Compounds can be formed in reactions. Introduction to General Chemistry. B. A reaction between substances in which atoms of a simple substance replace the atoms of one of the elements in a complex substance

1. Reactions of the compound. DI Mendeleev defined a compound as a reaction “in which one of two substances occurs. So, in the reactions of a compound from several reacting substances of a relatively simple composition, one substance of a more complex composition is obtained

A + B + C = D

Compound reactions include the combustion of simple substances (sulfur, phosphorus, carbon) in air. For example, carbon burns in air C + O2 \u003d CO2 (of course, this reaction proceeds gradually, first carbon monoxide CO is formed). As a rule, these reactions are accompanied by the release of heat, i.e. lead to the formation of more stable and less energy-rich compounds - are exothermic.

The reactions of compounding simple substances are always of a redox character. Compound reactions occurring between complex substances can occur as without changing the valence

CaCO3 + CO2 + H2O \u003d Ca (HCO3) 2

so also refer to the number of redox

2FеСl2 + Сl2 \u003d 2FеСl3.

2. Decomposition reactions. Chemical decomposition reactions, according to Mendeleev, “constitute cases opposite to the combination, that is, those in which one substance gives two, or, in general, a given number of substances - a greater number of them.

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

A \u003d B + C + D

The decomposition products of a complex substance can be both simple and complex substances. An example of a decomposition reaction is the chemical reaction of the decomposition of chalk (or limestone under the influence of temperature): CaCO3 \u003d CaO + CO2. The decomposition reaction usually requires heating. Such processes are endothermic, i.e. flow with heat absorption. Of the decomposition reactions proceeding without changing the valence states, the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids should be noted

CuSO4 5H2O \u003d CuSO4 + 5H2O,

Cu (OH) 2 \u003d CuO + H2O,

H2SiO3 \u003d SiO2 + H2O.

The decomposition reactions of a redox character include the decomposition of oxides, acids and salts formed by elements in higher oxidation states

2SO3 \u003d 2SO2 + O2,

4HNO3 \u003d 2H2O + 4NO2O + O2O,

2AgNO3 \u003d 2Ag + 2NO2 + O2,

(NH4) 2Cr2O7 \u003d Cr2O3 + N2 + 4H2O.

Redox decomposition reactions are especially characteristic for nitric acid salts.

Decomposition reactions in organic chemistry, in contrast to decomposition reactions in inorganic chemistry, have their own specifics. They can be considered as the reverse processes of joining, since as a result, multiple bonds or cycles are most often formed.

Decomposition reactions in organic chemistry are called cracking

C18H38 \u003d C9H18 + C9H20

or dehydrogenation C4H10 \u003d C4H6 + 2H2.

In the other two types of reactions, the number of reagents is equal to the number of products.

3. Substitution reactions. Their distinguishing feature is the interaction of a simple substance with a complex one. Such reactions also exist in organic chemistry. However, the concept of "substitution" in organic matter is broader than in inorganic chemistry. If in the molecule of the starting substance any atom or functional group is replaced by another atom or group, these are also substitution reactions, although from the point of view of inorganic chemistry, the process looks like an exchange reaction.

In substitution reactions, usually a simple substance interacts with a complex one, forming another simple substance and another complex A + BC \u003d AB + C

For example, dropping a steel nail into a solution of copper sulfate, we get iron sulfate (iron displaced copper from its salt) Fe + CuSO4 \u003d FeSO4 + Cu.

These reactions in the overwhelming majority belong to the redox

2Аl + Fe2O3 \u003d 2Fе + Аl2О3,

Zn + 2HCl \u003d ZnCl2 + H2,

2KBr + Cl2 \u003d 2KCl + Br2,

2KLO3 + l2 \u003d 2KlO3 + Cl2

Examples of substitution reactions not accompanied by a change in the valence states of atoms are extremely few.

It should be noted the reaction of silicon dioxide with salts of oxygen-containing acids, which correspond to gaseous or volatile anhydrides

CaCO3 + SiO2 \u003d CaSiO3 + CO2,

Ca3 (PO4) 2 + 3SiO2 \u003d 3CaSiO3 + P2O5.

Sometimes these reactions are considered as exchange reactions

CH4 + Cl2 \u003d CH3Cl + HCl.

4. Reactions of exchange (including neutralization). Exchange reactions are called reactions between two compounds that exchange their constituent parts with each other

AB + CD \u003d AD + CB

A large number of them occur in aqueous solutions. An example of a chemical exchange reaction is the neutralization of an acid with an alkali

NaOH + HCl \u003d NaCl + H2O.

Here, in the reagents (substances on the left), the hydrogen ion from the HCl compound is exchanged with the sodium ion from the NaOH compound, resulting in a solution of sodium chloride in water.

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

ZnO + Н2SО4 \u003d ZnSО4 + Н2О,

AgNO3 + KBr \u003d AgBr + KNO3,

CrCl3 + 3NaOH \u003d Cr (OH) 3 + 3NaCl.

A special case of these exchange reactions is the neutralization reaction

HCl + KOH \u003d KCl + H2O.

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

NaHCO3 + HCl \u003d NaCl + H2O + CO2,

Ca (HCO3) 2 + Ca (OH) 2 \u003d 2CaCO3 ↓ + 2H2O,

CH3COONa + H3PO4 \u003d CH3COOH + NaH2PO4.

However, many reactions do not fit into this simple scheme. For example, the chemical reaction between potassium permanganate (potassium permanganate) and sodium iodide cannot be attributed to any of these types. Such reactions are usually called redox reactions, for example

2KMnO4 + 10NaI + 8H2SO4 \u003d 2MnSO4 + K2SO4 + 5Na2SO4 + 5I2 + 8H2O.

Redox in inorganic chemistry includes all substitution reactions and those decomposition reactions and compounds in which at least one simple substance is involved. In a more generalized version (taking into account organic chemistry), all reactions involve simple substances. And, conversely, all exchange reactions belong to the reactions proceeding without changing the oxidation states of the elements that form the reactants and reaction products.

2. Classification of reactions by phase characteristics

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

1. Gas reactions:

2. Reactions in solutions:

NaOH (p-p) + HCl (p-p) \u003d NaCl (p-p) + H2O (g).

3. Reactions between solids:

CaO (tv) + SiO2 (tv) \u003d CaSiO3 (tv).

3. Classification of reactions by the number of phases

Phase is understood as a set of homogeneous parts of a system with identical physical and chemical properties and separated from each other by an interface.

Many processes, without which it is impossible to imagine our life (such as respiration, digestion, photosynthesis and the like), are associated with various chemical reactions organic compounds (and inorganic). Let's look at their main types and take a closer look at the process called connection (connection).

What is called a chemical reaction

First of all, it is worth giving a general definition of this phenomenon. The phrase under consideration means various reactions of substances of varying complexity, as a result of which different from the initial products are formed. The substances involved in this process are called "reagents".

In writing, the chemical reaction of organic compounds (and inorganic) is written using specialized equations. Outwardly, they are a bit like mathematical addition examples. However, instead of the equal sign ("\u003d"), arrows ("→" or "⇆") are used. In addition, there can sometimes be more substances on the right side of the equation than on the left. Everything before the arrow is a substance before the start of the reaction (left side of the formula). Everything after it (right side) are compounds formed as a result of the chemical process that has occurred.

As an example of a chemical equation, we can consider water for hydrogen and oxygen under the action of an electric current: 2H 2 O → 2H 2 + O 2. Water is the starting reagent, and oxygen and hydrogen are products.

As another, but already more complex example of the chemical reaction of compounds, we can consider a phenomenon familiar to every housewife who has baked sweets at least once. It's about quenching baking soda with vinegar. The occurring action is illustrated by the following equation: NaHCO 3 + 2 CH 3 COOH → 2CH 3 COONa + CO 2 + H 2 O. It is clear from it that the interaction of sodium bicarbonate and vinegar forms the sodium salt of acetic acid, water and carbon dioxide.

By its nature, it occupies an intermediate place between physical and nuclear.

Unlike the former, the compounds involved in chemical reactions are capable of changing their composition. That is, from the atoms of one substance, several others can be formed, as in the above equation for the decomposition of water.

Unlike nuclear reactions, chemical reactions do not affect the nuclei of atoms of interacting substances.

What are the types of chemical processes

The distribution of the reactions of compounds by type occurs according to different criteria:

• Reversibility / irreversibility.
• The presence / absence of catalytic substances and processes.
• By absorption / release of heat (endothermic / exothermic reactions).
• By the number of phases: homogeneous / heterogeneous and two hybrid varieties.
• By changing the oxidation states of the interacting substances.

Types of chemical processes in the way of interaction

This criterion is special. With its help, four types of reactions are distinguished: compound, substitution, decomposition (splitting) and exchange.

The name of each of them corresponds to the process that it describes. That is, they unite, in substitution, they change to other groups, in decomposition, several are formed from one reagent, and in exchange, the participants in the reaction exchange atoms with each other.

Types of processes by the method of interaction in organic chemistry

Despite the great complexity, the reactions of organic compounds follow the same principle as inorganic ones. However, they have slightly different names.

So, the reactions of compound and decomposition are called "addition", as well as "elimination" (elimination) and directly organic decomposition (in this section of chemistry there are two types of decomposition processes).

Other reactions of organic compounds are substitution (the name does not change), rearrangement (exchange), and redox processes. Despite the similarity of the mechanisms of their course, in organics they are more multifaceted.

Chemical reaction of a compound

Having considered the various types of processes in which substances enter in organic and inorganic chemistry, it is worth dwelling in more detail on the compound.

This reaction differs from all the others in that, regardless of the number of reagents at its beginning, in the end they all combine into one.

As an example, we can recall the process of slaking lime: CaO + H 2 O → Ca (OH) 2. In this case, the reaction of the compound of calcium oxide (quicklime) with hydrogen oxide (water) occurs. The result is calcium hydroxide (slaked lime) and warm steam. By the way, this means that this process is really exothermic.

Compound reaction equation

The process under consideration can be schematically depicted as follows: A + BV → ABC. In this formula, ABC is a newly formed A - a simple reagent, and BV is a variant of a complex compound.

It should be noted that this formula is also typical for the process of joining and joining.

Examples of the reaction under consideration is the interaction of sodium oxide and carbon dioxide (NaO 2 + CO 2 (t 450-550 ° C) → Na 2 CO 3), as well as sulfur oxide with oxygen (2SO 2 + O 2 → 2SO 3).

Also, several complex compounds are capable of reacting with each other: AB + VG → ABVG. For example, the same sodium oxide and hydrogen oxide: NaO 2 + H 2 O → 2NaOH.

Reaction conditions in inorganic compounds

As shown in the previous equation, substances of varying degrees of complexity are capable of entering the interaction under consideration.

In this case, for simple reagents of inorganic origin, redox reactions of the compound are possible (A + B → AB).

As an example, we can consider the process of obtaining trivalent. For this, a compound reaction is carried out between chlorine and ferum (iron): 3Cl 2 + 2Fe → 2FeCl 3.

If we are talking about the interaction of complex inorganic substances (AB + VG → ABVG), the processes in them can occur, both affecting and not affecting their valence.

As an illustration of this, it is worth considering the example of the formation of calcium bicarbonate from carbon dioxide, hydrogen oxide (water) and white food coloring E170 (calcium carbonate): CO 2 + H 2 O + CaCO 3 → Ca (CO 3) 2. In this case, it has place is a classic connection reaction. During its implementation, the valence of the reactants does not change.

A slightly more perfect (than the first) chemical equation 2FeCl 2 + Cl 2 → 2FeCl 3 is an example of a redox process when a simple and complex inorganic reagents interact: gas (chlorine) and salt (ferric chloride).

Types of addition reactions in organic chemistry

As already mentioned in the fourth paragraph, in substances of organic origin, the considered reaction is called "addition". As a rule, complex substances with a double (or triple) bond take part in it.

For example, the reaction between dibromium and ethylene, leading to the formation of 1,2-dibromoethane: (C 2 H 4) CH 2 \u003d CH 2 + Br 2 → (C₂H₄Br₂) BrCH 2 - CH 2 Br. By the way, signs similar to equals and minus ("\u003d" and "-"), in this equation show the connections between the atoms of a complex substance. This is a feature of recording the formulas of organic substances.

Depending on which of the compounds act as reagents, there are several varieties of the considered addition process:

• Hydrogenation (hydrogen H molecules are added at multiple bonds).
• Hydrohalogenation (hydrogen halide is added).
• Halogenation (addition of halogens Br 2, Cl 2 and the like).
• Polymerization (the formation of high molecular weight substances from several low molecular weight compounds).

Examples of the addition reaction (connection)

After listing the varieties of the process under consideration, it is worth learning in practice some examples of the compound reaction.

As an illustration of hydrogenation, one can pay attention to the equation of interaction of propene with hydrogen, which will result in propane: (C 3 H 6) CH 3 —CH \u003d CH 2 + H 2 → (C 3 H 8) CH 3 —CH 2 —CH 3.

In organic chemistry, the reaction of a compound (addition) can occur between hydrochloric acid (inorganic substance) and ethylene to form chloroethane: (C 2 H 4) CH 2 \u003d CH 2 + HCl → CH 3 - CH 2 —Cl (C 2 H 5 Cl ). The equation presented is an example of hydrohalogenation.

With regard to halogenation, it can be illustrated by the reaction between dichlorine and ethylene leading to the formation of 1,2-dichloroethane: (C 2 H 4) CH 2 \u003d CH 2 + Cl 2 → (C₂H₄Cl₂) ClCH 2 -CH 2 Cl.

Many nutrients are formed through organic chemistry. The reaction of connection (addition) of ethylene molecules with a radical initiator of polymerization under the influence of ultraviolet radiation is a confirmation of this: n CH 2 \u003d CH 2 (R and UV light) → (-CH 2 -CH 2 -) n. The substance formed in this way is well known to everyone under the name of polyethylene.

Various types of packaging, bags, dishes, pipes, insulating substances and much more are made from this material. A feature of this substance is the possibility of its recycling. Polyethylene owes its popularity to the fact that it does not decompose, which is why environmentalists have a negative attitude towards it. However, in last years a method was found for the safe disposal of polyethylene products. For this, the material is treated with nitric acid (HNO 3). After that, certain types of bacteria are able to decompose this substance into safe components.

The connection (addition) reaction plays important role in nature and human life. In addition, it is often used by scientists in laboratories to synthesize new substances for various important research.

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 converting 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 on the left and right sides of the chemical equation is the same. Usually, the formulas of the starting materials are written on the left side of the equation, and the formulas for the products are on the right. The equality of the number of atoms of each element in the left and right sides of the equation is achieved by arranging 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 below) the equal sign.

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

Classification of chemical reactions by the number and composition of the starting and formed substances

According to this classification, chemical reactions are subdivided 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, 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.

2FеСl 2 + Сl 2 \u003d 2FеСl 3

The reactions of the compound are in most cases exothermic, i.e. proceed with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox (ORR), i.e. proceed 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 belongs to the OVR.

Reactions as a result of which several other new substances (complex or simple) are formed from one complex substance are referred to as decomposition reactions... In general, the chemical decomposition equation 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 occur on heating (1,4,5). Decomposition by electric current possible (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 belong to OVR. OVR decomposition reactions include the decomposition of oxides, acids, and salts formed by elements in the highest oxidation states (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)

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

For example:

2Аl + Fe 2 O 3 \u003d 2Fе + Аl 2 О 3 (1)

Zn + 2HCl \u003d ZnCl 2 + H 2 (2)

2КВr + Сl 2 \u003d 2КСl + Вr 2 (3)

2KSlO 3 + l 2 \u003d 2KlO 3 + Сl 2 (4)

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

Ca 3 (PO 4) 2 + 3SiO 2 \u003d 3CaSiO 3 + P 2 O 5 (6)

СН 4 + Сl 2 \u003d СН 3 Сl + НСl (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 call the reactions occurring 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 of the chemical exchange reaction will look like this:

AB + CD \u003d 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)

AgNО 3 + КВr \u003d АgВr ↓ + КNО 3 (4)

СrСl 3 + ЗNаОН \u003d Сr (ОН) 3 ↓ + ЗNаСl (5)

Metabolic reactions are not redox reactions. A special case of 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 by changes in oxidation states

Depending on the change in the oxidation states of the elements that make up the reagents and reaction products, all chemical reactions are subdivided into redox (1, 2) and proceeding without a change in the oxidation state (3, 4).

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

Mg 0 - 2e \u003d Mg 2+ (reducing agent)

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

FeS 2 + 8HNO 3 (conc) \u003d Fe (NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)

Fe 2+ -e \u003d Fe 3+ (reducing agent)

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

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

Ca (OH) 2 + H 2 SO 4 \u003d 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 conventionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during the reaction is called the heat effect of the reaction. If the equation indicates the amount of released or absorbed heat, then such equations are called thermochemical.

N 2 + 3H 2 \u003d 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

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

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

A + B ↔ AB

For example:

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

Examples of irreversible reactions include the following reactions:

2KSlO 3 → 2KSl + 3O 2

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O

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

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 accelerates 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 (catalyst MnO 2)

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

Chemical reaction - This is the "transformation" of one or more substances into another substance, with a different structure and chemical composition. The resulting substance or substances are called "reaction products". During chemical reactions, nuclei and electrons form new compounds (redistributed), but their number does not change and the isotopic composition of chemical elements remains the same.

All chemical reactions are divided into simple and complex.

By the number and composition of the starting and obtained substances, simple chemical reactions can be divided into several main types.

Decomposition reactions are reactions in which several other substances are obtained from one complex substance. At the same time, the formed substances can be both simple and complex. As a rule, the course of a chemical decomposition reaction requires heating (this is an endothermic process, heat absorption).

For example, when malachite powder is heated, three new substances are formed: copper oxide, water and carbon dioxide:

Cu 2 CH 2 O 5 \u003d 2CuO + H 2 O + CO 2

malachite → copper oxide + water + carbon dioxide

If only decomposition reactions occurred in nature, then all complex substances that can decompose would decompose and chemical phenomena could no longer be realized. But there are other reactions as well.

In the reactions of a compound from several simple or complex substances, one complex substance is obtained. It turns out that the compound reactions are the opposite of decomposition reactions.

For example, when copper is heated in air, it becomes covered with a black coating. Copper is converted to copper oxide:

2Cu + O 2 \u003d 2CuO

copper + oxygen → copper oxide

Chemical reactions between simple and complex substances in which the atoms that make up a simple substance replace the atoms of one of the elements of a complex substance are called substitution reactions.

For example, if an iron nail is immersed in a solution of copper chloride (CuCl 2), it (the nail) begins to be covered with copper released on its surface. And by the end of the reaction, the solution turns greenish from blue: instead of copper chloride, it now contains ferric chloride:

Fe + CuCl 2 \u003d Cu + FeCl 2

Iron + copper chloride → copper + ferric chloride

The copper atoms in copper chloride are replaced by iron atoms.

An exchange reaction is a reaction in which two complex substances exchange their constituent parts. Most often, such reactions take place in aqueous solutions.

In the reactions of metal oxides with acids, two complex substances - oxide and acid - exchange their constituents: oxygen atoms - for acid residues, and hydrogen atoms - for metal atoms.

For example, if copper oxide (CuO) is combined with sulfuric acid H 2 SO 4 and heated, a solution is obtained from which copper sulfate can be isolated:

CuO + H 2 SO 4 \u003d CuSO 4 + H 2 O

copper oxide + sulfuric acid → copper sulfate + water

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During chemical reactions from some substances, others are obtained (not to be confused with nuclear reactions, in which one chemical element turns into another).

Any chemical reaction is described by a chemical equation:

Reagents → Reaction Products

The arrow indicates the direction of the reaction.

For example:

In this reaction, methane (CH 4) reacts with oxygen (O 2), resulting in the formation of carbon dioxide (CO 2) and water (H 2 O), or rather, water vapor. This is what happens in your kitchen when you light your gas burner. The equation should be read like this: one molecule of methane gas reacts with two molecules of oxygen gas, resulting in one molecule of carbon dioxide and two molecules of water (water vapor).

The numbers in front of the components of a chemical reaction are called reaction coefficients.

Chemical reactions are endothermic (with energy absorption) and exothermic (with the release of energy). Combustion of methane is a typical example of an exothermic reaction.

There are several types of chemical reactions. The most common:

• compound reactions;
• decomposition reactions;
• single substitution reactions;
• double substitution reactions;
• oxidation reactions;
• redox reactions.

Compound reactions

In compound reactions, at least two elements form one product:

2Na (t) + Cl 2 (g) → 2NaCl (t) - the formation of table salt.

Attention should be paid to the essential nuance of the reactions of the compound: depending on the conditions of the reaction or the proportions of the reagents entering into the reaction, different products may result. For example, under normal conditions of combustion of coal, carbon dioxide is obtained:
C (t) + O 2 (g) → CO 2 (g)

If the amount of oxygen is insufficient, then deadly carbon monoxide is formed:
2C (t) + O 2 (g) → 2CO (g)

Decomposition reactions

These reactions are, as it were, essentially opposite to the reactions of the compound. As a result of the decomposition reaction, the substance decomposes into two (3, 4 ...) simpler elements (compounds):

• 2H 2 O (l) → 2H 2 (g) + O 2 (g) - decomposition of water
• 2H 2 O 2 (l) → 2H 2 (g) O + O 2 (g) - decomposition of hydrogen peroxide

Single substitution reactions

As a result of single substitution reactions, the more active element replaces the less active one in the compound:

Zn (t) + CuSO 4 (p-p) → ZnSO 4 (p-p) + Cu (t)

The zinc in the copper sulfate solution displaces the less active copper, resulting in a zinc sulfate solution.

The degree of activity of metals by increasing activity:

• The most active are alkali and alkaline earth metals

The ionic equation of the above reaction will be:

Zn (t) + Cu 2+ + SO 4 2- → Zn 2+ + SO 4 2- + Cu (t)

The ionic bond CuSO 4, when dissolved in water, decomposes into a copper cation (charge 2+) and sulfate anion (charge 2-). As a result of the substitution reaction, a zinc cation is formed (which has the same charge as the copper cation: 2-). Note that the sulfate anion is present in both sides of the equation, so it can be abbreviated by all the rules of mathematics. As a result, we get the ion-molecular equation:

Zn (t) + Cu 2+ → Zn 2+ + Cu (t)

Double substitution reactions

In double substitution reactions, two electrons are replaced. Such reactions are also called exchange reactions... Such reactions take place in solution with the formation of:

• insoluble solid (precipitation reaction);
• water (neutralization reaction).

Precipitation reactions

When mixing a solution of silver nitrate (salt) with a solution of sodium chloride, silver chloride is formed:

Molecular Equation: KCl (p-p) + AgNO 3 (p-p) → AgCl (t) + KNO 3 (p-p)

Ionic equation: K + + Cl - + Ag + + NO 3 - → AgCl (t) + K + + NO 3 -

Molecular ion equation: Cl - + Ag + → AgCl (s)

If the compound is soluble, it will be ionic in solution. If the compound is insoluble, it will precipitate forming a solid.

Neutralization reactions

These are reactions of interaction between acids and bases, as a result of which water molecules are formed.

For example, the reaction of mixing a solution of sulfuric acid and a solution of sodium hydroxide (lye):

Molecular Equation: H 2 SO 4 (p-p) + 2NaOH (p-p) → Na 2 SO 4 (p-p) + 2H 2 O (g)

Ionic equation: 2H + + SO 4 2- + 2Na + + 2OH - → 2Na + + SO 4 2- + 2H 2 O (g)

Molecular ionic equation: 2H + + 2OH - → 2H 2 O (l) or H + + OH - → H 2 O (l)

Oxidation reactions

These are reactions of interaction of substances with gaseous oxygen in the air, in which, as a rule, a large amount of energy is released in the form of heat and light. A typical oxidation reaction is combustion. At the very beginning of this page is the reaction of the interaction of methane with oxygen:

CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (g)

Methane refers to hydrocarbons (compounds of carbon and hydrogen). When a hydrocarbon reacts with oxygen, a lot of thermal energy is released.

Redox reactions

These are reactions in which there is an exchange of electrons between the atoms of the reactants. The reactions considered above are also redox reactions:

• 2Na + Cl 2 → 2NaCl - compound reaction
• CH 4 + 2O 2 → CO 2 + 2H 2 O - oxidation reaction
• Zn + CuSO 4 → ZnSO 4 + Cu - single substitution reaction

Redox reactions with a large number of examples of solving equations by the electronic balance method and the half-reaction method are described in the section