Chemical equations classification of chemical reactions. Reversible and irreversible chemical reactions

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 sides indicates that the total number of atoms of the substances participating 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 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, 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 the transfer of an electron pair occurs 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 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.

Department of Education of the Ivanovo Region

Regional State Budgetary Vocational Educational Institution

Southern Technological College

METHODOLOGICAL DEVELOPMENT

OPEN LESSON IN CHEMISTRY

On the topic:

« Classification of chemical reactions»

Lecturer: Vdovin Yu.A.

Well:I

Group: 39-40

Yuzha - 2017

Lesson topic:	Classification of chemical reactions
Lesson Objectives:	Expand and deepen knowledge of chemical reactions, compare them with other types of phenomena. Learn to highlight the essential features that can be used as the basis for the classification of chemical reactions. Consider the classification of chemical reactions according to various criteria.
Lesson objectives:	1. Educational - to systematize, generalize and deepen students' knowledge of chemical reactions and their classification, develop independent work skills, the ability to write reaction equations and set coefficients, indicate types of reactions, draw conclusions and generalizations. 2. Developing - to develop a culture of speech using chemical terms and formulas, the development of cognitive abilities, thinking, attention. 3. Educational - education of independence, perseverance, attentiveness, tolerance.
Lesson type:	Combined
Equipment and reagents:	Reagents: Ammonium nitrate, sodium hydroxide, ammonium hydroxide, copper (II) sulfate, sodium carbonate, hydrochloric acid, potassium hexacyanoferrate (III), iron (III) chloride, potassium permanganate, sulfuric acid, ethanol. Equipment: Test tubes, bottles with solutions, pipettes, stands, Petri dish, porcelain evaporation dish, glass rod, cotton wool, metal tray.
Teaching methods	Verbal (conversation, explanation) Problem-based learning methods, laboratory experience.
Forms of work:	individual, frontal.

Lesson plan:

During the classes:

1. Organizational moment (1 min)

A) greeting;

B) Safety precautions;

2. Motivation (2 min)

Introduction:

A huge number of reactions take place in the world around us. Here we are just sitting, standing, going somewhere, and in every cell of our body every second there are tens and hundreds of thousands of transformations of one substance into another.

Almost as good as a living organism and inanimate matter. Somewhere now, right at this moment, a chemical cycle is taking place: some molecules disappear, others arise, and these processes never stop.

If all of them suddenly ceased, the world would become silent. How to keep in mind the diversity of chemical processes, how to practically navigate them? How do biologists manage to navigate the diversity of living organisms? (creating a problematic situation).

Suggested answer: In any science, a classification technique is used that allows one to divide the entire set of objects into groups according to common features.

Let's formulate the topic of the lesson: Classification of chemical reactions.

Any lesson should have goals.

Let's formulate the goals of today's lesson?

What should we consider?

What is worth learning?

Consider possible classifications of chemical reactions.

Learn to highlight the signs by which the classification of reactions is made.

What is the use of classifying chemical reactions?

Suggested answer: It helps to generalize, structure knowledge about chemical processes, highlight something in common and predict, based on existing knowledge, something else unknown, but similar to the known.

And where can knowledge of the classification of chemical reactions be applied in your practice?

Suggested answer: some classes of chemical reactions can be useful to us in practical activities. For example, such an important phenomenon for you as electroplating is based on redox processes. I think the concept of "Galvanic cells" is painfully familiar to you!

In addition, knowledge of the class of chemical reaction of a process can help in managing this process.

3. Actualization of knowledge (6 min)

A) Task with cards on the difference between physical processes and chemical reactions (2 min).

The task is performed by a student on a magnetic board and in parallel with a group presentation.

Take a look at these phenomena known to all of you. Divide them into groups. Name the groups and define each group.

B) Repetition of safety precautions

Conducting laboratory experiments (3 min)

And how can we know that we have a chemical reaction going on?

I. Classification of reactions according to the number and composition of reactants

A. Reactions occurring without changing the qualitative composition of the substance . These are numerous allotropic transformations of simple substances (for example, oxygen ↔ ozone (3O 2 ↔ 2O 3), white tin ↔ gray tin); transition with a change in temperature of some solids from one crystalline state to another - polymorphic transformations(for example, red crystals of mercury (II) iodide, when heated, turn into a yellow substance of the same composition, when cooled, the reverse process occurs); isomerization reactions (for example, NH 4 OCN ↔ (NH 2) 2 CO), etc.

B. Reactions occurring with a change in the composition of the reactants.

Connection reactions Reactions in which two or more starting materials form one new compound. Source substances can be both simple and complex, for example:

4P + 5O 2 \u003d 2P 2 O 5; 4NO 2 + O 2 + 2H 2 O \u003d 4HNO 3; CaO + H 2 O \u003d Ca (OH) 2.

Decomposition reactions are reactions in which two or more new substances are formed from one initial complex substance. Substances formed in reactions of this type can be both simple and complex, for example:

2HI \u003d H 2 + I 2; CaCO 3 \u003d CaO + CO 2; (CuOH) 2 CO 3 \u003d CuO + H 2 O + CO 2.

Substitution reactions- These are processes in which atoms of a simple substance replace the atoms of an element in a complex substance. Since a simple substance is necessarily involved in substitution reactions as one of the reagents, almost all transformations of this type are redox, for example:

Zn + H 2 SO 4 \u003d H 2 + ZnSO 4; 2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3; H 2 S + Br 2 \u003d 2HBr + S.

Exchange reactions are reactions in which two compounds exchange their constituents. Exchange reactions can proceed directly between two reagents without the participation of a solvent, for example: H 2 SO 4 + 2KOH \u003d K 2 SO 4 + 2H 2 O; SiO 2 (tv) + 4HF (g) \u003d SiF 4 + 2H 2 O.

Exchange reactions occurring in electrolyte solutions are called ion exchange reactions. Such reactions are possible only if one of the formed substances is a weak electrolyte, is released from the reaction sphere in the form of a gas or a sparingly soluble substance (Berthollet's rule):

AgNO 3 + HCl \u003d AgCl ↓ + HNO 3, or Ag + + Cl - \u003d AgCl ↓;

NH 4 Cl + KOH \u003d KCl + NH 3 + H 2 O, or NH 4 + + OH - \u003d H 2 O + NH 3;

NaOH + HCl \u003d NaCl + H 2 O, or H + + OH - \u003d H 2 O.

II. Classification of reactions by thermal effect

BUT. Reactions proceeding with the release of thermal energy –exothermic reactions (+ Q).

B. Reactions proceeding with the absorption of heat –endothermic reactions (-Q).

thermal effect Reaction refers to the amount of heat that is released or absorbed as a result of a chemical reaction. The reaction equation in which its thermal effect is indicated is called thermochemical. It is convenient to give the value of the heat effect of the reaction per 1 mol of one of the participants in the reaction, therefore, in thermochemical equations one can often find fractional coefficients:

1/2N 2 (g) + 3/2H 2 (g) = NH 3 (g) + 46.2 kJ / mol.

Exothermic are all combustion reactions, the vast majority of oxidation and combination reactions. Decomposition reactions usually require energy.

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 are widespread: the flow of electric current through a metal conductor, forging and melting of metal, the release of heat, the transformation of 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 absorb heat from the 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 bichromate, oxygen, ozone, hydrogen peroxide, nitric, sulfuric, selenic acids, hypochlorites, perchlorates, chlorates, aqua regia, a mixture of concentrated nitric and hydrofluoric acids, anode in 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 sides 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 potassium permanganate solution, 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 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...

	Sign of a leak	Reaction scheme
	The appearance of an odor
	Precipitation
	Dissolution of the precipitate
	Gas evolution
	Color change
	light emission
	Selection or heat absorption

Connection reactions	Reactions in which two or more substances form one compound
Decomposition reactions	Reactions in which several new substances are formed from a complex substance.
Substitution reactions	Reactions in which atoms of a simple substance replace the atoms of one of the elements in a complex substance.
Exchange reactions	Reactions in which two compounds exchange their constituents.
exothermic reactions	Reactions that proceed with the release of heat.
Endothermic reactions	Reactions proceeding with the absorption of heat.
catalytic reactions	Reactions that take place with the participation of a catalyst.
Non-catalytic reactions	Reactions that take place without a catalyst.
redox	Reactions that occur with a change in the oxidation states of the elements that form the substances involved in the reaction.
Reversible reactions	Chemical reactions that occur simultaneously in two opposite directions - forward and reverse.
irreversible reactions	Chemical reactions, as a result of which the initial substances are almost completely converted into final products.
Homogeneous reactions	Reactions that take place in a homogeneous medium, such as a mixture of gases or solutions.
heterogeneous reactions	Reactions that occur between substances in a heterogeneous environment.