Lecture: Classification chemical reactions in inorganic and organic chemistry

Types of chemical reactions in inorganic chemistry

A) Classification by the number of initial substances:

Decomposition - as a result of this reaction, from one existing complex substance, two or more simple, as well as complex substances are formed.

Example: 2H 2 O 2 → 2H 2 O + O 2

Compound - this is such a reaction in which two or more simple, as well as complex substances, form one, but more complex.

Example: 4Al+3O 2 → 2Al 2 O 3

substitution - This is a certain chemical reaction that takes place between some simple, as well as complex substances. The atoms of a simple substance, in this reaction, are replaced by atoms of one of the elements found in a complex substance.

Example: 2КI + Cl2 → 2КCl + I 2

Exchange - this is such a reaction in which two substances of complex structure exchange their parts.

Example: HCl + KNO 2 → KCl + HNO 2

B) Classification by thermal effect:

exothermic reactions - These are certain chemical reactions in which heat is released.
Examples:

S + O 2 → SO 2 + Q

2C 2 H 6 + 7O 2 → 4CO 2 + 6H 2 O + Q

Endothermic reactions are certain chemical reactions in which heat is absorbed. As a rule, these are decomposition reactions.

Examples:

CaCO 3 → CaO + CO 2 - Q
2KClO 3 → 2KCl + 3O 2 - Q

The heat released or absorbed in a chemical reaction is called thermal effect.

Chemical equations in which the heat effect of a reaction is indicated are called thermochemical.

C) Classification by reversibility:

Reversible reactions are reactions that proceed under the same conditions in mutually opposite directions.

Example: 3H 2 + N 2 ⇌ 2NH 3

irreversible reactions - these are reactions that proceed only in one direction, as well as culminating in the complete consumption of all starting materials. In these reactions, isolate gas, sediment, water.
Example: 2KClO 3 → 2KCl + 3O 2

D) Classification according to the change in the degree of oxidation:

Redox reactions - in the course of these reactions, a change in the degree of oxidation occurs.

Example: Сu + 4HNO 3 → Cu(NO 3) 2 + 2NO 2 + 2H 2 O.

Not redox - reactions without changing the oxidation state.

Example: HNO 3 + KOH → KNO 3 + H 2 O.

E) Phase classification:

Homogeneous reactions – reactions occurring in one phase, when the starting materials and reaction products have the same state of aggregation.

Example: H 2 (gas) + Cl 2 (gas) → 2HCL

heterogeneous reactions - reactions occurring at the phase interface, in which the reaction products and the starting materials have a different state of aggregation.
Example: CuO+ H 2 → Cu+H 2 O

Classification by catalyst use:

A catalyst is a substance that speeds up a reaction. A catalytic reaction proceeds in the presence of a catalyst, a non-catalytic reaction without a catalyst.
Example: 2H 2 0 2 MnO2 → 2H 2 O + O 2 catalyst MnO 2

The interaction of alkali with acid proceeds without a catalyst.
Example: KOH + HCl → KCl + H 2 O

Inhibitors are substances that slow down a reaction.
Catalysts and inhibitors themselves are not consumed during the reaction.

Types of chemical reactions in organic chemistry

substitution - this is a reaction during which one atom / group of atoms is replaced in the original molecule with other atoms / groups of atoms.
Example: CH 4 + Cl 2 → CH 3 Cl + Hcl

Accession are reactions in which several molecules of a substance combine into one. Addition reactions include:

• Hydrogenation is a reaction in which hydrogen is added to a multiple bond.

Example: CH 3 -CH \u003d CH 2 (propene) + H 2 → CH 3 -CH 2 -CH 3 (propane)

Hydrohalogenation is a reaction that adds a hydrogen halide.

Example: CH 2 \u003d CH 2 (ethene) + Hcl → CH 3 -CH 2 -Cl (chloroethane)

Alkynes react with hydrogen halides (hydrogen chloride, hydrogen bromide) in the same way as alkenes. Attachment in a chemical reaction takes place in 2 stages, and is determined by the Markovnikov rule:

When protic acids and water are added to unsymmetrical alkenes and alkynes, a hydrogen atom is attached to the most hydrogenated carbon atom.

The mechanism of this chemical reaction. Formed in the 1st, fast stage, the p-complex in the 2nd slow stage gradually turns into an s-complex - a carbocation. In the 3rd stage, the stabilization of the carbocation occurs - that is, the interaction with the bromine anion:

I1, I2 - carbocations. P1, P2 - bromides.

Halogenation A reaction in which a halogen is added. Halogenation is also called all processes, as a result of which halogen atoms are introduced into organic compounds. This concept used in " broad sense". In accordance with this concept, the following chemical reactions based on halogenation are distinguished: fluorination, chlorination, bromination, iodination.

Halogen-containing organic derivatives are considered the most important compounds that are used both in organic synthesis and as target products. Halogen derivatives of hydrocarbons are considered to be starting products in in large numbers nucleophilic substitution reactions. Concerning practical use compounds containing halogen, they are used in the form of solvents, such as chlorine-containing compounds, refrigerants - chlorofluoro derivatives, freons, pesticides, pharmaceuticals, plasticizers, monomers for plastics.

Hydration– addition reactions of a water molecule to a multiple bond.

Polymerization - this is a special type of reaction in which molecules of a substance having a relatively small molecular weight join each other, subsequently forming molecules of a substance with a high molecular weight.

Lecture 2

Chemical reactions. Classification of chemical reactions.

Redox reactions

Substances interacting with each other undergo various changes and transformations. For example, coal burns to form carbon dioxide. Beryllium, interacting with atmospheric oxygen, turns into beryllium oxide.

Phenomena in which some substances are converted into others that differ from the original in composition and properties and at the same time there is no change in the composition of the nuclei of atoms are called chemical. Oxidation of iron, combustion, obtaining metals from ores - all these are chemical phenomena.

A distinction must be made between chemical and physical phenomena.

During physical phenomena, the form or physical state of a substance changes or new substances are formed due to changes in the composition of the nuclei of atoms. For example, when gaseous ammonia interacts with liquid nitrogen, ammonia first passes into a liquid and then into a solid state. This is not a chemical, but a physical phenomenon, because. the composition of the substance does not change. Some phenomena leading to education. New substances are classified as physical. Such, for example, are nuclear reactions, as a result of which atoms of others are formed from the nuclei of one element.

Physical phenomena, because and chemical widespread: percolation electric current along a metal conductor, forging and melting metal, release of heat, turning water into ice or steam. Etc.

Chemical phenomena are always accompanied by physical ones. For example, during the combustion of magnesium, heat and light are released, in a galvanic cell, as a result of a chemical reaction, an electric current arises.

In accordance with the atomic and molecular theory and the law of conservation of the mass of a substance, from the atoms of substances that have entered into a reaction, new substances are formed, both simple and complex, and the total number of atoms of each element always remains constant.

Chemical phenomena occur due to the flow of chemical reactions.

Chemical reactions are classified according to various criteria.

1. On the basis of the release or absorption of heat. Reactions that release heat are called exothermic. For example, the reaction of the formation of hydrogen chloride from hydrogen and chlorine:

H 2 + CI 2 \u003d 2HCI + 184.6 kJ

Reactions that take place with the absorption of heat from environment are called endothermic. For example, the reaction of the formation of nitric oxide (II) from nitrogen and oxygen, which proceeds at high temperature:

N 2 + O 2 \u003d 2NO - 180.8 kJ

The amount of heat released or absorbed as a result of the reaction is called the thermal effect of the reaction. The branch of chemistry that studies the thermal effects of chemical reactions is called thermochemistry. We will talk about this in detail when studying the section "Energy of Chemical Reactions".

2. According to the change in the number of initial and final substances, the reactions are divided into the following types: connection, decomposition and exchange .

Reactions in which two or more substances form one new substance are called compound reactions :

For example, the interaction of hydrogen chloride with ammonia:

HCI + NH3 = NH4CI

Or burning magnesium:

2Mg + O2 = 2MgO

Reactions in which several new substances are formed from one substance are called decomposition reactions .

For example, the decomposition reaction of hydrogen iodide

2HI \u003d H 2 + I 2

Or decomposition of potassium permanganate:

2KmnO 4 \u003d K2mnO 4 + mnO 2 + O 2

Reactions between simple and complex substances, as a result of which the atoms of a simple substance replace the atoms of one of the elements of a complex substance are called substitution reactions.

For example, replacing lead with zinc in lead(II) nitrate:

Pb (NO 3) 2 + Zn \u003d Zn (NO 3) 2 + Pb

Or displacing bromine with chlorine:

2NaBr + CI 2 = 2NaCI + Br 2

Reactions in which two substances exchange their constituents to form two new substances are called exchange reactions . For example, the interaction of aluminum oxide with sulfuric acid:

AI2O3 + 3H3SO4 = AI2(SO4)3 + 3H3O

Or the interaction of calcium chloride with silver nitrate:

CaCI 2 + AgNO 3 \u003d Ca (NO 3) 2 + AgCI

3. On the basis of reversibility, reactions are divided into reversible and irreversible.

4. On the basis of a change in the oxidation state of the atoms that make up the reactants, there are reactions that occur without changing the oxidation state of atoms and redox reactions (with a change in the oxidation state of atoms).

Redox reactions. The most important oxidizing and reducing agents. Methods for selecting coefficients in reactions

redox

All chemical reactions can be divided into two types. The first type includes reactions that occur without changing the oxidation states of the atoms that make up the reactants.

For example

HNO 3 + NaOH = NaNO 3 + H3O

BaCI 2 + K 2 SO4 = BaSO 4 + 2KCI

The second type includes chemical reactions that occur with a change in the oxidation states of all or some elements:

2KCIO 3 = 2KICI+3O2

2KBr+CI2=Br 2 +2KCI

Here, in the first reaction, the atoms of chlorine and oxygen change their oxidation state, and in the second, the atoms of bromine and chlorine.

Reactions that occur with a change in the oxidation state of the atoms that make up the reactants are called redox reactions.

The change in oxidation state is associated with the pulling or movement of electrons.

The main provisions of the theory of redox

reactions:

1. Oxidation is the process of giving off electrons by an atom, molecule or ion.

AI - 3e - = AI 3+ H 2 - 2e - = 2H +

2. Recovery is the process of adding electrons to an atom, molecule or ion.

S + 2e - \u003d S 2- CI 2 + 2e - \u003d 2CI -

3.Atoms, molecules or ions that donate electrons are called reducing agents. During the reaction they are oxidized

4.Atoms, molecules or ions that accept electrons are called oxidizing agents. During the reaction, they are restored.

Oxidation is always accompanied by reduction, and vice versa, reduction is always associated with oxidation, which can be expressed by the equation:

Reducing agent – ​​e – = Oxidizing agent

Oxidizer + e - = Reductant

Therefore, redox reactions are a unity of two opposite processes of oxidation and reduction.

The number of electrons given away by the reducing agent is always equal to the number of electrons attached by the oxidizing agent.

Reducing agents and oxidizing agents can be both simple substances, i.e. consisting of one element or complex. Typical reducing agents are atoms in the outer energy level of which there are from one to three electrons. This group includes metals. Reducing properties can also be exhibited by non-metals, such as hydrogen, carbon, boron, etc.

In chemical reactions, they donate electrons according to the scheme:

E - ne - \u003d E n +

In periods with an increase in the ordinal number of the element, the reducing properties of simple substances decrease, while the oxidizing ones increase and become maximum for halogens. For example, in the third period, sodium is the most active reducing agent, and chlorine is the oxidizing agent.

In the elements of the main subgroups, the reducing properties increase with an increase in the serial number and the oxidizing properties weaken. Elements of the main subgroups of groups 4 - 7 (non-metals) can both give and receive electrons, i.e. exhibit reducing and oxidizing properties. An exception is fluorine, which exhibits only oxidizing properties, because has the highest electronegativity. The elements of the secondary subgroups have a metallic character, because the outer level of their atoms contains 1-2 electrons. Therefore, their simple substances are reducing agents.

The oxidizing or reducing properties of complex substances depend on the degree of oxidation of the atom of a given element.

For example, KMnO 4, MnO 2, MnSO 4,

In the first compound, manganese has a maximum oxidation state and can no longer increase it, therefore it can only be an oxidizing agent.

In the third compound, manganese has a minimum oxidation state; it can only be a reducing agent.

The most important reducing agents : metals, hydrogen, coal, carbon monoxide, hydrogen sulfide, stannous chloride, nitrous acid, aldehydes, alcohols, glucose, formic and oxalic acids, hydrochloric acid, electrolysis cathode.

The most important oxidizers : halogens, potassium permanganate, potassium dichromate, oxygen, ozone, hydrogen peroxide, nitric, sulfuric, selenic acids, hypochlorites, perchlorates, chlorates, aqua regia, a mixture of concentrated nitric and hydrofluoric acid, an anode during electrolysis.

Drawing up equations of redox reactions

1.Method of electronic balance. In this method, the oxidation states of atoms in the initial and final substances are compared, guided by the rule that the number of electrons given up by the reducing agent is equal to the number of electrons attached by the oxidizing agent. To draw up an equation, you need to know the formulas of the reactants and reaction products. The latter are determined either on the basis of the known properties of the elements or empirically.

Copper, forming a copper ion, gives up two electrons., Its oxidation state increases from 0 to +2. The palladium ion, by attaching two electrons, changes the oxidation state from +2 to 0. Therefore, palladium nitrate is an oxidizing agent.

If both the initial substances and the products of their interaction are established, then writing the reaction equation is reduced, as a rule, to finding and arranging the coefficients. The coefficients are determined by the electronic balance method using electronic equations. We calculate how the reducing agent and oxidizing agent change their oxidation state, and reflect this in electronic equations:

Cu 0 -2e - = Cu 2+ 1

Pd +2 +2e - =Pd 0 1

From the above electronic equations it can be seen that with a reducing agent and an oxidizing agent, the coefficients are equal to 1.

Final reaction equation:

Cu + Pd(NO 3) 2 = Cu(NO 3) 2 + Pd

To check the correctness of the formulated equation, we count the number of atoms in the right and left parts of the equation. The last thing we check is oxygen.

reduction reaction proceeding according to the scheme:

KMnO 4 + H 3 PO 3 + H 2 SO 4 →MnSO 4 + H 3 PO 4 + K 2 SO 4 + H 2 O

Solution If both the initial substances and the products of their interaction are given in the condition of the problem, then writing the reaction equation is reduced, as a rule, to finding and arranging the coefficients. The coefficients are determined by the electronic balance method using electronic equations. We calculate how the reducing agent and oxidizing agent change their oxidation state, and reflect this in electronic equations:

reducing agent 5 │ Р 3+ - 2ē ═ R 5+ oxidation process

oxidizing agent 2 │Mn +7 + 5 ē ═ Mn 2+ recovery process

The total number of electrons donated by reduction must be equal to the number of electrons that the oxidizing agent adds. The common smallest multiple for given and received electrons is 10. Dividing this number by 5, we get a factor of 2 for the oxidant and its reduction product. The coefficients in front of substances whose atoms do not change their oxidation state are found by selection. The reaction equation will look like

2KМnO 4 + 5H 3 PO 3 + 3H 2 SO 4 ═2MnSO 4 + 5H 3 PO 4 + K 2 SO 4 + 3H 2 Oh

Half-reaction method or ion-electron method. As the name itself indicates, this method is based on the compilation of ionic equations for the oxidation process and the reduction process.

When hydrogen sulfide is passed through an acidified solution of potassium permanganate, the crimson color disappears and the solution becomes cloudy.

Experience shows that the turbidity of the solution occurs as a result of the formation of sulfur:

H 2 S  S + 2H +

This scheme is equalized by the number of atoms. To equalize by the number of charges, two electrons must be subtracted from the left side, after which you can replace the arrow with an equal sign

H 2 S - 2e - \u003d S + 2H +

This is the first half-reaction - the process of oxidation of the hydrogen sulfide reducing agent.

The discoloration of the solution is associated with the transition of MnO 4 - (crimson color) to Mn 2+ (light pink color). This can be expressed by the diagram

MnO 4 - Mn 2+

In an acidic solution, oxygen, which is part of MnO 4 - together with hydrogen ions, eventually forms water. Therefore, the transition process is written as

MnO 4 - + 8H + Mn 2+ + 4H 2 O

In order to replace the arrow with an equal sign, the charges must also be equalized. Since the initial substances have seven positive charges, and the final two positive charges, then to fulfill the equality conditions, five electrons must be added to the left side of the circuit

MnO 4 - + 8H + + 5e - Mn 2+ + 4H 2 O

This is a half-reaction - the process of reducing the oxidizing agent, i.e. permanganate ion.

To compile the general reaction equation, it is necessary to add the equations of half-reactions term by term, first, by equalizing the numbers of given and received electrons. In this case, according to the rule of finding the least multiple, the corresponding factors are determined by which the field equations are multiplied

H 2 S - 2e - \u003d S + 2H + 5

MnO 4 - + 8H + + 5e - Mn 2+ + 4H 2 O 2

5H 2 S + 2MnO 4 - + 16H + \u003d 5S + 10H + + 2Mn 2+ + 8H 2 O

After reducing by 10H+ we get

5H 2 S + 2MnO 4 - + 6H + \u003d 5S + 2Mn 2+ + 8H 2 O or in molecular form

2k + + 3SO 4 2- = 2k + + 3SO 4 2-

5H 2 S + 2KMnO 4 + 3H 2 SO 4 \u003d 5S + 2MnSO 4 + K 2 SO 4 + 8H 2 O

Let's compare both methods. The advantage of the half-reaction method in comparison with the electron balance method is that it uses not hypothetical ions, but really existing ones. Indeed, there are no Mn +7, Cr +6, S +6, S +4 ions in the solution; MnO 4– , Cr 2 O 7 2– , CrO 4 2– , SO 4 2– . With the half-reaction method, it is not necessary to know all the substances formed; they appear in the reaction equation when deriving it.

Classification of redox reactions

There are usually three types of redox reactions: intermolecular, intramolecular and disproportionation reactions .

Intermolecular reactions are reactions in which the oxidizing agent and the reducing agent are in different substances. This also includes reactions between different substances in which atoms of the same element have different oxidation states:

2H 2 S + H 2 SO 3 \u003d 3S + 3H 2 O

5HCI + HCIO 3 = 5CI 2 + 3H 2 O

Intramolecular reactions are those reactions in which the oxidizing agent and reducing agent are in the same substance. In this case, an atom with a more positive oxidation state oxidizes an atom with a lower oxidation state. Such reactions are reactions of chemical decomposition. For example:

2NaNO 3 \u003d 2NaNO 2 + O 2

2KCIO 3 = 2KCI + 3O 2

This also includes the decomposition of substances in which atoms of the same element have different oxidation states:

NH 4 NO 3 \u003d N 2 O + 2H 2 O

The course of disproportionation reactions is accompanied by a simultaneous increase and decrease in the degree of oxidation of atoms of the same element. In this case, the starting substance forms compounds, one of which contains atoms with a higher, and the other with a lower degree of oxidation. These reactions are possible for substances with an intermediate oxidation state. An example is the conversion of potassium manganate in which manganese has an intermediate oxidation state of +6 (from +7 to +4). The solution of this salt has a beautiful dark green color (the color of the MnO ion 4 chemical Chemical experiment on inorganic chemistry in the system of problem-based learning Diploma work >> Chemistry

Tasks" 27. Classification chemical reactions. Reactions, which go without changing the composition. 28. Classification chemical reactions who go...

The chemical properties of substances are revealed in a variety of chemical reactions.

Transformations of substances, accompanied by a change in their composition and (or) structure, are called chemical reactions. The following definition is often found: chemical reaction The process of transformation of initial substances (reagents) into final substances (products) is called.

Chemical reactions are written using chemical equations and schemes containing the formulas of the starting materials and reaction products. In chemical equations, unlike schemes, the number of atoms of each element is the same on the left and right sides, which reflects the law of conservation of mass.

On the left side of the equation, the formulas of the starting substances (reagents) are written, on the right side - the substances obtained as a result of a chemical reaction (reaction products, final substances). The equal sign connecting the left and right side, 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 ratios between the reactants and reaction products.

Chemical equations may contain additional information about the features of the reaction. If a chemical reaction proceeds under the influence of external influences (temperature, pressure, radiation, etc.), this is indicated by the appropriate symbol, usually above (or “under”) the equals sign.

A huge number of chemical reactions can be grouped into several types of reactions, which are characterized by well-defined features.

As classification features the following can be selected:

1. The number and composition of the starting materials and reaction products.

2. Aggregate state of reactants and reaction products.

3. The number of phases in which the participants in the reaction are.

4. The nature of the transferred particles.

5. The possibility of the reaction proceeding in the forward and reverse directions.

6. The sign of the thermal effect separates all reactions into: exothermic reactions proceeding with the exo-effect - the release of energy in the form of heat (Q> 0, ∆H<0):

C + O 2 \u003d CO 2 + Q

and endothermic reactions proceeding with the endo effect - the absorption of energy in the form of heat (Q<0, ∆H >0):

N 2 + O 2 \u003d 2NO - Q.

Such reactions are thermochemical.

Let us consider in more detail each of the types of reactions.

Classification according to the number and composition of reagents and final substances

1. Connection reactions

In the reactions of a compound from several reacting substances of a relatively simple composition, one substance of a more complex composition is obtained:

As a rule, these reactions are accompanied by heat release, i.e. lead to the formation of more stable and less energy-rich compounds.

The reactions of the combination of simple substances are always redox in nature. Connection reactions occurring between complex substances can occur both without a change in valency:

CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2,

and be classified as redox:

2FeCl 2 + Cl 2 = 2FeCl 3.

2. Decomposition reactions

Decomposition reactions lead to the formation of several compounds from one complex substance:

A = B + C + D.

The decomposition products of a complex substance can be both simple and complex substances.

Of the decomposition reactions that occur without changing the valence states, the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids should be noted:

	t o
4HNO 3	=	2H 2 O + 4NO 2 O + O 2 O.

2AgNO 3 \u003d 2Ag + 2NO 2 + O 2,
(NH 4) 2Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O.

Particularly characteristic are the redox reactions of decomposition for salts of nitric acid.

Decomposition reactions in organic chemistry are called cracking:

C 18 H 38 \u003d C 9 H 18 + C 9 H 20,

or dehydrogenation

C 4 H 10 \u003d C 4 H 6 + 2H 2.

3. Substitution reactions

In substitution reactions, usually a simple substance interacts with a complex one, forming another simple substance and another complex one:

A + BC = AB + C.

These reactions in the vast majority belong to redox reactions:

2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3,

Zn + 2HCl \u003d ZnCl 2 + H 2,

2KBr + Cl 2 \u003d 2KCl + Br 2,

2KSlO 3 + l 2 = 2KlO 3 + Cl 2.

Examples of substitution reactions that are not accompanied by a change in the valence states of atoms are extremely few. It should be noted the reaction of silicon dioxide with salts of oxygen-containing acids, which correspond to gaseous or volatile anhydrides:

CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2,

Ca 3 (RO 4) 2 + ZSiO 2 \u003d ZCaSiO 3 + P 2 O 5,

Sometimes these reactions are considered as exchange reactions:

CH 4 + Cl 2 = CH 3 Cl + Hcl.

4. Exchange reactions

Exchange reactions Reactions between two compounds that exchange their constituents are called:

AB + CD = AD + CB.

If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts:

ZnO + H 2 SO 4 \u003d ZnSO 4 + H 2 O,

AgNO 3 + KBr = AgBr + KNO 3,

CrCl 3 + ZNaOH = Cr(OH) 3 + ZNaCl.

A special case of these exchange reactions is neutralization reactions:

Hcl + KOH \u003d KCl + H 2 O.

Usually, these reactions obey the laws of chemical equilibrium and proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous, volatile substance, precipitate, or low-dissociation (for solutions) compound:

NaHCO 3 + Hcl \u003d NaCl + H 2 O + CO 2,

Ca (HCO 3) 2 + Ca (OH) 2 \u003d 2CaCO 3 ↓ + 2H 2 O,

CH 3 COONa + H 3 RO 4 \u003d CH 3 COOH + NaH 2 RO 4.

5. Transfer reactions.

In transfer reactions, an atom or a group of atoms passes from one structural unit to another:

AB + BC \u003d A + B 2 C,

A 2 B + 2CB 2 = DIA 2 + DIA 3.

For example:

2AgCl + SnCl 2 \u003d 2Ag + SnCl 4,

H 2 O + 2NO 2 \u003d HNO 2 + HNO 3.

Classification of reactions according to phase features

Depending on the state of aggregation of the reacting substances, the following reactions are distinguished:

1. Gas reactions

H 2 + Cl 2		2HCl.

2. Reactions in solutions

NaOH (p-p) + Hcl (p-p) \u003d NaCl (p-p) + H 2 O (l)

3. Reactions between solids

	t o
CaO (tv) + SiO 2 (tv)	=	CaSiO 3 (TV)

Classification of reactions according to the number of phases.

A phase is understood as a set of homogeneous parts of a system with the same physical and chemical properties and separated from each other by an interface.

From this point of view, the whole variety of reactions can be divided into two classes:

1. Homogeneous (single-phase) reactions. These include reactions occurring in the gas phase, and a number of reactions occurring in solutions.

2. Heterogeneous (multiphase) reactions. These include reactions in which the reactants and products of the reaction are in different phases. For example:

gas-liquid phase reactions

CO 2 (g) + NaOH (p-p) = NaHCO 3 (p-p).

gas-solid-phase reactions

CO 2 (g) + CaO (tv) \u003d CaCO 3 (tv).

liquid-solid-phase reactions

Na 2 SO 4 (solution) + BaCl 3 (solution) \u003d BaSO 4 (tv) ↓ + 2NaCl (p-p).

liquid-gas-solid-phase reactions

Ca (HCO 3) 2 (solution) + H 2 SO 4 (solution) \u003d CO 2 (r) + H 2 O (l) + CaSO 4 (tv) ↓.

Classification of reactions according to the type of particles carried

1. Protolytic reactions.

To protolytic reactions include chemical processes, the essence of which is the transfer of a proton from one reactant to another.

This classification is based on the protolytic theory of acids and bases, according to which an acid is any substance that donates a proton, and a base is a substance that can accept a proton, for example:

Protolytic reactions include neutralization and hydrolysis reactions.

2. Redox reactions.

These include reactions in which the reactants exchange electrons, while changing the oxidation state of the atoms of the elements that make up the reactants. For example:

Zn + 2H + → Zn 2 + + H 2 ,

FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O,

The vast majority of chemical reactions are redox, they play an extremely important role.

3. Ligand exchange reactions.

These include reactions during which an electron pair is transferred with the formation of a covalent bond by the donor-acceptor mechanism. For example:

Cu(NO 3) 2 + 4NH 3 = (NO 3) 2,

Fe + 5CO = ,

Al(OH) 3 + NaOH = .

A characteristic feature of ligand-exchange reactions is that the formation of new compounds, called complex ones, occurs without a change in the oxidation state.

4. Reactions of atomic-molecular exchange.

This type of reactions includes many of the substitution reactions studied in organic chemistry, which proceed according to the radical, electrophilic, or nucleophilic mechanism.

Reversible and irreversible chemical reactions

Such chemical processes are called reversible, the products of which are able to react with each other under the same conditions in which they are obtained, with the formation of starting substances.

For reversible reactions, the equation is usually written as follows:

Two oppositely directed arrows indicate that under the same conditions, both forward and reverse reactions occur simultaneously, for example:

CH 3 COOH + C 2 H 5 OH CH 3 COOS 2 H 5 + H 2 O.

Irreversible are such chemical processes, the products of which are not able to react with each other with the formation of starting substances. Examples of irreversible reactions are the decomposition of Bertolet salt when heated:

2KSlO 3 → 2KSl + ZO 2,

or oxidation of glucose with atmospheric oxygen:

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O.

Chemical reactions should be distinguished from nuclear reactions. As a result of chemical reactions, the total number of atoms of each chemical element and its isotopic composition do not change. Nuclear reactions are another matter - the processes of transformation of atomic nuclei as a result of their interaction with other nuclei or elementary particles, for example, the transformation of aluminum into magnesium:

27 13 Al + 1 1 H \u003d 24 12 Mg + 4 2 He

The classification of chemical reactions is multifaceted, that is, it can be based on various signs. But under any of these signs, reactions both between inorganic and between organic substances can be attributed.

Consider the classification of chemical reactions according to various criteria.

I. According to the number and composition of the reactants

Reactions that take place without changing the composition of substances.

In inorganic chemistry, such reactions include the processes of obtaining allotropic modifications of one chemical element, for example:

C (graphite) ↔ C (diamond)
S (rhombic) ↔ S (monoclinic)
R (white) ↔ R (red)
Sn (white tin) ↔ Sn (grey tin)
3O 2 (oxygen) ↔ 2O 3 (ozone)

In organic chemistry, this type of reactions can include isomerization reactions that occur without changing not only the qualitative, but also the quantitative composition of the molecules of substances, for example:

1. Isomerization of alkanes.

The reaction of isomerization of alkanes is of great practical importance, since hydrocarbons of the isostructure have a lower ability to detonate.

2. Isomerization of alkenes.

3. Isomerization of alkynes (reaction of A. E. Favorsky).

CH 3 - CH 2 - C \u003d - CH ↔ CH 3 - C \u003d - C- CH 3

ethylacetylene dimethylacetylene

4. Isomerization of haloalkanes (A. E. Favorsky, 1907).

5. Isomerization of ammonium cyanite upon heating.

For the first time, urea was synthesized by F. Wehler in 1828 by isomerization of ammonium cyanate when heated.

Reactions that go with a change in the composition of a substance

There are four types of such reactions: compounds, decompositions, substitutions and exchanges.

1. Connection reactions are such reactions in which one complex substance is formed from two or more substances

In inorganic chemistry, the whole variety of compound reactions can be considered, for example, using the example of reactions for obtaining sulfuric acid from sulfur:

1. Obtaining sulfur oxide (IV):

S + O 2 \u003d SO - one complex substance is formed from two simple substances.

2. Obtaining sulfur oxide (VI):

SO 2 + 0 2 → 2SO 3 - one complex substance is formed from a simple and complex substance.

3. Obtaining sulfuric acid:

SO 3 + H 2 O \u003d H 2 SO 4 - one complex is formed from two complex substances.

An example of a compound reaction in which one complex substance is formed from more than two starting materials is the final stage in the production of nitric acid:

4NO 2 + O 2 + 2H 2 O \u003d 4HNO 3

In organic chemistry, compound reactions are commonly referred to as "addition reactions". The whole variety of such reactions can be considered on the example of a block of reactions characterizing the properties of unsaturated substances, for example, ethylene:

1. Hydrogenation reaction - hydrogen addition:

CH 2 \u003d CH 2 + H 2 → H 3 -CH 3

ethene → ethane

2. Hydration reaction - addition of water.

3. Polymerization reaction.

2. Decomposition reactions are such reactions in which several new substances are formed from one complex substance.

In inorganic chemistry, the whole variety of such reactions can be considered in the block of reactions for obtaining oxygen by laboratory methods:

1. Decomposition of mercury (II) oxide - two simple ones are formed from one complex substance.

2. Decomposition of potassium nitrate - from one complex substance, one simple and one complex are formed.

3. Decomposition of potassium permanganate - from one complex substance, two complex and one simple are formed, that is, three new substances.

In organic chemistry, decomposition reactions can be considered on the block of reactions for the production of ethylene in the laboratory and in industry:

1. The reaction of dehydration (water splitting) of ethanol:

C 2 H 5 OH → CH 2 \u003d CH 2 + H 2 O

2. Dehydrogenation reaction (hydrogen splitting) of ethane:

CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2

or CH 3 -CH 3 → 2C + ZH 2

3. Cracking reaction (splitting) of propane:

CH 3 -CH 2 -CH 3 → CH 2 \u003d CH 2 + CH 4

3. Substitution reactions are such reactions as a result of which the atoms of a simple substance replace the atoms of an element in a complex substance.

In inorganic chemistry, an example of such processes is a block of reactions that characterize the properties of, for example, metals:

1. Interaction of alkali or alkaline earth metals with water:

2Na + 2H 2 O \u003d 2NaOH + H 2

2. Interaction of metals with acids in solution:

Zn + 2HCl = ZnCl 2 + H 2

3. Interaction of metals with salts in solution:

Fe + CuSO 4 = FeSO 4 + Cu

4. Metalthermy:

2Al + Cr 2 O 3 → Al 2 O 3 + 2Cr

The subject of study of organic chemistry is not simple substances, but only compounds. Therefore, as an example of a substitution reaction, we give the most characteristic property of saturated compounds, in particular methane, the ability of its hydrogen atoms to be replaced by halogen atoms. Another example is the bromination of an aromatic compound (benzene, toluene, aniline).

C 6 H 6 + Br 2 → C 6 H 5 Br + HBr

benzene → bromobenzene

Let us pay attention to the peculiarity of the substitution reaction in organic substances: as a result of such reactions, not a simple and complex substance is formed, as in inorganic chemistry, but two complex substances.

In organic chemistry, substitution reactions also include some reactions between two complex substances, for example, the nitration of benzene. It is formally an exchange reaction. The fact that this is a substitution reaction becomes clear only when considering its mechanism.

4. Exchange reactions are such reactions in which two complex substances exchange their constituent parts

These reactions characterize the properties of electrolytes and proceed in solutions according to the Berthollet rule, that is, only if a precipitate, gas, or a low-dissociating substance (for example, H 2 O) is formed as a result.

In inorganic chemistry, this can be a block of reactions characterizing, for example, the properties of alkalis:

1. Neutralization reaction that goes with the formation of salt and water.

2. The reaction between alkali and salt, which goes with the formation of gas.

3. The reaction between alkali and salt, which goes with the formation of a precipitate:

СuSO 4 + 2KOH \u003d Cu (OH) 2 + K 2 SO 4

or in ionic form:

Cu 2+ + 2OH - \u003d Cu (OH) 2

In organic chemistry, one can consider a block of reactions characterizing, for example, the properties of acetic acid:

1. The reaction proceeding with the formation of a weak electrolyte - H 2 O:

CH 3 COOH + NaOH → Na (CH3COO) + H 2 O

2. The reaction that goes with the formation of gas:

2CH 3 COOH + CaCO 3 → 2CH 3 COO + Ca 2+ + CO 2 + H 2 O

3. The reaction proceeding with the formation of a precipitate:

2CH 3 COOH + K 2 SO 3 → 2K (CH 3 COO) + H 2 SO 3

2CH 3 COOH + SiO → 2CH 3 COO + H 2 SiO 3

II. By changing the oxidation states of chemical elements that form substances

On this basis, the following reactions are distinguished:

1. Reactions that occur with a change in the oxidation states of elements, or redox reactions.

These include many reactions, including all substitution reactions, as well as those reactions of combination and decomposition in which at least one simple substance participates, for example:

1. Mg 0 + H + 2 SO 4 \u003d Mg + 2 SO 4 + H 2

2. 2Mg 0 + O 0 2 = Mg +2 O -2

Complex redox reactions are compiled using the electron balance method.

2KMn +7 O 4 + 16HCl - \u003d 2KCl - + 2Mn +2 Cl - 2 + 5Cl 0 2 + 8H 2 O

In organic chemistry, the properties of aldehydes can serve as a striking example of redox reactions.

1. They are reduced to the corresponding alcohols:

Aldecides are oxidized to the corresponding acids:

2. Reactions that take place without changing the oxidation states of chemical elements.

These include, for example, all ion exchange reactions, as well as many compound reactions, many decomposition reactions, esterification reactions:

HCOOH + CHgOH = HSOCH 3 + H 2 O

III. By thermal effect

According to the thermal effect, the reactions are divided into exothermic and endothermic.

1. Exothermic reactions proceed with the release of energy.

These include almost all compound reactions. A rare exception is the endothermic reactions of the synthesis of nitric oxide (II) from nitrogen and oxygen and the reaction of gaseous hydrogen with solid iodine.

Exothermic reactions that proceed with the release of light are referred to as combustion reactions. The hydrogenation of ethylene is an example of an exothermic reaction. It runs at room temperature.

2. Endothermic reactions proceed with the absorption of energy.

Obviously, almost all decomposition reactions will apply to them, for example:

1. Calcination of limestone

2. Butane cracking

The amount of energy released or absorbed as a result of the reaction is called the thermal effect of the reaction, and the equation of a chemical reaction indicating this effect is called the thermochemical equation:

H 2 (g) + C 12 (g) \u003d 2HC 1 (g) + 92.3 kJ

N 2 (g) + O 2 (g) \u003d 2NO (g) - 90.4 kJ

IV. According to the state of aggregation of reacting substances (phase composition)

According to the state of aggregation of the reacting substances, there are:

1. Heterogeneous reactions - reactions in which the reactants and reaction products are in different states of aggregation (in different phases).

2. Homogeneous reactions - reactions in which the reactants and reaction products are in the same state of aggregation (in one phase).

V. According to the participation of the catalyst

According to the participation of the catalyst, there are:

1. Non-catalytic reactions that take place without the participation of a catalyst.

2. Catalytic reactions taking place with the participation of a catalyst. Since all biochemical reactions occurring in the cells of living organisms proceed with the participation of special biological catalysts of protein nature - enzymes, they are all catalytic or, more precisely, enzymatic. It should be noted that more than 70% of chemical industries use catalysts.

VI. Towards

By direction there are:

1. Irreversible reactions proceed under given conditions in only one direction. These include all exchange reactions accompanied by the formation of a precipitate, gas or a low-dissociating substance (water) and all combustion reactions.

2. Reversible reactions under these conditions proceed simultaneously in two opposite directions. Most of these reactions are.

In organic chemistry, the sign of reversibility is reflected in the names - antonyms of processes:

Hydrogenation - dehydrogenation,

Hydration - dehydration,

Polymerization - depolymerization.

All esterification reactions are reversible (the opposite process, as you know, is called hydrolysis) and hydrolysis of proteins, esters, carbohydrates, polynucleotides. The reversibility of these processes underlies the most important property of a living organism - metabolism.

VII. According to the mechanism of flow, there are:

1. Radical reactions take place between the radicals and molecules formed during the reaction.

As you already know, in all reactions, old chemical bonds are broken and new chemical bonds are formed. The method of breaking the bond in the molecules of the starting substance determines the mechanism (path) of the reaction. If the substance is formed by a covalent bond, then there can be two ways to break this bond: hemolytic and heterolytic. For example, for the molecules of Cl 2 , CH 4 , etc., a hemolytic rupture of bonds is realized, it will lead to the formation of particles with unpaired electrons, that is, free radicals.

Radicals are most often formed when bonds are broken in which the shared electron pairs are distributed approximately equally between atoms (non-polar covalent bond), but many polar bonds can also be broken in a similar way, in particular when the reaction takes place in the gas phase and under the influence of light , as, for example, in the case of the processes discussed above - the interaction of C 12 and CH 4 - . Radicals are highly reactive, as they tend to complete their electron layer by taking an electron from another atom or molecule. For example, when a chlorine radical collides with a hydrogen molecule, it breaks the shared electron pair that binds the hydrogen atoms and forms a covalent bond with one of the hydrogen atoms. The second hydrogen atom, becoming a radical, forms a common electron pair with the unpaired electron of the chlorine atom from the collapsing Cl 2 molecule, resulting in a chlorine radical that attacks a new hydrogen molecule, etc.

Reactions, which are a chain of successive transformations, are called chain reactions. For the development of the theory of chain reactions, two outstanding chemists - our compatriot N. N. Semenov and the Englishman S. A. Hinshelwood were awarded the Nobel Prize.
The substitution reaction between chlorine and methane proceeds similarly:

Most of the combustion reactions of organic and inorganic substances, the synthesis of water, ammonia, the polymerization of ethylene, vinyl chloride, etc. proceed according to the radical mechanism.

2. Ionic reactions take place between ions already present or formed during the reaction.

Typical ionic reactions are interactions between electrolytes in solution. Ions are formed not only during the dissociation of electrolytes in solutions, but also under the action of electrical discharges, heating or radiation. γ-rays, for example, convert water and methane molecules into molecular ions.

According to another ionic mechanism, there are reactions of addition of hydrogen halides, hydrogen, halogens to alkenes, oxidation and dehydration of alcohols, replacement of alcohol hydroxyl by halogen; reactions characterizing the properties of aldehydes and acids. Ions in this case are formed by heterolytic breaking of covalent polar bonds.

VIII. According to the type of energy

initiating the reaction, there are:

1. Photochemical reactions. They are initiated by light energy. In addition to the above photochemical processes of HCl synthesis or the reaction of methane with chlorine, they include the production of ozone in the troposphere as a secondary atmospheric pollutant. In this case, nitric oxide (IV) acts as the primary one, which forms oxygen radicals under the action of light. These radicals interact with oxygen molecules, resulting in ozone.

The formation of ozone goes on as long as there is enough light, since NO can interact with oxygen molecules to form the same NO 2 . The accumulation of ozone and other secondary air pollutants can lead to photochemical smog.

This type of reaction also includes the most important process that occurs in plant cells - photosynthesis, the name of which speaks for itself.

2. Radiation reactions. They are initiated by high-energy radiation - x-rays, nuclear radiation (γ-rays, a-particles - He 2+, etc.). With the help of radiation reactions, very fast radiopolymerization, radiolysis (radiation decomposition), etc. are carried out.

For example, instead of a two-stage production of phenol from benzene, it can be obtained by the interaction of benzene with water under the action of radiation. In this case, radicals [OH] and [H] are formed from water molecules, with which benzene reacts to form phenol:

C 6 H 6 + 2 [OH] → C 6 H 5 OH + H 2 O

Rubber vulcanization can be carried out without sulfur using radiovulcanization, and the resulting rubber will be no worse than traditional rubber.

3. Electrochemical reactions. They are initiated by an electric current. In addition to the electrolysis reactions well known to you, we will also indicate the reactions of electrosynthesis, for example, the reactions of the industrial production of inorganic oxidants

4. Thermochemical reactions. They are initiated by thermal energy. These include all endothermic reactions and many exothermic reactions that require an initial supply of heat, that is, the initiation of the process.

The above classification of chemical reactions is reflected in the diagram.

The classification of chemical reactions, like all other classifications, is conditional. Scientists agreed to divide the reactions into certain types according to the signs they identified. But most chemical transformations can be attributed to different types. For example, let's characterize the ammonia synthesis process.

This is a compound reaction, redox, exothermic, reversible, catalytic, heterogeneous (more precisely, heterogeneous catalytic), proceeding with a decrease in pressure in the system. To successfully manage the process, all of the above information must be taken into account. A specific chemical reaction is always multi-qualitative, it is characterized by different features.

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 proceeding 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 chemical reactions are called, 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 a 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 called 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

By type of break chemical bond in the original 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: