Characteristics of classification features. Classification of chemical reactions underlying industrial chemical-technological processes Reversible and irreversible chemical reactions
And steel classification
- quality;
- chemical composition;
- appointment;
- microstructure;
- strength.
Steel quality
By chemical composition
carbon steels permanent impurities
Table 1.3.
CARBON STEEL
alloying elements additives or additives
Alloy steels low-alloyed(up to 2.5 wt.%), doped(from 2.5 to 10 wt.%) and highly alloyed "chrome"
According to the purpose of steel
Structural low-( or few-) And medium carbon.
instrumentalhigh carbon.
And (with special properties - ).
And
And increased heat resistance fast cutting steels.
ordinary quality,
Structural steels,
tool steel,
6) bearing (ball bearing) become,
7) high speed steel(high-alloyed, high-quality tool steels with a high tungsten content).
8) automatic, i.e.increased (or high) machinability, become.
An analysis of the composition of historically established marking groups of steels shows that the marking systems used make it possible to encode five classification features, namely: quality, chemical composition, purpose, degree of deoxidation, as well as way to get blanks(automatic or, in rare cases, foundries). The connection between marking groups and steel classes is illustrated in the lower part of the block diagram in Fig. 1.
SYSTEM OF MARKING GROUPS, MARKING RULES AND EXAMPLES OF STEEL GRADES
| CARBON | REGULAR QUALITY | |||||||
| steel group | Delivery guarantee | STAMPS | ||||||
| BUT | by chemical composition | St0 | St1 | St2 | StZ | St4 | St5 | St6 |
| B | by mechanical properties | Bst0 | Bst1 | Bst2 | BSTZ | Bst4 | Bst5 | Bst6 |
| IN | mechanical properties and chemical composition | ESPO | VST1 | VST2 | VSTZ | VST4 | VST5 | VST6 |
| Carbon concentration, wt. % | 0,23 | 0,06-0,12 | 0,09-0,15 | 0,14-0,22 | 0,18-0,27 | 0,28-0,37 | 0,38-0,49 | |
| QUALITY HIGH QUALITY | STRUCTURAL | EXAMPLES OF STAMPS | ||||||
| Grade: two-digit number of hundredths of a percent of carbon + an indication of the degree of deoxidation | 05 08kp 10 15 18kp 20A 25ps ZOA 35 40 45 50 55 ... 80 85 Notes: 1) the absence of an indicator of the degree of deoxidation means “sp”; 2) "A" at the end of the grade indicates that the steel is high quality | |||||||
| INSTRUMENTAL | STAMPS | |||||||
| Brand: symbol "U" + number TETHS OF A PERCENTAGE OF CARBON | U7 U7A U8 UVA U9 U9A U10 U10A U12 U12A | |||||||
| ALLOYED | HIGH QUALITY HIGH QUALITY EXTRA HIGH QUALITY | STRUCTURAL | EXAMPLES OF STAMPS | |||||
| Grade: two-digit number of HUNDREDTHS of a percentage of carbon + symbol of an alloying element + whole number of its percent | 09G2 10KhSND 18G2AFps 20Kh 40G 45KhN 65S2VA 110G13L 2) brand 110G13L - one of the few in which the number of hundredths of a percent of carbon is three-digit | |||||||
| INSTRUMENTAL | EXAMPLES OF STAMPS | |||||||
| Grade: number of TENSES of percent carbon + alloying element symbol+ whole number of its percent | ZKh2N2MF 4KhV2S 5KhNM 7X3 9KhVG X KhV4 9Kh4MZF2AGST-SH 2) "-SH" at the end of the brand shows that the steel is of especially high quality, obtained, for example, by the method electroslag remelting (but not only) |
Carbon structural steels of ordinary quality
Specific steels of the specified marking group are designated using a two-letter combination "St" which is the key (backbone) in the considered marking group. The steel grades of this group are immediately recognizable by this symbol.
The symbol "St" without a space is followed by a number indicating room brands from «0» before "6".
An increase in the grade number corresponds to an increase in the carbon content in steel, but does not indicate its specific value. Permissible limits of carbon concentration in steels of each grade are shown in Table. 1.5. Carbon content in ordinary carbon steels does not exceed 0.5 wt.%. Such steels are hypoeutectoid according to the structural criterion, and, therefore, structural according to their purpose.
After the number, one of three letter combinations follows: “kp”, “ps”, “sp”, indicating the degree of steel deoxidation.
The symbol "St" may be preceded by capital letters "A", "B" or "C", or there may be no symbols. In this way, information is transmitted about steel belonging to one of the so-called "delivery groups": A, B or IN, - depending on which of the normalized indicators of steel is guaranteed by the supplier.
Steel group BUT comes with a guarantee of the chemical composition, or the permissible values of the concentration of carbon and impurities specified by GOST. The letter "A" is often not put on the stamp and its absence default stands for chemical composition guarantee. The consumer of steel, having no information about the mechanical properties, can form them by appropriate heat treatment, the choice of modes of which requires knowledge of the chemical composition.
Steel group B comes with a guarantee of the required mechanical properties. The consumer of steel can determine its optimal use in structures according to the known characteristics of mechanical properties without prior heat treatment.
Steel group IN comes with a guarantee of both chemical composition and mechanical properties. It is used by the consumer mainly to create welded structures. Knowledge of the mechanical properties makes it possible to predict the behavior of the loaded structure in areas far from the welds, and knowledge of the chemical composition makes it possible to predict and, if necessary, correct the mechanical properties of the welds themselves by heat treatment.
Stamp Recording Examples ordinary quality carbon steel look like this: Vst3ps, Bst6sp, St1kp .
Ball bearing steels
Steels for bearings have their own marking, according to their purpose they form a special group structural steels, although in composition and properties they are close to tool steels. The term "ball bearing" defines their narrow scope - rolling bearings (not only ball bearings, but also roller and needle bearings). For its marking, the abbreviation "SHH" was proposed - ball bearing chromium, followed by a number tenths of a percent medium concentration chrome. Of the previously well-known brands SHKH6, SHKH9 and SHKH15, the SHKH15 brand remained in use. The difference between ball bearing steel and similar tool steel lies in more stringent requirements for the amount of non-metallic inclusions and uniform distribution of carbides in the microstructure.
The improvement of ShKh15 steel by introducing additional alloying additives (silicon and manganese) into it was reflected in a peculiar way in the marking - distribution to specific a system of later rules for the designation of alloying elements in the composition of alloyed steels: SHKH15SG, SHKH20SG.
High speed steels
High-speed steels are specifically marked with the initial letter of the Russian alphabet "R", corresponding to the first sound in the English word rapid - fast, quick. This is followed by an integer percentage of tungsten. As already mentioned, the most common brand of high speed steel used to be P18.
Due to the scarcity and high cost of tungsten, there was a transition to tungsten-molybdenum steel R6M5 without nitrogen and R6AM5 with nitrogen. Similar to bearing steels, there has been a merger (a kind of "hybridization") of the two marking systems. The development and development of new high-speed steels with cobalt and vanadium enriched the arsenal of "hybrid" grades: R6AM5F3, R6M4K8, 11R3AM3F2 - and also led to the emergence of generally tungsten-free high-speed steels, which are marked in a specific system (R0M5F1, R0M2F3), and in a completely new way - 9X6M3F3AGST-Sh, 9X4M3F2AGST-Sh.
Cast iron classification
Cast irons are called alloys of iron with carbon, having in their composition more than 2.14 wt.% C.
Cast irons are smelted for conversion into steel (conversion), to obtain ferroalloys that play the role of alloying additives, and also as high-tech alloys for castings (casting).
Carbon can be in cast iron in the form of two high-carbon phases - cementite (Fe 3 C) and graphite, and sometimes both in the form of cementite and graphite. Cast iron, in which only cementite is present, gives a light, shiny fracture and is therefore called white. The presence of graphite gives the cast iron fracture a gray color. However, not every cast iron with graphite belongs to the class of so-called gray cast irons. Between white and gray cast irons lies the class half-hearted cast irons.
half-hearted cast irons are called cast irons, in the structure of which, despite graphitization, ledeburite cementite is at least partially preserved, which means that ledeburite itself is present - a eutectic structural component having a specific form.
TO gray include cast irons in which ledeburite cementite has completely disintegrated, and the latter has disappeared from the structure. Gray cast iron consists of graphite inclusions And metal base. This metal base is pearlitic (eutectoid), ferritic-pearlitic (hypo-eutectoid) or ferritic (low carbon) steel. The specified sequence of types of the metal base of gray cast irons corresponds to an increasing degree of decomposition of cementite, which is part of perlite.
Anti-friction cast irons
Brand examples: ASF-1, ASF-2, ASF-3.
Special alloyed heat resistant, corrosion resistant And heat resistant cast irons:
EXAMPLES OF SPECIAL GRAY IRON GRADES
Classification and labeling
sintered hard alloys
Metal-ceramic hard alloys are alloys made by powder metallurgy (cermet) and consisting of carbides of refractory metals: WC, TiC, TaC, connected by a plastic metal binder, most often with cobalt.
At present, three groups of hard alloys are produced in Russia: tungsten, titanium-tungsten and titanium-tantalum-tungsten, – containing as a binder cobalt.
Due to the high cost of tungsten, hard alloys have been developed that do not contain tungsten carbide at all. As a solid phase, they contain only titanium carbide or titanium carbonitride– Ti(NC). The role of the plastic ligament is performed by nickel-molybdenum matrix. The classification of hard alloys is represented by a block diagram.
In accordance with the five classes of cermet hard alloys, the existing marking rules form five marking groups.
Tungsten ( sometimes called tungsten-cobalt) hard alloys
Examples: VK3, VK6, VK8, VK10.
Titanium tungsten ( sometimes called titanium-tungsten-cobalt) hard alloys
Examples: T30K4, T15K6, T5K10, T5K12.
Titanium tantalum tungsten ( sometimes called titanium-tantalum-tungsten-cobalt) hard alloys
Examples: TT7K12, TT8K6, TT10K8, TT20K9.
Sometimes, at the end of the brand, letters or letter combinations are added through a hyphen, characterizing the dispersion of carbide particles in the powder:
CLASSIFICATION OF HARD CERAMIC ALLOYS
Foreign analogues of some domestic alloy steel grades are shown in Table 1.1.
Table 1.1.
Foreign analogues of a number of domestic grades of alloyed steels
| Russia, GOST | Germany, DIN * | USA, ASTM* | Japan, LS * |
| 15X | 15Cr3 | SCr415 | |
| 40X | 41Cr4 | SCg440 | |
| 30XM | 25CrMo4 | SCM430, SCM2 | |
| 12HG3A | 14NiCr10** | – | SNC815 |
| 20HGNM | 21NiCrMo2 | SNCM220 | |
| 08X13 | X7Cr13 ** | 410S | SUS410S |
| 20X13 | Х20Сг13 | SUS420J1 | |
| 12X17 | X8Cr17 | 430 (51430 ***) | SUS430 |
| 12X18H9 | X12CrNi8 9 | SUS302 | |
| 08X18H10T | Х10CrNiTi18 9 | .321 | SUS321 |
| 10Х13СУ | X7CrA133 ** | 405 ** (51405) *** | SUS405** |
| 20Х25Н20С2 | Х15CrNiSi25 20 | 30314,314 | SCS18, SUH310 ** |
* DIN (Deutsche Industrienorm), ASTM (American Societi for Testing Materials), JIS (Japanese industrial Standard).
** Steel similar in composition; *** SAE standard
Characteristics of classification features
And steel classification
The modern classification features of steels include the following:
- quality;
- chemical composition;
- appointment;
- metallurgical features of production;
- microstructure;
- traditional way of hardening;
- the traditional way of obtaining blanks or parts;
- strength.
Let's briefly characterize each of them.
Steel quality is determined primarily by the content of harmful impurities - sulfur and phosphorus - and is characterized by 4 categories (see table. 1.2).
By chemical composition steels are conditionally divided into carbon (non-alloyed) steels and alloyed ones.
carbon steels do not contain specially introduced alloying elements. The elements contained in carbon steels, except for carbon, are among the so-called permanent impurities. Their concentration should be within the limits determined by the relevant state standards (GOSTs). Table 1.3. average concentration limits for some elements are given, allowing these elements to be classified as impurities rather than alloying elements. Specific limits for the content of impurities in carbon steels are given by GOSTs.
Table 1.3.
LIMITING CONCENTRATIONS OF SOME ELEMENTS, ALLOWING THEM TO BE CONSIDERED PERMANENT IMPURITIES
CARBON STEEL
alloying elements, sometimes called alloying additives or additives, are specially introduced into steel to obtain the required structure and properties.
Alloy steels are subdivided according to the total concentration of alloying elements, except for carbon, into low-alloyed(up to 2.5 wt.%), doped(from 2.5 to 10 wt.%) and highly alloyed(more than 10 wt.%) when the content in the latter of iron is not less than 45 wt.%. Usually, the introduced alloying element gives the alloy steel the corresponding name: "chrome"- doped with chromium, "silicon" - with silicon, "chromium-silicon" - with chromium and silicon at the same time, etc.
In addition, iron-based alloys are also distinguished, when the iron content of the material is less than 45%, but it is more than any other alloying element.
According to the purpose of steel subdivided into structural and instrumental.
Structural steels used for the manufacture of various machine parts, mechanisms and structures in mechanical engineering, construction and instrument making are considered. They must have the necessary strength and toughness, as well as, if required, a set of special properties (corrosion resistance, paramagnetism, etc.). As a rule, structural steels are low-( or few-) And medium carbon. Hardness is not a decisive mechanical characteristic for them.
instrumental called steels used for processing materials by cutting or pressure, as well as for the manufacture of measuring tools. They must have high hardness, wear resistance, strength and a number of other specific properties, for example, heat resistance. Necessary condition obtaining high hardness is an increased carbon content, so tool steels, with rare exceptions, are always high carbon.
Within each of the groups there is a more detailed division according to purpose. Structural steels are divided into construction, engineering And special application steels(with special properties - heat-resistant, heat-resistant, corrosion-resistant, non-magnetic).
Tool steels are divided into cutting tool steels, die steels And steel for measuring instruments.
A common operational property of tool steels is high hardness, which ensures the resistance of the tool to deformation and abrasion of its surface. At the same time, a specific requirement is imposed on steels for cutting tools - to maintain high hardness at elevated temperatures (up to 500 ... 600ºС), which develop in the cutting edge at high cutting speeds. The indicated ability of steel is called its heat resistance (or red hardness). According to the specified criterion, steels for cutting tools are divided into non-heat-resistant, semi-heat-resistant, heat-resistant And increased heat resistance. The last two groups are known in the art under the name fast cutting steels.
From die steels, in addition to high hardness, high toughness is required, since the die tool works under shock loading conditions. In addition, the tool for hot stamping, in contact with heated metal blanks, can heat up during prolonged work. Therefore, steels for hot stamping must also be heat resistant.
Measuring tool steels, in addition to high wear resistance, ensuring dimensional accuracy over a long service life, must guarantee tool dimensional stability regardless of operating temperature conditions. In other words, they should have a very small thermal expansion coefficient.
The chemical properties of substances are revealed in a variety of chemical reactions.
Transformations of substances, accompanied by a change in their composition and (or) structure, are called chemical reactions. The following definition is often found: chemical reaction The process of transformation of initial substances (reagents) into final substances (products) is called.
Chemical reactions are written using chemical equations and schemes containing the formulas of the starting materials and reaction products. In chemical equations, unlike schemes, the number of atoms of each element is the same on the left and right sides, which reflects the law of conservation of mass.
On the left side of the equation, the formulas of the starting substances (reagents) are written, on the right side - the substances obtained as a result of a chemical reaction (reaction products, final substances). The equal sign connecting the left and right sides indicates that the total number of atoms of the substances participating in the reaction remains constant. This is achieved by placing integer stoichiometric coefficients in front of the formulas, showing the quantitative ratios between the reactants and reaction products.
Chemical equations may contain additional information about the features of the reaction. If a chemical reaction proceeds under the influence of external influences (temperature, pressure, radiation, etc.), this is indicated by the appropriate symbol, usually above (or “under”) the equals sign.
A huge number of chemical reactions can be grouped into several types of reactions, which are characterized by well-defined features.
As classification features the following can be selected:
1. The number and composition of the starting materials and reaction products.
2. Aggregate state of reactants and reaction products.
3. The number of phases in which the participants in the reaction are.
4. The nature of the transferred particles.
5. The possibility of the reaction proceeding in the forward and reverse directions.
6. The sign of the thermal effect separates all reactions into: exothermic reactions proceeding with the exo-effect - the release of energy in the form of heat (Q> 0, ∆H<0):
C + O 2 \u003d CO 2 + Q
And endothermic reactions proceeding with the endo effect - the absorption of energy in the form of heat (Q<0, ∆H >0):
N 2 + O 2 \u003d 2NO - Q.
Such reactions are thermochemical.
Let us consider in more detail each of the types of reactions.
Classification according to the number and composition of reagents and final substances
1. Connection reactions
In the reactions of a compound from several reacting substances of a relatively simple composition, one substance of a more complex composition is obtained:
As a rule, these reactions are accompanied by heat release, i.e. lead to the formation of more stable and less energy-rich compounds.
The reactions of the combination of simple substances are always redox in nature. Connection reactions occurring between complex substances can occur both without a change in valency:
CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2,
and be classified as redox:
2FeCl 2 + Cl 2 = 2FeCl 3.
2. Decomposition reactions
Decomposition reactions lead to the formation of several compounds from one complex substance:
A = B + C + D.
The decomposition products of a complex substance can be both simple and complex substances.
Of the decomposition reactions that occur without changing the valence states, it should be noted the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids:
| t o | ||
| 4HNO 3 | = | 2H 2 O + 4NO 2 O + O 2 O. |
2AgNO 3 \u003d 2Ag + 2NO 2 + O 2,
(NH 4) 2Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O.
Particularly characteristic are the redox reactions of decomposition for salts of nitric acid.
Decomposition reactions in organic chemistry are called cracking:
C 18 H 38 \u003d C 9 H 18 + C 9 H 20,
or dehydrogenation
C 4 H 10 \u003d C 4 H 6 + 2H 2.
3. Substitution reactions
In substitution reactions, usually a simple substance interacts with a complex one, forming another simple substance and another complex one:
A + BC = AB + C.
These reactions in the vast majority belong to redox reactions:
2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3,
Zn + 2HCl \u003d ZnCl 2 + H 2,
2KBr + Cl 2 \u003d 2KCl + Br 2,
2KSlO 3 + l 2 = 2KlO 3 + Cl 2.
Examples of substitution reactions that are not accompanied by a change in the valence states of atoms are extremely few. It should be noted the reaction of silicon dioxide with salts of oxygen-containing acids, which correspond to gaseous or volatile anhydrides:
CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2,
Ca 3 (RO 4) 2 + ZSiO 2 \u003d ZCaSiO 3 + P 2 O 5,
Sometimes these reactions are considered as exchange reactions:
CH 4 + Cl 2 = CH 3 Cl + Hcl.
4. Exchange reactions
Exchange reactions Reactions between two compounds that exchange their constituents are called:
AB + CD = AD + CB.
If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts:
ZnO + H 2 SO 4 \u003d ZnSO 4 + H 2 O,
AgNO 3 + KBr = AgBr + KNO 3,
CrCl 3 + ZNaOH = Cr(OH) 3 + ZNaCl.
A special case of these exchange reactions is neutralization reactions:
Hcl + KOH \u003d KCl + H 2 O.
Usually, these reactions obey the laws of chemical equilibrium and proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous, volatile substance, precipitate, or low-dissociation (for solutions) compound:
NaHCO 3 + Hcl \u003d NaCl + H 2 O + CO 2,
Ca (HCO 3) 2 + Ca (OH) 2 \u003d 2CaCO 3 ↓ + 2H 2 O,
CH 3 COONa + H 3 RO 4 \u003d CH 3 COOH + NaH 2 RO 4.
5. Transfer reactions.
In transfer reactions, an atom or a group of atoms passes from one structural unit to another:
AB + BC \u003d A + B 2 C,
A 2 B + 2CB 2 = DIA 2 + DIA 3.
For example:
2AgCl + SnCl 2 \u003d 2Ag + SnCl 4,
H 2 O + 2NO 2 \u003d HNO 2 + HNO 3.
Classification of reactions according to phase features
Depending on the state of aggregation of the reacting substances, the following reactions are distinguished:
1. Gas reactions
| H 2 + Cl 2 | 2HCl. |
2. Reactions in solutions
NaOH (p-p) + Hcl (p-p) \u003d NaCl (p-p) + H 2 O (l)
3. Reactions between solids
| t o | ||
| CaO (tv) + SiO 2 (tv) | = | CaSiO 3 (TV) |
Classification of reactions according to the number of phases.
A phase is understood as a set of homogeneous parts of a system with the same physical and chemical properties and separated from each other by an interface.
From this point of view, the whole variety of reactions can be divided into two classes:
1. Homogeneous (single-phase) reactions. These include reactions occurring in the gas phase, and a number of reactions occurring in solutions.
2. Heterogeneous (multiphase) reactions. These include reactions in which the reactants and products of the reaction are in different phases. For example:
gas-liquid phase reactions
CO 2 (g) + NaOH (p-p) = NaHCO 3 (p-p).
gas-solid-phase reactions
CO 2 (g) + CaO (tv) \u003d CaCO 3 (tv).
liquid-solid-phase reactions
Na 2 SO 4 (solution) + BaCl 3 (solution) \u003d BaSO 4 (tv) ↓ + 2NaCl (p-p).
liquid-gas-solid-phase reactions
Ca (HCO 3) 2 (solution) + H 2 SO 4 (solution) \u003d CO 2 (r) + H 2 O (l) + CaSO 4 (tv) ↓.
Classification of reactions according to the type of particles carried
1. Protolytic reactions.
TO protolytic reactions include chemical processes, the essence of which is the transfer of a proton from one reactant to another.
This classification is based on the protolytic theory of acids and bases, according to which an acid is any substance that donates a proton, and a base is a substance that can accept a proton, for example:
Protolytic reactions include neutralization and hydrolysis reactions.
2. Redox reactions.
These include reactions in which the reactants exchange electrons, while changing the oxidation state of the atoms of the elements that make up the reactants. For example:
Zn + 2H + → Zn 2 + + H 2 ,
FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O,
The vast majority of chemical reactions are redox, they play an extremely important role.
3. Ligand exchange reactions.
These include reactions during which an electron pair is transferred with the formation of a covalent bond by the donor-acceptor mechanism. For example:
Cu(NO 3) 2 + 4NH 3 = (NO 3) 2,
Fe + 5CO = ,
Al(OH) 3 + NaOH = .
A characteristic feature of ligand-exchange reactions is that the formation of new compounds, called complex ones, occurs without a change in the oxidation state.
4. Reactions of atomic-molecular exchange.
This type of reactions includes many of the substitution reactions studied in organic chemistry, which proceed according to the radical, electrophilic, or nucleophilic mechanism.
Reversible and irreversible chemical reactions
Such chemical processes are called reversible, the products of which are able to react with each other under the same conditions in which they are obtained, with the formation of starting substances.
For reversible reactions, the equation is usually written as follows:
Two oppositely directed arrows indicate that under the same conditions, both forward and reverse reactions occur simultaneously, for example:
CH 3 COOH + C 2 H 5 OH CH 3 COOS 2 H 5 + H 2 O.
Irreversible are such chemical processes, the products of which are not able to react with each other with the formation of starting substances. Examples of irreversible reactions are the decomposition of Bertolet salt when heated:
2KSlO 3 → 2KSl + ZO 2,
or oxidation of glucose with atmospheric oxygen:
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O.
Chemical reactions should be distinguished from nuclear reactions. As a result of chemical reactions, the total number of atoms of each chemical element and its isotopic composition do not change. Nuclear reactions are another matter - the processes of transformation of atomic nuclei as a result of their interaction with other nuclei or elementary particles, for example, the transformation of aluminum into magnesium:
27 13 Al + 1 1 H \u003d 24 12 Mg + 4 2 He
The classification of chemical reactions is multifaceted, that is, it can be based on various signs. But under any of these signs, reactions both between inorganic and between organic substances can be attributed.
Consider the classification of chemical reactions according to various criteria.
I. According to the number and composition of the reactants
Reactions that take place without changing the composition of substances.
In inorganic chemistry, such reactions include the processes of obtaining allotropic modifications of one chemical element, for example:
C (graphite) ↔ C (diamond)
S (rhombic) ↔ S (monoclinic)
R (white) ↔ R (red)
Sn (white tin) ↔ Sn (grey tin)
3O 2 (oxygen) ↔ 2O 3 (ozone)
In organic chemistry, this type of reactions can include isomerization reactions that occur without changing not only the qualitative, but also the quantitative composition of the molecules of substances, for example:
1. Isomerization of alkanes.
The reaction of isomerization of alkanes is of great practical importance, since hydrocarbons of the isostructure have a lower ability to detonate.
2. Isomerization of alkenes.
3. Isomerization of alkynes (reaction of A. E. Favorsky).
CH 3 - CH 2 - C \u003d - CH ↔ CH 3 - C \u003d - C- CH 3
ethylacetylene dimethylacetylene
4. Isomerization of haloalkanes (A. E. Favorsky, 1907).
5. Isomerization of ammonium cyanite upon heating.
For the first time, urea was synthesized by F. Wehler in 1828 by isomerization of ammonium cyanate when heated.
Reactions that go with a change in the composition of a substance
There are four types of such reactions: compounds, decompositions, substitutions and exchanges.
1. Connection reactions are such reactions in which one complex substance is formed from two or more substances
In inorganic chemistry, the whole variety of compound reactions can be considered, for example, using the example of reactions for obtaining sulfuric acid from sulfur:
1. Obtaining sulfur oxide (IV):
S + O 2 \u003d SO - one complex substance is formed from two simple substances.
2. Obtaining sulfur oxide (VI):
SO 2 + 0 2 → 2SO 3 - one complex substance is formed from a simple and complex substance.
3. Obtaining sulfuric acid:
SO 3 + H 2 O \u003d H 2 SO 4 - one complex is formed from two complex substances.
An example of a compound reaction in which one complex substance is formed from more than two starting materials is the final stage in the production of nitric acid:
4NO 2 + O 2 + 2H 2 O \u003d 4HNO 3
In organic chemistry, compound reactions are commonly referred to as "addition reactions". The whole variety of such reactions can be considered on the example of a block of reactions characterizing the properties of unsaturated substances, for example, ethylene:
1. Hydrogenation reaction - hydrogen addition:
CH 2 \u003d CH 2 + H 2 → H 3 -CH 3
ethene → ethane
2. Hydration reaction - addition of water.
3. Polymerization reaction.
2. Decomposition reactions are such reactions in which several new substances are formed from one complex substance.
In inorganic chemistry, the whole variety of such reactions can be considered in the block of reactions for obtaining oxygen by laboratory methods:
1. Decomposition of mercury (II) oxide - two simple ones are formed from one complex substance.
2. Decomposition of potassium nitrate - from one complex substance, one simple and one complex are formed.
3. Decomposition of potassium permanganate - from one complex substance, two complex and one simple are formed, that is, three new substances.
In organic chemistry, decomposition reactions can be considered on the block of reactions for the production of ethylene in the laboratory and in industry:
1. The reaction of dehydration (water splitting) of ethanol:
C 2 H 5 OH → CH 2 \u003d CH 2 + H 2 O
2. Dehydrogenation reaction (hydrogen splitting) of ethane:
CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2
or CH 3 -CH 3 → 2C + ZH 2
3. Cracking reaction (splitting) of propane:
CH 3 -CH 2 -CH 3 → CH 2 \u003d CH 2 + CH 4
3. Substitution reactions are such reactions as a result of which the atoms of a simple substance replace the atoms of an element in a complex substance.
In inorganic chemistry, an example of such processes is a block of reactions that characterize the properties of, for example, metals:
1. Interaction of alkali or alkaline earth metals with water:
2Na + 2H 2 O \u003d 2NaOH + H 2
2. Interaction of metals with acids in solution:
Zn + 2HCl = ZnCl 2 + H 2
3. Interaction of metals with salts in solution:
Fe + CuSO 4 = FeSO 4 + Cu
4. Metalthermy:
2Al + Cr 2 O 3 → Al 2 O 3 + 2Cr
The subject of study of organic chemistry is not simple substances, but only compounds. Therefore, as an example of a substitution reaction, we give the most characteristic property of saturated compounds, in particular methane, the ability of its hydrogen atoms to be replaced by halogen atoms. Another example is the bromination of an aromatic compound (benzene, toluene, aniline).
C 6 H 6 + Br 2 → C 6 H 5 Br + HBr
benzene → bromobenzene
Let us pay attention to the peculiarity of the substitution reaction in organic substances: as a result of such reactions, not a simple and complex substance is formed, as in inorganic chemistry, but two complex substances.
In organic chemistry, substitution reactions also include some reactions between two complex substances, for example, the nitration of benzene. It is formally an exchange reaction. The fact that this is a substitution reaction becomes clear only when considering its mechanism.
4. Exchange reactions are such reactions in which two complex substances exchange their constituent parts
These reactions characterize the properties of electrolytes and proceed in solutions according to the Berthollet rule, that is, only if a precipitate, gas, or a low-dissociating substance (for example, H 2 O) is formed as a result.
In inorganic chemistry, this can be a block of reactions characterizing, for example, the properties of alkalis:
1. Neutralization reaction that goes with the formation of salt and water.
2. The reaction between alkali and salt, which goes with the formation of gas.
3. The reaction between alkali and salt, which goes with the formation of a precipitate:
СuSO 4 + 2KOH \u003d Cu (OH) 2 + K 2 SO 4
or in ionic form:
Cu 2+ + 2OH - \u003d Cu (OH) 2
In organic chemistry, one can consider a block of reactions characterizing, for example, the properties of acetic acid:
1. The reaction proceeding with the formation of a weak electrolyte - H 2 O:
CH 3 COOH + NaOH → Na (CH3COO) + H 2 O
2. The reaction that goes with the formation of gas:
2CH 3 COOH + CaCO 3 → 2CH 3 COO + Ca 2+ + CO 2 + H 2 O
3. The reaction proceeding with the formation of a precipitate:
2CH 3 COOH + K 2 SO 3 → 2K (CH 3 COO) + H 2 SO 3
2CH 3 COOH + SiO → 2CH 3 COO + H 2 SiO 3
II. By changing the oxidation states of chemical elements that form substances
On this basis, the following reactions are distinguished:
1. Reactions that occur with a change in the oxidation states of elements, or redox reactions.
These include many reactions, including all substitution reactions, as well as those reactions of combination and decomposition in which at least one simple substance participates, for example:
1. Mg 0 + H + 2 SO 4 \u003d Mg + 2 SO 4 + H 2
2. 2Mg 0 + O 0 2 = Mg +2 O -2
Complex redox reactions are compiled using the electron balance method.
2KMn +7 O 4 + 16HCl - \u003d 2KCl - + 2Mn +2 Cl - 2 + 5Cl 0 2 + 8H 2 O
In organic chemistry, the properties of aldehydes can serve as a striking example of redox reactions.
1. They are reduced to the corresponding alcohols:
Aldecides are oxidized to the corresponding acids:
2. Reactions that take place without changing the oxidation states of chemical elements.
These include, for example, all ion exchange reactions, as well as many compound reactions, many decomposition reactions, esterification reactions:
HCOOH + CHgOH = HSOCH 3 + H 2 O
III. By thermal effect
According to the thermal effect, the reactions are divided into exothermic and endothermic.
1. Exothermic reactions proceed with the release of energy.
These include almost all compound reactions. A rare exception is the endothermic reactions of the synthesis of nitric oxide (II) from nitrogen and oxygen and the reaction of gaseous hydrogen with solid iodine.
Exothermic reactions that proceed with the release of light are referred to as combustion reactions. The hydrogenation of ethylene is an example of an exothermic reaction. It runs at room temperature.
2. Endothermic reactions proceed with the absorption of energy.
Obviously, almost all decomposition reactions will apply to them, for example:
1. Calcination of limestone
2. Butane cracking
The amount of energy released or absorbed as a result of the reaction is called the thermal effect of the reaction, and the equation of a chemical reaction indicating this effect is called the thermochemical equation:
H 2 (g) + C 12 (g) \u003d 2HC 1 (g) + 92.3 kJ
N 2 (g) + O 2 (g) \u003d 2NO (g) - 90.4 kJ
IV. According to the state of aggregation of reacting substances (phase composition)
According to the state of aggregation of the reacting substances, there are:
1. Heterogeneous reactions - reactions in which the reactants and reaction products are in different states of aggregation (in different phases).
2. Homogeneous reactions - reactions in which the reactants and reaction products are in the same state of aggregation (in one phase).
V. According to the participation of the catalyst
According to the participation of the catalyst, there are:
1. Non-catalytic reactions that take place without the participation of a catalyst.
2. Catalytic reactions taking place with the participation of a catalyst. Since all biochemical reactions occurring in the cells of living organisms proceed with the participation of special biological catalysts of protein nature - enzymes, they are all catalytic or, more precisely, enzymatic. It should be noted that more than 70% of chemical industries use catalysts.
VI. Towards
By direction there are:
1. Irreversible reactions proceed under given conditions in only one direction. These include all exchange reactions accompanied by the formation of a precipitate, gas or a low-dissociating substance (water) and all combustion reactions.
2. Reversible reactions under these conditions proceed simultaneously in two opposite directions. Most of these reactions are.
In organic chemistry, the sign of reversibility is reflected in the names - antonyms of processes:
Hydrogenation - dehydrogenation,
Hydration - dehydration,
Polymerization - depolymerization.
All esterification reactions are reversible (the opposite process, as you know, is called hydrolysis) and hydrolysis of proteins, esters, carbohydrates, polynucleotides. The reversibility of these processes underlies the most important property of a living organism - metabolism.
VII. According to the mechanism of flow, there are:
1. Radical reactions take place between the radicals and molecules formed during the reaction.
As you already know, in all reactions, old chemical bonds are broken and new chemical bonds are formed. The method of breaking the bond in the molecules of the starting substance determines the mechanism (path) of the reaction. If the substance is formed by a covalent bond, then there can be two ways to break this bond: hemolytic and heterolytic. For example, for the molecules of Cl 2 , CH 4 , etc., a hemolytic rupture of bonds is realized, it will lead to the formation of particles with unpaired electrons, that is, free radicals.
Radicals are most often formed when bonds are broken in which the shared electron pairs are distributed approximately equally between atoms (non-polar covalent bond), but many polar bonds can also be broken in a similar way, in particular when the reaction takes place in the gas phase and under the influence of light , as, for example, in the case of the processes discussed above - the interaction of C 12 and CH 4 - . Radicals are highly reactive, as they tend to complete their electron layer by taking an electron from another atom or molecule. For example, when a chlorine radical collides with a hydrogen molecule, it breaks the shared electron pair that binds the hydrogen atoms and forms a covalent bond with one of the hydrogen atoms. The second hydrogen atom, becoming a radical, forms a common electron pair with the unpaired electron of the chlorine atom from the collapsing Cl 2 molecule, resulting in a chlorine radical that attacks a new hydrogen molecule, etc.
Reactions, which are a chain of successive transformations, are called chain reactions. For the development of the theory of chain reactions, two outstanding chemists - our compatriot N. N. Semenov and the Englishman S. A. Hinshelwood were awarded the Nobel Prize.
The substitution reaction between chlorine and methane proceeds similarly:
Most of the combustion reactions of organic and inorganic substances, the synthesis of water, ammonia, the polymerization of ethylene, vinyl chloride, etc. proceed according to the radical mechanism.
2. Ionic reactions take place between ions already present or formed during the reaction.
Typical ionic reactions are interactions between electrolytes in solution. Ions are formed not only during the dissociation of electrolytes in solutions, but also under the action of electrical discharges, heating or radiation. γ-rays, for example, convert water and methane molecules into molecular ions.
According to another ionic mechanism, reactions of addition of hydrogen halides, hydrogen, halogens to alkenes, oxidation and dehydration of alcohols, replacement of alcohol hydroxyl by halogen occur; reactions characterizing the properties of aldehydes and acids. Ions in this case are formed by heterolytic breaking of covalent polar bonds.
VIII. According to the type of energy
initiating the reaction, there are:
1. Photochemical reactions. They are initiated by light energy. In addition to the above photochemical processes of HCl synthesis or the reaction of methane with chlorine, they include the production of ozone in the troposphere as a secondary atmospheric pollutant. In this case, nitric oxide (IV) acts as the primary one, which forms oxygen radicals under the action of light. These radicals interact with oxygen molecules, resulting in ozone.
The formation of ozone goes on as long as there is enough light, since NO can interact with oxygen molecules to form the same NO 2 . The accumulation of ozone and other secondary air pollutants can lead to photochemical smog.
This type of reaction also includes the most important process that occurs in plant cells - photosynthesis, the name of which speaks for itself.
2. Radiation reactions. They are initiated by high-energy radiation - x-rays, nuclear radiation (γ-rays, a-particles - He 2+, etc.). With the help of radiation reactions, very fast radiopolymerization, radiolysis (radiation decomposition), etc. are carried out.
For example, instead of a two-stage production of phenol from benzene, it can be obtained by the interaction of benzene with water under the action of radiation. In this case, radicals [OH] and [H] are formed from water molecules, with which benzene reacts to form phenol:
C 6 H 6 + 2 [OH] → C 6 H 5 OH + H 2 O
Rubber vulcanization can be carried out without sulfur using radiovulcanization, and the resulting rubber will be no worse than traditional rubber.
3. Electrochemical reactions. They are initiated by an electric current. In addition to the electrolysis reactions well known to you, we also indicate the reactions of electrosynthesis, for example, the reactions of the industrial production of inorganic oxidants
4. Thermochemical reactions. They are initiated by thermal energy. These include all endothermic reactions and many exothermic reactions that require an initial supply of heat, that is, the initiation of the process.
The above classification of chemical reactions is reflected in the diagram.
The classification of chemical reactions, like all other classifications, is conditional. Scientists agreed to divide the reactions into certain types according to the signs they identified. But most chemical transformations can be attributed to different types. For example, let's characterize the ammonia synthesis process.
This is a compound reaction, redox, exothermic, reversible, catalytic, heterogeneous (more precisely, heterogeneous catalytic), proceeding with a decrease in pressure in the system. To successfully manage the process, all of the above information must be taken into account. A specific chemical reaction is always multi-qualitative, it is characterized by different features.
CHEMICAL-TECHNOLOGICAL PROCESS AND ITS CONTENT
The chemical-technological process is a set of operations that make it possible to obtain the target product from the feedstock. All these operations are part of three main stages, characteristic of almost every chemical-technological process.
At the first stage, the operations necessary to prepare the initial reagents for a chemical reaction are carried out. The reagents are transferred, in particular, to the most reactive state. For example, it is known that the rate of chemical reactions strongly depends on temperature, so often the reagents are heated before the reaction. Gaseous raw materials are subjected to compression to a certain pressure to increase the efficiency of the process and reduce the size of the equipment. To eliminate side effects and obtain a high quality product, the feedstock is subjected to purification from impurities using methods based on the difference in physical properties (solubility in various solvents, density, condensation and crystallization temperatures, etc.). In the purification of raw materials and reaction mixtures, the phenomena of heat and mass transfer, hydromechanical processes are widely used. Chemical cleaning methods can also be used, based on chemical reactions, as a result of which unnecessary impurities are converted into easily separable substances.
Appropriately prepared reagents in the next stage are subjected to chemical interaction, which may consist of several stages. In the intervals between these stages, it is sometimes necessary to reuse heat and mass transfer and other physical processes. For example, in the production of sulfuric acid, sulfur dioxide is partially oxidized to trioxide, then the reaction mixture is cooled, sulfur trioxide is extracted from it by absorption, and it is again directed to oxidation.
As a result of chemical reactions, a mixture of products (target, by-products, by-products) and unreacted reagents is obtained. The final operations of the last stage are associated with the separation of this mixture, for which hydromechanical, heat and mass transfer processes are again used, for example: filtration, centrifugation, rectification, absorption, extraction, etc. The reaction products are sent to the finished product warehouse or for further processing; unreacted raw materials are reused in the process, organizing its recycling.
At all stages, and especially at the final ones, the recovery of secondary material and energy resources is also carried out. The flows of gaseous and liquid substances entering the environment are subjected to purification and neutralization from hazardous impurities. Solid waste is either sent for further processing or placed for storage in environmentally friendly conditions.
Thus, the chemical-technological process as a whole is a complex system consisting of single interconnected processes (elements) and interacting with the environment.
The elements of a chemical-technological system are the above processes of heat and mass transfer, hydromechanical, chemical, etc. They are considered as single processes of chemical technology.
An important subsystem of a complex chemical-technological process is a chemical process.
A chemical process is one or more chemical reactions accompanied by the phenomena of heat, mass and momentum transfer that affect both each other and the course of a chemical reaction.
Analysis of individual processes, their mutual influence allows us to develop a technological regime.
A technological regime is a set of technological parameters (temperature, pressure, concentrations of reagents, etc.) that determine the operating conditions of an apparatus or a system of apparatuses (technological scheme).
Optimal process conditions are a combination of the main parameters (temperature, pressure, composition of the initial reaction mixture, etc.), which makes it possible to obtain the highest product yield at a high speed or to ensure the lowest cost, subject to the conditions for the rational use of raw materials and energy and minimizing possible damage to the environment. environment.
Single processes take place in various apparatuses - chemical reactors, absorption and distillation columns, heat exchangers, etc. Separate apparatuses are connected in a process flow diagram.
Technological scheme is a rationally constructed system of single devices connected by various types of connections (direct, reverse, serial, parallel), which allows obtaining a given product of a given quality from natural raw materials or semi-finished products.
Technological schemes are open and closed, may contain bypass (bypass) flows and recycles, allowing to increase the efficiency of the chemical-technological system as a whole.
The development and construction of a rational technological scheme is an important task of chemical technology.
Classification of chemical reactions underlying industrial chemical-technological processes
In modern chemistry, a large number of different chemical reactions are known. Many of them are carried out in industrial chemical reactors and, therefore, become the object of study in chemical engineering.
To facilitate the study of phenomena close in nature, it is customary in science to classify them according to common features. Depending on what signs are taken as a basis, there are several types of classification of chemical reactions.
An important type of classification is the classification by mechanism for the reaction. There are simple (single-stage) and complex (multi-stage) reactions, in particular, parallel, sequential and series-parallel.
Simple reactions are called, for the implementation of which it is required to overcome only one energy barrier (one stage).
Complex reactions include several parallel or sequential steps (simple reactions).
Real one-step reactions are extremely rare. However, some complex reactions passing through a series of intermediate stages can conveniently be considered formally simple. This is possible in cases where intermediate reaction products are not detected under the conditions of the problem under consideration.
Reaction classification by molecularity takes into account how many molecules are involved in the elementary act of the reaction; distinguish between mono-, bi- and trimolecular reactions.
The form of the kinetic equation (the dependence of the reaction rate on the concentrations of the reagents) makes it possible to classify in order of reaction. The reaction order is the sum of the exponents of the concentrations of the reactants in the kinetic equation. There are reactions of the first, second, third, fractional orders.
Chemical reactions are also by thermal effect. When exothermic reactions occur, accompanied by the release of heat ( Q> 0), the enthalpy of the reaction system decreases ( ∆H < 0); при протекании эндотермических реакций, сопровождающихся поглощением теплоты (Q< 0), the enthalpy of the reaction system increases ( ∆H> 0).
For the choice of the design of a chemical reactor and methods for controlling the conduct of the process, it is essential phase composition reaction system.
Depending on how many (one or more) phases form the initial reagents and reaction products, chemical reactions are divided into homophasic and heterophasic.
Homophasic reactions are those in which the reactants, stable intermediates, and reaction products are all within the same phase.
Reactions are called heterophasic in which the starting reagents, stable intermediates, and reaction products form more than one phase.
Depending on the leak zones reactions are divided into homogeneous and heterogeneous reactions.
The concepts of "homogeneous" and "heterogeneous" reactions do not coincide with the concepts of "homophasic" and "heterophasic" processes. The homogeneity and heterogeneity of a reaction reflects, to a certain extent, its mechanism: whether the reaction proceeds in the bulk of a single phase or at the phase interface. The homophasic and heterophasic nature of the process only makes it possible to judge the phase composition of the reaction participants.
In the case of homogeneous reactions, the reactants and products are in the same phase (liquid or gaseous) and the reaction proceeds in the volume of this phase. For example, the oxidation of nitric oxide with atmospheric oxygen in the production of nitric acid is a gas-phase reaction, while esterification reactions (obtaining esters from organic acids and alcohols) are liquid-phase.
When heterogeneous reactions occur, at least one of the reactants or products is in a phase state that differs from the phase state of the other participants, and the phase interface must be taken into account when analyzing it. For example, the neutralization of an acid with an alkali is a homophasic homogeneous process. The catalytic synthesis of ammonia is a homophasic heterogeneous process. The oxidation of hydrocarbons in the liquid phase with gaseous oxygen is a heterophasic process, but the ongoing chemical reaction is homogeneous. The slaking of lime CaO + H 2 O Ca (OH) 2, in which all three participants in the reaction form separate phases, and the reaction proceeds at the interface between water and calcium oxide, is a heterophase heterogeneous process.
Depending on whether or not special substances, catalysts, are used to change the reaction rate, they are distinguished catalytic And non-catalytic reactions and, accordingly, chemical-technological processes. The vast majority of chemical reactions on which industrial chemical-technological processes are based are catalytic reactions.
