The element phosphorus forms a number of oxides, the most important being phosphorus(III) oxide. P2O3 and phosphorus(V) oxide P2O5 .

Phosphorus (III) oxide, or phosphorous anhydride (P2O3) obtained by the slow oxidation of phosphorus, burning it in a lack of oxygen. It is a waxy crystalline white mass with a melting point of 22.5 °C. Poisonous.

Chemical properties:

1) reacts with cold water, thus forming phosphorous acid H3PO3;

2) interacting with alkalis, forms salts - phosphites;

3) is a strong reducing agent.

Interacting with oxygen, it is oxidized to phosphorus (V) oxide P2O5.

Phosphorus oxide (V), or phosphoric anhydride (P2O5) obtained by burning phosphorus in air or oxygen. It is a white crystalline powder with a melting point of 36 °C.

Chemical properties:

1) interacting with water, forms ortho-phosphoric acid H3PO4;

2) having properties acid oxide, reacts with basic oxides and hydroxides;

3) capable of absorbing water vapor.

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Chemical properties.
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Receipt.
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Phosphoric acids.
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Mineral fertilizers
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Salts of sodium and potassium
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Oxides and acids of phosphorus.

Bohr Chemistry.

Electronic configuration:

Boron is relatively uncommon in nature. TO major natural compounds boron includes boric acid and salts boric acids(most known borax ).

Boron is located in the third group of the periodic system, but according to its properties most similar not with other elements of this group, but with an element of the fourth group - silicon(manifestation of "diagonal similarity").

Like silicon, boron forms compounds with metals, many of which are characterized by high hardness and high melting points.

Free boron receive reduction of boric anhydride with magnesium (boron is released in the form of an amorphous powder contaminated with impurities).

Pure crystalline boron is obtained thermal decomposition or reduction of its halides, as well as decomposition hydrogen compounds boron. It is black in color and simple substances second only to diamond in hardness.

Water has no effect on boron; concentrated sulfuric and nitric acids oxidize it to boric acid. For instance:

B + 3HN = + 3N

At room temperature boron combines only with fluorine, it does not oxidize in air.

If amorphous boron is heated to 700 °C, it ignites and burns with a reddish flame, turning into an oxide; while a large amount of heat is released:

4V(K) + 30 2(g) = 2V 2 0 3 (k), ∆N = -2508 kJ.

At high temperature boron combines with many metals to form borides, such as magnesium boride Mg 3 B 2 .

Many borides are very hard and chemically stable, and retain these properties when high temperatures. They are also characterized by resilience.

With halogens boron also reacts when heated and forms substances of the general formula VG 3 . In these compounds, boron forms flat molecules with halogens with angles between G-C-D connections equal to 120°. Such a geometry of molecules is expected when considering the repulsion of electron pairs in the valence shell and on the basis of the BP2 hybridization of boron orbitals.

Boron halides, like other nonpolymeric boron compounds, are electron-deficient. So, in a boron trifluoride molecule, with a linear combination of 2s, 2px, 2py, 2r boron AO and one valence p-AO (with an unpaired electron) of each fluorine atom, 4 + 3 = 7 molecular orbitals are formed. These MOs have 3 + 3 = 6 electrons in pairs. Thus, in the boron trifluoride molecule, 7 - 6/2 = 4 MO remain unoccupied. One of the MOs, perpendicular to the plane of the molecule, does not participate in binding with fluorine atoms. But it has a rather low energy due to the relaxation (compression in this case, see Fig. 4.35 and 4.36) of the p-orbital of boron under the action of strongly electronegative fluorine. Therefore, the placement of an electron pair of another atom or ion on a given MO becomes energetically favorable, and the BF3 molecule can, therefore, be an electron pair acceptor. Indeed, BF3 combines in a donor-acceptor way with water, ammonia and other substances; the complex anion BF4 is also known. Formally this process can be represented as a diagram:

In all such compounds, the covalence and coordination number of boron are equal to four, and the boron atom forms tetrahedral structures determined by the energy minimum. This is due to the repulsion of 4 electron pairs of the valence shell or the sp3 hybridization of the boron atom.

Boranes (boranes). Under the action of hydrochloric acid on magnesium boride Mg3B2, a complex mixture of various borohydrides is obtained. From this mixture, the following borohydrides were isolated in pure form:

The main product of the interaction of magnesium boride with hydrochloric acid is tetraborane B 4 H 10 - a volatile liquid (bp 18 ° C), the vapor of which ignites in air. During storage, tetraborane gradually decomposes with the formation of the simplest of the obtained borohydrides, diborane B>H6. The latter is a gas that condenses into a liquid at -92.5 °C. In air, it does not ignite, but with water, like other borohydrides, it immediately decomposes with the elimination of hydrogen and the formation of boric acid H 3 BO 3:

B 2 H 6 + 6H 2 0 - 2H 3 IN 3 + 6H 2.

The boron atoms in the borohydrogen molecules are connected to each other by hydrogen "bridges", for example:

The dashed line and the dotted line in this diagram show three-center bonds: here a common pair of electrons occupies a molecular orbital covering three atoms - a "bridge" hydrogen atom and both boron atoms. Such an orbital is formed due to the overlapping of the ls orbital of the hydrogen atom with the sp 3 hybrid orbitals of two boron atoms. The four "terminal" hydrogen atoms are connected to the boron atoms by conventional two-center two-electron bonds. Thus, of the twelve valence electrons present in the atoms that make up the diborane molecule, eight participate in the formation of two-center B-H bonds, and four form two three-center B-H-B bonds.

Boron oxide, or boric anhydride, B 2 0 3 can be obtained either by direct combination of boron with oxygen or by calcining boric acid. Boric anhydride is very fire resistant and is not reduced by coal even at white heat. It dissolves in water, eventually forming orthoboric acid and liberating heat:

Bg0 3 (k) + ZN 2 0 (g) \u003d 2HzB0 3 (p),
DN = -76.5 kJ.

Orthoboric acid H 3 BO 3 is a white crystals, shiny flakes of which dissolve in hot water. In an aqueous solution, orthoboric acid is in equilibrium with other boron acids:

As a rule, of various acids of the same element that differ from each other in the degree of hydration (i.e., the number of bound water molecules), the most hydrated form is the most stable in aqueous solutions. Of these boron acids, orthoboric acid is the most stable in aqueous solutions. Therefore, in those cases when any boric acid should be obtained in the course of the reaction, its most stable form, orthoboric acid, is always isolated in aqueous solutions. When the solution is cooled, boric acid crystallizes out, since in cold water she is insoluble.

Orthoboric acid is one of the very weak acids. Its less hydrated forms (HBO 2, H 2 B 4 O 7) are also weak acids, but somewhat stronger than orthoboric acid. This rule about the greater strength of less hydrated forms of the same acid holds in other cases. Therefore, among the weak acids of boron, tetraboric acid is somewhat stronger.

As a result, when you try to neutralize an aqueous solution of orthoboric acid with alkali, you get not an orthoborate, but a tetraborate of an alkaline element:

4H 3 B0 3 + 2NaOH \u003d Na 2 B 4 0 7 + 7H z O.

Salts of boric acids borates are mostly derivatives not of orthoboric acid H3BO3, but of tetraboric and other boric acids that are poorer in water.

Boron compounds with nitrogen have two polymorphic modifications: diamond-like and graphite-like. The graphite-like modification of boron nitride has a graphite structure in which boron atoms alternate with nitrogen atoms, both in the planes formed by six-membered rings and in the planes perpendicular to the layers.

Ticket 2.

2. Oxides and their hydrated forms of elements of the V group.

Nitrogen, phosphorus, arsenic, antimony and bismuth belong to the main subgroup of group V of the periodic system.

These elements, having five electrons on the outer electron shell of the atom, are generally characterized as non-metals. Due to the presence of five outer electrons, the highest positive degree oxidation of the elements of this subgroup is equal to -1-5, and negative -3.

nitrogen oxides.

Nitrogen forms a series of oxides with oxygen; they can all be obtained from nitric acid or its salts.

Nitric oxide (I), or nitrous oxide, N 2 O is obtained by heating ammonium nitrate:

NH 4 NO 3 \u003d N 2 0 + 2 H 2 0.

Nitric oxide (I) is a colorless gas that is used as an anesthetic.

Nitric oxide (I) is a thermodynamically unstable compound. The standard Gibbs energy of its formation is positive. However, due to the high strength of bonds in the N 2 0 molecule, the activation energies of reactions occurring with the participation of this substance are high. In particular, the activation energy for the decomposition of N 2 0 is high. Therefore, nitric oxide (I) is stable at room temperature. However, at elevated temperatures, it decomposes into nitrogen and oxygen; decomposition proceeds faster, the higher the temperature.

Neither with water, nor with acids, nor with alkali, nitric oxide (I) reacts.

Nitric oxide(II ), or nitric oxide, NO is a colorless gas that is difficult to liquefy.

According to its chemical properties, nitric oxide (II) is one of the indifferent oxides, since it does not form any acid.

Like N20, nitric (II) OXIDE is thermodynamically unstable. At room temperature, NO does not decompose because its molecules are strong enough. Only at temperatures above 1000 °C does its decomposition into nitrogen and oxygen begin to proceed at a noticeable rate.

In the laboratory, nitric oxide (II) is usually obtained by reacting 30-35% nitric acid with copper:

3Cu + 8HNO 3 \u003d 3Cu (N0 3) 2 + 2NOt + 4H 2 0.

In industry, it is an intermediate product in the production of nitric acid.

Nitric oxide (II) is characterized by redox duality. Under the action of strong oxidizing agents, it is oxidized, and in the presence of strong reducing agents, it is reduced. For example, it is easily oxidized by atmospheric oxygen to nitrogen dioxide:

2NO + 0 2 = 2N0 2 .

At the same time the mixture equal volumes N0 and H 2 explode when heated: 2NO (r) + 2H 2 (G) \u003d N 2 (r) 4- 2H 2 0 (G), AN \u003d -655 kJ.

Nitrogen dioxide (or dioxide) N0 2 is a brown poisonous gas. The change in the color of nitrogen dioxide with increasing temperature is also accompanied by a change in its molecular weight.

Nitrogen dioxide is a very energetic oxidizing agent. Many substances can burn in an atmosphere of NO 2, taking away oxygen from it.

When dissolved in water, NO 2 reacts with water, forming nitric and nitrous acids: 2N0 2 + H 2 0 \u003d HNO 3 + HNO 2

Therefore, in practice, the interaction of nitrogen dioxide with water, especially hot water, proceeds according to the equation 6N0 2 + 2H 2 0 = 4HN0 3 + 2N0, which can be obtained by adding the two previous equations, if you first multiply the first of them by three.

In the presence of air, the resulting nitric oxide is immediately oxidized to nitrogen dioxide, so that in this case NO 2 is eventually completely converted into nitric acid:

4N0 2 + 0 2 + 2Н 2 0 = 4HN0 3

If nitrogen dioxide is dissolved in alkalis, then a mixture of salts of nitric and nitrous acids is formed, for example: 2N0 2 + 2NaOH \u003d NaN0 3 + NaN0 2 + H 2 0.

Nitric oxide (III), or nitrous anhydride, N 2 O 3 is a dark blue liquid, already at low temperatures decomposing into N0 and NO 2 . A mixture of equal volumes of N0 and NO 2 upon cooling again forms N 2 O 3: NO + NO 2<=* N 2 O 3

Nitric oxide (III) corresponds to nitrous acid HNO 2 .

Nitric oxide (V), or nitric anhydride, N 2 O 5 - white crystals, already at room temperature gradually decomposing into NO 2 and O 2 - It can be obtained by the action of phosphoric anhydride on nitric acid: 2HNO 3 + P 2 O 5 - N 2 O 5 + 2HPO 3

Nitric oxide (V) is a very strong oxidizing agent. In water, nitric oxide (V) dissolves well with the formation of nitric acid.

in solid state.

Acids:

Nitrous acid.

When a solution of some nitrite is exposed to dilute sulfuric acid, free nitrous acid is obtained:

2NaN0 2 + H 2 S0 4 = Na 2 S0 4 + 2HN0 2 .

It is among the weak ones and is known only in highly dilute aqueous solutions. When the solution is concentrated or when it is heated, nitrous acid decomposes:

2HN0 2 \u003d N0 + N0 2 + H 2 0.

HN0 2 exhibits redox duality. Under the action of reducing agents, it is reduced (usually to NO), and in reactions with oxidizing agents, it is oxidized to HNO3. The following reactions are examples:

2HN0 2 + 2KI + H 2 S0 4 = 2N0 + I 2 + K 2 S0 4 + 2H 2 0:

5HN0 2 + 2KMn0 4 + 3H 2 S0 4 = 5HN0 3 + 2MnS0 4 + K 2 S0 4 + 3H 2 0.

Nitric acid.

Read ticket number 4.

Oxides and acids of phosphorus.

The most important oxides of phosphorus are P 2 03 and P 2 0 5 .

Phosphorus (III) oxide, or phosphorous anhydride, P 2 0z is obtained by the slow oxidation of phosphorus or when phosphorus burns out with insufficient access to oxygen. Its molecular weight at low temperatures corresponds to the formula P 4 0 6. Under the action of cold water, phosphorus (III) oxide slowly interacts with it, forming phosphorous acid H 3 PO 3. Both phosphorus(III) oxide and phosphorous acid have strong reducing properties.

Phosphorus(V) oxide, or phosphoric anhydride, P 2 0 5 is formed during the combustion of phosphorus in air or in oxygen in the form of a white voluminous snow-like mass.

Phosphorus(V) oxide greedily combines with water and therefore is used as a very strong dehydrating agent. In air, phosphorus oxide (V), attracting moisture, quickly turns into a deliquescent mass of metaphosphoric acid.

Phosphoric acids.

Phosphorus oxide (V) responds to several acids. The most important of these is orthophosphoric acid HzPO 4 , usually called simply phosphoric. Other phosphoric acids are polymeric compounds. In the anion of all phosphoric acids, the phosphorus atom, which is in the state of sp 3 hybridization, is surrounded by four oxygen atoms located at the vertices of the tetrahedron. Orthophosphoric acid is built from isolated tetrahedra; in other phosphoric acids, the PO 4 tetrahedra are combined through oxygen atoms into aggregates containing from two to a very large number - about 10 5 - phosphorus atoms.

Orthophosphoric acid is not a strong acid. Being tribasic, it forms three series of salts: medium and acid salts with one or two hydrogen atoms in the acid residue. Medium salts of phosphoric acid are called orthophosphates or simply phosphates, acid salts are called hydrophosphates:

In the laboratory, phosphoric acid can be obtained by oxidation of phosphorus with 30% HNO 3 . The reaction proceeds according to the equation:

ZR + 5HN0 3 + 2H 2 0 \u003d ZN 3 RO 4 + 5N0

In industry, phosphoric acid is obtained by two methods: extraction and thermal. The extraction method is based on the treatment of natural phosphates with sulfuric acid:

Ca 3 (P0 4) 2 + 3H 2 S0 4 \u003d 3CaS0 4 + 2H 3 P0 4

All other phosphoric acids are PO4 tetrahedra compounds. Most of these acids are not isolated in the free state, but are known as mixtures, in aqueous solutions, or as salts.

Phosphorus oxides. Phosphoric anhydride P;05 ("the simplest" formula) is the most stable phosphorus oxide under normal conditions. It is a white crystalline (or glassy) highly hygroscopic substance of composition P4O10. Each phosphorus atom is surrounded by four oxygen atoms: P4O,0 actively interacts with water, and also takes it away from other compounds, forming, depending on the conditions, either metaphosphoric HPOj, or orthophosphoric HjPO*, or pyrophosphoric H4P2O7 acid [see. reactions (12.5)]. That is why RchO|0 is widely used as a desiccant of various substances from water vapor. Phosphoric anhydride is described by the simplest formula P2O3 and the true formula P4Ob: Phosphorus in P4Ob is coordinatively unsaturated and therefore unstable. Phosphorus (III) dioxide is a white waxy mass formed during the oxidation of phosphorus under conditions of lack of oxygen [see. reaction equation (16.4)]. The interaction of P06 with water leads to the formation of phosphorous acid P406 + 6H20 \u003d 4H3P03 Gaseous HC1 decomposes P4Ob: Phosphoric acids - under this name they combine acids containing phosphorus atoms in the +5 oxidation state. Of the three phosphoric acids, orthophosphoric acid HjPO4 (often referred to simply as phosphoric acid) is of greatest practical importance - a white solid = 44.4 ° C), highly soluble in water. In an aqueous solution, it dissociates in steps, forming three types of anions (dihydrophosphates H2PO4~, hydrophosphates HP042~ and phosphates P043~) ​​and being in equilibrium with them in accordance with the dissociation constants: This is very clearly illustrated by the diagram in Fig. 16.2, showing the proportion of each of the particles depending on the acidity of the solution (pH of the solution). For example, the proportion of the acid itself a (H3P04), determined by the formula, prevails if. And vice versa, only at pH>p£s does the proportion of phosphate ions P043 begin to predominate. Intermediate ions dominate at pH values ​​that are between pK2 and pK3. All dihydrogen phosphates are soluble in water. Of the hydrophosphates and phosphates, only alkali metal and ammonium salts are soluble in water (see solubility table). Salts of phosphoric acid - valuable mineral fertilizers. The most common among them are superphosphate, precipitate and phosphate rock. Simple superphosphate is a mixture of calcium dihydrogen phosphate Ca (H2P04) 2 and "ballast" CaS04. It is obtained by treating phosphorites and apatites with sulfuric acid. In the treatment of mineral phosphates with phosphoric acid, Ryas. 16.2. The proportion of equilibrium "gasters" depending on the pH of the solution during multistage dissociation of H3PO4 is double superphosphate Ca(H2PO4)2. When phosphoric acid is quenched with lime, a precipitate is obtained. Complex fertilizers are important (i.e., containing both nitrogen and phosphorus; or nitrogen, phosphorus and potassium). Of these, ammophos is the most useful - a mixture of NHJH2P04 and (NH ^ HPCV Just like orthophosphoric acid, pyrophosphoric H4P2O7 (four-basic) is an acid of medium strength. Salts of this acid - pyrophosphates - hydrolyze in aqueous solutions and give a weakly alkaline medium. Pyrophosphoric acid is obtained either by heating 100% orthophosphoric acid, or by adding phosphorus (V) dioxide to the latter: Unlike other phosphoric acids, metaphosphoric HPO3 is a strong acid and, as a result, its salts, metaphosphates, are not hydrolyzed by water. absorption of water by phosphorus (V) dioxide: “Taking away” water from 100% non-HN03, phosphorus (V) dioxide also forms metaphosphoric acid: When boiling a solution of metaphosphoric acid, orthophosphoric acid is formed: water forms phosphorous acid HjP03: Despite the fact that the acid contains three hydrogen atoms, it is dibasic, since the third hydrogen atom “non-acidic”, i.e., does not dissociate in aqueous solutions and is not replaced (see. structure). Phosphorous acid - an acid of medium strength forms two series of salts - phosphites or hydrophosphites. Also known is the monobasic phosphorous acid H3PO2 of medium strength (I0 ~ 2) f salts - hypo-phosphites.