Chemistry decomposition of chemical elements. What is a decomposition reaction in chemistry? Decomposition reaction examples

Chemical reactions (chemical phenomena)- these are processes as a result of which others are formed from some substances, differing from the original ones in composition or structure. During the course of chemical reactions, there is no change in the number of atoms of one or another element, the interconversion of isotopes.

The classification of chemical reactions is multifaceted, it can be based on various features: the number and composition of reactants and reaction products, thermal effect, reversibility, etc.

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 thermal 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.

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 of chemical reactions themselves without suspecting (and maybe suspecting) cooks do when they prepare 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 the combustion of simple substances such as sulfur or phosphorus in air (perhaps, such a 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 a chemical reaction of decomposition can be a reaction of decomposition of chalk, during which the chalk itself is formed quicklime and carbon dioxide.

Chemical substitution reaction

The substitution reaction is carried out by the interaction a simple substance with complex. Let's give an example of a chemical substitution reaction: if you lower a steel nail into a solution with copper sulphate, 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 what it looks like chemical equation 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, e.g. during humid air covered with a brown coating, as a result of a chemical reaction of the interaction of iron and).
Precipitation (if suddenly through mortar pass carbon dioxide, then we get the precipitation of a white insoluble precipitate of calcium carbonate).
Gas evolution (if you drop 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; different substances and different reactions have their own signs.

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

This state is called chemical equilibrium. chemical system, at which several chemical reactions take place and the rates in each pair of forward and reverse reactions are equal to each other. 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 what they call change. internal energy systems as a result of a chemical reaction and the transformation of the starting substances (reactants) into reaction products in amounts corresponding to the chemical reaction equation under the following conditions:

only possible job in this case, there is only work against external pressure.
the starting materials and products obtained as a result of a chemical reaction have the same temperature.

Chemical reactions, video

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

(photochemical reactions), electric current (electrode processes), ionizing radiation (radiation-chemical reactions), mechanical action (mechanochemical reactions), in low-temperature plasma (plasma-chemical reactions), etc. The interaction of molecules with each other occurs along a chain route: association - electronic isomerization - dissociation, in which the active particles are radicals, ions, coordinatively unsaturated compounds. The rate of a chemical reaction is determined by the concentration of active particles and the difference between the energies of the bond being broken and formed.

Chemical processes occurring in matter differ both from physical processes and from nuclear transformations. In physical processes, each of the participating substances retains its composition unchanged (although substances can form mixtures), but they can change their external form or state of aggregation.

AT chemical processes(chemical reactions) new substances are obtained with properties different from the reagents, but atoms of new elements are never formed. In the atoms of the elements participating in the reaction, modifications of the electron shell necessarily occur.

In nuclear reactions, changes occur in the atomic nuclei of all participating elements, which leads to the formation of atoms of new elements.

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There are a large number of signs by which chemical reactions can be classified.

1. By the presence of a phase boundary, all chemical reactions are divided into homogeneous and heterogeneous

A chemical reaction occurring within the same phase is called homogeneous chemical reaction . A chemical reaction that occurs at a phase boundary is called heterogeneous chemical reaction . In a multi-step chemical reaction, some steps may be homogeneous while others may be heterogeneous. Such reactions are called homogeneous-heterogeneous .

Depending on the number of phases that form the starting substances and reaction products, chemical processes can be homophasic (the starting substances and products are within the same phase) and heterophase (the starting substances and products form several phases). The homo- and heterophasic nature of a reaction is unrelated to whether the reaction is homo- or heterogeneous. Therefore, four types of processes can be distinguished:

Homogeneous reactions (homophasic) . In reactions of this type, the reaction mixture is homogeneous, and the reactants and products belong to the same phase. An example of such reactions are ion exchange reactions, for example, neutralization of an acid solution with an alkali solution:

N a O H + H C l → N a C l + H 2 O (\displaystyle \mathrm (NaOH+HCl\rightarrow NaCl+H_(2)O) )

Heterogeneous homophasic reactions . The components are within the same phase, however, the reaction proceeds at the phase boundary, for example, on the surface of the catalyst. An example would be the hydrogenation of ethylene on a nickel catalyst:

C 2 H 4 + H 2 → C 2 H 6 (\displaystyle \mathrm (C_(2)H_(4)+H_(2)\rightarrow C_(2)H_(6)) )

Homogeneous heterophase reactions . The reactants and products in such a reaction exist within several phases, but the reaction proceeds in one phase. Thus, the oxidation of hydrocarbons in the liquid phase with gaseous oxygen can take place.
Heterogeneous heterophase reactions . In this case, the reactants are in a different phase state, the reaction products can also be in any phase state. The reaction process takes place at the phase boundary. An example is the reaction of carbonic acid salts (carbonates) with Bronsted acids:

M g C O 3 + 2 H C l → M g C l 2 + C O 2 + H 2 O (\displaystyle \mathrm (MgCO_(3)+2HCl\rightarrow MgCl_(2)+CO_(2)\uparrow +H_(2 )O) )

2. By changing the oxidation states of the reagents

In this case, distinguish

Redox reactions, in which the atoms of one element (oxidizing agent) are recovering , that is lower their oxidation state, and the atoms of another element (reductant) are oxidized , that is increase their oxidation state. A special case of redox reactions are the proportionate reactions, in which the oxidizing and reducing agents are atoms of the same element in different oxidation states.

An example of a redox reaction is the combustion of hydrogen (reductant) in oxygen (oxidizer) to form water:

2 H 2 + O 2 → 2 H 2 O (\displaystyle \mathrm (2H_(2)+O_(2)\rightarrow 2H_(2)O) )

An example of a proportionate reaction is the decomposition reaction of ammonium nitrate when heated. The oxidizing agent in this case is nitrogen (+5) of the nitro group, and the reducing agent is nitrogen (-3) of the ammonium cation:

N H 4 N O 3 → N 2 O + 2 H 2 O (< 250 ∘ C) {\displaystyle \mathrm {NH_{4}NO_{3}\rightarrow N_{2}O\uparrow +2H_{2}O\qquad (<250{}^{\circ }C)} }

They do not belong to redox reactions in which there is no change in the oxidation states of atoms, for example:

B a C l 2 + N a 2 S O 4 → B a S O 4 ↓ + 2 N a C l (\displaystyle \mathrm (BaCl_(2)+Na_(2)SO_(4)\rightarrow BaSO_(4)\downarrow +2NaCl) )

3. According to the thermal effect of the reaction

All chemical reactions are accompanied by the release or absorption of energy. When chemical bonds are broken in reagents, energy is released, which mainly goes to the formation of new chemical bonds. In some reactions, the energies of these processes are close, and in this case the total thermal effect of the reaction approaches zero. In other cases, we can distinguish:

exothermic reactions that go with heat release,(positive thermal effect) for example, the above hydrogen combustion
endothermic reactions in which heat is absorbed(negative thermal effect) from the environment.

The thermal effect of the reaction (reaction enthalpy, Δ r H), which is often very important, can be calculated according to the Hess law if the enthalpies of formation of reactants and products are known. When the sum of the enthalpies of the products is less than the sum of the enthalpies of the reactants (Δ r H< 0) наблюдается heat generation, otherwise (Δ r H > 0) - absorption.

4. According to the type of transformations of the reacting particles

Chemical reactions are always accompanied by physical effects: absorption or release of energy, change in the color of the reaction mixture, etc. It is these physical effects that are often used to judge the course of chemical reactions.

Connection reaction - a chemical reaction in which two or more more of the initial substances, only one new one is formed. Both simple and complex substances can enter into such reactions.

decomposition reaction A chemical reaction that produces several new substances from one substance. Only complex compounds enter into reactions of this type, and their products can be both complex and simple substances.

substitution reaction - a chemical reaction in which the atoms of one element, which are part of a simple substance, replace the atoms of another element in its complex compound. As follows from the definition, in such reactions one of the starting materials must be simple and the other complex.

Exchange reactions A reaction in which two compounds exchange their constituents

5. On the basis of the direction of flow, chemical reactions are divided into irreversible and reversible

irreversible refers to chemical reactions that proceed in only one direction. from left to right"), as a result of which the starting substances are converted into reaction products. Such chemical processes are said to proceed "to the end." These include combustion reactions, as well as reactions accompanied by the formation of poorly soluble or gaseous substances reversible called chemical reactions occurring simultaneously in two opposite directions ("from left to right" and "from right to left"). In the equations of such reactions, the equal sign is replaced by two oppositely directed arrows. Among two simultaneously occurring reactions, there are direct( flows from left to right) and reverse(flows "from right to left"). Since in the course of a reversible reaction the starting materials are both consumed and formed, they are not completely converted into reaction products. Therefore, about reversible reactions They say they don't go all the way. As a result, a mixture of initial substances and reaction products is always formed.

6. On the basis of the participation of catalysts, chemical reactions are divided into catalytic and non-catalytic

catalytic are called reactions occurring in the presence of catalysts. In the equations of such reactions chemical formula catalysts are indicated above the sign of equality or reversibility, sometimes together with the designation of the flow conditions (temperature t, pressure p). Many reactions of decomposition and combination belong to reactions of this type.

Types of reactions: All chemical reactions are divided into simple and complex. Simple chemical reactions, in turn, are usually divided into four types: compound reactions, decomposition reactions, substitution reactions and exchange reactions.

D. I. Mendeleev defined a compound as a reaction, “in which one of two substances occurs. An example compound chemical reaction heating of powders of iron and sulfur can serve, - in this case, iron sulfide is formed: Fe + S = FeS. Combination reactions include the processes of combustion of simple substances (sulfur, phosphorus, carbon, ...) in air. For example, carbon burns in air C + O 2 \u003d CO 2 (of course, this reaction proceeds gradually, first it forms carbon monoxide CO). Combustion reactions are always accompanied by the release of heat - they are exothermic.

Decomposition chemical reactions, according to Mendeleev, “cases are inverse to the connection, that is, those in which one substance gives two, or, in general, a given number of substances is a greater number of them. An example of a decomposition reaction between the two is the chemical reaction of the decomposition of chalk (or limestone under the influence of temperature): CaCO 3 → CaO + CO 2. The decomposition reaction generally requires heating. Such processes are endothermic, that is, they proceed with the absorption of heat.

In reactions of the other two types, the number of reactants is equal to the number of products. If a simple substance and a complex substance interact, then this chemical reaction is called chemical substitution reaction: For example, dropping a steel nail into the solution blue vitriol we get ferrous sulfate (here iron displaced copper from its salt) Fe + CuSO 4 → FeSO 4 + Cu.

Reactions between two complex substances in which they exchange their parts are referred to as chemical reactions of exchange. A large number of them are in aqueous solutions. An example of a chemical exchange reaction is the neutralization of an acid with an alkali: NaOH + HCl → NaCl + H 2 O. 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 the formation of a sodium chloride solution in water

Reaction types and their mechanisms are shown in the table:

compound chemical reactions

Example:
S + O 2 → SO 2

From several simple or complex substances, one complex substance is formed

decomposition chemical reactions

Example:
2HN 3 → H 2 + 3N 2

From a complex substance, several simple or complex substances are formed

chemical substitution reactions

Example:
Fe + CuSO 4 → Cu + FeSO 4

An atom of a simple substance replaces one of the atoms of a complex

ion exchange chemical reactions

Example:
H 2 SO 4 + 2NaCl → Na 2 SO 4 + 2HCl

Compounds exchange their constituents

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

2KMnO 4 + 10NaI + 8H 2 SO 4 → 2MnSO 4 + K 2 SO 4 + 5Na 2 SO 4 + 5I 2 + 8H 2 O.

Signs of chemical reactions

Signs of chemical reactions. They can be used to judge whether a chemical reaction between the reagents has passed or not. These signs include the following:

Color change (for example, light iron is covered in moist air with a brown coating of iron oxide - a chemical reaction of the interaction of iron with oxygen).
- Precipitation (for example, if carbon dioxide is passed through a lime solution (calcium hydroxide solution), a white insoluble precipitate of calcium carbonate will fall out).
- Emission of gas (for example, if you drop citric acid on baking soda, carbon dioxide will be released).
- The formation of weakly dissociated substances (for example, reactions in which one of the reaction products is water).
- The glow of the solution.
An example of the glow of a solution is a reaction using such a reagent as a solution of luminol (luminol is a complex Chemical substance, which can emit light during chemical reactions).

Redox reactions

Redox reactions- constitute a special class of chemical reactions. Their characteristic feature is a change in the oxidation state of at least a pair of atoms: the oxidation of one (loss of electrons) and the reduction of the other (addition of electrons).

Compounds that lower their oxidation state - oxidizers, and increasing the degree of oxidation - reducing agents. For example:

2Na + Cl 2 → 2NaCl,
- here the oxidizing agent is chlorine (it attaches electrons to itself), and the reducing agent is sodium (it gives up electrons).

The substitution reaction NaBr -1 + Cl 2 0 → 2NaCl -1 + Br 2 0 (typical for halogens) also refers to redox reactions. Here, chlorine is an oxidizing agent (accepts 1 electron), and sodium bromide (NaBr) is a reducing agent (a bromine atom gives up an electron).

The decomposition reaction of ammonium dichromate ((NH 4) 2 Cr 2 O 7) also refers to redox reactions:

(N -3 H 4) 2 Cr 2 +6 O 7 → N 2 0 + Cr 2 +3 O 3 + 4H 2 O

Another common classification of chemical reactions is their separation according to the thermal effect. Separate endothermic reactions and exothermic reactions. Endothermic reactions - chemical reactions accompanied by the absorption of ambient heat (remember cooling mixtures). Exothermic (vice versa) - chemical reactions accompanied by the release of heat (for example, combustion).

Dangerous chemical reactions : "BOMB IN THE SHELL" - funny or not so?!

There are some chemical reactions that occur spontaneously when reactants are mixed. In this case, rather dangerous mixtures are formed that can explode, ignite or poison. Here is one of them!
Strange phenomena were observed in some American and English clinics. From time to time, sounds resembling pistol shots were heard from the sinks, and in one case the drain pipe suddenly exploded. Fortunately, no one was hurt. The investigation showed that the culprit was a very weak (0.01%) sodium azide NaN 3 solution, which was used as a preservative for saline solutions.

Excess azide solution was poured into the sinks for many months or even years - sometimes up to 2 liters per day.

By itself, sodium azide - a salt of hydroazide acid HN 3 - does not explode. However, azides of heavy metals (copper, silver, mercury, lead, etc.) are very unstable crystalline compounds that explode upon friction, impact, heating, and exposure to light. An explosion can occur even under a layer of water! Lead azide Pb (N 3) 2 is used as an initiating explosive, which undermines the main mass of explosives. For this, only two tens of milligrams of Pb (N 3) 2 are enough. This compound is more explosive than nitroglycerin, and the speed of detonation (propagation of an explosive wave) during an explosion reaches 45 km / s - 10 times greater than that of TNT.

But where could heavy metal azides come from in clinics? It turned out that in all cases the drain pipes under the sinks were made of copper or brass (such pipes bend easily, especially after heating, so they are convenient to install in drain system). The sodium azide solution poured into the sinks, flowing through such tubes, gradually reacted with their surface, forming copper azide. I had to change the tubes to plastic ones. When such a replacement was carried out in one of the clinics, it turned out that the removed copper tubes heavily loaded with solid matter. The specialists who were engaged in "mine clearance", in order not to take risks, blew up these tubes on the spot, folding them into a metal tank weighing 1 ton. The explosion was so strong that it moved the tank several centimeters!

Physicians were not very interested in the nature of the chemical reactions leading to the formation of explosives. No description of this process has been found in the chemical literature either. But it can be assumed, based on the strong oxidizing properties of HN 3, that such a reaction took place: the N-3 anion, oxidizing copper, formed one N2 molecule and a nitrogen atom, which became part of ammonia. This corresponds to the reaction equation: 3NaN 3 +Cu + 3H 2 O → Cu(N 3) 2 + 3NaOH + N 2 +NH 3.

Everyone who deals with soluble metal azides, including chemists, has to reckon with the danger of a bomb forming in a sink, since azides are used to obtain highly pure nitrogen, in organic synthesis, as a blowing agent (foaming agent for the production of gas-filled materials: foam plastics, porous rubber, etc.). In all such cases, it must be ensured that the drain pipes are plastic.

Relatively recently, azides have found a new application in the automotive industry. In 1989, airbags appeared in some models of American cars. Such a pillow containing sodium azide is almost invisible when folded. In a head-on collision, the electric fuse leads to a very rapid decomposition of the azide: 2NaN 3 =2Na+3N 2 . 100 g of powder release about 60 liters of nitrogen, which in about 0.04 s inflates the pillow in front of the driver's chest, thereby saving his life.

During chemical reactions, other substances are obtained from one substance (not to be confused with nuclear reactions, in which one chemical element is converted 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 exactly the kind of reaction that happens in your kitchen when you light a 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 (steam).

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 energy release). The 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.

Connection reactions

In a compound reaction, at least two elements form one product:

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

Attention should be paid to an essential nuance of compound reactions: depending on the conditions of the reaction or the proportions of the reactants that enter into the reaction, different products can be its result. For example, under normal combustion conditions hard coal produces carbon dioxide:
C (t) + O 2 (g) → CO 2 (g)

If there is not enough oxygen, then deadly carbon monoxide is formed:
2C (t) + O 2 (g) → 2CO (g)

Decomposition reactions

These reactions are, as it were, opposite in essence to the reactions of the compound. As a result of the decomposition reaction, the substance breaks up into two (3, 4...) more simple element(connections):

2H 2 O (g) → 2H 2 (g) + O 2 (g)- water decomposition
2H 2 O 2 (g) → 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 element in the compound:

Zn (t) + CuSO 4 (solution) → ZnSO 4 (solution) + 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 in ascending order of activity:

The most active are alkali and alkaline earth metals.

The ionic equation for 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 an anion sulfate (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 on both sides of the equation, i.e., by all the rules of mathematics, it can be reduced. The result is an ion-molecular equation:

Zn (t) + Cu 2+ → Zn 2+ + Cu (t)

Double substitution reactions

In double substitution reactions, two electrons are already replaced. Such reactions are also called exchange reactions. These reactions take place in solution to form:

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

Precipitation reactions

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

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

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

Molecular-ionic equation: Cl - + Ag + → AgCl (t)

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

Neutralization reactions

These are reactions 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 (l)

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

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

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, the reaction of the interaction of methane with oxygen is given:

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 heat energy is released.

Redox reactions

These are reactions in which electrons are exchanged between the atoms of the reactants. The reactions discussed 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

The most detailed redox reactions with large quantity examples of solving equations by the electron balance method and the half-reaction method are described in the section