Chemical properties substances are detected in a variety of chemical reactions.
Transformations of substances accompanied by changes in their composition and (or) structure are called chemical reactions. The following definition is often found: chemical reaction is the process of converting starting substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations and diagrams containing the formulas of the starting substances and reaction products. IN chemical equations, unlike the diagrams, 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 the 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 involved in the reaction remains constant. This is achieved by placing integer stoichiometric coefficients in front of the formulas, showing the quantitative relationships between the reactants and reaction products.
Chemical equations may contain additional information about the characteristics of the reaction. If a chemical reaction occurs under the influence of external influences (temperature, pressure, radiation, etc.), this is indicated by the appropriate symbol, usually above (or “below”) the equal sign.
Huge number chemical reactions can be grouped into several types of reactions, which have very specific characteristics.
As classification characteristics the following can be selected:
1. The number and composition of starting substances and reaction products.
2. Physical state reagents and reaction products.
3. The number of phases in which the reaction participants are located.
4. Nature of transferred particles.
5. Possibility of the reaction occurring in forward and reverse directions.
6. The sign of the thermal effect divides all reactions into: exothermic reactions occurring with exo-effect - release of energy in the form of heat (Q>0, ∆H<0):
C + O 2 = CO 2 + Q
And endothermic reactions occurring with the endo effect - the absorption of energy in the form of heat (Q<0, ∆H >0):
N 2 + O 2 = 2NO - Q.
Such reactions are referred to as thermochemical.
Let's take a closer look at each type of reaction.
Classification according to the number and composition of reagents and final substances
1. Compound reactions
When a compound reacts from several reacting substances of relatively simple composition, one substance of a more complex composition is obtained:
As a rule, these reactions are accompanied by the release of heat, i.e. lead to the formation of more stable and less energy-rich compounds.
Reactions of compounds of simple substances are always redox in nature. Compound reactions occurring between complex substances can occur without a change in valency:
CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2,
and also 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, noteworthy is the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids:
| t o | ||
| 4HNO3 | = | 2H 2 O + 4NO 2 O + O 2 O. |
2AgNO3 = 2Ag + 2NO2 + O2,
(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O.
Redox decomposition reactions are especially characteristic for nitric acid salts.
Decomposition reactions in organic chemistry are called cracking:
C 18 H 38 = C 9 H 18 + C 9 H 20,
or dehydrogenation
C4H10 = C4H6 + 2H2.
3. Substitution reactions
In substitution reactions, usually a simple substance reacts with a complex one, forming another simple substance and another complex one:
A + BC = AB + C.
These reactions overwhelmingly belong to redox reactions:
2Al + Fe 2 O 3 = 2Fe + Al 2 O 3,
Zn + 2HCl = ZnСl 2 + H 2,
2KBr + Cl 2 = 2KCl + Br 2,
2КlO 3 + l 2 = 2KlO 3 + Сl 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 = CaSiO 3 + CO 2,
Ca 3 (PO 4) 2 + 3SiO 2 \u003d 3СаSiO 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 are reactions between two compounds that exchange their constituents with each other:
AB + CD = AD + CB.
If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of the atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts:
ZnO + H 2 SO 4 = ZnSO 4 + H 2 O,
AgNO 3 + KBr = AgBr + KNO 3,
CrCl 3 + ZNaON = Cr(OH) 3 + ZNaCl.
A special case of these exchange reactions is neutralization reactions:
HCl + KOH = KCl + H 2 O.
Typically, 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-dissociating (for solutions) compound:
NaHCO 3 + HCl = NaCl + H 2 O + CO 2,
Ca(HCO 3) 2 + Ca(OH) 2 = 2CaCO 3 ↓ + 2H 2 O,
CH 3 COONa + H 3 PO 4 = CH 3 COOH + NaH 2 PO 4.
5. Transfer reactions.
In transfer reactions, an atom or group of atoms moves from one structural unit to another:
AB + BC = A + B 2 C,
A 2 B + 2CB 2 = DIA 2 + DIA 3.
For example:
2AgCl + SnCl 2 = 2Ag + SnCl 4,
H 2 O + 2NO 2 = HNO 2 + HNO 3.
Classification of reactions according to phase characteristics
Depending on the state of aggregation of the reacting substances, the following reactions are distinguished:
1. Gas reactions
| H2+Cl2 | 2HCl. |
2. Reactions in solutions
NaOH(solution) + HCl(p-p) = NaCl(p-p) + H 2 O(l)
3. Reactions between solids
| t o | ||
| CaO(tv) + SiO 2 (tv) | = | CaSiO 3 (sol) |
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 entire 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 reaction products 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) = CaCO 3 (tv).
liquid-solid-phase reactions
Na 2 SO 4 (solution) + BaCl 3 (solution) = BaSO 4 (tv) ↓ + 2NaCl (p-p).
liquid-gas-solid-phase reactions
Ca(HCO 3) 2 (solution) + H 2 SO 4 (solution) = CO 2 (r) + H 2 O (l) + CaSO 4 (sol)↓.
Classification of reactions according to the type of particles transferred
1. Protolytic reactions.
TO protolytic reactions include chemical processes, the essence of which is the transfer of a proton from one reacting substance 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 reacting substances exchange electrons, thereby changing the oxidation states of the atoms of the elements that make up the reacting substances. 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 reactions; they play an extremely important role.
3. Ligand exchange reactions.
These include reactions during which the transfer of an electron pair occurs with the formation of a covalent bond via a 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 complexes, occurs without changing the oxidation state.
4. Reactions of atomic-molecular exchange.
This type of reaction includes many of the substitution reactions studied in organic chemistry that occur via a radical, electrophilic or nucleophilic mechanism.
Reversible and irreversible chemical reactions
Reversible chemical processes are those whose products are capable of reacting with each other under the same conditions in which they were obtained to form the 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 COOC 2 H 5 + H 2 O.
Irreversible chemical processes are those whose products are not able to react with each other to form the starting substances. Examples of irreversible reactions include the decomposition of Berthollet salt when heated:
2КlО 3 → 2Кl + ЗО 2,
or oxidation of glucose by atmospheric oxygen:
C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O.
Classification of inorganic substances with examples of compounds
Now let's analyze the classification scheme presented above in more detail.
As we see, first of all, all inorganic substances are divided into simple And complex:
Simple substances These are substances that are formed by atoms of only one chemical element. For example, simple substances are hydrogen H2, oxygen O2, iron Fe, carbon C, etc.
Among simple substances there are metals, nonmetals And noble gases:
Metals formed by chemical elements located below the boron-astatine diagonal, as well as all elements located in side groups.
Noble gases formed by chemical elements of group VIIIA.
Nonmetals are formed respectively by chemical elements located above the boron-astatine diagonal, with the exception of all elements of side subgroups and noble gases located in group VIIIA:
The names of simple substances most often coincide with the names of the chemical elements whose atoms they are formed from. However, for many chemical elements the phenomenon of allotropy is widespread. Allotropy is the phenomenon when one chemical element is capable of forming several simple substances. For example, in the case of the chemical element oxygen, the existence of molecular compounds with the formulas O 2 and O 3 is possible. The first substance is usually called oxygen in the same way as the chemical element whose atoms it is formed, and the second substance (O 3) is usually called ozone. The simple substance carbon can mean any of its allotropic modifications, for example, diamond, graphite or fullerenes. The simple substance phosphorus can be understood as its allotropic modifications, such as white phosphorus, red phosphorus, black phosphorus.
Complex substances
Complex substances are substances formed by atoms of two or more chemical elements.
For example, complex substances are ammonia NH 3, sulfuric acid H 2 SO 4, slaked lime Ca (OH) 2 and countless others.
Among complex inorganic substances, there are 5 main classes, namely oxides, bases, amphoteric hydroxides, acids and salts:
Oxides - complex substances formed by two chemical elements, one of which is oxygen in the oxidation state -2.
The general formula of oxides can be written as E x O y, where E is the symbol of a chemical element.
Nomenclature of oxides
The name of the oxide of a chemical element is based on the principle:
For example:
Fe 2 O 3 - iron (III) oxide; CuO—copper(II) oxide; N 2 O 5 - nitric oxide (V)
You can often find information that the valency of an element is indicated in parentheses, but this is not the case. So, for example, the oxidation state of nitrogen N 2 O 5 is +5, and the valency, oddly enough, is four.
If a chemical element has a single positive oxidation state in compounds, then the oxidation state is not indicated. For example:
Na 2 O - sodium oxide; H 2 O - hydrogen oxide; ZnO - zinc oxide.
Oxides classification
Oxides, according to their ability to form salts when interacting with acids or bases, are divided accordingly into salt-forming And non-salt-forming.
There are few non-salt-forming oxides; they are all formed by nonmetals in the oxidation state +1 and +2. The list of non-salt-forming oxides should be remembered: CO, SiO, N 2 O, NO.
Salt-forming oxides, in turn, are divided into basic, acidic And amphoteric.
Basic oxides These are oxides that, when reacting with acids (or acid oxides), form salts. Basic oxides include metal oxides in the oxidation state +1 and +2, with the exception of the oxides BeO, ZnO, SnO, PbO.
Acidic oxides These are oxides that, when reacting with bases (or basic oxides), form salts. Acidic oxides are almost all oxides of non-metals with the exception of non-salt-forming CO, NO, N 2 O, SiO, as well as all metal oxides in high oxidation states (+5, +6 and +7).
Amphoteric oxides are called oxides that can react with both acids and bases, and as a result of these reactions form salts. Such oxides exhibit a dual acid-base nature, that is, they can exhibit the properties of both acidic and basic oxides. Amphoteric oxides include metal oxides in the oxidation states +3, +4, as well as the oxides BeO, ZnO, SnO, and PbO as exceptions.
Some metals can form all three types of salt-forming oxides. For example, chromium forms the basic oxide CrO, the amphoteric oxide Cr 2 O 3 and the acidic oxide CrO 3.
As you can see, the acid-base properties of metal oxides directly depend on the degree of oxidation of the metal in the oxide: the higher the degree of oxidation, the more pronounced the acidic properties.
Reasons
Reasons - compounds with the formula Me(OH) x, where x most often equal to 1 or 2.
Exceptions: Be(OH) 2, Zn(OH) 2, Sn(OH) 2 and Pb(OH) 2 are not bases, despite the oxidation state of the metal +2. These compounds are amphoteric hydroxides, which will be discussed in more detail in this chapter.
Classification of bases
Bases are classified according to the number of hydroxyl groups in one structural unit.
Bases with one hydroxo group, i.e. type MeOH is called monoacid bases, with two hydroxo groups, i.e. type Me(OH) 2, respectively, diacid etc.
Bases are also divided into soluble (alkalis) and insoluble.
Alkalies include exclusively hydroxides of alkali and alkaline earth metals, as well as thallium hydroxide TlOH.
Nomenclature of bases
The name of the foundation is based on the following principle:
For example:
Fe(OH) 2 - iron (II) hydroxide,
Cu(OH) 2 - copper (II) hydroxide.
In cases where the metal in complex substances has a constant oxidation state, it is not required to indicate it. For example:
NaOH - sodium hydroxide,
Ca(OH) 2 - calcium hydroxide, etc.
Acids
Acids - complex substances whose molecules contain hydrogen atoms that can be replaced by a metal.
The general formula of acids can be written as H x A, where H are hydrogen atoms that can be replaced by a metal, and A is the acidic residue.
For example, acids include compounds such as H2SO4, HCl, HNO3, HNO2, etc.
Classification of acids
According to the number of hydrogen atoms that can be replaced by a metal, acids are divided into:
- O base acids: HF, HCl, HBr, HI, HNO 3 ;
- d basic acids: H 2 SO 4, H 2 SO 3, H 2 CO 3;
- T rehobasic acids: H 3 PO 4 , H 3 BO 3 .
It should be noted that the number of hydrogen atoms in the case of organic acids most often does not reflect their basicity. For example, acetic acid with the formula CH 3 COOH, despite the presence of 4 hydrogen atoms in the molecule, is not tetra- but monobasic. The basicity of organic acids is determined by the number of carboxyl groups (-COOH) in the molecule.
Also, based on the presence of oxygen in the molecules, acids are divided into oxygen-free (HF, HCl, HBr, etc.) and oxygen-containing (H 2 SO 4, HNO 3, H 3 PO 4, etc.). Oxygen-containing acids are also called oxoacids.
You can read more about the classification of acids.
Nomenclature of acids and acid residues
The following list of names and formulas of acids and acid residues is a must-learn.
In some cases, a number of the following rules can make memorization easier.
As can be seen from the table above, the construction of systematic names of oxygen-free acids is as follows:
For example:
HF—hydrofluoric acid;
HCl—hydrochloric acid;
H 2 S is hydrosulfide acid.
The names of acidic residues of oxygen-free acids are based on the principle:
For example, Cl - - chloride, Br - - bromide.
The names of oxygen-containing acids are obtained by adding various suffixes and endings to the name of the acid-forming element. For example, if the acid-forming element in an oxygen-containing acid has the highest oxidation state, then the name of such an acid is constructed as follows:
For example, sulfuric acid H 2 S +6 O 4, chromic acid H 2 Cr +6 O 4.
All oxygen-containing acids can also be classified as acid hydroxides because they contain hydroxyl groups (OH). For example, this can be seen from the following graphical formulas of some oxygen-containing acids:
Thus, sulfuric acid can otherwise be called sulfur (VI) hydroxide, nitric acid - nitrogen (V) hydroxide, phosphoric acid - phosphorus (V) hydroxide, etc. In this case, the number in brackets characterizes the degree of oxidation of the acid-forming element. This version of the names of oxygen-containing acids may seem extremely unusual to many, but occasionally such names can be found in real KIMs of the Unified State Examination in Chemistry in tasks on the classification of inorganic substances.
Amphoteric hydroxides
Amphoteric hydroxides - metal hydroxides exhibiting a dual nature, i.e. capable of exhibiting both the properties of acids and the properties of bases.
Metal hydroxides in oxidation states +3 and +4 are amphoteric (as are oxides).
Also, as exceptions, amphoteric hydroxides include the compounds Be(OH) 2, Zn(OH) 2, Sn(OH) 2 and Pb(OH) 2, despite the oxidation state of the metal in them +2.
For amphoteric hydroxides of tri- and tetravalent metals, the existence of ortho- and meta-forms is possible, differing from each other by one water molecule. For example, aluminum(III) hydroxide can exist in the ortho form Al(OH)3 or the meta form AlO(OH) (metahydroxide).
Since, as already mentioned, amphoteric hydroxides exhibit both the properties of acids and the properties of bases, their formula and name can also be written differently: either as a base or as an acid. For example:
Salts
Salts - these are complex substances that contain metal cations and anions of acid residues.
For example, salts include compounds such as KCl, Ca(NO 3) 2, NaHCO 3, etc.
The definition presented above describes the composition of most salts, however, there are salts that do not fall under it. For example, instead of metal cations, the salt may contain ammonium cations or its organic derivatives. Those. salts include compounds such as, for example, (NH 4) 2 SO 4 (ammonium sulfate), + Cl - (methyl ammonium chloride), etc.
Also contradicting the definition of salts above is the class of so-called complex salts, which will be discussed at the end of this topic.
Classification of salts
On the other hand, salts can be considered as products of the replacement of hydrogen cations H + in an acid with other cations, or as products of the replacement of hydroxide ions in bases (or amphoteric hydroxides) with other anions.
With complete replacement, the so-called average or normal salt. For example, with complete replacement of hydrogen cations in sulfuric acid with sodium cations, an average (normal) salt Na 2 SO 4 is formed, and with complete replacement of hydroxide ions in the base Ca (OH) 2 with acidic residues of nitrate ions, an average (normal) salt is formed Ca(NO3)2.
Salts obtained by incomplete replacement of hydrogen cations in a dibasic (or more) acid with metal cations are called acidic. Thus, when hydrogen cations in sulfuric acid are incompletely replaced by sodium cations, the acid salt NaHSO 4 is formed.
Salts that are formed by incomplete replacement of hydroxide ions in two-acid (or more) bases are called bases. O strong salts. For example, with incomplete replacement of hydroxide ions in the base Ca(OH) 2 with nitrate ions, a base is formed O clear salt Ca(OH)NO3.
Salts consisting of cations of two different metals and anions of acidic residues of only one acid are called double salts. So, for example, double salts are KNaCO 3, KMgCl 3, etc.
If a salt is formed by one type of cations and two types of acid residues, such salts are called mixed. For example, mixed salts are the compounds Ca(OCl)Cl, CuBrCl, etc.
There are salts that do not fall under the definition of salts as products of the replacement of hydrogen cations in acids with metal cations or products of the replacement of hydroxide ions in bases with anions of acidic residues. These are complex salts. For example, complex salts are sodium tetrahydroxozincate and tetrahydroxoaluminate with the formulas Na 2 and Na, respectively. Complex salts can most often be recognized among others by the presence of square brackets in the formula. However, you need to understand that in order for a substance to be classified as a salt, it must contain some cations other than (or instead of) H +, and the anions must contain some anions other than (or instead of) OH -. For example, the compound H2 does not belong to the class of complex salts, since when it dissociates from cations, only hydrogen cations H + are present in the solution. Based on the type of dissociation, this substance should rather be classified as an oxygen-free complex acid. Likewise, the OH compound does not belong to salts, because this compound consists of cations + and hydroxide ions OH -, i.e. it should be considered a comprehensive foundation.
Nomenclature of salts
Nomenclature of medium and acid salts
The name of medium and acid salts is based on the principle:
If the oxidation state of a metal in complex substances is constant, then it is not indicated.
The names of acid residues were given above when considering the nomenclature of acids.
For example,
Na 2 SO 4 - sodium sulfate;
NaHSO 4 - sodium hydrogen sulfate;
CaCO 3 - calcium carbonate;
Ca(HCO 3) 2 - calcium bicarbonate, etc.
Nomenclature of basic salts
The names of the main salts are based on the principle:
For example:
(CuOH) 2 CO 3 - copper (II) hydroxycarbonate;
Fe(OH) 2 NO 3 - iron (III) dihydroxonitrate.
Nomenclature of complex salts
The nomenclature of complex compounds is much more complicated, and to pass the Unified State Exam you do not need to know much about the nomenclature of complex salts.
You should be able to name complex salts obtained by reacting alkali solutions with amphoteric hydroxides. For example:
*The same colors in the formula and name indicate the corresponding elements of the formula and name.
Trivial names of inorganic substances
By trivial names we mean the names of substances that are not related, or weakly related, to their composition and structure. Trivial names are determined, as a rule, either by historical reasons or by the physical or chemical properties of these compounds.
List of trivial names of inorganic substances that you need to know:
| Na 3 | cryolite |
| SiO2 | quartz, silica |
| FeS 2 | pyrite, iron pyrite |
| CaSO 4 ∙2H 2 O | gypsum |
| CaC2 | calcium carbide |
| Al 4 C 3 | aluminum carbide |
| KOH | caustic potassium |
| NaOH | caustic soda, caustic soda |
| H2O2 | hydrogen peroxide |
| CuSO 4 ∙5H 2 O | copper sulfate |
| NH4Cl | ammonia |
| CaCO3 | chalk, marble, limestone |
| N2O | laughing gas |
| NO 2 | brown gas |
| NaHCO3 | baking (drinking) soda |
| Fe3O4 | iron scale |
| NH 3 ∙H 2 O (NH 4 OH) | ammonia |
| CO | carbon monoxide |
| CO2 | carbon dioxide |
| SiC | carborundum (silicon carbide) |
| PH 3 | phosphine |
| NH 3 | ammonia |
| KClO3 | Bertholet's salt (potassium chlorate) |
| (CuOH)2CO3 | malachite |
| CaO | quicklime |
| Ca(OH)2 | slaked lime |
| transparent aqueous solution of Ca(OH) 2 | lime water |
| suspension of solid Ca(OH) 2 in its aqueous solution | lime milk |
| K2CO3 | potash |
| Na 2 CO 3 | soda ash |
| Na 2 CO 3 ∙10H 2 O | crystal soda |
| MgO | magnesia |
And classification of steels
- quality;
- chemical composition;
- purpose;
- microstructure;
- strength.
Steel quality
By chemical composition
Carbon steels permanent impurities
Table 1.3.
CARBON STEEL
Alloying elements additives or additives
Alloy steels low alloy(up to 2.5 wt.%), alloyed(from 2.5 to 10 wt.%) and highly alloyed "chrome"
By purpose steel
Structural low-( or few-) And medium carbon.
Instrumentalhigh carbon.
And (with special properties - ).
And
And increased heat resistance high-speed steels
Ordinary quality
Structural steels,
Tool steels,
6) bearing (ball bearing) steel,
7) high-speed steels(high-alloy, high-quality tool steels with high tungsten content).
8) automatic, i.e.increased (or high) machinability, steel.
An analysis of the composition of historically developed steel marking groups shows that the marking systems used make it possible to encode five classification characteristics, namely: quality, chemical composition, purpose, degree of deoxidation, and also method of obtaining blanks(automatic or, in rare cases, foundry). The relationship between marking groups and steel classes is illustrated by the lower part of the block diagram in Fig. 1.
SYSTEM OF MARKING GROUPS, MARKING RULES AND EXAMPLES OF STEEL GRADES
| CARBON | ORDINARY QUALITY | |||||||
| Steel group | Delivery guarantee | BRANDS | ||||||
| A | by chemical composition | St0 | St1 | St2 | StZ | St4 | St5 | St6 |
| B | by mechanical properties | BSt0 | BSt1 | BSt2 | BStZ | BSt4 | BSt5 | BSt6 |
| IN | by mechanical properties and chemical composition | VSTO | 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 BRANDS | ||||||
| Brand: two-digit number of HUNDREDTHS of a percent carbon + 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 mark indicates that the steel is high quality | |||||||
| INSTRUMENTAL | BRANDS | |||||||
| Brand: symbol “U” + number TENTHS OF PERCENT Carbon | U7 U7A U8 UVA U9 U9A U10 U10A U12 U12A | |||||||
| DOPED | QUALITY HIGH QUALITY EXTRA HIGH QUALITY | STRUCTURAL | EXAMPLES OF BRANDS | |||||
| Brand: two-digit number of HUNDREDTHS of a percent carbon + alloying element symbol + an integer number of its percent | 09G2 10KHSND 18G2AFps 20Kh 40G 45KhN 65S2VA 110G13L Notes: 1) the number “1” is not included as an indicator of concentration ≤ 1 wt.% of the alloying element; 2) grade 110G13L is one of the few in which the number of hundredths of a percent of carbon is three-digit | |||||||
| INSTRUMENTAL | EXAMPLES OF BRANDS | |||||||
| Brand: number of TENTHS of a percent carbon + alloying element symbol+ an integer number of its percent | ЗХ2Н2МФ 4ХВ2С 5ХНМ 7X3 9ХВГ X ХВ4 9Х4МЗФ2AGСТ-Ш Notes: 1) the number “10” is not used as an indicator of “ten tenths” of mass% carbon; 2) “-Ш” at the end of the mark indicates 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 key (system-forming) in the marking group under consideration. The steel grades of this group are immediately recognizable by this symbol.
The symbol "St" without a space is followed by a number indicating number brands – from «0» to "6".
An increase in the grade number corresponds to an increase in the carbon content in steel, but does not indicate its specific value. The permissible limits of carbon concentration in steels of each grade are shown in table. 1.5. Carbon content in carbon steels of ordinary quality does not exceed 0.5 wt.%. Such steels are hypoeutectoid according to the structural criterion, and, therefore, structural in purpose.
The number is followed by one of three letter combinations: “kp”, “ps”, “sp” - indicating the degree of deoxidation of the steel.
The "St" symbol may be preceded by a capital letter "A", "B" or "C", or may not have any symbols. In this way, information is transmitted about whether the steel belongs to one of the so-called “delivery groups”: A, B or IN, – depending on which of the standardized steel indicators is guaranteed by the supplier.
Steel group A comes with a guarantee of the chemical composition, or the permissible values of carbon concentration and impurities specified by GOST. The letter “A” is often not included in stamps and its absence default means a guarantee of the chemical composition. The consumer of steel, without information about the mechanical properties, can form them through 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 based on known characteristics of mechanical properties without preliminary 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 a 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.
Examples of recording stamps ordinary quality carbon steel look like this: VSt3ps, BSt6sp, St1kp .
Ball bearing steels
Steels for bearings have their own markings and constitute a special group according to their intended purpose. structural steels, although in composition and properties they are close to tool steels. The term “ball bearing” defines their narrow field of application – rolling bearings (not only ball bearings, but also roller and needle bearings). To mark it, the abbreviation “SHH” was proposed - ball bearing chromium, – followed by a number tenths of a percent average concentration chromium. Of the previously widely known brands ShKh6, ShKh9 and ShKh15, the ShKh15 brand remains in use. The difference between ball bearing steel and similar tool steel is in more stringent requirements for the number of non-metallic inclusions and the uniform distribution of carbides in the microstructure.
The improvement of ShKh15 steel by introducing additional alloying additives (silicon and manganese) was uniquely reflected in the marking - spreading to specific system of later rules for designating alloying elements in alloy steels: ShKh15SG, ShKh20SG.
High speed steels
High-speed steels are specifically marked with the initial letter of the Russian alphabet “P”, corresponding to the first sound in the English word rapid – fast, fast. This is followed by an integer percentage of tungsten. As already mentioned, the previously most common grade of high-speed steel was 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 two marking systems. The development and development of new high-speed steels with cobalt and vanadium has 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 both in a specific system (R0M5F1, R0M2F3) and in a completely new way – 9Kh6M3F3AGST-Sh, 9Kh4M3F2AGST-Sh.
Classification of cast irons
Cast irons are alloys of iron and carbon containing more than 2.14 wt.% C.
Cast irons are smelted for processing into steel (conversion), to produce ferroalloys, which act as alloying additives, and also as high-tech alloys for producing castings (foundry).
Carbon can be present in cast iron in the form of two high-carbon phases - cementite (Fe 3 C) and graphite, and sometimes simultaneously 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 cast iron fractures a gray color. However, not all cast iron with graphite belongs to the class of so-called gray cast iron Between white and gray cast iron there is a class half-hearted cast iron
Half-hearted Cast irons are cast irons, in the structure of which, despite graphitization, ledeburite cementite is at least partially preserved, and, therefore, ledeburite itself is present - a eutectic structural component that has a specific form.
TO gray include cast irons in which ledeburite cementite has completely disintegrated, and the latter is no longer present in the structure. Gray cast iron consists of graphite inclusions And metal base. This metal base is pearlitic (eutectoid), ferritic-pearlitic (hypoeutectoid) or ferritic (low carbon) steel. The indicated sequence of types of metal base of gray cast iron corresponds to an increasing degree of decomposition of cementite, which is part of perlite.
Anti-friction cast irons
Examples of brands: ASF-1, ASF-2, ASF-3.
Special alloy heat resistant, corrosion resistant And heat resistant cast irons:
EXAMPLES OF GRADES OF SPECIAL GRAY CAST IRONS
Classification and labeling
metal-ceramic hard alloys
Metal-ceramic hard alloys are alloys made by powder metallurgy (metal-ceramics) and consisting of carbides of refractory metals: WC, TiC, TaC, joined by a plastic metal binder, most often cobalt.
Currently, hard alloys of three groups are produced in Russia: tungsten, titanium-tungsten and titanium-tungsten, – containing as a connective 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 a plastic ligament is performed by nickel-molybdenum matrix. The classification of hard alloys is presented in a block diagram.
In accordance with the five classes of metal-ceramic 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 grades of alloy steels are given in Table 1.1.
Table 1.1.
Foreign analogues of a number of domestic grades of alloy steels
| Russia, GOST | Germany, DIN * | USA, ASTM* | Japan, LS * |
| 15X | 15Cr3 | SCr415 | |
| 40X | 41Сг4 | SСг440 | |
| 30ХМ | 25CrMo4 | SСМ430,SСМ2 | |
| 12ХГ3А | 14NiCr10** | – | SNC815 |
| 20ХГНМ | 21NiCrMo2 | SNСМ220 | |
| 08X13 | Х7Сr1З** | 410S | SUS410S |
| 20X13 | Х20Сг13 | SUS420J1 | |
| 12X17 | Х8Сг17 | 430 (51430 ***) | SUS430 |
| 12Х18Н9 | Х12СгNi8 9 | SUS302 | |
| 08Х18Н10Т | Х10CrNiTi18 9 | .321 | SUS321 |
| 10Х13СУ | Х7CrA133** | 405 ** (51405) *** | SUS405** |
| 20Х25Н20С2 | Х15CrNiSi25 20 | 30314,314 | SСS18, SUH310** |
* DIN (Deutsche Industrienorm), ASTM (American Society for Testing Materials), JIS (Japanese industrial Standard).
** Steel, similar in composition; *** SAE standard
Characteristics of classification features
And classification of steels
Modern classification characteristics of steels include the following:
- quality;
- chemical composition;
- purpose;
- metallurgical features of production;
- microstructure;
- traditional method of strengthening;
- traditional method of obtaining blanks or parts;
- strength.
Let us briefly describe 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 conventionally divided into carbon (unalloyed) steels and alloyed ones.
Carbon steels do not contain specially introduced alloying elements. The elements contained in carbon steels, other than carbon, are among the so-called permanent impurities. Their concentration must be within the limits determined by the relevant state standards (GOSTs). In table 1.3. averaged limiting concentration values of some elements are given, which make it possible to classify these elements as impurities rather than alloying elements. Specific limits for the content of impurities in carbon steels are given by GOST standards.
Table 1.3.
LIMITING CONCENTRATIONS OF SOME ELEMENTS THAT ALLOW THEM TO BE CONSIDERED AS 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 divided according to the total concentration of alloying elements, except carbon, into low alloy(up to 2.5 wt.%), alloyed(from 2.5 to 10 wt.%) and highly alloyed(more than 10 wt.%) with an iron content in the latter of at least 45 wt.%. Usually the introduced alloying element gives the alloy steel its corresponding name: "chrome"– alloyed with chromium, “silicon” – with silicon, “chrome-silicon” – with chromium and silicon at the same time, etc.
In addition, iron-based alloys are also distinguished when the composition of the material contains less than 45% iron, but more than any other alloying element.
By purpose steel divided 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, and also, if required, a set of special properties (corrosion resistance, paramagnetism, etc.). Typically, structural steels are low-( or few-) And medium carbon. Hardness is not a decisive mechanical characteristic for them.
Instrumental are called steels used for processing materials by cutting or pressing, as well as for the manufacture of measuring instruments. They must have high hardness, wear resistance, strength and a number of other specific properties, for example, heat resistance. A necessary condition for obtaining high hardness is an increased carbon content, therefore tool steels, with rare exceptions, are always high carbon.
Within each group there is a more detailed division according to purpose. Structural steels are divided into construction, engineering And steel for special applications(with special properties - heat resistant, heat resistant, corrosion resistant, non-magnetic).
Tool steels are divided into steels for cutting tools, die steels And steel for measuring instruments.
A common performance property of tool steels is high hardness, which ensures the tool’s resistance to deformation and abrasion of its surface. At the same time, steels for cutting tools are subject to a specific requirement - to maintain high hardness at elevated temperatures (up to 500...600ºС), which develop in the cutting edge at high cutting speeds. The specified ability of steel is called its heat resistance (or red resistance). 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 technology as high-speed steels
Die steels, in addition to high hardness, require high toughness, since the die tool operates under shock loading conditions. In addition, the tool for hot stamping, in contact with heated metal workpieces, can heat up during prolonged operation. Therefore, steels for hot stamping must also be heat-resistant.
Steels for measuring tools, in addition to high wear resistance, ensuring dimensional accuracy over a long service life, must guarantee dimensional stability of the tools regardless of operating temperature conditions. In other words, they must have a very small coefficient of thermal expansion.
Chemical reactions must 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 a different matter - 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 = 24 12 Mg + 4 2 He
The classification of chemical reactions is multifaceted, that is, it can be based on various characteristics. But any of these characteristics can include reactions between both inorganic and organic substances.
Let's consider the classification of chemical reactions according to various criteria.
I. According to the number and composition of reacting substances
Reactions that occur 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 (orhombic) ↔ S (monoclinic)
P (white) ↔ P (red)
Sn (white tin) ↔ Sn (gray tin)
3O 2 (oxygen) ↔ 2O 3 (ozone)
In organic chemistry, this type of reaction can include isomerization reactions, which occur without changing not only the qualitative, but also the quantitative composition of the molecules of substances, for example:
1. Isomerization of alkanes.
The isomerization reaction of alkanes is of great practical importance, since hydrocarbons of 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= - CH ↔ CH 3 - C= - C- CH 3
ethyl acetylene dimethyl acetylene
4. Isomerization of haloalkanes (A. E. Favorsky, 1907).
5. Isomerization of ammonium cyanite when heated.
Urea was first synthesized by F. Wöhler in 1828 by isomerizing ammonium cyanate when heated.
Reactions that occur with a change in the composition of a substance
Four types of such reactions can be distinguished: combination, decomposition, substitution and exchange.
1. Compound reactions are 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 the production of sulfuric acid from sulfur:
1. Preparation of sulfur oxide (IV):
S + O 2 = SO - from two simple substances one complex substance is formed.
2. Preparation of sulfur oxide (VI):
SO 2 + 0 2 → 2SO 3 - one complex substance is formed from simple and complex substances.
3. Preparation of sulfuric acid:
SO 3 + H 2 O = H 2 SO 4 - one complex substance is formed from two complex substances.
An example of a compound reaction in which one complex substance is formed from more than two initial substances is the final stage of producing nitric acid:
4NO2 + O2 + 2H2O = 4HNO3
In organic chemistry, compound reactions are commonly called “addition reactions.” The whole variety of such reactions can be considered using the example of a block of reactions characterizing the properties of unsaturated substances, for example ethylene:
1. Hydrogenation reaction - addition of hydrogen:
CH 2 =CH 2 + H 2 → H 3 -CH 3
ethene → ethane
2. Hydration reaction - addition of water.
3. Polymerization reaction.
2. Decomposition reactions are 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 producing 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 substance are formed, that is, three new substances.
In organic chemistry, decomposition reactions can be considered in the block of reactions for the production of ethylene in the laboratory and in industry:
1. Reaction of dehydration (elimination of water) of ethanol:
C 2 H 5 OH → CH 2 =CH 2 + H 2 O
2. Dehydrogenation reaction (elimination of hydrogen) of ethane:
CH 3 -CH 3 → CH 2 =CH 2 + H 2
or CH 3 -CH 3 → 2C + ZN 2
3. Propane cracking (splitting) reaction:
CH 3 -CH 2 -CH 3 → CH 2 =CH 2 + CH 4
3. Substitution reactions are reactions in which atoms of a simple substance replace atoms of some element in a complex substance.
In inorganic chemistry, an example of such processes is a block of reactions characterizing the properties, for example, of metals:
1. Interaction of alkali or alkaline earth metals with water:
2Na + 2H 2 O = 2NaOH + H 2
2. Interaction of metals with acids in solution:
Zn + 2HCl = ZnСl 2 + H 2
3. Interaction of metals with salts in solution:
Fe + CuSO 4 = FeSO 4 + Cu
4. Metallothermy:
2Al + Cr 2 O 3 → Al 2 O 3 + 2Сr
The subject of the study of organic chemistry is not simple substances, but only compounds. Therefore, as an example of a substitution reaction, we present 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 a 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 reactions in which two complex substances exchange their components
These reactions characterize the properties of electrolytes and in solutions proceed according to Berthollet’s rule, that is, only if the result is the formation of a precipitate, gas or slightly dissociating substance (for example, H 2 O).
In inorganic chemistry, this can be a block of reactions that characterize, for example, the properties of alkalis:
1. Neutralization reaction that occurs with the formation of salt and water.
2. The reaction between alkali and salt, which occurs with the formation of gas.
3. The reaction between alkali and salt, resulting in the formation of a precipitate:
CuSO 4 + 2KOH = Cu(OH) 2 + K 2 SO 4
or in ionic form:
Cu 2+ + 2OH - = Cu(OH) 2
In organic chemistry, we can consider a block of reactions that characterize, for example, the properties of acetic acid:
1. The reaction that occurs with the formation of a weak electrolyte - H 2 O:
CH 3 COOH + NaOH → Na(CH3COO) + H 2 O
2. Reaction that occurs with the formation of gas:
2CH 3 COOH + CaCO 3 → 2CH 3 COO + Ca 2+ + CO 2 + H 2 O
3. The reaction that occurs 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
Based on this feature, 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 is involved, for example:
1. Mg 0 + H + 2 SO 4 = Mg +2 SO 4 + H 2
2. 2Mg 0 + O 0 2 = Mg +2 O -2
Complex redox reactions are composed using the electron balance method.
2KMn +7 O 4 + 16HCl - = 2KCl - + 2Mn +2 Cl - 2 + 5Cl 0 2 + 8H 2 O
In organic chemistry, a striking example of redox reactions is the properties of aldehydes.
1. They are reduced to the corresponding alcohols:
Aldekydes are oxidized to the corresponding acids:
2. Reactions that occur 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 = HCOOCH 3 + H 2 O
III. By thermal effect
Based on the thermal effect, reactions are divided into exothermic and endothermic.
1. Exothermic reactions occur with the release of energy.
These include almost all compound reactions. A rare exception is the endothermic reaction of the synthesis of nitric oxide (II) from nitrogen and oxygen and the reaction of hydrogen gas with solid iodine.
Exothermic reactions that occur with the release of light are classified as combustion reactions. The hydrogenation of ethylene is an example of an exothermic reaction. It runs at room temperature.
2. Endothermic reactions occur with the absorption of energy.
Obviously, these will include almost all decomposition reactions, for example:
1. Limestone firing
2. Butane cracking
The amount of energy released or absorbed as a result of a 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) = 2HC 1(g) + 92.3 kJ
N 2 (g) + O 2 (g) = 2NO (g) - 90.4 kJ
IV. According to the state of aggregation of the reacting substances (phase composition)
According to the state of aggregation of the reacting substances, they are distinguished:
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 the same phase).
V. By catalyst participation
Based on the participation of the catalyst, they are distinguished:
1. Non-catalytic reactions occurring without the participation of a catalyst.
2. Catalytic reactions occurring with the participation of a catalyst. Since all biochemical reactions occurring in the cells of living organisms occur with the participation of special biological catalysts of a 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. By direction
According to the direction they are distinguished:
1. Irreversible reactions occur under given conditions in only one direction. These include all exchange reactions accompanied by the formation of a precipitate, gas or slightly dissociating substance (water) and all combustion reactions.
2. Reversible reactions under these conditions occur simultaneously in two opposite directions. The overwhelming majority of such reactions are.
In organic chemistry, the sign of reversibility is reflected by the names - antonyms of the processes:
Hydrogenation - dehydrogenation,
Hydration - dehydration,
Polymerization - depolymerization.
All reactions of esterification (the opposite process, as you know, is called hydrolysis) and hydrolysis of proteins, esters, carbohydrates, and polynucleotides are reversible. The reversibility of these processes underlies the most important property of a living organism - metabolism.
VII. According to the mechanism of flow they are distinguished:
1. Radical reactions occur 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 a substance is formed by a covalent bond, then there can be two ways to break this bond: hemolytic and heterolytic. For example, for molecules Cl 2, CH 4, etc., hemolytic cleavage of bonds occurs; 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 shared approximately equally between the atoms (non-polar covalent bond), but many polar bonds can also be broken in a similar way, particularly 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 very reactive because 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 causes the shared electron pair bonding the hydrogen atoms to break and forms a covalent bond with one of the hydrogen atoms. The second hydrogen atom, having become a radical, forms a common electron pair with the unpaired electron of the chlorine atom from the collapsing Cl 2 molecule, resulting in the formation of a chlorine radical that attacks a new hydrogen molecule, etc.
Reactions that represent 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 combustion reactions of organic and inorganic substances, the synthesis of water, ammonia, polymerization of ethylene, vinyl chloride, etc., proceed by the radical mechanism.
2. Ionic reactions occur between ions that are 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 with halogen occur; reactions characterizing the properties of aldehydes and acids. In this case, ions are formed by the heterolytic cleavage of polar covalent bonds.
VIII. According to the type of energy
initiating the reaction are distinguished:
1. Photochemical reactions. They are initiated by light energy. In addition to the photochemical processes of HCl synthesis or the reaction of methane with chlorine discussed above, these include the production of ozone in the troposphere as a secondary atmospheric pollutant. The primary role in this case is nitric oxide (IV), which under the influence of light forms oxygen radicals. These radicals interact with oxygen molecules, resulting in ozone.
Ozone formation occurs 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 occurring 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 rapid radiopolymerization, radiolysis (radiation decomposition), etc. are carried out.
For example, instead of the two-stage production of phenol from benzene, it can be obtained by reacting benzene with water under the influence 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
Vulcanization of rubber 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 well-known electrolysis reactions, we will also indicate electrosynthesis reactions, for example, reactions for the industrial production of inorganic oxidizers
4. Thermochemical reactions. They are initiated by thermal energy. These include all endothermic reactions and many exothermic reactions, the initiation of which requires an initial supply of heat, that is, initiation of the process.
The classification of chemical reactions discussed above is reflected in the diagram.
The classification of chemical reactions, like all other classifications, is conditional. Scientists agreed to divide reactions into certain types according to the characteristics they identified. But most chemical transformations can be classified into different types. For example, let's characterize the process of ammonia synthesis.
This is a compound reaction, redox, exothermic, reversible, catalytic, heterogeneous (more precisely, heterogeneous-catalytic), occurring with a decrease in pressure in the system. To successfully manage the process, it is necessary to take into account all the information provided. A specific chemical reaction is always multi-qualitative and is characterized by different characteristics.
The chemical elements that make up living and inanimate nature are in constant motion, because the substances that consist of these elements are constantly changing.
Chemical reactions (from the Latin reaction - opposition, resistance) are the response of substances to the influence of other substances and physical factors (temperature, pressure, radiation, etc.).
However, this definition also corresponds to physical changes that occur with substances - boiling, melting, condensation, etc. Therefore, it is necessary to clarify that chemical reactions are processes as a result of which old chemical bonds are destroyed and new ones arise and, as a consequence, from From the original substances, new substances are formed.
Chemical reactions continuously occur both inside our body and in the world around us. Countless reactions are usually classified according to various criteria. Let's remember the signs from the 8th grade course that you are already familiar with. To do this, let's turn to laboratory experiment.
Laboratory experiment No. 3
Substitution of iron for copper in a solution of copper (II) sulfate
Pour 2 ml of copper (II) sulfate solution into a test tube and place a thumbtack or paper clip in it. What are you observing? Write the reaction equations in molecular and ionic forms. Consider redox processes. Based on the molecular equation, classify this reaction into one or another group of reactions based on the following characteristics:
Now check yourself. CuSO 4 + Fe = FeSO 4 + Cu.
|
We have come to a very important concept in chemistry - “the rate of a chemical reaction.” It is known that some chemical reactions occur very quickly, others over significant periods of time. When a solution of silver nitrate is added to a solution of sodium chloride, a white cheesy precipitate precipitates almost instantly:
AgNO 3 + NaCl = NaNO 3 + AgCl↓.
Reactions occur at enormous speeds, accompanied by an explosion (Fig. 11, 1). On the contrary, stalactites and stalagmites slowly grow in stone caves (Fig. 11, 2), steel products corrode (rust) (Fig. 11, 3), palaces and statues are destroyed by acid rain (Fig. 11, 4).
Rice. 11.
Chemical reactions occurring at enormous speeds (1) and very slowly (2-4)
| The rate of a chemical reaction is the change in the concentration of reactants per unit time: V p = C 1 - C 2 /t. |
In turn, concentration is understood as the ratio of the amount of a substance (as you know, it is measured in moles) to the volume that it occupies (in liters). From here it is not difficult to derive the unit of measurement for the rate of a chemical reaction - 1 mol/(l s).
A special branch of chemistry studies the rate of chemical reactions, which is called chemical kinetics.
Knowing its laws allows you to control a chemical reaction, making it proceed faster or slower.
What factors determine the rate of a chemical reaction?
1. Nature of reactants. Let's turn to the experiment.
Laboratory experiment No. 4
Dependence of the rate of a chemical reaction on the nature of the reactants using the example of the interaction of acids with metals
| Pour 1-2 ml of hydrochloric acid into two test tubes and place: in the 1st - a zinc granule, in the 2nd - a piece of iron of the same size. The nature of which reagent affects the rate of interaction of the acid with the metal? Why? Write down the reaction equations in molecular and ionic forms. Consider them from the point of view of oxidation-reduction. Next, place identical zinc granules in two other test tubes and add acid solutions of the same concentration to them: in the 1st - hydrochloric acid, in the 2nd - acetic acid. The nature of which reagent affects the rate of interaction of the acid with the metal? Why? Write down the reaction equations in molecular and ionic forms. Consider them from the point of view of oxidation-reduction. |
2. Concentration of reactants. Let's turn to the experiment.
Laboratory experiment No. 5
Dependence of the rate of a chemical reaction on the concentration of reactants using the example of the interaction of zinc with hydrochloric acid of various concentrations
It's easy to conclude: The higher the concentration of reactants, the higher the rate of interaction between them.
The concentration of gaseous substances for homogeneous production processes is increased by increasing the pressure. For example, this is done in the production of sulfuric acid, ammonia, and ethyl alcohol.
The factor of dependence of the rate of a chemical reaction on the concentration of reacting substances is taken into account not only in production, but also in other areas of human activity, for example in medicine. Patients with lung diseases, in whom the rate of interaction of blood hemoglobin with oxygen in the air is low, breathe easier with the help of oxygen pillows.
3. Contact area of reacting substances. An experiment illustrating the dependence of the rate of a chemical reaction on this factor can be performed using the following experiment.
Laboratory experiment No. 6
Dependence of the rate of a chemical reaction on the area of contact of the reacting substances
For heterogeneous reactions: the larger the contact area of the reacting substances, the higher the reaction rate.
You could verify this from personal experience. To light a fire, you put small wood chips under the wood, and under them - crumpled paper, from which the whole fire caught fire. On the contrary, extinguishing a fire with water involves reducing the area of contact of burning objects with air.
In production, this factor is specially taken into account; the so-called fluidized bed is used. To increase the reaction rate, the solid substance is crushed almost to the state of dust, and then a second substance, usually gaseous, is passed through it from below. Passing it through a finely divided solid creates a boiling effect (hence the name of the method). The fluidized bed is used, for example, in the production of sulfuric acid and petroleum products.
Laboratory experiment No. 7
Fluidized bed modeling
4. Temperature. Let's turn to the experiment.
Laboratory experiment No. 8
Dependence of the rate of a chemical reaction on the temperature of the reacting substances using the example of the interaction of copper (II) oxide with a solution of sulfuric acid at different temperatures
It is easy to conclude: the higher the temperature, the greater the reaction rate.
The first Nobel Prize laureate, the Dutch chemist J. X. van't Hoff, formulated the rule:
In production, as a rule, high-temperature chemical processes are used: in the smelting of cast iron and steel, the melting of glass and soap, the production of paper and petroleum products, etc. (Fig. 12).
Rice. 12.
High-temperature chemical processes: 1 - iron smelting; 2 - glass melting; 3 - production of petroleum products
The fifth factor on which the speed of a chemical reaction depends is catalysts. You will meet him in the next paragraph.
New words and concepts
- Chemical reactions and their classification.
- Signs of classification of chemical reactions.
- The rate of a chemical reaction and the factors on which it depends.
Tasks for independent work
- What is a chemical reaction? What is the essence of chemical processes?
- Give a complete classification description of the following chemical processes:
- a) combustion of phosphorus;
- b) the interaction of a sulfuric acid solution with aluminum;
- c) neutralization reactions;
- d) the formation of nitric oxide (IV) from nitric oxide (II) and oxygen.
- Based on personal experience, give examples of chemical reactions that occur at different rates.
- What is the rate of a chemical reaction? What factors does it depend on?
- Give examples of the influence of various factors on biochemical and industrial chemical processes.
- Based on personal experience, give examples of the influence of various factors on chemical reactions that occur in everyday life.
- Why is food stored in the refrigerator?
- The chemical reaction was started at a temperature of 100 °C, then raised to 150 °C. The temperature coefficient of this reaction is 2. How many times will the rate of the chemical reaction increase?
