Dependence of nitrogen density on temperature. Properties of cryogenic liquids at cryogenic temperatures

DEFINITION

Nitrogen- non-metal. Under normal conditions, it is a colorless gas that can condense into a colorless liquid(density liquid nitrogen equal to 0.808 g/cm 3), boiling, unlike liquid oxygen, at a lower temperature (-195.75 o C) than liquid oxygen.

In the solid state it appears as white crystals.

Nitrogen is poorly soluble in water (worse than oxygen), but it is highly soluble in liquid sulfur dioxide.

Chemical composition and structure of the liquid nitrogen molecule

Under normal conditions, nitrogen is a colorless gas consisting of N 2 molecules. There is a triple bond between the nitrogen atoms in the molecule, as a result of which its molecule is extremely strong. Molecular nitrogen is chemically inactive and weakly polarized.

Let us consider the formation of a nitrogen molecule (Fig. 1), the electron cloud of which has the shape of an elongated figure eight. When two nitrogen atoms approach, their electron clouds overlap. Such overlap is possible only when the electrons have antiparallel spins. In the region of cloud overlap, the electron density increases, as a result of which the attractive forces between atoms increase. Number of shared electron pairs in a nitrogen molecule is equal to one (one electron from each atom). The molecule has a covalent (non-polar) type of bond.

Rice. 1. The structure of the nitrogen molecule.

Brief description of the chemical properties and density of liquid nitrogen

Under normal conditions, nitrogen is a chemically passive element; does not react with acids, alkalis, ammonia hydrate, halogens, sulfur. Reacts to a small extent with hydrogen and oxygen under the action of an electric discharge (1, 2). In the presence of moisture, it reacts with lithium at room temperature (3). When heated, it reacts with magnesium, calcium, aluminum and other metals (4, 5, 6).

N 2 + 3H 2 ↔ 2NH 3 (1);

N 2 + O 2 ↔ 2NO (2);

N 2 + 6Li = 2Li 3 N (3);

N 2 + 3Mg = Mg 3 N_2 (4);

N 2 + 3Ca = Ca 3 N 2 (5);

N 2 + 2Al = 2AlN (6).

The reactions of nitrogen with fluorine and carbon, as in the case of hydrogen or oxygen, occur under the action of an electric discharge:

N 2 + 3F 2 = 2NF 3 ;

N 2 + 2C↔C 2 N 2.

When heated to a temperature of 500-600 o C, nitrogen reacts with lithium hydride (7), but if the temperature range is 300-350 o C, then a reaction with calcium carbide (8) is possible:

N 2 + 3LiH = Li 3 N + NH 3 (7);

N 2 + CaC 2 = Ca(CN) 2 (8).

Examples of problem solving

EXAMPLE 1

NITROGEN, N (lat. Nitrogenium * a. nitrogen; n. Stickstoff; f. azote, nitrogene; i. nitrogeno), — chemical element Group V periodic table Mendeleev, atomic number 7, atomic mass 14.0067. Discovered in 1772 by the English explorer D. Rutherford.

Properties of nitrogen

Under normal conditions, nitrogen is a colorless and odorless gas. Natural nitrogen consists of two stable isotopes: 14 N (99.635%) and 15 N (0.365%). The nitrogen molecule is diatomic; the atoms are connected by a covalent triple bond NN. Diameter of a nitrogen molecule, defined in different ways, 3.15-3.53 A. The nitrogen molecule is very stable - the dissociation energy is 942.9 kJ/mol.

Molecular nitrogen

Molecular nitrogen constants: f melting - 209.86°C, f boiling - 195.8°C; density nitrogen gas 1.25 kg/m3, liquid - 808 kg/m3.

Characteristics of nitrogen

In the solid state, nitrogen exists in two modifications: cubic a-shape with a density of 1026.5 kg/m 3 and a hexagonal b-form with a density of 879.2 kg/m 3. Heat of fusion 25.5 kJ/kg, heat of evaporation 200 kJ/kg. Surface tension liquid nitrogen in contact with air 8.5.10 -3 N/m; permittivity 1.000538. Solubility of nitrogen in water (cm 3 per 100 ml of H 2 O): 2.33 (0°C), 1.42 (25°C) and 1.32 (60°C). External electron shell The nitrogen atom consists of 5 electrons. The oxidation states of nitrogen vary from 5 (in N 2 O 5) to -3 (in NH 3).

Nitrogen compound

Nitrogen at normal conditions can react with transition metal compounds (Ti, V, Mo, etc.), forming complexes or being reduced to form ammonia and hydrazine. Nitrogen interacts with active metals such as when heated to relatively low temperatures. Nitrogen reacts with most other elements at high temperatures and in the presence of catalysts. Nitrogen compounds with: N 2 O, NO, N 2 O 5 have been well studied. Nitrogen combines with C only at high temperatures and in the presence of catalysts; this produces ammonia NH 3 . Nitrogen does not directly interact with halogens; therefore, all nitrogen halides are obtained only indirectly, for example, nitrogen fluoride NF 3 - by interaction with ammonia. Nitrogen does not combine directly with sulfur either. When hot water reacts with nitrogen, cyanogen (CN) 2 is formed. When ordinary nitrogen is exposed to electric discharges, as well as during electric discharges in the air, active nitrogen can be formed, which is a mixture of nitrogen molecules and atoms with an increased energy reserve. Active nitrogen interacts very energetically with oxygen, hydrogen, vapor, and some metals.

Nitrogen is one of the most common elements on Earth, and the bulk of it (about 4.10 15 tons) is concentrated in a free state in. Every year at volcanic activity 2.10 6 tons of nitrogen are released into the atmosphere. A small part of nitrogen is concentrated in (average content in the lithosphere 1.9.10 -3%). Natural nitrogen compounds - ammonium chloride and various nitrates(saltpeter). Nitrogen nitrides can only be formed when high temperatures and pressures, which apparently took place in the earliest stages of the Earth's development. Large accumulations of saltpeter are found only in dry desert climates (, etc.). Small quantities bound nitrogen are found in (1-2.5%) and (0.02-1.5%), as well as in the waters of rivers, seas and oceans. Nitrogen accumulates in soils (0.1%) and living organisms (0.3%). Nitrogen is part of protein molecules and many natural organic compounds.

Nitrogen cycle in nature

In nature, there is a nitrogen cycle, which includes the cycle of molecular atmospheric nitrogen in the biosphere, the cycle of chemically bound nitrogen in the atmosphere, the cycle of buried organic matter surface nitrogen in the lithosphere with its return back to the atmosphere. Nitrogen for industry was previously extracted entirely from natural saltpeter deposits, the number of which is very limited in the world. Particularly large deposits of nitrogen in the form of sodium nitrate are found in Chile; saltpeter production in some years amounted to more than 3 million tons.

DEFINITION

Under normal conditions nitrogen- a colorless gas consisting of N 2 molecules.

Strong intramolecular bonding, small size and non-polarity of the nitrogen molecule are the reasons for the weak intermolecular interaction, so nitrogen has low temperatures boiling and melting.

Nitrogen is odorless. Slightly soluble in water and slightly lighter than air: the mass of 1 liter of nitrogen is 1.25 g.

The most important nitrogen constants are presented in the table below:

Table 1. Physical properties and nitrogen density.

The nitrogen molecule N2 in the ground state has the following electronic configuration: [σ(2s) 2 ][ [σ * (2s) 2 ][π(2p y) 2 ][π(2p x) 2 ][π(2p z) 2 ]. The ground state nitrogen atom has three unpaired electrons. This indicates that the valency of nitrogen is three. However, despite the fact that nitrogen is not characterized by the presence of an excited state, this element is capable of exhibiting valences I, II, IV and V in its compounds.

Natural abundance of nitrogen

Nitrogen can be found in nature in the form of a simple substance in nature (the nitrogen content in the air is -78 (vol.)%). In the form of amino acids and nucleic acids Nitrogen is included in animal and plant organisms. From natural minerals industrial value have Chilean nitrate (NaNO 3) and potassium nitrate (KNO 3). General content nitrogen in earth's crust(including the hydrosphere and atmosphere) is 0.04% (mass.).

Brief description of the chemical properties and density of nitrogen

Molecular nitrogen has extremely low reactivity, which is due to the presence of a strong triple bond in the N2 molecule, as well as the non-polarity of the molecule. Indeed, the ionization energy of a nitrogen molecule, 1402 kJ/mol, is close to the ionization energy of an argon atom, 1520 kJ/mol; in other words, nitrogen is a poor reducing agent.

The electron affinity of molecular nitrogen is -3.6 eV, so at room temperature it reacts only with some strong reducing agents, such as lithium. The process proceeds through the stage of one-electron reduction with the formation of unstable pernitride Li + N 2 -, which during the reaction turns into nitride:

6Li + N 2 = 2Li 3 N.

To form magnesium nitride Mg 3 N 2 from simple substances requires heating to 300 o C. Nitrides active metals represent ionic compounds, hydrolyzed by water to form ammonia.

IN electrical discharge in a nitrogen molecule, an electron transition occurs from the orbital σ(2p z) to π * (2p z). Such excited molecules quickly return to their normal state, emitting a yellow glow. Sometimes it lasts for several minutes after the discharge stops. In an excited state, nitrogen is highly reactive. Indeed, in an electrical discharge it reacts with oxygen:

N 2 + O 2 ↔ 2NO.

The reaction with hydrogen occurs at elevated temperatures (400 o C) and pressure (200 atm) in the presence of a catalyst (Fe):

N 2 + 3H 2 ↔ 2NH 3 .

Nitrogen does not interact with sulfur and halogens.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

The chemical element nitrogen has the symbol N, atomic number 7 and atomic mass 14. In the elemental state, nitrogen forms very stable diatomic molecules N 2 with strong interatomic bonds.

Nitrogen molecule, its size and gas properties

The nitrogen molecule is formed threefold covalent bond between two nitrogen atoms and has chemical formula N 2. The size of the molecules of most substances in general, and nitrogen in particular, is a rather difficult value to determine, and even the concept itself is not unambiguous. To understand the operating principles of equipment that separates air components, the best concept is kinetic diameter molecule, which is defined as the smallest dimension of a molecule. Nitrogen N2, as well as oxygen O2, are diatomic molecules, in shape more similar to cylinders than to spheres - therefore, one of their dimensions, which can conventionally be called “length,” is larger than the other, which can conventionally be called “diameter.” Even the kinetic diameter of the nitrogen molecule is not unambiguously determined, however, there are both theoretical and experimentally data on the kinetic diameter of nitrogen and oxygen molecules (we present data on oxygen because oxygen is the second main integral part atmospheric air, and it is from this that nitrogen is required to be purified when it is obtained in the process of air separation), including:
- N 2 3.16Å and O 2 2.96Å - from viscosity data
- N 2 3.14Å and O 2 2.90Å - from data on van der Waals forces

Nitrogen N 2 melts, that is, passes from the solid phase to the liquid phase, at a temperature of -210°C, and evaporates (boils), that is, passes from a liquid to a gaseous state, at a temperature of -195.79°C.

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Nitrogen gas is inert gas, colorless, tasteless, odorless, non-flammable and non-toxic. The density of nitrogen under normal atmospheric conditions (that is, at a temperature of 0°C and an absolute pressure of 101325 Pa) is 1.251 kg/m³. Nitrogen does not react with virtually any other substances (with the exception of rare reactions of nitrogen binding with lithium and magnesium). Also, on the contrary, the Haber process is widely used in industry, in the production of fertilizers, in which, in the presence of a catalyst, iron trioxide Fe 3 O 4, nitrogen is bonded with hydrogen at high temperature and pressure.

Nitrogen constitutes the main part earth's atmosphere both by volume (78.3%) and by mass (75.47%). Nitrogen is present in all living organisms, in dead organisms, in waste products of organisms, in protein molecules, nucleic acids and amino acids, urea, uric acid and others. organic molecules. In nature, there are also nitrogen-containing minerals: nitrate (potassium nitrate - potassium nitrate KNO 3, ammonium nitrate - ammonium nitrate NH 4 NO 3, sodium nitrate - sodium nitrate NaNO 3, magnesium nitrate, barium nitrate, etc.), ammonia compounds (for example, ammonium chloride NH 4 Cl, etc.) and other, mostly quite rare, minerals.

The table shows the density of nitrogen and its thermophysical properties in gaseous state depending on temperature and pressure. The thermophysical properties of nitrogen are given at temperatures from 0 to 1000°C and pressure from 1 to 100 atmospheres.

As can be seen from the table, such properties of nitrogen as thermal diffusivity and kinematic viscosity strongly depend on temperature. With increasing pressure, these properties of nitrogen decrease their values, while nitrogen density increases significantly. For example, when atmospheric pressure and a temperature of 0°C, the density of nitrogen is 1.21 kg/m 3 , and with an increase in pressure 100 times, the density of nitrogen increases to 122.8 kg/m 3 at the same temperature.

The specific heat capacity of nitrogen increases with increasing temperature of this gas. As pressure increases, the specific heat capacity of nitrogen also increases. For example, at a temperature of 0°C and atmospheric pressure The specific heat capacity of nitrogen is 1039 J/(kg deg), and when this gas is compressed to a pressure of 100 atmospheres, it will be 1242 J/(kg deg) at the same temperature.

It should be noted that at high temperatures (about 1000°C) the influence of pressure on the value specific heat capacity nitrogen is reduced. So, at a temperature of 1000°C and a pressure of 1 and 100 atm. the heat capacity value will be equal to 1215 and 1219 J/(kg deg) respectively.

The table gives following properties nitrogen:

• nitrogen density γ , kg/m 3 ;
• specific heat C p , kJ/(kg deg);
• thermal conductivity coefficient λ , W/(m deg);
• dynamic viscosity μ , ;
• thermal diffusivity a , m 2 /s;
• kinematic viscosity ν , m 2 /s;
• Prandtl number Pr .

Density of dissociated nitrogen at high temperatures.

The table gives the density values ​​of nitrogen in the dissociated and ionized state at pressures from 0.2 to 100 atmospheres at high temperatures. The density of nitrogen in the gaseous state is given in the temperature range 5000...40000 K in the dimension kg/m 3.

The density of nitrogen decreases with increasing temperature and increases with increasing gas pressure. Meaning specific gravity nitrogen (its density) in the table ranges from 0.00043 to 6.83 kg/m 3. For example, at atmospheric pressure and a temperature of 5000 K (4727 ° C), the density of nitrogen is 0.0682 kg/m 3. When nitrogen is heated to a temperature of 40,000 K, its density decreases to a value of 0.00213 kg/m 3.

Note: Be careful! The density of nitrogen in the table is indicated in powers of 10 3. Don't forget to divide by 1000.

Thermal conductivity of nitrogen in liquid and gaseous states

The table shows the thermal conductivity of nitrogen in liquid and gaseous states depending on temperature and pressure.
The thermal conductivity of nitrogen (dimension W/(m deg)) is indicated in the temperature range from -193 to 1127 °C and pressure from 1 to 600 atmospheres.

Thermal conductivity of dissociated nitrogen at high temperatures.

The table gives the thermal conductivity values ​​of dissociated nitrogen at pressures from 0.001 to 100 atmospheres and high temperatures.
The thermal conductivity of nitrogen in the gaseous state is given in the temperature range 2000...6000 K in the dimension W/(m deg).

The value of the thermal conductivity coefficient of nitrogen increases with increasing temperature and generally decreases with increasing pressure of this gas. The thermal conductivity of dissociated nitrogen under the conditions considered in the table varies from 0.126 to 6.142 W/(m deg).

Be careful! The thermal conductivity of nitrogen in the table is indicated to the power of 10 3. Don't forget to share table value by 1000.

Thermal conductivity of liquid nitrogen at the saturation line.

The table shows the values ​​of the thermal conductivity coefficient of liquid nitrogen on the saturation line at low temperatures.
The thermal conductivity of liquid nitrogen is indicated at temperatures of 90...120 K (-183...-153°C).

According to the table, it can be seen that the thermal conductivity of nitrogen in liquid state decreases with increasing temperature.

Note: Be careful! The thermal conductivity of nitrogen in the table is indicated to the power of 10 3. Don't forget to divide by 1000.

Dynamic viscosity of nitrogen depending on temperature and pressure

The table shows nitrogen values ​​depending on temperature and pressure.
The dynamic viscosity of nitrogen (dimension Pa s) is indicated in the temperature range from 80 to 6000 K and pressure from 1 to 400 atmospheres and from 0.001 to 100 atmospheres.

At a nitrogen temperature of 3600 K, it begins to partially dissociate. As the temperature of azoate gas increases, its dynamic viscosity increases. As the temperature of liquid nitrogen increases, the value of its dynamic viscosity also increases.

Note: Be careful! The viscosity of nitrogen in the table is indicated in powers of 10 6. Don't forget to divide by 10 6 .

Sources:

1. Physical quantities. Directory. A.P. Babichev, N.A. Babushkina, A.M. Bratkovsky and others; Ed. I.S. Grigorieva, E.Z. Meilikhova. - M.: Energoatomizdat, 1991. - 1232 p.