to impurities. Glows in the dark when heated and when exposed to UV radiation. Increases the fluidity ("fusibility") of slags in the production of metals, which is the reason for its use as a flux.
Calcium chloride CaCl 2- colorless crist. in-in well r-rimoe in water. Forms hydrated CaCl 2 *6H 2 O. Anhydrous ("fused") calcium chloride is a good drying agent.
Calcium nitrate Ca(NO 3) 2- ("calcium nitrate") colorless. crist. in-in well r-rimoe in water. Component pyrotechnic compositions, giving the flame a red-orange color.
Calcium carbide CaС 2- reacts with water, forming acetylene, for example: CaС 2 + H 2 O \u003d C 2 H 2 + Ca (OH) 2
Application:
Metallic calcium is used as a strong reducing agent in the production of some difficult-to-recover metals ("calcium term"): chromium, rare earth elements, thorium, uranium, etc. In the metallurgy of copper, nickel, special steels and bronzes, calcium and its alloys are used to remove harmful impurities of sulfur, phosphorus, excess carbon.
Calcium is also used to bind small amounts of oxygen and nitrogen in the production of high vacuum and purification of inert gases.
Neutron-excess ions 48 Ca are used for the synthesis of new chemical elements, such as element No. 114, . Another isotope of calcium, 45 Ca, is used as a radioactive tracer in research. biological role calcium and its migration in the environment.
The main field of application of numerous calcium compounds is the production of building materials (cement, building mixtures, drywall, etc.).
Calcium is one of the macronutrients in the composition of living organisms, forming compounds necessary for building both the internal skeleton of vertebrates and the external skeleton of many invertebrates, egg shells. Calcium ions are also involved in the regulation of intracellular processes, cause blood clotting. Lack of calcium in childhood leads to rickets, in the elderly - to osteoporosis. Dairy products, buckwheat, nuts serve as a source of calcium, and vitamin D contributes to its absorption. In case of calcium deficiency, various preparations are used: calcex, calcium chloride solution, calcium gluconate, etc.
The mass fraction of calcium in the human body is 1.4-1.7%, daily requirement 1-1.3 g (depending on age). Excess calcium intake can lead to hypercalcemia - the deposition of its compounds in internal organs formation of blood clots in blood vessels. Sources:
Calcium (element) // Wikipedia. URL: http://ru.wikipedia.org/wiki/Calcium (date of access: 3.01.2014).
Popular library of chemical elements: Calcium. // URL: http://n-t.ru/ri/ps/pb020.htm (3.01.2014).
DEFINITION
calcium hydroxide(slaked lime, portlandite) is a white substance (Fig. 1), which, when heated, decomposes without melting.
It is poorly soluble in water (a dilute alkaline solution is formed).
calcium hydroxide - strong base, slightly soluble in water; 1 l of water dissolves at 20 o C only 1.56 g of Ca(OH) 2 . A saturated solution of calcium hydroxide is called lime water and has alkaline reaction. In air, lime water quickly becomes cloudy due to the absorption of carbon dioxide and the formation of insoluble calcium carbonate.
Rice. 1. Calcium hydroxide. Appearance.
The main characteristics of calcium hydroxide are shown in the table below:
Obtaining calcium hydroxide
If burnt lime (calcium oxide) is doused with water, then the water is absorbed by porous pieces of lime and reacts with it with the release of a significant amount of heat. In this case, part of the water turns into steam, and pieces of lime crumble into a loose mass of calcium hydroxide:
CaO + H 2 O \u003d Ca (OH) 2 + 65 kJ.
Chemical properties of calcium hydroxide
Calcium hydroxide exhibits basic properties, i.e. reacts with non-metals (1, 2), acid oxides (3, 4), acids (5, 6) and salts (7):
2Ca(OH) 2 + 2Cl 2 = Ca(ClO) 2 + CaCl 2 + 2H 2 O (1);
3Ca(OH) 2 + 6H 2 O + 2P 4 = 3Ca(PH 2 O 2) 2 + 2PH 3 (2);
Ca(OH) 2 + CO 2 = CaCO 3 ↓ + H 2 O (3);
Ca(OH) 2 + SO 2 = CaSO 3 ↓ + H 2 O (4);
Ca(OH) 2 + 2HCl dilute = CaCl 2 + 2H 2 O (5);
Ca(OH) 2 + H 2 SO 4 (conc) = CaSO 4 ↓ + 2H 2 O (6);
Ca(OH) 2 + 2NaClO = Ca(ClO) 2 ↓ + 2NaOH (7).
When calcium hydroxide is heated to a temperature of 520 - 580 o C, it decomposes:
Ca (OH) 2 \u003d CaO + H 2 O.
Application of calcium hydroxide
Calcium hydroxide is used in the construction industry. Its mixture with sand and water is called lime mortar and serves to hold bricks together when laying walls. Calcium hydroxide is also used as a plaster. Its hardening occurs first due to the evaporation of water, and then as a result of the absorption of carbon dioxide from the air by slaked lime and the formation of calcium carbonate.
Examples of problem solving
EXAMPLE 1
Ca(OH)2 is calcium hydroxide (from the Latin Calcium hydroxide) and is a fairly common chemical. It is by nature considered a strong base. It is a fine-grained yellowish powder or colorless crystals. Able to decompose when heated, resulting in the release of calcium oxide. It is poorly soluble in water. Wherein water solution calcium hydroxide in its chemical properties is an average base. In the presence of metals, it can release hydrogen, which is recognized as an explosive gas.
Calcium hydroxide, when taken into the body through the mouth or as a result of inhalation of an aerosol, can be absorbed into tissues and accumulate in them. At a normal room temperature of 20-22 degrees, this substance practically does not evaporate, but when sprayed, its particles can be hazardous to health. Getting on the skin Airways or the mucous membranes of the eyes, calcium hydroxide is irritating, even corrosive. Prolonged contact with skin may cause dermatitis. Lung tissue may also be affected by chronic exposure to calcium hydroxide particles.
This chemical compound has many trivial names, such as (it is obtained by quenching calcium oxide with plain water), lime water (it is a clear aqueous solution). Other names: fluff (calcium hydroxide in the form of dry powder) and milk of lime (saturated aqueous suspension). Often or lime is also called calcium oxide.
calcium hydroxide, Chemical properties which are considered aggressive with respect to other substances, are obtained by lime slaking, that is, as a result of interaction ( chemical reaction) calcium oxide and water. Schematically, this reaction looks like this:
CaO + H2O = Ca(OH)2
The resulting aqueous solution is characterized by an alkaline reaction of the medium. Like all typical calcium reacts with:
1. inorganic acids with the formation of typical calcium salts
H2SO4 + Ca(OH)2 = CaSO4 + 2H2O
2. carbon dioxide, which is dissolved in water, so the aqueous solution becomes cloudy very quickly in air, and a white insoluble precipitate is formed - calcium carbonate
CO2 + Ca(OH)2 = CaCO3 + H2O
3. carbon monoxide when the temperature rises to 400 degrees Celsius
CO (t°) + Ca(OH)2 = CaCO3 + H2
4. salts, as a result, a white precipitate also falls out - calcium sulfate
Na2SO3 + Ca(OH)2 = CaSO3 + 2NaOH
The use of calcium hydroxide is very popular. Surely, everyone knows that walls of rooms, tree trunks are treated with lime, and it is also used as a component of building lime mortar. The use of calcium hydroxide in construction has been known since ancient times. And now it is included in the composition of the plaster, silicate brick and concrete are produced from it, the compositions of which are almost the same as the mortar. The main difference lies in the method of preparation of these same solutions.
Calcium hydroxide is used for softening for the manufacture of calcareous inorganic fertilizers, causticization of potassium and sodium carbonate. Also, this substance is indispensable for tanning leather in the textile industry, for obtaining various calcium compounds, as well as for neutralizing acidic solutions, including. Based on it, organic acids are obtained.
Calcium hydroxide has found its application in the food industry, where it is better known as food supplement E526, used as an acidity regulator, hardener and thickener. In the sugar industry, it is used for the desaccharification of molasses.
In laboratory and demonstration experiments, lime water is an indispensable indicator of the detection of carbon dioxide during chemical reactions. Plants are treated with milk of lime in order to combat diseases and pests.
Calcium hydroxide is a strong base chemical. What are its features and chemical properties will be considered in this article.
Characterization of calcium hydroxide
Crystalline calcium hydroxide is a white powder that decomposes when heated, but is practically insoluble in water. The formula of calcium hydroxide is Ca(OH) 2 . In ionic form, the equation for the formation of calcium hydroxide looks like this:
Rice. 1. Equation for the formation of calcium hydroxide.
Calcium hydroxide has other names: slaked lime, milk of lime, lime water
The molar mass of calcium hydroxide is 74.09 g/mol. This means that 74.09 g / mol of the amount of calcium hydroxide substance contains 6.02 * 10^23 atoms or molecules of this substance.
Calcium hydroxide is used for whitewashing in construction, disinfection of tree trunks, in the sugar industry, for tanning leather, to obtain bleach. A doughy mixture of slaked lime with cement and sand is used in construction.
Rice. 2. Calcium hydroxide.
Chemical properties of calcium hydroxide
Calcium hydroxide, like all bases, reacts with acids:
Ca (OH) 2 (calcium hydroxide) + H 2 SO 4 (sulfuric acid) \u003d CaSo 4 (salt - calcium sulfate) + 2H 2 O (water).
Calcium hydroxide is also able to form compounds with carbon dioxide. A solution of this substance in air becomes cloudy, since calcium hydroxide, like other strong bases, interacts with carbon dioxide dissolved in water:
Ca (OH) 2 + CO 2 (calcium hydroxide) \u003d CaCO 3 (calcium carbonate) + H 2 O (water)
When heated to 400 degrees, calcium hydroxide reacts with carbon monoxide:
Ca (OH) 2 (calcium hydroxide) + CO (carbon monoxide) \u003d CaCO 3 (calcium carbonate) + H 2 (hydrogen).
Calcium hydroxide can interact with salts, resulting in a precipitate:
Ca (OH) 2 (calcium hydroxide) + Na 2 SO 3 (sodium sulfite) \u003d CaSO 3 (calcium sulfite) + 2NaOH (sodium hydroxide).
At a temperature of 520-580 degrees, calcium hydroxide is subject to a decomposition reaction. As a result, calcium oxide and water are formed:
Rice. 3. Slaked lime.
Ca (OH) 2 (calcium hydroxide) \u003d CaO (calcium oxide) + H 2 O (water).
Calcium hydroxide is obtained by chemical reaction of calcium oxide (quicklime) with water. This process is called lime slaking. The lime slaking reaction equation is as follows:
CaO (calcium oxide) + H 2 O (water) \u003d Ca (OH) 2 (calcium hydroxide).
What have we learned?
Calcium hydroxide is a strong base, slightly soluble in water. Like any chemical element it has a number of properties - it is able to react with carbon dioxide, salts, and also decomposes when high temperature. Calcium hydroxide is used in construction and industry.
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Calcium oxide (CaO) - quicklime or burnt lime- a white fire-resistant substance formed by crystals. It crystallizes in a cubic face-centered crystal lattice. Melting point - 2627 ° C, boiling point - 2850 ° C.
It is called burnt lime because of the method of its production - the burning of calcium carbonate. Roasting is carried out in high shaft kilns. Limestone and fuel are laid in layers in the furnace, and then kindled from below. When heated, calcium carbonate decomposes to form calcium oxide:
Since the concentrations of substances in solid phases are unchanged, the equilibrium constant of this equation can be expressed as follows: K=.
In this case, the gas concentration can be expressed using its partial pressure, that is, the equilibrium in the system is established at a certain pressure of carbon dioxide.
Substance dissociation pressure is the equilibrium partial pressure of a gas resulting from the dissociation of a substance.
To provoke the formation of a new portion of calcium, it is necessary to increase the temperature or remove part of the resulting CO2, and the partial pressure will decrease. By maintaining a constant lower partial pressure than the dissociation pressure, a continuous calcium production process can be achieved. To do this, when burning lime in kilns, make good ventilation.
Receipt:
1) in the interaction of simple substances: 2Ca + O2 = 2CaO;
2) during thermal decomposition of hydroxide and salts: 2Ca(NO3)2 = 2CaO + 4NO2? +O2?.
Chemical properties:
1) interacts with water: CaO + H2O = Ca(OH)2;
2) reacts with non-metal oxides: CaO + SO2 = CaSO3;
3) dissolves in acids, forming salts: CaO + 2HCl = CaCl2 + H2O.
Calcium hydroxide (Ca (OH) 2 - slaked lime, fluff)- a white crystalline substance, crystallizes in a hexagonal crystal lattice. It is a strong base, poorly soluble in water.
lime water- a saturated solution of calcium hydroxide, having an alkaline reaction. It becomes cloudy in air as a result of the absorption of carbon dioxide, forming calcium carbonate.
Receipt:
1) is formed when calcium and calcium oxide are dissolved in the input: CaO + H2O \u003d Ca (OH) 2 + 16 kcal;
2) when calcium salts interact with alkalis: Ca(NO3)2 + 2NaOH = Ca(OH)2 + 2NaNO3.
Chemical properties:
1) when heated to 580 ° C, it decomposes: Ca (OH) 2 \u003d CaO + H2O;
2) reacts with acids: Ca(OH)2 + 2HCl = CaCl2 + 2H2O.
58. Water hardness and ways to eliminate it
Since calcium is widely distributed in nature, its salts are found in large quantities in natural waters. Water containing magnesium and calcium salts is called hard water. If salts are present in water in small quantities or absent, then water is called soft. In hard water, soap does not foam well, since calcium and magnesium salts form insoluble compounds with it. It doesn't digest food well. When boiling, scale forms on the walls of steam boilers, which does not conduct heat well, causes an increase in fuel consumption and wear of the boiler walls. Hard water cannot be used in a number of technological processes (dyeing). Scale formation: Ca + 2HCO3 \u003d H2O + CO2 + CaCO3?.
The factors listed above indicate the need to remove calcium and magnesium salts from the water. The process of removing these salts is called water softening, is one of the phases of water treatment (water treatment).
Water treatment– water treatment used for various household and technological processes.
Water hardness is divided into:
1) carbonate hardness (temporary), which is caused by the presence of calcium and magnesium bicarbonates and is eliminated by boiling;
2) non-carbonate hardness (constant), which is caused by the presence of sulfites and chlorides of calcium and magnesium in water, which are not removed during boiling, therefore it is called constant hardness.
The formula is correct: Total hardness = Carbonate hardness + Non-carbonate hardness.
General hardness is eliminated by adding chemicals or using cation exchangers. To completely eliminate hardness, water is sometimes distilled.
When applying the chemical method, soluble calcium and magnesium salts are converted into insoluble carbonates:
A more modern process for removing water hardness - using cation exchangers.
Cation exchangers- complex substances (natural compounds of silicon and aluminum, high-molecular organic compounds), the general formula of which is Na2R, where R- complex acid residue.
When water passes through a layer of cation exchanger, Na ions (cations) are exchanged for Ca and Mg ions: Ca + Na2R = 2Na + CaR.
Ca ions from the solution pass into the cation exchanger, and Na ions pass from the cation exchanger into the solution. To restore the used cation exchanger, it must be washed with a solution of common salt. In this case, the reverse process occurs: 2Na + 2Cl + CaR = Na2R + Ca + 2Cl.
