The formula of calcium hydroxide is structural chemical. Slaked lime: formula and chemical composition of limestone Formation of calcium hydroxide

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 the gas resulting from the dissociation of the 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) in the interaction of calcium salts 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 foams poorly, 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 poorly conducts heat, 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.

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 is a 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 is alkaline. 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

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 at high temperatures. Calcium hydroxide is used in construction and industry.

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Calcium- an element of the 4th period and the PA group of the Periodic System, serial number 20. The electronic formula of the atom is [ 18 Ar] 4s 2, oxidation states +2 and 0. Refers to alkaline earth metals. It has a low electronegativity (1.04), exhibits metallic (basic) properties. Forms (as a cation) numerous salts and binary compounds. Many calcium salts are sparingly soluble in water. In nature - sixth in terms of chemical abundance, the element (the third among metals) is in a bound form. A vital element for all organisms. The lack of calcium in the soil is replenished by the application of lime fertilizers (CaCO 3 , CaO, calcium cyanamide CaCN 2, etc.). Calcium, calcium cation and its compounds color the flame of a gas burner in a dark orange color ( qualitative detection).

Calcium Ca

Silver-white metal, soft, ductile. In humid air, it tarnishes and becomes covered with a film of CaO and Ca(OH) 2. Very reactive; ignites when heated in air, reacts with hydrogen, chlorine, sulfur and graphite:

Reduces other metals from their oxides (an industrially important method is calciumthermy):

Receipt calcium in industry:

Calcium is used to remove non-metal impurities from metal alloys, as a component of light and antifriction alloys, to isolate rare metals from their oxides.

Calcium oxide CaO

basic oxide. The technical name is quicklime. White, highly hygroscopic. Has an ionic structure Ca 2+ O 2- . Refractory, thermally stable, volatile on ignition. Absorbs moisture and carbon dioxide from the air. Reacts vigorously with water (high exo- effect), forms a strongly alkaline solution (hydroxide precipitation is possible), the process is called lime slaking. Reacts with acids, metal and non-metal oxides. It is used for the synthesis of other calcium compounds, in the production of Ca(OH) 2 , CaC 2 and mineral fertilizers, as a flux in metallurgy, a catalyst in organic synthesis, a component of binders in construction.

Equations of the most important reactions:

Receipt Cao in industry– limestone roasting (900-1200 °С):

CaCO3 = CaO + CO2

Calcium hydroxide Ca(OH) 2

basic hydroxide. The technical name is slaked lime. White, hygroscopic. It has an ionic structure Ca 2+ (OH -) 2. Decomposes on moderate heat. Absorbs moisture and carbon dioxide from the air. Slightly soluble in cold water (an alkaline solution is formed), even less so in boiling water. A clear solution (lime water) quickly becomes cloudy due to the precipitation of hydroxide (the suspension is called milk of lime). A qualitative reaction to the Ca 2+ ion is the passage of carbon dioxide through lime water with the appearance of a precipitate of CaCO 3 and its transition into solution. Reacts with acids and acid oxides, enters into ion exchange reactions. It is used in the production of glass, bleaching lime, lime mineral fertilizers, for causticizing soda and softening fresh water, as well as for the preparation of lime mortars - pasty mixtures (sand + slaked lime + water), serving as a binder for stone and brickwork, finishing ( plastering) walls and other construction purposes. The hardening ("seizure") of such solutions is due to the absorption of carbon dioxide from the air.

The lesson is devoted to the study of substances that are of great practical importance in human life, namely in such an area of ​​​​his life as construction. The teacher will talk about the methods of obtaining, properties and application of calcium oxide and hydroxide.

Topic: Substances and their transformations

Lesson: Calcium oxide and hydroxide. Properties and application

Even in ancient times, people noticed that if you burn limestone, chalk or marble, you get a white powder with special properties. The main component of chalk, marble and limestone is a substance called calcium carbonate. Its chemical formula is CaCO 3. When burning limestone, a reaction occurs, the equation of which is:

CaCO 3 \u003d CaO + CO 2

Rice. 1. Minerals based on calcium carbonate

Calcium oxide can also be obtained by direct combustion of calcium in an oxygen atmosphere:

2Ca + O 2 \u003d 2CaO

In this case, the reaction of the combination of calcium with oxygen proceeds with the formation of calcium oxide.

The properties of the resulting calcium oxide are still used in construction. Calcium oxide is the nomenclature name for CaO. In addition to the nomenclature, this substance has several historical names. As you already know, calcium oxide can be obtained by burning limestone, which is why one of its historical names is burnt lime.

If water is added to the resulting calcium oxide, the water will hiss as if it is hot. Therefore, burnt lime was called "boiling". When in contact with water, calcium oxide is quenched, as it were, giving off heat. Therefore, the ongoing process was called quenching, and calcium oxide was called quicklime.

The water vapor formed during quenching loosens the quicklime, as it were, it becomes overgrown with fluff. In this regard, the slaked lime obtained by interacting with water has also become known as fluff.

What happens when quicklime is slaked? It has been established that one molecule of calcium oxide interacts with one molecule of water and only one new substance is formed - slaked lime. This reaction belongs to the type of compound.

CaO + H 2 O \u003d Ca (OH) 2

It is customary to write the formula for slaked lime as follows: Ca (OH) 2. The nomenclature name for this substance is calcium hydroxide:

A mixture of slaked lime and water is called lime mortar, which is used in construction. Since calcium hydroxide is slightly soluble in water, the lime mortar contains a precipitate of calcium hydroxide and the solution itself (lime water).

The use of lime mortar in construction for a strong connection of stones is associated with its hardening in air.

Thus, the whole process of obtaining and using calcium oxide can be represented as a diagram (Fig. 2).

Rice. 2. Obtaining and using calcium oxide

When calcium carbonate is calcined, quicklime is formed - calcium oxide. When mixed with water, calcium oxide turns into slaked lime - calcium hydroxide. A mixture of calcium hydroxide and water, which is sparingly soluble in water, is called lime mortar. When standing in air, the lime mortar reacts with carbon dioxide and turns back into calcium carbonate.

The equation of the reaction corresponding to the process of solidification of lime mortar:

Ca (OH) 2 + CO 2 \u003d CaCO 3 + H 2 O

The hardening of the lime mortar occurs because an insoluble substance is formed - calcium carbonate.

1. Collection of tasks and exercises in chemistry: 8th grade: to textbook. P.A. Orzhekovsky and others. “Chemistry. Grade 8 / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M .: AST: Astrel, 2006. (p. 92-96)

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 84-86)

3. Chemistry. 8th grade. Proc. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. – M.: Astrel, 2013. (§27)

4. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005. (§33)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. edited by V.A. Volodin, leading. scientific ed. I. Leenson. – M.: Avanta+, 2003.

Additional web resources

1. Oxide and calcium hydroxide ().

Homework

1) with. 84-86 №№ 1,2,8 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

2) p.155-156 No. 2, A1, A2 from the textbook P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova "Chemistry: 8th grade", 2013