The classification of chemical reactions in inorganic and organic chemistry is carried out on the basis of various classifying features, details of which are given in the table below.
By changing the oxidation state of elements
The first sign of classification is by changing the degree of oxidation of the elements that form the reactants and products.
a) redox
b) without changing the oxidation state
redox called reactions accompanied by a change in the oxidation states of the chemical elements that make up the reagents. Redox in inorganic chemistry includes all substitution reactions and those decomposition and compound reactions in which at least one simple substance is involved. Reactions that proceed without changing the oxidation states of the elements that form the reactants and reaction products include all exchange reactions.
According to the number and composition of reagents and products
Chemical reactions are classified according to the nature of the process, i.e., according to the number and composition of reactants and products.
Connection reactions called chemical reactions, as a result of which complex molecules are obtained from several simpler ones, for example:
4Li + O 2 = 2Li 2 O
Decomposition reactions called chemical reactions, as a result of which simple molecules are obtained from more complex ones, for example:
CaCO 3 \u003d CaO + CO 2
Decomposition reactions can be viewed as processes inverse to compound.
substitution reactions chemical reactions are called, as a result of which an atom or group of atoms in a molecule of a substance is replaced by another atom or group of atoms, for example:
Fe + 2HCl \u003d FeCl 2 + H 2
Their distinguishing feature is the interaction of a simple substance with a complex one. Such reactions exist in organic chemistry.
However, the concept of "substitution" in organics is broader than in inorganic chemistry. If any atom or functional group in the molecule of the original substance is replaced by another atom or group, these are also substitution reactions, although from the point of view of inorganic chemistry, the process looks like an exchange reaction.
- exchange (including neutralization).
Exchange reactions call chemical reactions that occur without changing the oxidation states of the elements and lead to the exchange of the constituent parts of the reagents, for example:
AgNO 3 + KBr = AgBr + KNO 3
Run in the opposite direction if possible.
If possible, proceed in the opposite direction - reversible and irreversible.
reversible called chemical reactions occurring at a given temperature simultaneously in two opposite directions with commensurate speeds. When writing the equations of such reactions, the equal sign is replaced by oppositely directed arrows. The simplest example of a reversible reaction is the synthesis of ammonia by the interaction of nitrogen and hydrogen:
N 2 + 3H 2 ↔2NH 3
irreversible are reactions that proceed only in the forward direction, as a result of which products are formed that do not interact with each other. Irreversible include chemical reactions that result in the formation of slightly dissociated compounds, a large amount of energy is released, as well as those in which the final products leave the reaction sphere in gaseous form or in the form of a precipitate, for example:
HCl + NaOH = NaCl + H2O
2Ca + O 2 \u003d 2CaO
BaBr 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaBr
By thermal effect
exothermic are chemical reactions that release heat. The symbol for the change in enthalpy (heat content) is ΔH, and the thermal effect of the reaction is Q. For exothermic reactions, Q > 0, and ΔH< 0.
endothermic called chemical reactions that take place with the absorption of heat. For endothermic reactions Q< 0, а ΔH > 0.
Coupling reactions will generally be exothermic reactions, and decomposition reactions will be endothermic. A rare exception is the reaction of nitrogen with oxygen - endothermic:
N2 + O2 → 2NO - Q
By phase
homogeneous called reactions occurring in a homogeneous medium (homogeneous substances, in one phase, for example, g-g, reactions in solutions).
heterogeneous called reactions occurring in an inhomogeneous medium, on the contact surface of the reacting substances that are in different phases, for example, solid and gaseous, liquid and gaseous, in two immiscible liquids.
By using a catalyst
A catalyst is a substance that speeds up a chemical reaction.
catalytic reactions proceed only in the presence of a catalyst (including enzymatic ones).
Non-catalytic reactions run in the absence of a catalyst.
By type of rupture
According to the type of chemical bond breaking in the initial molecule, homolytic and heterolytic reactions are distinguished.
homolytic called reactions in which, as a result of breaking bonds, particles are formed that have an unpaired electron - free radicals.
Heterolytic called reactions that proceed through the formation of ionic particles - cations and anions.
- homolytic (equal gap, each atom receives 1 electron)
- heterolytic (unequal gap - one gets a pair of electrons)
Radical(chain) chemical reactions involving radicals are called, for example:
CH 4 + Cl 2 hv → CH 3 Cl + HCl
Ionic called chemical reactions that take place with the participation of ions, for example:
KCl + AgNO 3 \u003d KNO 3 + AgCl ↓
Electrophilic refers to heterolytic reactions of organic compounds with electrophiles - particles that carry a whole or fractional positive charge. They are divided into reactions of electrophilic substitution and electrophilic addition, for example:
C 6 H 6 + Cl 2 FeCl3 → C 6 H 5 Cl + HCl
H 2 C \u003d CH 2 + Br 2 → BrCH 2 -CH 2 Br
Nucleophilic refers to heterolytic reactions of organic compounds with nucleophiles - particles that carry an integer or fractional negative charge. They are subdivided into nucleophilic substitution and nucleophilic addition reactions, for example:
CH 3 Br + NaOH → CH 3 OH + NaBr
CH 3 C (O) H + C 2 H 5 OH → CH 3 CH (OC 2 H 5) 2 + H 2 O
Classification of organic reactions
The classification of organic reactions is given in the table:
All substances can be divided into simple (consisting of atoms of one chemical element) and complex (consisting of atoms of different chemical elements). Elementary substances are divided into metals and nonmetals.
Metals have a characteristic “metallic” luster, malleability, malleability, can be rolled into sheets or drawn into wire, have good thermal and electrical conductivity. At room temperature, all metals except mercury are in a solid state.
Non-metals do not have luster, are brittle, and do not conduct heat and electricity well. At room temperature, some non-metals are in a gaseous state.
Compounds are divided into organic and inorganic.
Organic compounds are commonly referred to as carbon compounds. Organic compounds are part of biological tissues and are the basis of life on Earth.
All other connections are called inorganic (rarely mineral). Simple carbon compounds (CO, CO 2 and a number of others) are usually referred to as inorganic compounds, they are usually considered in the course of inorganic chemistry.
Classification of inorganic compounds
Inorganic substances are divided into classes either by composition (binary and multi-element; oxygen-containing, nitrogen-containing, etc.) or by functional features.
Salts, acids, bases, and oxides are among the most important classes of inorganic compounds isolated according to their functional characteristics.
salt are compounds that dissociate in solution into metal cations and acid residues. Examples of salts are, for example, barium sulfate BaSO 4 and zinc chloride ZnCl 2 .
acids- substances that dissociate in solutions with the formation of hydrogen ions. Examples of inorganic acids are hydrochloric (HCl), sulfuric (H 2 SO 4), nitric (HNO 3), phosphoric (H 3 PO 4) acids. The most characteristic chemical property of acids is their ability to react with bases to form salts. According to the degree of dissociation in dilute solutions, acids are divided into strong acids, acids of medium strength and weak acids. According to the redox ability, oxidizing acids (HNO 3) and reducing acids (HI, H 2 S) are distinguished. Acids react with bases, amphoteric oxides and hydroxides to form salts.
Foundations- substances that dissociate in solutions with the formation of only hydroxide anions (OH 1-). Water-soluble bases are called alkalis (KOH, NaOH). A characteristic property of bases is the interaction with acids to form salt and water.
oxides are compounds of two elements, one of which is oxygen. There are basic, acidic and amphoteric oxides. Basic oxides are formed only by metals (CaO, K 2 O), they correspond to bases (Ca (OH) 2, KOH). Acid oxides are formed by non-metals (SO 3, P 2 O 5) and metals that exhibit a high degree of oxidation (Mn 2 O 7), they correspond to acids (H 2 SO 4, H 3 PO 4, HMnO 4). Amphoteric oxides, depending on the conditions, exhibit acidic and basic properties, interact with acids and bases. These include Al 2 O 3 , ZnO, Cr 2 O 3 and a number of others. There are oxides that exhibit neither basic nor acidic properties. Such oxides are called indifferent (N 2 O, CO, etc.)
Classification of organic compounds
Carbon in organic compounds, as a rule, forms stable structures based on carbon-carbon bonds. In its ability to form such structures, carbon is unmatched by other elements. Most organic molecules consist of two parts: a fragment that remains unchanged during the reaction, and a group that undergoes transformations. In this regard, the belonging of organic substances to one or another class and a number of compounds is determined.
An unchanged fragment of a molecule of an organic compound is usually considered as the backbone of the molecule. It may be hydrocarbon or heterocyclic in nature. In this regard, four large series of compounds can be conventionally distinguished: aromatic, heterocyclic, alicyclic and acyclic.
In organic chemistry, additional series are also distinguished: hydrocarbons, nitrogen-containing compounds, oxygen-containing compounds, sulfur-containing compounds, halogen-containing compounds, organometallic compounds, organosilicon compounds.
As a result of the combination of these fundamental series, compound series are formed, for example: "Acyclic hydrocarbons", "Aromatic nitrogen-containing compounds".
The presence of certain functional groups or atoms of elements determines whether the compound belongs to the corresponding class. Among the main classes of organic compounds, alkanes, benzenes, nitro and nitroso compounds, alcohols, phenols, furans, ethers, and a large number of others are distinguished.
Types of chemical bonds
A chemical bond is an interaction that holds two or more atoms, molecules, or any combination of them. By its very nature, a chemical bond is an electrical force of attraction between negatively charged electrons and positively charged atomic nuclei. The magnitude of this attractive force depends mainly on the electronic configuration of the outer shell of atoms.
The ability of an atom to form chemical bonds is characterized by its valency. The electrons involved in the formation of a chemical bond are called valence electrons.
There are several types of chemical bonds: covalent, ionic, hydrogen, metallic.
At education covalent bond there is a partial overlap of electron clouds of interacting atoms, electron pairs are formed. The covalent bond is the stronger, the more the interacting electron clouds overlap.
Distinguish between polar and non-polar covalent bonds.
If a diatomic molecule consists of identical atoms (H 2 , N 2), then the electron cloud is distributed in space symmetrically with respect to both atoms. This covalent bond is called non-polar (homeopolar). If a diatomic molecule consists of different atoms, then the electron cloud is shifted towards the atom with a higher relative electronegativity. This covalent bond is called polar (heteropolar). Examples of compounds with such a bond are HCl, HBr, HJ.
In the examples considered, each of the atoms has one unpaired electron; when two such atoms interact, a common electron pair is created - a covalent bond arises. An unexcited nitrogen atom has three unpaired electrons; due to these electrons, nitrogen can participate in the formation of three covalent bonds (NH 3). A carbon atom can form 4 covalent bonds.
The overlapping of electron clouds is possible only if they have a certain mutual orientation, while the overlapping region is located in a certain direction with respect to the interacting atoms. In other words, a covalent bond is directional.
The energy of covalent bonds is in the range of 150–400 kJ/mol.
The chemical bond between ions, carried out by electrostatic attraction, is called ionic bond . An ionic bond can be viewed as the limit of a polar covalent bond. Unlike a covalent bond, an ionic bond is neither directional nor saturable.
An important type of chemical bonding is the bonding of electrons in a metal. Metals are made up of positive ions, which are held at the nodes of the crystal lattice, and free electrons. When a crystal lattice is formed, the valence orbitals of neighboring atoms overlap and electrons move freely from one orbital to another. These electrons no longer belong to a particular metal atom, they are in giant orbitals that extend throughout the crystal lattice. A chemical bond resulting from the binding of positive ions of the metal lattice by free electrons is called metallic.
There can be weak bonds between molecules (atoms) of substances. One of the most important - hydrogen bond , which may be intermolecular and intramolecular. A hydrogen bond occurs between the hydrogen atom of a molecule (it is partially positively charged) and a strongly electronegative element of the molecule (fluorine, oxygen, etc.).
The hydrogen bond energy is much less than the covalent bond energy and does not exceed 10 kJ/mol. However, this energy is sufficient to create associations of molecules that make it difficult for the molecules to separate from each other. Hydrogen bonds play an important role in biological molecules (proteins and nucleic acids) and largely determine the properties of water.
Van der Waals forces are also considered weak ties. They are due to the fact that any two neutral molecules (atoms) at very close distances are weakly attracted due to the electromagnetic interactions of the electrons and nuclei of one molecule with the electrons and nuclei of the other.
♦ According to the number and composition of the starting and obtained substances, chemical reactions are:
- Connections- from two or more substances one complex substance is formed:
Fe + S = FeS
(when iron and sulfur powders are heated, iron sulfide is formed) - expansions- two or more substances are formed from one complex substance:
2H 2 O \u003d 2H 2 + O 2
(water decomposes into hydrogen and oxygen when an electric current is passed) - Substitutions- atoms of a simple substance replace one of the elements in a complex substance:
Fe + CuCl 2 = Cu↓ + FeCl 2
(iron displaces copper from copper(II) chloride solution) - exchange- 2 complex substances exchange constituent parts:
HCl + NaOH = NaCl + H2O
(neutralization reaction - hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water)
♦ Reactions that proceed with the release of energy (heat) are called exothermic. These include combustion reactions, such as sulfur:
S + O 2 \u003d SO 2 + Q
Sulfur oxide (IV) is formed, energy release is denoted by + Q
Reactions that require energy, i.e., proceeding with the absorption of energy, are called endothermic. An endothermic reaction is the decomposition of water under the action of an electric current:
2H 2 O \u003d 2H 2 + O 2 - Q
♦ Reactions accompanied by a change in the oxidation states of elements, i.e., the transition of electrons, are called redox:
Fe 0 + S 0 \u003d Fe +2 S -2
The opposite are electronic static reactions, often called simply reactions that occur without changing the oxidation state. These include all exchange reactions:
H +1 Cl -1 + Na +1 O -2 H +1 = Na +1 Cl -1 + H 2 +1 O -2
(Recall that the degree of oxidation in substances consisting of two elements is numerically equal to valence, the sign is placed before the number)
2. Experience. Carrying out reactions confirming the qualitative composition of the proposed salt, for example, copper sulfate (II)
The qualitative composition of salt is proved by reactions accompanied by precipitation or evolution of gas with a characteristic odor or color. Precipitation occurs when insoluble substances are obtained (determined from the solubility table). Gases are released when weak acids (many require heating) or ammonium hydroxide are formed.
The presence of a copper ion can be proved by adding sodium hydroxide, a blue precipitate of copper (II) hydroxide precipitates:
CuSO 4 + 2NaOH \u003d Cu (OH) 2 ↓ + Na 2 SO 4
Additionally, copper (II) hydroxide can be decomposed when heated, black oxide of copper (II) is formed:
Cu(OH) 2 \u003d CuO + H 2 O
The presence of a sulfate ion is proved by the precipitation of a white crystalline precipitate, insoluble in concentrated nitric acid, when a soluble barium salt is added:
CuSO 4 + BaCl 2 = BaSO 4 ↓ + CuCl 2
Introduction
1. General concept of a chemical reaction
2. Classification of chemical reactions
Conclusion
Bibliography
Introduction
The most interesting thing about the world around us is that it is constantly changing.
concept « chemical reaction » - the second main concept of chemistry. Every second, an innumerable number of reactions take place in the world, as a result of which one substance turns into another. We can observe some reactions directly, for example, the rusting of iron objects, blood clotting, and the combustion of automobile fuel.
At the same time, the vast majority of reactions remain invisible, but they determine the properties of the world around us.
In order to realize one's place in the world and learn how to manage it, a person must deeply understand the nature of these reactions and the laws that they obey. The task of modern chemistry is to study the functions of substances in complex chemical and biological systems, to analyze the relationship between the structure of a substance and its functions, and to synthesize substances with given functions.
So, there are a lot of chemical reactions taking place around a person, they take place constantly. What needs to be done in order not to get confused in the whole variety of chemical reactions? Learn to classify them and identify the essential features of classes.
The purpose of this work: to consider the concept of "chemical reaction" and to systematize and generalize knowledge about the classification of chemical reactions.
The work consists of an introduction, two chapters, a conclusion and a list of references. The total amount of work is 14 pages.
1. General concept of a chemical reaction
A chemical reaction is the transformation of one substance into another. However, this definition needs a significant addition.
So, for example, in a nuclear reactor or in an accelerator, some substances are also converted into others, but such transformations are not called chemical. What is the matter here? Nuclear reactions take place in a nuclear reactor. They lie in the fact that the nuclei of elements, when colliding with high-energy particles (they can be neutrons, protons and nuclei of other elements), are broken into fragments, which are the nuclei of other elements. It is also possible to merge the nuclei with each other. These new nuclei then receive electrons from the environment and thus the formation of two or more new substances is completed. All these substances are some elements of the Periodic system. Unlike nuclear reactions, in chemical reactions cores are not affected atoms. All changes occur only in the outer electron shells. Some chemical bonds are broken and others are formed.
Thus, chemical reactions called phenomena in which some substances with a certain composition and properties are converted into other substances - with a different composition and other properties. At the same time, no changes occur in the composition of atomic nuclei.
Let's highlight the signs and conditions of chemical reactions (Fig. 1, 2).
Figure 1 - Signs of chemical reactions
Figure 2 - Conditions for conducting chemical reactions
Consider a typical chemical reaction: the combustion of natural gas (methane) in the oxygen of the air (this reaction can be observed at home, who has a gas stove) in Figure 3.
Figure 3 - Combustion of natural gas (methane) in atmospheric oxygen
Methane CH 4 and oxygen O 2 react with each other to form carbon dioxide CO 2 and water H 2 O. In this case, the bonds between the C and H atoms in the methane molecule and between the oxygen atoms in the O 2 molecule are broken. In their place, new bonds arise between the atoms C and O, H and O.
Figure 3 clearly shows that for the successful implementation of the response to one take a methane molecule two oxygen molecules. However, it is not very convenient to write down a chemical reaction using drawings of molecules, therefore, abbreviated formulas of substances are used to write chemical reactions - such a record is called chemical reaction equation.
Figure 4 - Reaction equation
The equation for the chemical reaction shown in Figure 3 is as follows
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O
The number of atoms of different elements in the left and right sides of the equation is the same. On the left side one a carbon atom in the methane molecule (CH 4), and in the right - the same we find the carbon atom in the composition of the CO 2 molecule. all four we will definitely find hydrogen atoms from the left side of the equation and in the right - in the composition of water molecules.
In a chemical reaction equation, to equalize the number of identical atoms in different parts of the equation, odds, which are recorded before substance formulas.
Consider another reaction - the conversion of calcium oxide CaO (quicklime) into calcium hydroxide Ca(OH) 2 (slaked lime) under the action of water (Fig. 5).
Figure 5 - Calcium oxide CaO attaches a water molecule H 2 O
with the formation of calcium hydroxide Ca (OH) 2
Unlike mathematical equations, chemical equations cannot interchange the left and right sides. Substances on the left side of a chemical reaction equation are called reagents, and on the right reaction products .
If we swap the left and right parts in the equation from Figure 5, we get the equation completely different chemical reaction
Ca (OH) 2 \u003d CaO + H 2 O
If the reaction between CaO and H 2 O (Fig. 4) starts spontaneously and proceeds with the release of a large amount of heat, then the last reaction, where Ca(OH) 2 serves as a reagent, requires strong heating. We also add that the reactants and products may not necessarily be molecules, but also atoms - if some element or elements in a pure form participate in the reaction, for example
H 2 + CuO \u003d Cu + H 2 O
Thus, we have come to the classification of chemical reactions, which we will consider in the next chapter.
2. Classification of chemical reactions
In the process of studying chemistry, one has to meet with classifications of chemical reactions according to various criteria (Table 1).
Table 1 - Classification of chemical reactions
| By thermal effect | exothermic- flow with the release of energy 4P + 5O 2 \u003d 2P 2 O 5 + Q; CH 4 + 2O 2 → CO 2 + 2H 2 O + Q |
| Endothermic- proceed with the absorption of energy Cu(OH) 2 CuO + H 2 O - Q; C 8 H 18 C 8 H 16 + H 2 - Q | |
| By the number and composition of the original and formed substances | Decomposition reactions- several simpler ones are formed from one complex substance: CaCO 3 CaO + CO 2 C 2 H 5 OH → C 2 H 4 + H 2 O |
| Connection reactions- one complex substance is formed from several simple or complex substances: 2H 2 + O 2 → 2H 2 OC 2 H 4 + H 2 → C 2 H 6 | |
| Substitution reactions- atoms of a simple substance replace the atoms of one of the elements in a complex substance: Zn + 2HCl \u003d ZnCl 2 + H 2 CH 4 + Cl 2 → CH 3 Cl + HCl | |
| Exchange reactions- two complex substances exchange constituent parts: AgNO 3 + HCl \u003d AgCl ↓ + HNO 3 HCOOH + CH 3 OH → HCOOCH 3 + H 2 O | |
| By state of aggregation of reactants | Heterogeneous- the initial substances and reaction products are in different states of aggregation: Fe (t) + CuCl 2 (solution) → Cu (t) + FeCl 2 (solution) 2Na (t) + 2C 2 H 5 OH (l) → 2C 2 H 5 ONa (solution) + H 2(g) |
| homogeneous- the starting materials and reaction products are in the same state of aggregation: H 2 (g) + Cl 2 (g) \u003d 2HCl (g) C 2 H 5 OH (l) + CH 3 COOH (l) → CH 3 COOC 2 H 5 (l) + H 2 O (l) | |
| By the presence of a catalyst | catalytic 2H 2 O 2 2H 2 O + O 2 C 2 H 4 + H 2 C 2 H 4 |
| Non-catalytic S + O 2 SO 2 C 2 H 2 + 2Cl 2 → C 2 H 2 Cl 4 | |
| By direction | irreversible- flow under these conditions in only one direction: H 2 SO 4 + BaCl 2 → BaSO 4 + 2HCl CH 4 + 2O 2 → CO 2 + 2H 2 O |
| reversible- flow under these conditions simultaneously in two opposite directions: 3H 2 + N 2 ↔ 2NH 3; C 2 H 4 + H 2 ↔ C 2 H 6 | |
| By change in the oxidation state of the atoms of the elements | redox- reactions that occur with a change in the oxidation state: Fe 0 + 2H +1 Cl -1 → Fe 2+ Cl 2 -1 + H 2 0 H +1 C 0 O -2 H +1 + H 2 → C -2 H 3 +1 O -2 H +1 |
| Non-oxidizing-reducing- reactions that go on without changing the oxidation state: S + 4 O 4 -2 + H 2 O → H 2 + S + 4 O 4 -2 CH 3 NH 2 + HCl → (CH 3 NH 3) Cl |
As you can see, there are various ways to classify chemical reactions, of which we will consider the following in more detail.
A variety of chemical reactions, the number of which cannot be calculated, cannot be covered by a single universal classification, therefore they are divided according to certain common features. Under any of these signs, reactions can be attributed both between inorganic and between organic substances.
First, there are reactions without changing the composition of the substance and reactions with a change in composition.
Reactions that take place without changing the composition of substances:
AlCl3,t
CH3-CH2-CH2-CH3 > CH3-CH-CH3
Reactions that occur with a change in the composition of substances:
6 CO2 + 6 H2O = C6H12O6 + 6 O2
In organic chemistry, this type of reaction includes isomerization reactions. Thus, the isomerization of alkanes is carried out to obtain gasoline with a high octane number.
For chemical processes occurring between inorganic reagents, the following classifications are most often used:
1. The number and composition of the starting materials and reaction products.
2. Aggregate state of reactants and reaction products.
3. The number of phases in which the participants in the reaction are.
4. The nature of the transferred particles.
5. The possibility of the reaction proceeding in the forward and reverse directions.
6. Thermal effect sign
Various classification methods are often combined with each other (Fig. 1).
Figure 1 - Sign of the classification of chemical reactions
Let us consider in more detail each of the types of chemical reactions.
1. Classification according to the number and composition of reagents and final substances (Table 1).
Table 1 - Types of chemical reactions and their mechanisms
1. Reactions of connection. D.I. Mendeleev defined a compound as a reaction, “in which one of two substances occurs. So, in the reactions of a compound from several reacting substances of a relatively simple composition, one substance of a more complex composition is obtained
Combination reactions include the processes of combustion of simple substances (sulfur, phosphorus, carbon) in air. For example, carbon burns in air C + O 2 \u003d CO 2 (of course, this reaction proceeds gradually, carbon monoxide CO is first formed). As a rule, these reactions are accompanied by heat release, i.e. lead to the formation of more stable and less energy-rich compounds - are exothermic.
The reactions of the combination of simple substances are always redox in nature. Connection reactions occurring between complex substances can occur both without a change in valence
CaCO3 + CO2 + H2O = Ca (HCO3) 2
and be classified as a redox
2FeCl2 + Cl2 = 2FeCl3.
2. Decomposition reactions. Chemical reactions of decomposition, according to Mendeleev, “comprise cases inverse to compound, that is, those in which one substance gives two, or, in general, a given number of substances is a greater number of them.
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. An example of a decomposition reaction is the chemical reaction of decomposition of chalk (or limestone under the influence of temperature): CaCO 3 \u003d CaO + CO 2. The decomposition reaction generally requires heating. Such processes are endothermic, i.e. flow with the absorption of heat. Of the decomposition reactions that occur without changing the valence states, it should be noted the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids
CuSO4 5H2O = CuSO4 + 5H2O,
Cu(OH)2 = CuO + H2O,
H2SiO3 = SiO2 + H2O.
Decomposition reactions of a redox nature include the decomposition of oxides, acids and salts formed by elements in higher oxidation states
2SO3 = 2SO2 + O2,
4HNO3 = 2H2O + 4NO2O + O2O,
2AgNO3 = 2Ag + 2NO2 + O2,
(NH4) 2Cr2O7 = Cr2O3 + N2 + 4H2O.
Particularly characteristic are the redox reactions of decomposition for salts of nitric acid.
Decomposition reactions in organic chemistry, in contrast to decomposition reactions in inorganic chemistry, have their own specifics. They can be considered as the reverse processes of addition, since the result most often is the formation of multiple bonds or cycles.
Decomposition reactions in organic chemistry are called cracking
С18H38 = С9H18 + С9H20
or dehydrogenation C4H10 = C4H6 + 2H2.
In reactions of the other two types, the number of reactants is equal to the number of products.
3. Substitution reactions. Their distinguishing feature is the interaction of a simple substance with a complex one. Such reactions exist in organic chemistry. However, the concept of "substitution" in organics is broader than in inorganic chemistry. If any atom or functional group in the molecule of the original substance is replaced by another atom or group, these are also substitution reactions, although from the point of view of inorganic chemistry, the process looks like an exchange reaction.
In substitution reactions, usually a simple substance interacts with a complex one, forming another simple substance and another complex one. A + BC = AB + C
For example, by dipping a steel nail into a solution of copper sulfate, we get iron sulfate (iron displaced copper from its salt) Fe + CuSO 4 \u003d FeSO 4 + Cu.
These reactions are predominantly redox reactions.
2Al + Fe2O3 = 2Fe + Al2O3,
Zn + 2HCl = ZnCl2 + H2,
2KBr + Cl2 = 2KCl + Br2,
2KSlO3 + l2 = 2KlO3 + Cl2.
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
CaCO3 + SiO2 = CaSiO3 + CO2,
Ca3(PO4)2 + 3SiO2 = 3CaSiO3 + P2O5.
Sometimes these reactions are considered as exchange reactions.
CH4 + Cl2 = CH3Cl + HCl.
4. Exchange reactions (including neutralization). Exchange reactions are reactions between two compounds that exchange their constituents with each other.
AB + CD = AD + CB
A large number of them occur in aqueous solutions. An example of a chemical exchange reaction is the neutralization of an acid with an alkali.
NaOH + HCl \u003d NaCl + H 2 O.
Here, in the reagents (substances on the left), a hydrogen ion from the HCl compound is exchanged with a sodium ion from the NaOH compound, resulting in a solution of common salt in water.
If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts.
ZnO + H2SO4 = ZnSO4 + H2O,
AgNO3 + KBr = AgBr + KNO3,
CrCl3 + ZNaOH = Cr(OH)3 + ZNaCl.
A special case of these exchange reactions - neutralization reactions
HCl + KOH = KCl + H2O.
Usually, these reactions obey the laws of chemical equilibrium and proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous, volatile substance, precipitate, or low-dissociating (for solutions) compound
NaHCO3 + Hcl \u003d NaCl + H2O + CO2 ^,
Ca (HCO3) 2 + Ca (OH) 2 \u003d 2CaCO3v + 2H2O,
CH3COONa + H3PO4 = CH3COOH + NaH2PO4.
However, many reactions do not fit into the above simple scheme. For example, a chemical reaction between potassium permanganate (potassium permanganate) and sodium iodide cannot be attributed to any of the indicated types. Such reactions are usually referred to as redox reactions, for example
2KMnO 4 +10NaI+ 8H2SO4=2MnSO4+K2SO4+5Na2SO4+5I2+8H2O.
Redox reactions in inorganic chemistry include all substitution reactions and those decomposition and compound reactions in which at least one simple substance is involved. In a more generalized version (already taking into account organic chemistry), all reactions involving simple substances. And, conversely, reactions that go on without changing the oxidation states of the elements that form the reactants and reaction products include all exchange reactions.
2. 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:
2. Reactions in solutions:
NaOH (p-p) + Hcl (p-p) = NaCl (p-p) + H2O (l).
3. Reactions between solids:
CaO (tv) + SiO2 (tv) \u003d CaSiO3 (tv).
