Chemical properties substances are detected in various 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 linking the left and right sides indicates that total 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.

Huge number 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 valence:

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

and be classified as redox:

2FeCl 2 + Cl 2 = 2FeCl 3.

2. Decomposition reactions

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

A = B + C + D.

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

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

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

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

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

Decomposition reactions in organic chemistry are called cracking:

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

or dehydrogenation

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

3. Substitution reactions

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

A + BC = AB + C.

These reactions in the vast majority belong to redox reactions:

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

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

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

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

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

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

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

Sometimes these reactions are considered as exchange reactions:

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

4. Exchange reactions

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

AB + CD = AD + CB.

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

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

AgNO 3 + KBr = AgBr + KNO 3,

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

A special case of these exchange reactions is neutralization reactions:

Hcl + KOH \u003d KCl + H 2 O.

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

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

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

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

5. Transfer reactions.

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

AB + BC \u003d A + B 2 C,

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

For example:

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

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

Classification of reactions according to phase features

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

1. Gas reactions

H 2 + Cl 2		2HCl.

2. Reactions in solutions

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

3. Reactions between solids

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

Classification of reactions according to the number of phases.

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

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

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

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

gas-liquid phase reactions

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

gas-solid-phase reactions

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

liquid-solid-phase reactions

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

liquid-gas-solid-phase reactions

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

Classification of reactions according to the type of particles carried

1. Protolytic reactions.

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

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

Protolytic reactions include neutralization and hydrolysis reactions.

2. Redox reactions.

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

Zn + 2H + → Zn 2 + + H 2 ,

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

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

3. Ligand exchange reactions.

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

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

Fe + 5CO = ,

Al(OH) 3 + NaOH = .

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

4. Reactions of atomic-molecular exchange.

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

Reversible and irreversible chemical reactions

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

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

Two oppositely directed arrows indicate that under the same conditions, both forward and reverse reactions proceed 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.

The chemical elements that make up animate and inanimate nature are in constant motion, because the substances that consist of these elements are constantly changing.

Chemical reactions (from Latin reaction - counteraction, repulse) - this is the response of substances to the influence of other substances and physical factors (temperature, pressure, radiation, etc.).

However, this definition also corresponds to the physical changes that occur with substances - boiling, melting, condensation, etc. Therefore, it is necessary to clarify that chemical reactions are processes that destroy old chemical bonds and create new ones and, as a result, from new substances are formed.

Chemical reactions are constantly taking place both inside our body and in the world around us. Countless reactions are usually classified according to various criteria. Let's recall from the 8th grade course the signs with which you are already familiar. To do this, we turn to a laboratory experiment.

Lab experience #3
Substitution of iron for copper in copper (II) sulfate solution

Pour 2 ml of copper (II) sulfate solution into a test tube and place a push pin or paper clip in it. What are you watching? Write down the reaction equations in molecular and ionic forms. Consider redox processes. Based on the molecular equation, assign this reaction to one or another group of reactions based on the following features: “the number and composition of the starting materials and reaction products” (as you probably remember, on this basis, reactions of combination, decomposition, substitution and exchange, including neutralization reactions, are distinguished); “direction” (recall that, according to this criterion, reactions are divided into two groups: reversible and irreversible); "thermal effect" (distinguish between endo- and exothermic reactions, including combustion reactions); "change in the oxidation states of the elements that form the substances involved in the reaction" (redox and without changing the oxidation states); "aggregate state of reacting substances" (homogeneous and heterogeneous); "participation of a catalyst" (non-catalytic and catalytic, including enzymatic). Now check yourself. CuSO 4 + Fe \u003d FeSO 4 + Cu. This is a substitution reaction, since a new simple and a new complex substance are formed from the initial simple and complex substances. This reaction is irreversible as it proceeds in only one direction. This reaction is probably exothermic, i.e., it proceeds with a slight release of heat (you can draw such a conclusion based on the fact that this reaction does not require heating of the contents of the test tube). This is a redox reaction, since copper and iron changed their oxidation states: (oxidizer) Cu 2+ + 2ё → Сu 0 (reduction) (reducing agent) Fe 0 - 2ё → Fe 2+ (oxidation) This reaction is heterogeneous, as it takes place between a solid and a solution. The reaction proceeds without the participation of a catalyst - non-catalytic. (Remember from the 8th grade course what substances are called catalysts. That's right, these are substances that speed up a chemical reaction.)

We have come to a very important concept in chemistry - "the rate of a chemical reaction." It is known that some chemical reactions proceed very quickly, others - for considerable periods of time. When a solution of silver nitrate is added to a solution of sodium chloride, a white cheesy precipitate almost instantly precipitates:

AgNO 3 + NaCl \u003d NaNO 3 + AgCl ↓.

Reactions proceed with great speeds, accompanied by an explosion (Fig. 11, 1). On the contrary, stalactites and stalagmites slowly grow in stone caves (Fig. 11, 2), steel products corrode (rust) (Fig. 11, 3), palaces and statues are destroyed under the action of acid rains (Fig. 11, 4).

Rice. eleven.
Chemical reactions occurring at great speeds (1) and very slowly (2-4)

The rate of a chemical reaction is understood as the change in the concentration of reactants per unit time: V p \u003d C 1 - C 2 /t.

In turn, concentration is understood as the ratio of the amount of a substance (as you know, it is measured in moles) to the volume that it occupies (in liters). From here it is not difficult to derive the unit of measurement of the rate of a chemical reaction - 1 mol / (l s).

The study of the rate of a chemical reaction is a special branch of chemistry called chemical kinetics.

Knowing its patterns allows you to control a chemical reaction, making it proceed faster or slower.

What factors affect the rate of a chemical reaction?

1. Nature of the reactants. Let's turn to the experiment.

Laboratory experiment No. 4
The dependence of the rate of a chemical reaction on the nature of the reactants on the example of the interaction of acids with metals

Pour 1-2 ml of hydrochloric acid into two test tubes and place: in the 1st - a zinc granule, in the 2nd - a piece of iron of the same size. The nature of what reagent affects the rate of interaction between an acid and a metal? Why? Write down the reaction equations in molecular and ionic forms. Consider them from the standpoint of oxidation-reduction. Then place in two other test tubes on the same zinc granule and add solutions of acids of the same concentration to them: in the 1st - hydrochloric acid, in the 2nd - acetic. The nature of what reagent affects the rate of interaction between an acid and a metal? Why? Write down the reaction equations in molecular and ionic forms. Consider them from the standpoint of oxidation-reduction.

2. Concentration of reactants. Let's turn to the experiment.

Laboratory experiment No. 5
Dependence of the rate of a chemical reaction on the concentration of reactants on the example of the interaction of zinc with hydrochloric acid of various concentrations

It is easy to conclude: the higher the concentration of reactants, the higher the rate of interaction between them.

The concentration of gaseous substances for homogeneous production processes is increased by increasing the pressure. For example, this is done in the production of sulfuric acid, ammonia, ethyl alcohol.

The factor of the dependence of the rate of a chemical reaction on the concentration of reacting substances is taken into account not only in production, but also in other areas of human life, for example, in medicine. Patients with lung diseases, in whom the rate of interaction of blood hemoglobin with atmospheric oxygen is low, facilitate breathing with the help of oxygen pillows.

3. Contact area of ​​reactants. An experiment illustrating the dependence of the rate of a chemical reaction on this factor can be performed using the following experiment.

Laboratory experiment No. 6
The dependence of the rate of a chemical reaction on the area of ​​contact of the reactants

For heterogeneous reactions: the larger the contact area of ​​the reactants, the faster the reaction rate.

You could see this from personal experience. To kindle a fire, you put small chips under the firewood, and under them - crumpled paper, from which the whole fire caught fire. On the contrary, extinguishing a fire with water is to reduce the area of ​​​​contact of burning objects with air.

In production, this factor is taken into account on purpose, the so-called fluidized bed is used. To increase the rate of the reaction, the solid is crushed almost to the state of dust, and then a second substance, usually gaseous, is passed through it from below. Passing it through a finely divided solid creates a boiling effect (hence the name of the method). The fluidized bed is used, for example, in the production of sulfuric acid and petroleum products.

Laboratory experiment No. 7
Fluidized Bed Modeling

4. Temperature. Let's turn to the experiment.

Laboratory experiment No. 8
The dependence of the rate of a chemical reaction on the temperature of the reacting substances on the example of the interaction of copper oxide (II) with a solution of sulfuric acid at different temperatures

It is easy to conclude that the higher the temperature, the faster the reaction rate.

The first Nobel Prize winner, the Dutch chemist J. X. Van't Hoff, formulated the rule:

In production, as a rule, high-temperature chemical processes are used: in the smelting of iron and steel, the melting of glass and soap, the production of paper and petroleum products, etc. (Fig. 12).

Rice. 12.
High-temperature chemical processes: 1 - iron smelting; 2 - glass melting; 3 - production of petroleum products

The fifth factor on which the rate of a chemical reaction depends is catalysts. You will meet him in the next paragraph.

New words and concepts

1. Chemical reactions and their classification.
2. Signs of the classification of chemical reactions.
3. The rate of a chemical reaction and the factors on which it depends.

Tasks for independent work

1. What is a chemical reaction? What is the essence of chemical processes?
2. Give a complete classification of the following chemical processes:
   • a) the burning of phosphorus;
   • b) the interaction of a solution of sulfuric acid with aluminum;
   • c) neutralization reactions;
   • d) the formation of nitric oxide (IV) from nitric oxide (II) and oxygen.
3. Based on personal experience, give examples of chemical reactions occurring at different rates.
4. What is the rate of a chemical reaction? What factors does it depend on?
5. Give examples of the influence of various factors on biochemical and industrial chemical processes.
6. Based on personal experience, give examples of the influence of various factors on chemical reactions that occur in everyday life.
7. Why is food stored in the refrigerator?
8. The chemical reaction was started at a temperature of 100°C, then raised to 150°C. The temperature coefficient of this reaction is 2. How many times will the rate of the chemical reaction increase?

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 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.