DEFINITION
Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.
Most often, chemical reactions are understood as the process of converting initial substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations containing the formulas of the starting materials and reaction products. According to the law of conservation of mass, the number of atoms of each element on the left and right sides chemical equation the same. Usually, the formulas of the starting materials are written on the left side of the equation, and the formulas for the products are on the right. The equality of the number of atoms of each element in the left and right sides of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.
Chemical equations may contain additional information about the features of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or “below”) the equal sign.
All chemical reactions can be grouped into several classes, which have certain characteristics.
Classification of chemical reactions by the number and composition of the starting and formed substances
According to this classification, chemical reactions are subdivided into reactions of combination, decomposition, substitution, exchange.
As a result compound reactions one new substance is formed from two or more (complex or simple) substances. In general terms, the equation for such a chemical reaction will look like this:
For instance:
CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2
SO 3 + H 2 O \u003d H 2 SO 4
2Mg + O 2 \u003d 2MgO.
2FеСl 2 + Сl 2 \u003d 2FеСl 3
The reactions of the compound are in most cases exothermic, i.e. proceed with the release of heat. If the reaction involves simple substances, then such reactions are most often redox (ORR), i.e. proceed with a change in the oxidation states of the elements. It is impossible to say unequivocally whether the reaction of a compound between complex substances belongs to the OVR.
Reactions as a result of which several other new substances (complex or simple) are formed from one complex substance are referred to as decomposition reactions... In general terms, the chemical decomposition equation will look like this:
For instance:
CaCO 3 CaO + CO 2 (1)
2H 2 O \u003d 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O \u003d CuSO 4 + 5H 2 O (3)
Cu (OH) 2 \u003d CuO + H 2 O (4)
H 2 SiO 3 \u003d SiO 2 + H 2 O (5)
2SO 3 \u003d 2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O (7)
Most decomposition reactions occur on heating (1,4,5). Decomposition by electric current possible (2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) proceeds without changing the oxidation states of the elements, i.e. these reactions do not belong to OVR. The decomposition reactions include the decomposition of oxides, acids, and salts formed by elements in higher oxidation states (6).
Decomposition reactions occur in organic chemistry, but under other names - cracking (8), dehydrogenation (9):
C 18 H 38 \u003d C 9 H 18 + C 9 H 20 (8)
C 4 H 10 \u003d C 4 H 6 + 2H 2 (9)
When substitution reactions a simple substance interacts with a complex substance, forming a new simple and new complex substance. In general terms, the chemical substitution equation will look like this:
For instance:
2Аl + Fe 2 O 3 \u003d 2Fе + Аl 2 О 3 (1)
Zn + 2HCl \u003d ZnCl 2 + H 2 (2)
2КВr + Сl 2 \u003d 2КСl + Вr 2 (3)
2KSlO 3 + l 2 \u003d 2KlO 3 + Сl 2 (4)
CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2 (5)
Ca 3 (PO 4) 2 + 3SiO 2 \u003d 3CaSiO 3 + P 2 O 5 (6)
СН 4 + Сl 2 \u003d СН 3 Сl + НСl (7)
Substitution reactions are mostly redox reactions (1 - 4, 7). Examples of decomposition reactions in which there is no change in oxidation states are few (5, 6).
Exchange reactions call the reactions occurring between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions located in aqueous solution... In general, the equation for the chemical exchange reaction will look like this:
AB + CD \u003d AD + CB
For instance:
CuO + 2HCl \u003d CuCl 2 + H 2 O (1)
NaOH + HCl \u003d NaCl + H 2 O (2)
NaHCO 3 + HCl \u003d NaCl + H 2 O + CO 2 (3)
AgNО 3 + КВr \u003d АgВr ↓ + КNО 3 (4)
СrСl 3 + ЗNаОН \u003d Сr (ОН) 3 ↓ + ЗNаСl (5)
Metabolic reactions are not redox reactions. A special case These exchange reactions are neutralization reactions (reactions of interaction of acids with alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5), or a low-dissociating compound, most often water (1, 2).
Classification of chemical reactions by changes in oxidation states
Depending on the change in the oxidation states of the elements that make up the reagents and reaction products, all chemical reactions are subdivided into redox (1, 2) and proceeding without a change in the oxidation state (3, 4).
2Mg + CO 2 \u003d 2MgO + C (1)
Mg 0 - 2e \u003d Mg 2+ (reducing agent)
C 4+ + 4e \u003d C 0 (oxidizing agent)
FeS 2 + 8HNO 3 (conc) \u003d Fe (NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)
Fe 2+ -e \u003d Fe 3+ (reducing agent)
N 5+ + 3e \u003d N 2+ (oxidizing agent)
AgNO 3 + HCl \u003d AgCl ↓ + HNO 3 (3)
Ca (OH) 2 + H 2 SO 4 \u003d CaSO 4 ↓ + H 2 O (4)
Classification of chemical reactions by thermal effect
Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conventionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during the reaction is called the heat effect of the reaction. If the equation indicates the amount of released or absorbed heat, then such equations are called thermochemical.
N 2 + 3H 2 \u003d 2NH 3 +46.2 kJ (1)
2Mg + O 2 \u003d 2MgO + 602.5 kJ (2)
N 2 + O 2 \u003d 2NO - 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
In the direction of the reaction, reversible (chemical processes whose products are capable of reacting with each other under the same conditions in which they were obtained, with the formation of initial substances) and irreversible (chemical processes whose products are unable to react with each other with the formation of initial substances ).
For reversible reactions, the equation in general form is usually written as follows:
A + B ↔ AB
For instance:
CH 3 COOH + C 2 H 5 OH↔ H 3 COOC 2 H 5 + H 2 O
Examples of irreversible reactions include the following reactions:
2KSlO 3 → 2KSl + 3O 2
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
Evidence of the irreversibility of the reaction can be the release of a gaseous substance, a precipitate or a low-dissociating compound, most often water, as reaction products.
Classification of chemical reactions by the presence of a catalyst
From this point of view, catalytic and non-catalytic reactions are distinguished.
A catalyst is a substance that accelerates a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:
2H 2 O 2 \u003d 2H 2 O + O 2 (catalyst MnO 2)
Often one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):
MeO + 2HF \u003d MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1
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Classification chemical reactions
Chemical reactions are chemical processes, as a result of which from some substances others are formed that differ from them in composition and (or) structure. During chemical reactions, a change in substances necessarily occurs, in which old bonds are broken and new bonds are formed between atoms. Signs of chemical reactions: Gas evolves Precipitates 3) Substances change color Heat, light is emitted or absorbed
Chemical reactions in inorganic chemistry
Chemical reactions in inorganic chemistry
Chemical reactions in inorganic chemistry 1. By changing oxidation states chemical elements: Redox reactions: Redox reactions are reactions that change the oxidation states of elements. Intermolecular is a reaction that changes the oxidation state of atoms in different molecules. -2 +4 0 2H 2 S + H 2 SO 3 → 3S + 3H 2 O +2 -1 +2.5 -2 2Na 2 S 2 O 3 + H 2 O 2 → Na 2 S 4 O 6 + 2NaOH
Chemical reactions in inorganic chemistry 1. According to the change in the oxidation states of the chemical elements that form the substance: Redox reactions: 2. Intramolecular - this is a reaction that changes the oxidation state different atoms in one molecule. -3 +5 t 0 +3 (NH4) 2 Cr 2 O 7 → N 2 + Cr 2 O 3 + 4H 2 O Disproportionation is a reaction that occurs with a simultaneous increase and decrease in the oxidation state of atoms of the same element. +1 +5 -1 3NaClO → NaClO 3 + 2NaCl
2 .1. Reactions proceeding without changing the composition of substances In inorganic chemistry, such reactions include the processes of obtaining allotropic modifications of one chemical element, for example: С (graphite) С (diamond) 3О 2 (oxygen) 2О 3 (ozone) Sn (white tin) Sn ( gray tin) S (rhombic) S (plastic) P (red) P (white) Chemical reactions in inorganic chemistry 2. By the number and composition of reactants:
Chemical reactions in inorganic chemistry 2. By the number and composition of reactants: 2.2. Reactions involving a change in the composition of a substance 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 using the example of the reaction of obtaining sulfuric acid from sulfur: a) obtaining sulfur oxide (IV): S + O 2 SO 2 - one complex is formed from two simple substances, b) obtaining sulfur oxide (VI ): 2 SO 2 + O 2 2SO 3 - from a simple and complex substances one complex is formed, c) obtaining sulfuric acid: SO 3 + H 2 O \u003d H 2 SO 4 - from two complex substances one complex is formed.
Chemical reactions in inorganic chemistry 2. According to the number and composition of reacting substances: 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 at the block of reactions for obtaining oxygen by laboratory methods: a) decomposition of mercury (II) oxide: 2HgO t 2Hg + O 2 - two simple ones are formed from one complex substance. b) decomposition of potassium nitrate: 2KNO 3 t 2KNO 2 + O 2 - from one complex substance, one simple and one complex are formed. c) decomposition of potassium permanganate: 2 KMnO 4 → t K 2 MnO 4 + MnO 2 + O 2 - from one complex substance, two complex and one simple are formed.
Chemical reactions in inorganic chemistry 2. According to the number and composition of reactants: 3. Substitution reactions are such reactions as a result of which the atoms of a simple substance replace the atoms of some element in a complex substance. In inorganic chemistry, an example of such processes is a block of reactions characterizing the properties of metals: a) the interaction of alkali or alkaline earth metals with water: 2 Na + 2H 2 O \u003d 2NaOH + H 2 Ca + 2H 2 O \u003d Ca (OH) 2 + H 2 b) interaction of metals with acids in solution: Zn + 2HCl \u003d ZnCl 2 + H 2 c) interaction of metals with salts in solution: Fe + Cu SO 4 \u003d FeSO 4 + Cu d) metallothermy: 2Al + Cr 2 O 3 t Al 2 O 3 + 2Cr
4. Exchange reactions are such reactions in which two complex substances exchange their constituent parts. These reactions characterize the properties of electrolytes and in solutions proceed according to Berthollet's rule, that is, only if the result is a precipitate, gas or a low-dissociating substance (for example , H 2 O). In the inorganic, this can be a block of reactions characterizing the properties of alkalis: a) a neutralization reaction proceeding with the formation of salt and water: NaOH + HNO 3 \u003d NaNO 3 + H 2 O or in ionic form: OH - + H + \u003d H 2 O b ) the reaction between alkali and salt, proceeding with the formation of gas: 2NH 4 Cl + Ca (OH) 2 \u003d CaCl 2 + 2NH 3 + 2 H 2 O c) the reaction between alkali and salt, proceeding with the formation of a precipitate: Cu SO 4 + 2KOH \u003d Cu (OH) 2 + K 2 SO 4 Chemical reactions in inorganic chemistry 2. By the number and composition of reactants:
Chemical reactions in inorganic chemistry 3. By thermal effect: 3.1. Exothermic reactions: Exothermic reactions are reactions that take place with the release of energy into the external environment. These include almost all compound reactions. Exothermic reactions that occur with the release of light are referred to as combustion reactions, for example: 4P + 5O 2 \u003d 2P 2 O 5 + Q 3.2. Endothermic reactions: Endothermic reactions are reactions that take place with the absorption of energy into the external environment. These include almost all decomposition reactions, for example: Calcination of limestone: CaCO 3 t CaO + CO 2 - Q
Chemical reactions in inorganic chemistry 4. Reversibility of the process: 4.1. Irreversible reactions: Irreversible reactions proceed in these conditions only in one direction. These reactions include all exchange reactions accompanied by the formation of a precipitate, gas or low-dissociating substance (water) and all combustion reactions: S + O 2 SO 2; 4 P + 5O 2 2P 2 O 5; Cu SO 4 + 2KOH Cu (OH) 2 + K 2 SO 4 4.2. Reversible reactions: Reversible reactions under these conditions proceed simultaneously in two opposite directions. The overwhelming majority of such reactions. For example: 2 SO 2 + O 2 2SO 3 N 2 + 3H 2 2NH 3
Catalysts are substances that take part in a chemical reaction and change its rate or direction, but at the end of the reaction remain unchanged qualitatively and quantitatively. 5.1. Non-catalytic reactions: Non-catalytic reactions are reactions that take place without the participation of a catalyst: 2HgO t 2Hg + O 2 2Al + 6HCl t 2AlCl 3 + 3H 2 5.2. Catalytic reactions: Catalytic reactions are reactions involving a catalyst: t , MnO 2 2KClO 3 → 2KCl + 3O 2 P, t CO + NaOH H-CO-ONa Chemical reactions in inorganic chemistry 5. Catalyst involvement
Chemical reactions in inorganic chemistry 6. The presence of a phase interface 6.1. Heterogeneous reactions: Heterogeneous reactions are reactions in which reactants and reaction products are in different states of aggregation (in different phases): FeO (t) + CO (g) Fe (t) + CO 2 (g) + Q 2 Al (t) + 3C u C l 2 (p-p) \u003d 3C u (t) + 2AlCl 3 (p-p) CaC 2 (t) + 2H 2 O (l) \u003d C 2 H 2 + Ca ( OH) 2 (solution) 6.2. Homogeneous reactions: Homogeneous reactions are reactions in which the reactants and reaction products are in the same state of aggregation (in one phase): 2C 2 H 6 (g) + 7O 2 (g) 4CO 2 (g) + 6H 2 O (g) 2 SO 2 (g) + O 2 (g) \u003d 2SO 3 (g) + QH 2 (g) + F 2 (g) \u003d 2HF (g)
In inorganic chemistry, chemical reactions are classified according to different criteria.
1. By changing the oxidation state on redox, going with a change in the oxidation state of elements and acid-base, which proceed without changing the oxidation state.
2. By the nature of the process.
Decomposition reactions are called chemical reactions in which simple molecules are obtained from more complex ones.
Compound reactions chemical reactions are called in which complex compounds are obtained from several simpler ones.
Substitution reactionschemical reactions are called in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms.
Exchange reactionsare called chemical reactions that occur without changing the oxidation state of the elements and lead to the exchange of the constituent parts of the reactants.
3. If possible, flow in the opposite direction to reversible and irreversible.
Some reactions, such as the reaction of ethanol combustion, are practically irreversible, i.e. conditions cannot be created for it to flow in the opposite direction.
However, there are many reactions that, depending on the process conditions, can proceed both in forward and reverse directions. Reactions capable of proceeding both forward and backward are called reversible.
4. By the type of bond breaking - homolytic (equal gap, each atom receives one electron) and heterolytic (unequal gap - one gets a pair of electrons).
5. Thermal effect exothermic(heat generation) and endothermic(heat absorption).
Compound reactions will generally be exothermic and decomposition reactions will be endothermic. A rare exception is the endothermic reaction of nitrogen with oxygen N 2 + O 2 \u003d 2NO - Q.
6. By the state of aggregation of the phases.
Homogeneous (the reaction takes place in one phase, without interfaces; reactions in gases or in solutions).
Heterogeneous (reactions taking place at the interface).
7. On the use of the catalyst.
A catalyst is a substance that accelerates a chemical reaction, but remains chemically unchanged.
Catalytic practically do not go without the use of a catalyst and non-catalytic.
Classification of organic reactions
|
Reaction type |
Radical |
Nucleophilic (N) |
Electrophilic (E) |
|
Substitution (S) |
Radical substitution (S R) |
Nucleophilic Substitution (S N) |
Electrophilic substitution (S E) |
|
Connection (A) |
Radical accession (A R) |
Nucleophilic addition (A N) |
Electrophilic connection (A E) |
|
Cleavage (E) (elimination) |
Radical cleavage (E R) |
Nucleophilic cleavage (E N) |
Electrophilic cleavage (E E) |
Heterolytic reactions of organic compounds with electrophiles - particles carrying a whole or fractional positive charge are called electrophilic. They are subdivided into electrophilic substitution and electrophilic addition reactions. For instance,
H 2 C \u003d CH 2 + Br 2 BrCH 2 - CH 2 Br
Nucleophilic are heterolytic reactions of organic compounds with nucleophiles - particles that carry a whole or fractional negative charge. They are subdivided into nucleophilic substitution and nucleophilic addition reactions. For instance,
CH 3 Br + NaOH CH 3 OH + NaBr
Chemical reactions involving radicals are called radical (chain) reactions, for example
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 - transformation processes atomic nuclei as a result of their interaction with other nuclei or elementary particles, for example the conversion of aluminum to 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 can be attributed reactions both between inorganic and between organic substances.
Consider the classification of chemical reactions according to various criteria.
I. By the number and composition of reactants
Reactions 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)
P (white) ↔ P (red)
Sn (white tin) ↔ Sn (gray tin)
3O 2 (oxygen) ↔ 2O 3 (ozone)
In organic chemistry, this type of reaction can be attributed to isomerization reactions, which proceed 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 the isostructural hydrocarbons are less capable of detonation.
2. Isomerization of alkenes.
3. Isomerization of alkynes (reaction of AE 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.
Urea was first synthesized by F. Wöhler in 1828 by isomerization of ammonium cyanate upon heating.
Reactions with a change in the composition of matter
Four types of such reactions can be distinguished: compound, decomposition, substitution, and exchange.
1. Reactions of a compound 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, by the example of the reactions of obtaining sulfuric acid from sulfur:
1. Obtaining sulfur oxide (IV):
S + O 2 \u003d SO - one complex is formed from two simple substances.
2. Obtaining sulfur oxide (VI):
SO 2 + 0 2 → 2SO 3 - from a simple and complex substances, one complex is formed.
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 initial ones, can be the final stage of obtaining 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 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 \u003d CH 2 + H 2 → H 3 -CH 3
ethen → ethane
2. The reaction of hydration is the 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 on the block of reactions for obtaining oxygen by laboratory methods:
1. Decomposition of mercury (II) oxide - two simple substances 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, that is, three new substances, are formed.
In organic chemistry, decomposition reactions can be considered on the block of reactions for producing ethylene in the laboratory and in industry:
1. Reaction of ethanol dehydration (elimination of water):
С 2 H 5 OH → CH 2 \u003d CH 2 + H 2 O
2. Reaction of dehydrogenation (elimination of hydrogen) of ethane:
CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2
or CH 3 -CH 3 → 2C + ZN 2
3. Reaction of cracking (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 characterizing 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 \u003d ZnCl 2 + H 2
3. Interaction of metals with salts in solution:
Fe + CuSO 4 \u003d FeSO 4 + Cu
4. Metallothermy:
2Аl + Сr 2 O 3 → Аl 2 O 3 + 2Сr
The subject of organic chemistry study is not simple substances, but only compounds. Therefore, as an example of a substitution reaction, we give the most characteristic property limiting 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 at organic matter: 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 reactions in which two complex substances exchange their constituent parts
These reactions characterize the properties of electrolytes and in solutions proceed according to Berthollet's rule, that is, only if the result is a precipitate, gas, or a low-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. The reaction of neutralization, proceeding with the formation of salt and water.
2. The reaction between alkali and salt, proceeding with the formation of gas.
3. The reaction between alkali and salt, proceeding with the formation of a precipitate:
CuSO 4 + 2KON \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 that characterize, for example, the properties of acetic acid:
1. The reaction proceeding with the formation of a weak electrolyte - Н 2 O:
CH 3 COOH + NaOH → Na (CH3COO) + H 2 O
2. The reaction proceeding 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 proceeding with a change in the oxidation states of elements, or redox reactions.
These include many reactions, including all substitution reactions, as well as those compound and decomposition reactions in which at least one simple substance is involved, for example:
1.Mg 0 + H + 2 SO 4 \u003d Mg +2 SO 4 + H 2
2.2Mg 0 + O 0 2 \u003d Mg +2 O -2
Complex redox reactions are composed using the electronic 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 proceeding 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 \u003d HCOOCH 3 + H 2 O
III. Thermal effect
According to the thermal effect, reactions are divided into exothermic and endothermic.
1. Exothermic reactions proceed with the release of energy.
These include almost all compound reactions. Endothermic reactions for the synthesis of nitrogen oxide (II) from nitrogen and oxygen and the reaction of gaseous hydrogen with solid iodine are a rare exception.
Exothermic reactions that occur 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. Firing limestone
2. Cracking of butane
The amount of energy released or absorbed as a result of the reaction is called the heat 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 reactants (phase composition)
According to the state of aggregation of the reacting substances, they are distinguished:
1. Heterogeneous reactions - reactions in which reactants and reaction products are in different states of aggregation (in different phases).
2. Homogeneous reactions - reactions in which reactants and reaction products are in the same state of aggregation (in one phase).
V. By the participation of the catalyst
By the participation of the catalyst, they are distinguished:
1. Non-catalytic reactions without the participation of a catalyst.
2. Catalytic reactions involving a catalyst. Since all biochemical reactions occurring in the cells of living organisms take place with the participation of special biological catalysts of a protein nature - enzymes, they all belong to catalytic or, more precisely, enzymatic. It should be noted that more than 70% of chemical industries use catalysts.
Vi. Towards
The direction is distinguished:
1. Irreversible reactions proceed in these conditions only in one direction. These include all exchange reactions accompanied by the formation of a precipitate, gas or low-dissociating substance (water) and all combustion reactions.
2. Reversible reactions under these conditions proceed simultaneously in two opposite directions. The overwhelming majority of such reactions.
In organic chemistry, the sign of reversibility is reflected by the names - the 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, polynucleotides are reversible. The reversibility of these processes underlies the most important property of a living organism - metabolism.
Vii. The mechanism of flow is distinguished:
1. Radical reactions occur between radicals and molecules formed during the reaction.
As you already know, with all reactions, the old are broken and new ones are formed. chemical bonds... The method of breaking the bond in the molecules of the initial substance determines the mechanism (path) of the reaction. If a substance is formed due to a covalent bond, then there can be two ways of breaking this bond: hemolytic and heterolytic. For example, for molecules 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 common electron pairs are distributed approximately equally between atoms (non-polar covalent bond), however, many polar bonds can also be broken in the same 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 above processes - 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 causes the rupture of the common electron pair that connects 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 an unpaired electron of the chlorine atom from the disintegrating Cl 2 molecule, as a result of which a chlorine radical appears, which attacks a new hydrogen molecule, etc.
Reactions representing 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 in a similar way:
By the radical mechanism, most of the combustion reactions of organic and inorganic substances, synthesis of water, ammonia, polymerization of ethylene, vinyl chloride, etc.
2. Ionic reactions take place between the ions already available or formed during the reaction.
Typical ionic reactions is the interaction between electrolytes in solution. Ions are formed not only during the dissociation of electrolytes in solutions, but also under the influence of electric discharges, heating, or radiation. Gamma rays, for example, convert water and methane molecules into molecular ions.
According to another ionic mechanism, reactions of addition to alkenes of hydrogen halides, hydrogen, halogens, oxidation and dehydration of alcohols, replacement of alcohol hydroxyl by halogen occur; reactions characterizing the properties of aldehydes and acids. In this case, ions are formed upon heterolytic rupture of covalent polar bonds.
VIII. By the type of energy
initiating reaction are distinguished:
1. Photochemical reactions. They are initiated by light energy. In addition to the above-considered photochemical processes of HCl synthesis or the reaction of methane with chlorine, these 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 influence of light. These radicals interact with oxygen molecules to produce ozone.
Ozone is formed all the time 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 atmospheric pollutants can lead to photochemical smog.
This type of reaction also includes the most important process taking place 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, α-particles - He 2+, etc.). With the help of radiation reactions, very fast radio polymerization, radiolysis (radiation decomposition), etc. are carried out.
For example, instead of the 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
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. Electric current initiates them. In addition to the electrolysis reactions that are well known to you, we also indicate the reactions of electrosynthesis, for example, the reaction industrial production inorganic oxidants
4. Thermochemical reactions. Thermal energy initiates them. These include all endothermic reactions and many exothermic reactions, for the beginning of which an initial supply of heat is required, that is, the initiation of the process.
The classification of chemical reactions considered above is reflected in the diagram.
The classification of chemical reactions, like all other classifications, is conditional. Scientists have agreed to divide reactions into certain types according to the characteristics they have identified. But most of the chemical transformations can be attributed to different types. For example, let's make a description of the ammonia synthesis process.
This is a compound reaction, redox, exothermic, reversible, catalytic, heterogeneous (more precisely, heterogeneous catalytic), proceeding with decreasing pressure in the system. All of the information given must be considered to successfully manage the process A specific chemical reaction is always multi-quality, it is characterized by different signs.
\u003e\u003e Chemistry: Types of chemical reactions in organic chemistry
Reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the whole variety of reactions of organic compounds cannot be reduced to the framework of the proposed classification (for example, combustion reactions). However, such a classification will help establish analogies with the classifications of reactions that occur between inorganic substances that are already familiar to you from the course of inorganic chemistry.
As a rule, the main organic compoundparticipating in the reaction is called the substrate, and the other component of the reaction is conventionally considered as a reagent.
Substitution reactions
Reactions resulting in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.
Saturated and aromatic compounds, such as, for example, alkanes, cycloalkanes or arenes, enter into substitution reactions.
Let us give examples of such reactions.
