Chemical reactions, their properties, types, conditions, etc., are one of the cornerstones of interesting science called chemistry. Let's try to figure out what a chemical reaction is and what its role is. So, a chemical reaction in chemistry is considered to be the transformation of one or more substances into other substances. At the same time, their nuclei do not change (unlike nuclear reactions), but there is a redistribution of electrons and nuclei, and, of course, new chemical elements appear.

Chemical reactions in nature and everyday life

You and I are surrounded by chemical reactions, moreover, we ourselves regularly carry them out by various household activities, when, for example, we light a match. Especially a lot chemical reactions themselves without suspecting it (and maybe suspecting) cooks do when they cook food.

Of course, also in natural conditions many chemical reactions take place: the eruption of a volcano, foliage and trees, but what can I say, almost any biological process can be attributed to examples of chemical reactions.

Types of chemical reactions

All chemical reactions can be divided into simple and complex. Simple chemical reactions, in turn, are divided into:

  • compound reactions,
  • decomposition reactions,
  • substitution reactions,
  • exchange reactions.

Chemical reaction of the compound

According to the very apt definition of the great chemist D. I. Mendeleev, the reaction of a compound takes place when “one of their two substances occurs.” An example of a chemical reaction of a compound can be the heating of powders of iron and sulfur, in which iron sulfide is formed from them - Fe + S = FeS. Another striking example of this reaction is combustion. simple substances, such as sulfur or phosphorus in air (perhaps a similar reaction can also be called a thermal chemical reaction).

Decomposition chemical reaction

It's simple, the decomposition reaction is the opposite of the compound reaction. It produces two or more substances from one substance. A simple example The chemical reaction of decomposition can be the reaction of decomposition of chalk, during which quicklime and carbon dioxide are formed from the actual chalk.

Chemical substitution reaction

The substitution reaction is carried out when a simple substance interacts with a complex one. Let's give an example of a chemical substitution reaction: if you lower a steel nail into a solution with blue vitriol, then during this simple chemical experience we get iron sulfate (iron will displace copper from salt). The equation for such a chemical reaction would look like this:

Fe + CuSO 4 → FeSO 4 + Cu

Chemical exchange reaction

Exchange reactions take place exclusively between complex chemicals, during which they change their parts. A lot of these reactions take place in various solutions. Neutralization of acid by bile good example chemical exchange reaction.

NaOH + HCl → NaCl + H 2 O

This is the chemical equation of this reaction, in which a hydrogen ion from the HCl compound is exchanged with a sodium ion from the NaOH compound. The consequence of this chemical reaction is the formation of a salt solution.

Signs of chemical reactions

According to the signs of the occurrence of chemical reactions, one can judge whether a chemical reaction between the reagents has passed or not. Here are examples of signs of chemical reactions:

  • Color change (light iron, for example, in humid air is covered with a brown coating, as a result of a chemical reaction between iron and iron).
  • Precipitation (if carbon dioxide is suddenly passed through a lime solution, we will get a precipitation of a white insoluble precipitate of calcium carbonate).
  • Gas release (if you drop it on baking soda citric acid, you get the release of carbon dioxide).
  • The formation of weakly dissociated substances (all reactions resulting in the formation of water).
  • The glow of the solution (an example here is the reactions that occur with a solution of luminol, which emits light during chemical reactions).

In general, it is difficult to distinguish which signs of chemical reactions are the main ones for different substances and different reactions characteristic features.

How to determine the sign of a chemical reaction

You can determine the sign of a chemical reaction visually (with a change in color, glow), or by the results of this very reaction.

The rate of a chemical reaction

The rate of a chemical reaction is usually understood as the change in the amount of one of the reactants per unit of time. Moreover, the rate of a chemical reaction is always a positive value. In 1865, the chemist N. N. Beketov formulated the law of mass action, which states that "the rate of a chemical reaction at any given time is proportional to the concentrations of reagents raised to powers equal to their stoichiometric coefficients."

Factors in the rate of a chemical reaction include:

  • the nature of the reactants
  • the presence of a catalyst
  • temperature,
  • pressure,
  • the surface area of ​​the reactants.

All of them have the most direct influence on the rate of a chemical reaction.

Equilibrium of a chemical reaction

Chemical equilibrium is such a state of a chemical system in which several chemical reactions occur and the rates in each pair of forward and reverse reactions are equal. Thus, the equilibrium constant of a chemical reaction is singled out - this is the value that determines for a given chemical reaction the ratio between the thermodynamic activities of the starting substances and products in the state chemical equilibrium. Knowing the equilibrium constant, you can determine the direction of a chemical reaction.

Conditions for the occurrence of chemical reactions

To initiate chemical reactions, it is necessary to create the appropriate conditions for this:

  • bringing substances into close contact.
  • heating substances to a certain temperature (the temperature of the chemical reaction must be appropriate).

Thermal effect of a chemical reaction

This is the name given to the change in the internal energy of the system as a result of the occurrence of a chemical reaction and the transformation of the starting materials (reactants) into reaction products in quantities corresponding to the chemical reaction equation under the following conditions:

  • the only possible work in this case is only work against external pressure.
  • starting materials and the products resulting from a chemical reaction have the same temperature.

Chemical reactions, video

And in conclusion, an interesting video about the most amazing chemical reactions.

DEFINITION

Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.

Most often, chemical reactions are understood as the process of transformation of initial substances (reagents) into final substances (products).

Chemical reactions are written using chemical equations containing the formulas of the starting materials and reaction products. According to the law of conservation of mass, the number of atoms of each element in the left and right sides chemical equation equally. Usually, the formulas of the starting substances are written on the left side of the equation, and the formulas of the products are written on the right. The equality of the number of atoms of each element in the left and right parts of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.

Chemical equations may contain additional information about the features of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or “under”) the equals sign.

All chemical reactions can be grouped into several classes, which have certain characteristics.

Classification of chemical reactions according to the number and composition of the initial and resulting substances

According to this classification, chemical reactions are divided into reactions of combination, decomposition, substitution, exchange.

As a result compound reactions from two or more (complex or simple) substances, one new substance is formed. IN general view The equation for such a chemical reaction will look like this:

For example:

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

SO 3 + H 2 O \u003d H 2 SO 4

2Mg + O 2 \u003d 2MgO.

2FeCl 2 + Cl 2 = 2FeCl 3

Combination reactions are in most cases exothermic, i.e. flow with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox (ORD), i.e. occur with a change in the oxidation states of the elements. It is impossible to say unequivocally whether the reaction of a compound between complex substances can be attributed to OVR.

Reactions in which several other new substances (complex or simple) are formed from one complex substance are classified as decomposition reactions. In general, the equation for a chemical decomposition reaction will look like this:

For example:

CaCO 3 CaO + CO 2 (1)

2H 2 O \u003d 2H 2 + O 2 (2)

CuSO 4 × 5H 2 O \u003d CuSO 4 + 5H 2 O (3)

Cu (OH) 2 \u003d CuO + H 2 O (4)

H 2 SiO 3 \u003d SiO 2 + H 2 O (5)

2SO 3 \u003d 2SO 2 + O 2 (6)

(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O (7)

Most decomposition reactions proceed with heating (1,4,5). May be decomposed by electric current(2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) proceeds without changing the oxidation states of the elements, i.e. these reactions do not apply to OVR. OVR decomposition reactions include the decomposition of oxides, acids and salts, formed by elements V higher degrees oxidation (6).

Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):

C 18 H 38 \u003d C 9 H 18 + C 9 H 20 (8)

C 4 H 10 \u003d C 4 H 6 + 2H 2 (9)

At substitution reactions a simple substance interacts with a complex one, forming a new simple and new complex substance. In general, the equation for a chemical substitution reaction will look like this:

For example:

2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3 (1)

Zn + 2HCl = ZnCl 2 + H 2 (2)

2KBr + Cl 2 \u003d 2KCl + Br 2 (3)

2KSlO 3 + l 2 = 2KlO 3 + Cl 2 (4)

CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2 (5)

Ca 3 (RO 4) 2 + ZSiO 2 = ZCaSiO 3 + P 2 O 5 (6)

CH 4 + Cl 2 = CH 3 Cl + Hcl (7)

Substitution reactions are mostly redox reactions (1 - 4, 7). Examples of decomposition reactions in which there is no change in oxidation states are few (5, 6).

Exchange reactions called the reactions that occur between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions in aqueous solution. In general, the equation for a chemical exchange reaction will look like this:

AB + CD = AD + CB

For example:

CuO + 2HCl \u003d CuCl 2 + H 2 O (1)

NaOH + HCl \u003d NaCl + H 2 O (2)

NaHCO 3 + HCl \u003d NaCl + H 2 O + CO 2 (3)

AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)

CrCl 3 + ZNaOH = Cr(OH) 3 ↓+ ZNaCl (5)

Exchange reactions are not redox. special case these exchange reactions are neutralization reactions (reactions of interaction of acids with alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5) or a low-dissociating compound, most often water (1, 2).

Classification of chemical reactions according to changes in oxidation states

Depending on the change in the oxidation states of the elements that make up the reactants and reaction products, all chemical reactions are divided into redox (1, 2) and those occurring without changing the oxidation state (3, 4).

2Mg + CO 2 \u003d 2MgO + C (1)

Mg 0 - 2e \u003d Mg 2+ (reductant)

C 4+ + 4e \u003d C 0 (oxidizing agent)

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

Fe 2+ -e \u003d Fe 3+ (reductant)

N 5+ + 3e \u003d N 2+ (oxidizing agent)

AgNO 3 + HCl \u003d AgCl ↓ + HNO 3 (3)

Ca(OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)

Classification of chemical reactions by thermal effect

Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conditionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during a reaction is called the heat of the reaction. If the equation indicates the amount of released or absorbed heat, then such equations are called thermochemical.

N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)

2Mg + O 2 \u003d 2MgO + 602.5 kJ (2)

N 2 + O 2 \u003d 2NO - 90.4 kJ (3)

Classification of chemical reactions according to the direction of the reaction

According to the direction of the reaction, there are reversible (chemical processes whose products are able to react with each other under the same conditions in which they are obtained, with the formation of starting substances) and irreversible (chemical processes, the products of which are not able to react with each other with the formation of starting substances ).

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

A + B ↔ AB

For example:

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

Examples of irreversible reactions are the following reactions:

2KSlO 3 → 2KSl + ZO 2

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O

Evidence of the irreversibility of the reaction can serve as the reaction products of a gaseous substance, a precipitate or a low-dissociating compound, most often water.

Classification of chemical reactions by the presence of a catalyst

From this point of view, catalytic and non-catalytic reactions are distinguished.

A catalyst is a substance that speeds up a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:

2H 2 O 2 \u003d 2H 2 O + O 2 (MnO 2 catalyst)

Often, one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):

MeO + 2HF \u003d MeF 2 + H 2 O, where Me is a metal.

Examples of problem solving

EXAMPLE 1

IN modern science distinguish between chemical and nuclear reactions that occur as a result of the interaction of the starting substances, which are commonly called reagents. As a result, other chemicals are formed, which are called products. All interactions occur under certain conditions (temperature, radiation, the presence of catalysts, etc.). The nuclei of atoms of the reactants of chemical reactions do not change. In nuclear transformations, new nuclei and particles are formed. There are several different signs by which the types of chemical reactions are determined.

The classification can be based on the number of initial and resulting substances. In this case, all types of chemical reactions are divided into five groups:

  1. Decompositions (several new ones are obtained from one substance), for example, decomposition when heated to potassium chloride and oxygen: KCLO3 → 2KCL + 3O2.
  2. Compounds (two or more compounds form one new one), interacting with water, calcium oxide turns into calcium hydroxide: H2O + CaO → Ca(OH)2;
  3. Substitutions (the number of products is equal to the number of starting substances in which one component is replaced by another), iron in copper sulfate, replacing copper, forms ferrous sulfate: Fe + CuSO4 → FeSO4 + Cu.
  4. Double exchange (molecules of two substances exchange the parts that leave them), metals in and exchange anions, forming precipitated silver iodide and cadium nitrate: KI + AgNO3 → AgI↓ + KNO3.
  5. Polymorphic transformation (there is a transition of a substance from one crystalline form to another), color iodide, when heated, turns into yellow mercury iodide: HgI2 (red) ↔ HgI2 (yellow).

If chemical transformations are considered on the basis of changes in the oxidation state of elements in the reacting substances, then the types of chemical reactions can be divided into groups:

  1. With a change in the degree of oxidation - redox reactions (ORD). As an example, consider the interaction of iron with hydrochloric acid: Fe + HCL → FeCl2 + H2, as a result, the oxidation state of iron (electron donating reducing agent) changed from 0 to -2, and hydrogen (electron accepting oxidizing agent) from +1 to 0.
  2. No change in oxidation state (i.e., no OVR). For example, the reactions of acid-base interaction of hydrogen bromide with sodium hydroxide: HBr + NaOH → NaBr + H2O, as a result of such reactions, salt and water are formed, and the oxidation states of the chemical elements included in the starting substances do not change.

If we consider the flow rate in the forward and reverse directions, then all types of chemical reactions can also be divided into two groups:

  1. Reversible - those that flow in two directions at the same time. Most reactions are reversible. An example is the dissolution of carbon dioxide in water with the formation of unstable carbonic acid, which decomposes into the starting substances: H2O + CO2 ↔ H2CO3.
  2. Irreversible - flow only in the forward direction, after the complete consumption of one of the starting substances, they are completed, after which only products and the starting substance, taken in excess, are present. Usually one of the products is either precipitated insoluble matter or evolved gas. For example, when sulfuric acid and barium chloride react: H2SO4 + BaCl2 + → BaSO4↓ + 2HCl, an insoluble

The types of chemical reactions in organic chemistry can be divided into four groups:

  1. Substitution (one atoms or groups of atoms are replaced by others), for example, when chloroethane reacts with sodium hydroxide, ethanol and sodium chloride are formed: C2H5Cl + NaOH → C2H5OH + NaCl, that is, the chlorine atom is replaced by a hydrogen atom.
  2. Attachment (two molecules react and form one), for example, bromine joins at the site of the double bond break in the ethylene molecule: Br2 + CH2=CH2 → BrCH2—CH2Br.
  3. Cleavage (a molecule decomposes into two or more molecules), for example, under certain conditions, ethanol decomposes into ethylene and water: C2H5OH → CH2=CH2 + H2O.
  4. Rearrangement (isomerization, when one molecule turns into another, but the qualitative and quantitative composition of the atoms in it does not change), for example, 3-chlororuten-1 (C4H7CL) turns into 1 chlorobutene-2 ​​(C4H7CL). Here the chlorine atom moved from the third carbon atom in the hydrocarbon chain to the first, and the double bond connected the first and second carbon atoms, and then began to connect the second and third atoms.

Other types of chemical reactions are also known:

  1. By flowing with absorption (endothermic) or release of heat (exothermic).
  2. According to the type of reactants or products formed. Interaction with water - hydrolysis, with hydrogen - hydrogenation, with oxygen - oxidation or combustion. The splitting off of water is dehydration, hydrogen is dehydrogenation, and so on.
  3. According to the conditions of interaction: in the presence under the influence of low or high temperature, when pressure changes, in the light and so on.
  4. According to the mechanism of the reaction: ionic, radical chain or chain reactions.

The concept of "compound reaction" is the antonym of the concept of "decomposition reaction". Try, using the opposition technique, to define the concept of "compound reaction". Right! You have the following wording.

Let's consider this type of reactions with the help of another, new for you, form of recording chemical processes - the so-called chains of transitions, or transformations. For example, schema

shows the conversion of phosphorus to phosphorus oxide (V) P 2 O 5 , which, in turn, is then converted to phosphoric acid H 3 PO 4 .

The number of arrows in the scheme of transformation of substances corresponds to the minimum number of chemical transformations - chemical reactions. In this example, these are two chemical processes.

1st process. Obtaining phosphorus oxide (V) Р 2 O 5 from phosphorus. Obviously, this is a reaction of the combination of phosphorus with oxygen.

Let's put some red phosphorus in a spoon for burning substances and set it on fire. Phosphorus burns with a bright flame, producing white smoke consisting of small particles of phosphorus (V) oxide:

4P + 5O 2 \u003d 2P 2 O 5.

2nd process. Let's put a spoonful of burning phosphorus into the flask. It is filled with dense smoke from phosphorus oxide (V). We take out a spoon from the flask, pour water into the flask and shake the contents, after closing the neck of the flask with a cork. The smoke gradually thins, dissolves in water, and finally disappears completely. If a little litmus is added to the solution obtained in the flask, it will turn red, which is evidence of the formation of phosphoric acid:

P 2 O 5 + ZN 2 O \u003d 2H 3 RO 4.

The reactions that are carried out to carry out the transitions under consideration proceed without the participation of a catalyst, therefore they are called non-catalytic. The reactions considered above proceed only in one direction, i.e., they are irreversible.

Let us analyze how many and what substances entered into the above reactions and how many and what substances were formed in them. In the first reaction, one complex substance was formed from two simple substances, and in the second - from two complex substances, each of which consists of two elements, one complex substance was formed, already consisting of three elements.

One complex substance can also be formed as a result of the reaction of the combination of complex and simple substances. For example, in the production of sulfuric acid from sulfur oxide (IV), sulfur oxide (VI) is obtained:

This reaction proceeds both in the forward direction, i.e., with the formation of a reaction product, and in the opposite direction, i.e., the reaction product decomposes into the starting substances, therefore, instead of the equal sign, they put the sign of reversibility.

This reaction involves a catalyst - vanadium (V) oxide V 2 O 5, which is indicated above the reversibility sign:

A complex substance can also be obtained in the reaction of the combination of three substances. For example, nitric acid is obtained by a reaction whose scheme is:

NO 2 + H 2 O + O 2 → HNO 3.

Consider how to choose the coefficients to equalize the scheme of this chemical reaction.

The number of nitrogen atoms does not need to be equalized: both in the left and in the right parts of the scheme, one nitrogen atom each. Equalize the number of hydrogen atoms - in front of the acid formula we write the coefficient 2:

NO 2 + H 2 O + O 2 → 2HNO 3.

but in this case, the equality of the number of nitrogen atoms will be violated - one nitrogen atom remains on the left side, and there are two of them on the right. We write the coefficient 2 in front of the formula for nitric oxide (IV):

2NO 2 + H 2 O + O 2 → 2HNO 3.

Let's count the number of oxygen atoms: there are seven on the left side of the reaction scheme, and six on the right side. To equalize the number of oxygen atoms (six atoms in each part of the equation), remember that before the formulas of simple substances, you can write the fractional coefficient 1/2:

2NO 2 + H 2 O + 1/2O 2 → 2HNO 3 .

Let's make the coefficients integers. To do this, we rewrite the equation by doubling the coefficients:

4NO 2 + 2Н 2 O + O 2 → 4HNO 3.

It should be noted that almost all compound reactions are exothermic reactions.

Laboratory experiment No. 15
Calcination of copper in the flame of an alcohol lamp

    Consider the copper wire (plate) given to you and describe it appearance. Ignite the wire, holding it with crucible tongs, in the upper part of the spirit lamp flame for 1 minute. Describe the conditions for the reaction. Describe a sign confirming that a chemical reaction has occurred. Write an equation for the reaction. Name the starting materials and products of the reaction.

    Explain whether the mass of the copper wire (plate) has changed after the end of the experiment. Justify your answer using knowledge of the law of conservation of mass of substances.

Keywords and phrases

  1. Combination reactions are antonyms for decomposition reactions.
  2. Catalytic (including enzymatic) and non-catalytic reactions.
  3. Chains of transitions, or transformations.
  4. Reversible and irreversible reactions.

Work with computer

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