Chemical reactions. Introduction to General Chemistry
Classification of chemical reactions in inorganic and organic chemistry carried out on the basis of various classifying features, information about which is given in the table below.
By changing the oxidation state of elements
The first sign of classification is based on a change in the oxidation state of the elements that form reagents and products.
a) redox
b) without changing the oxidation state
Redox are called reactions accompanied by a change in oxidation states chemical elements included in the reagents. Redox in inorganic chemistry includes all substitution reactions and those decomposition reactions and compounds in which at least one simple substance is involved. All exchange reactions belong to the reactions proceeding without changing the oxidation states of the elements that form the reactants and reaction products.
By the number and composition of reagents and products
Chemical reactions classified by the nature of the process, i.e. by the number and composition of reagents and products.
Compound reactions chemical reactions are called, as a result of which complex molecules are obtained from several simpler ones, for example:
4Li + O 2 = 2Li 2 O
Decomposition reactions chemical reactions are called, as a result of which simple molecules are obtained from more complex ones, for example:
CaCO 3 = CaO + CO 2
Decomposition reactions can be viewed as the inverse of compound.
Substitution reactions chemical reactions are called, as a result of which an atom or group of atoms in a molecule of a substance is replaced by another atom or group of atoms, for example:
Fe + 2HCl = FeCl 2 + H 2
Their distinctive feature- 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 are 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.
- exchange (including neutralization).
Exchange reactions are called chemical reactions that proceed without changing the oxidation states of the elements and lead to the exchange of the constituent parts of the reagents, for example:
AgNO 3 + KBr = AgBr + KNO 3
If possible, flow in the opposite direction
If possible, flow in the opposite direction - reversible and irreversible.
Reversible are called chemical reactions that occur at a given temperature simultaneously in two opposite directions with commensurate rates. When writing the equations of such reactions, the equal sign is replaced with oppositely directed arrows. The simplest example of a reversible reaction is the synthesis of ammonia by the interaction of nitrogen and hydrogen:
N 2 + 3H 2 ↔2NH 3
Irreversible are called reactions that proceed only in the forward direction, as a result of which products are formed that do not interact with each other. Irreversible include chemical reactions that result in the formation of low-dissociated compounds, the release of a large amount of energy, as well as those in which the final products leave the reaction sphere in a gaseous form or in the form of a precipitate, for example:
HCl + NaOH = NaCl + H2O
2Ca + O 2 = 2CaO
BaBr 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaBr
Thermal effect
Exothermic are called chemical reactions with the release of heat. Symbol changes in enthalpy (heat content) ΔH, and the heat effect of the reaction Q. For exothermic reactions, Q> 0, and ΔH< 0.
Endothermic are called chemical reactions that take place with heat absorption. For endothermic reactions Q< 0, а ΔH > 0.
Compound reactions will generally be exothermic and decomposition reactions will be endothermic. A rare exception is the reaction of nitrogen with oxygen - endothermic:
N2 + О2 → 2NO - Q
Phase
Homogeneous are called reactions that take place in a homogeneous medium (homogeneous substances, in one phase, for example, r-g, reactions in solutions).
Heterogeneous are called reactions that take place in a heterogeneous medium, on the contact surface of the reacting substances in different phases, for example, solid and gaseous, liquid and gaseous, in two immiscible liquids.
By using the catalyst
A catalyst is a substance that accelerates a chemical reaction.
Catalytic reactions proceed only in the presence of a catalyst (including enzymatic).
Non-catalytic reactions go in the absence of a catalyst.
By the type of disconnection
By break type chemical bond in the original molecule, homolytic and heterolytic reactions are distinguished.
Homolytic are called reactions in which, as a result of the breaking of bonds, particles are formed that have an unpaired electron - free radicals.
Heterolytic called the reactions proceeding through the formation of ionic particles - cations and anions.
- homolytic (equal gap, each atom receives 1 electron)
- heterolytic (unequal break - one gets a pair of electrons)
Radical(chain) chemical reactions involving radicals are called, for example:
CH 4 + Cl 2 hv → CH 3 Cl + HCl
Ionic chemical reactions involving ions are called, for example:
KCl + AgNO 3 = KNO 3 + AgCl ↓
Heterolytic reactions of organic compounds with electrophiles - particles carrying a whole or fractional positive charge are called electrophilic. They are classified into electrophilic substitution and electrophilic addition reactions, for example:
C 6 H 6 + Cl 2 FeCl3 → C 6 H 5 Cl + HCl
H 2 C = CH 2 + Br 2 → BrCH 2 –CH 2 Br
Nucleophilic are heterolytic reactions of organic compounds with nucleophiles - particles that carry a whole or fractional negative charge. They are classified into nucleophilic substitution and nucleophilic addition reactions, for example:
CH 3 Br + NaOH → CH 3 OH + NaBr
CH 3 C (O) H + C 2 H 5 OH → CH 3 CH (OC 2 H 5) 2 + H 2 O
Classification of organic reactions
The classification of organic reactions is shown in the table:
(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, coordination-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 from both physical processes and 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.
In chemical processes (chemical reactions), new substances are obtained with properties different from 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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Exists a large number of signs by which chemical reactions can be classified.
1.According to the presence of a phase boundary, all chemical reactions are subdivided into homogeneous and heterogeneous
A chemical reaction occurring within one phase is called homogeneous chemical reaction ... A chemical reaction occurring at the interface is called heterogeneous chemical reaction ... In a multistep chemical reaction, some steps may be homogeneous and others heterogeneous. Such reactions are called homogeneous-heterogeneous .
Depending on the number of phases that form the starting materials and reaction products, chemical processes can be homophase (starting materials and products are within the same phase) and heterophase (starting materials and products form several phases). The homo- and heterophase nature of the reaction is not related to whether the reaction is homo- or heterogeneous. Therefore, four types of processes can be distinguished:
- Homogeneous reactions (homophase) ... 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 is ion exchange reactions, for example, neutralization of an acid solution with an alkali solution:
- Heterogeneous homophase reactions ... The components are within the same phase, but the reaction proceeds at the interface, for example, on the catalyst surface. An example would be the hydrogenation of ethylene over a nickel catalyst:
- Homogeneous heterophase reactions ... The reagents 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 reagents are in a different phase state, the reaction products can also be in any phase state. The reaction process takes place at the interface. An example is the reaction of carbonic acid salts (carbonates) with Bronsted acids:
2.On a change in the oxidation states of reagents
In this case, distinguish
- Redox reactions, in which atoms of one element (oxidizing agent) recover , that is lower their oxidation state, and the atoms of another element (reducing agent) oxidized , that is increase their oxidation state... A special case of redox reactions are proportionation 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 (reducing agent) in oxygen (oxidizing agent) 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 counterproportionation reaction is the decomposition reaction of ammonium nitrate upon heating. 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 apply 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 SO 4 → B a SO 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, energy is released in the reagents, which is mainly spent on the formation of new chemical bonds. In some reactions, the energies of these processes are close, and in this case the overall heat effect of the reaction approaches zero. In other cases, we can distinguish:
- exothermic reactions that go with the release of heat,(positive thermal effect) e.g. above hydrogen combustion
- endothermic reactions during which heat is absorbed(negative thermal effect) from the environment.
The heat of reaction (enthalpy of reaction, Δ r H), which is often very important, can be calculated according to Hess's 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.By the type of transformations of reacting particles
Chemical reactions are always accompanied by physical effects: absorption or release of energy, a change in the color of the reaction mixture, etc. It is by these physical effects that chemical reactions are often judged.
Compound reaction - a chemical reaction, as a result of which only one new is formed from two or more initial substances. Both simple and complex substances can enter into such reactions.
Decomposition reaction - a chemical reaction, as a result of which several new substances are formed from one substance. In reaction of this type only complex compounds enter, and their products can be both complex and simple substances
Substitution reaction - a chemical reaction, as a result of 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 initial substances should be simple, and the other complex.
Exchange reactions - a reaction as a result of which two complex substances exchange their constituent parts
5.According to the direction of flow, chemical reactions are divided into irreversible and reversible
Irreversible are called chemical reactions proceeding in only one direction (" from left to right"), as a result of which the initial substances are converted into reaction products. Such chemical processes are said to proceed" to the end. " combustion reactions, and reactions accompanied by the formation of poorly soluble or gaseous substances Reversible chemical reactions are called, proceeding simultaneously in two opposite directions ("left to right" and "right to left"). In the equations of such reactions, the equal sign is replaced by two oppositely directed arrows. Among the two simultaneously occurring reactions are distinguished straight ( flows "from left to right") and reverse(proceeds "from right to left") Since in the course of a reversible reaction the starting substances are simultaneously consumed and formed, they are not completely converted into reaction products. Therefore, they say about reversible reactions that they do not proceed "completely." As a result, a mixture of initial substances and reaction products is always formed.
6. Based on the participation of catalysts, chemical reactions are divided into catalytic and non-catalytic
Catalytic are called reactions that occur in the presence of catalysts. In the equations of such reactions, the chemical formula of the catalyst is indicated above the equal or reversible sign, sometimes together with the designation of the conditions of occurrence (temperature t, pressure p). Many decomposition reactions and compounds belong to reactions of this type.
Decomposition reactions play an important role in the life of the planet. After all, it is they who contribute to the destruction of the waste of life of all biological organisms... In addition, this process daily helps the human body to assimilate various complex compounds by breaking them down into simple ones (catabolism). In addition to all of the above, this reaction promotes the formation of simple organic and inorganic substances from complex ones. Let's learn more about this process and also look at practical examples of the decomposition chemical reaction.
What are called reactions in chemistry, what types of them are and what they depend on
Before examining information on decomposition, it is worth learning about in general. This name means the ability of molecules of some substances to interact with others and thus form new compounds.
For example, if oxygen and two interact with each other, the result will be two molecules of hydrogen oxide, which we all know as water. This process can be written using the following chemical equation: 2H 2 + O 2 → 2H 2 O.
Although there are different criteria by which chemical reactions are distinguished (thermal effect, catalysts, presence / absence of phase boundaries, change in the oxidation states of reagents, reversibility / irreversibility), they are most often classified according to the type of transformation of the interacting substances.
Thus, four types of chemical processes are distinguished.
- Compound.
- Decomposition.
- Exchange.
- Substitution.
All of the above reactions are graphically written using equations. Their general scheme looks like this: A → B.
On the left side of this formula are the initial reagents, and on the right - the substances formed as a result of the reaction. As a rule, to start it requires exposure to temperature, electricity, or the use of catalytic additives. Their presence must also be indicated in the chemical equation.
decomposition (splitting)
This type of chemical process is characterized by the formation of two or more new compounds from molecules of one substance.
In simpler terms, the decomposition reaction can be compared to a house from a construction set. Having decided to build a car and a boat, the child disassembles the initial structure and builds the desired one from its parts. In this case, the structure of the elements of the constructor does not change, just as it happens with the atoms of the substance participating in the splitting.
What does the equation of the considered reaction look like?
Despite the fact that hundreds of connections are capable of separating into simpler components, all such processes occur according to the same principle. It can be depicted using the schematic formula: ABC → A + B + C.
In it, ABC is the initial compound that has undergone cleavage. A, B and C are substances formed from ABC atoms during the decomposition reaction.
Cleavage reactions
As mentioned above, in order to start a chemical process, it is often necessary to exert a certain effect on the reagents. Depending on the type of such stimulation, there are several types of decomposition:
The decomposition of potassium permanganate (KMnO4)
Having dealt with the theory, it is worth considering practical examples of the process of splitting substances.
The first of these will be the decay of KMnO 4 (commonly referred to as potassium permanganate) due to heating. The reaction equation looks like this: 2KMnO 4 (t 200 ° C) → K 2 MnO 4 + MnO 2 + O 2.
From the presented chemical formula, it can be seen that to activate the process, it is necessary to heat the initial reagent to 200 degrees Celsius. For better flow reaction potassium permanganate is placed in a vacuum vessel. From this we can conclude that this process is pyrolysis.
In laboratories and in manufacturing, it is carried out to obtain pure and controlled oxygen.
Potassium chlorate thermolysis (KClO3)
The decomposition reaction of berthollet's salt is another example of classical thermolysis in pure form.
The mentioned process goes through two stages and looks like this:
- 2 KClO 3 (t 400 ° С) → 3KClO 4 + KCl.
- KClO 4 (t from 550 ° С) → KCl + 2О2
Also, the thermolysis of potassium chlorate can be carried out at more low temperatures(up to 200 ° C) in one stage, but for this it is necessary that catalytic substances take part in the reaction - oxides of various metals (copper, ferum, mangan, etc.).
An equation of this kind will look like this: 2KClO 3 (t 150 ° С, MnO 2) → KCl + 2О 2.
Like potassium permanganate, berthollet's salt is used in laboratories and industry to produce pure oxygen.
Electrolysis and radiolysis of water (H20)
Another interesting practical example of this reaction is the decomposition of water. It can be produced in two ways:
- By exposure to hydrogen oxide electric current: H 2 O → H 2 + O 2. The considered method of obtaining oxygen is used by submariners on their submarines. Also in the future it is planned to use it to obtain hydrogen in large quantities... The main obstacle to this today is the huge energy costs required to stimulate the response. When a way is found to minimize them, electrolysis of water will become the main method of producing not only hydrogen, but also oxygen.
- It is also possible to split water when exposed to alpha radiation: H 2 O → H 2 O + + e -. As a result, the hydrogen oxide molecule loses one electron, ionizing. In this form, Н2О + reacts with other neutral water molecules, forming a highly reactive hydroxide radical: Н2О + Н2О + → Н2О + ОН. The lost electron, in turn, also reacts in parallel with neutral hydrogen oxide molecules, facilitating their decomposition into the H and OH radicals: H 2 O + e - → H + OH.
Decomposition of alkanes: methane
Considering different ways separation of complex substances, it is worth paying Special attention alkane decomposition reactions.
This name hides saturated hydrocarbons with the general formula C X H 2X + 2. In the molecules of the substances under consideration, all carbon atoms are connected by single bonds.
Representatives of this series are found in nature in all three states of aggregation (gas, liquid, solid).
All alkanes (the decomposition reaction of representatives of this series is below) are lighter than water and do not dissolve in it. However, they themselves are excellent solvents for other compounds.
Among the main chemical properties such substances (combustion, substitution, halogenation, dehydrogenation) - and the ability to split. However, this process can occur both completely and partially.
The above property can be considered by the example of the decomposition reaction of methane (the first member of the alkane series). This thermolysis occurs at 1000 ° C: CH 4 → C + 2H 2.
However, if the methane decomposition reaction is carried out at a higher temperature (1500 ° C), and then it is sharply reduced, this gas will not decompose completely, forming ethylene and hydrogen: 2CH 4 → C 2 H 4 + 3H 2.
Ethane decomposition
The second member of the alkane series under consideration is C 2 H 4 (ethane). Its decomposition reaction also occurs under the influence of high temperature (50 ° C) and in the complete absence of oxygen or other oxidizing agents. It looks like this: C 2 H 6 → C 2 H 4 + H 2.
The above reaction equation for the decomposition of ethane to hydrogen and ethylene cannot be considered pure pyrolysis. The fact is that this process occurs with the presence of a catalyst (for example, nickel metal Ni or water vapor), and this contradicts the definition of pyrolysis. Therefore, it is correct to speak of the above example of cleavage as a decomposition process that occurs during pyrolysis.
It should be noted that the considered reaction is widely used in industry to obtain the most produced organic compound in the world - ethylene gas. However, due to the explosiveness of C 2 H 6, this simplest alkene is more often synthesized from other substances.
Having considered the definitions, equations, types and various examples of the decomposition reaction, we can conclude that it plays a very important role not only for the human body and nature, but also for industry. Also with its help in laboratories it is possible to synthesize many useful material that helps scientists to conduct important
In modern science, chemical and nuclear reactions are distinguished that occur as a result of the interaction of initial substances, which are usually called reagents. As a result, other chemical substances which are called products. All interactions occur under certain conditions (temperature, radiation, the presence of catalysts, etc.). The atomic nuclei of the reactants of chemical reactions do not change. In nuclear transformations, new nuclei and particles are formed. There are several different criteria by which the types of chemical reactions are determined.
The classification can be based on the number of starting and resulting substances. In this case, all types of chemical reactions are divided into five groups:
- Decompositions (several new ones are obtained from one substance), for example, decomposition upon heating into potassium chloride and oxygen: KCLO3 → 2KCL + 3O2.
- Compounds (two or more compounds form one new), interacting with water, calcium oxide turns into calcium hydroxide: H2O + CaO → Ca (OH) 2;
- Substitutions (the number of products is equal to the number of starting materials in which one constituent part is replaced by another), iron in copper sulfate, replacing copper, forms ferrous sulfate: Fe + CuSO4 → FeSO4 + Cu.
- Double exchange (the 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.
- Polymorphic transformation (there is a transition of a substance from one crystalline form to another), color iodide when heated turns into mercury iodide yellow color: HgI2 (red) ↔ HgI2 (yellow).
If chemical transformations are considered on the basis of a change in the oxidation state of the elements in the reacting substances, then the types of chemical reactions can be divided into groups:
- With a change in the oxidation state - redox reactions (ORR). As an example, consider the interaction of iron with hydrochloric acid: Fe + HCL → FeCl2 + H2, as a result, the oxidation state of iron (reducing agent that donates electrons) changed from 0 to -2, and hydrogen (oxidizing agent that accepts electrons) changed from +1 to 0 ...
- No change in the oxidation state (i.e., not ORP). 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 initial substances do not change.
If we consider the rate of flow in the forward and reverse directions, then all types of chemical reactions can also be divided into two groups:
- Reversible - those that simultaneously flow in two directions. Most of the 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 materials: H2O + CO2 ↔ H2CO3.
- Irreversible - they flow only in the forward direction, after the complete consumption of one of the initial substances, they are completed, after which there are only products and the original substance taken in excess. Typically one of the products is either precipitated insoluble matter or evolved gas. For example, the interaction of sulfuric acid and barium chloride: H2SO4 + BaCl2 + → BaSO4 ↓ + 2HCl precipitates insoluble
The types of chemical reactions in organic chemistry can be divided into four groups:
- Substitution (some atoms or groups of atoms are replaced by others), for example, when chloroethane interacts with sodium hydroxide, ethanol and sodium chloride are formed: C2H5Cl + NaOH → C2H5OH + NaCl, that is, the chlorine atom is replaced by a hydrogen atom.
- Attachment (two molecules react and form one), for example, bromine is attached at the site of the break of the double bond in the ethylene molecule: Br2 + CH2 = CH2 → BrCH2 — CH2Br.
- Cleavage (a molecule decomposes into two or more molecules), for example, under certain conditions, ethanol decomposes into ethylene and water: C2H5OH → CH2 = CH2 + H2O.
- Rearrangement (isomerization, when one molecule transforms into another, but the qualitative and quantitative composition of the atoms in it does not change), for example, 3-chloruthene-1 (C4H7CL) is converted into 1 chlorobutene-2 (C4H7CL). Here the chlorine atom passed 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 known:
- By flowing with absorption (endothermic) or heat release (exothermic).
- By the type of interacting reagents or resulting products. Interaction with water - hydrolysis, with hydrogen - hydrogenation, with oxygen - oxidation or combustion. Elimination of water - dehydration, hydrogen - dehydrogenation, and so on.
- According to the conditions of interaction: in the presence under the influence of low or high temperature, with a change in pressure, in the light, etc.
- According to the mechanism of the reaction: ionic, radical-chain or chain 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 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 in the left and right sides 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 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 “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 resulting substances
According to this classification, chemical reactions are subdivided into reactions of combination, decomposition, substitution, exchange.
As a result compound reactions one new substance is formed from two or more (complex or simple) substances. V general view the equation for such a chemical reaction will look like this:
For example:
CaCO 3 + CO 2 + H 2 O = Ca (HCO 3) 2
SO 3 + H 2 O = H 2 SO 4
2Mg + O 2 = 2MgO.
2FеСl 2 + Сl 2 = 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 reactions (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 = 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O = CuSO 4 + 5H 2 O (3)
Cu (OH) 2 = CuO + H 2 O (4)
H 2 SiO 3 = SiO 2 + H 2 O (5)
2SO 3 = 2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 = 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. The decomposition reactions include the decomposition of oxides, acids, and salts formed by elements in higher oxidation states (6).
Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):
C 18 H 38 = C 9 H 18 + C 9 H 20 (8)
C 4 H 10 = C 4 H 6 + 2H 2 (9)
At 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 = 2Fе + Аl 2 О 3 (1)
Zn + 2HCl = ZnCl 2 + H 2 (2)
2KBr + Cl 2 = 2KCl + Br 2 (3)
2KSlO 3 + l 2 = 2KlO 3 + Сl 2 (4)
CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (5)
Ca 3 (PO 4) 2 + 3SiO 2 = 3CaSiO 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 no change in oxidation states occurs are few (5, 6).
Exchange reactions call the reactions that occur between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions located in aqueous solution... In general, the equation of the chemical exchange reaction will look like this:
AB + CD = AD + CB
For example:
CuO + 2HCl = CuCl 2 + H 2 O (1)
NaOH + HCl = NaCl + H 2 O (2)
NaHCO 3 + HCl = NaCl + H 2 O + CO 2 (3)
AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)
СrСl 3 + ЗNаОН = Сr (ОН) 3 ↓ + ЗNаСl (5)
Metabolic reactions are not redox. A 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 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 = 2MgO + C (1)
Mg 0 - 2e = Mg 2+ (reducing agent)
C 4+ + 4e = 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 = Fe 3+ (reducing agent)
N 5+ + 3e = N 2+ (oxidizing agent)
AgNO 3 + HCl = AgCl ↓ + HNO 3 (3)
Ca (OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)
Thermal classification of chemical reactions
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 amount of released or absorbed heat is indicated in the equation, then such equations are called thermochemical.
N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)
2Mg + O 2 = 2MgO + 602.5 kJ (2)
N 2 + O 2 = 2NO - 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
According to 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, the products of which are not able to react with each other to form ).
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 COOC 2 H 5 + H 2 O
Examples of irreversible reactions include the following reactions:
2KSlO 3 → 2KSl + 3O 2
С 6 Н 12 О 6 + 6О 2 → 6СО 2 + 6Н 2 О
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 the 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 the course of a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:
2H 2 O 2 = 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 = MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1