The classification of chemical reactions in inorganic and organic chemistry is carried out on the basis of various classification characteristics, information about which is given in the table below.
By changing the oxidation state of elements
The first sign of classification is based on the change in the oxidation state of the elements that form the reactants and products.
a) redox
b) without changing the oxidation state
Redox are called reactions accompanied by a change in the oxidation states of the chemical elements that make up the reagents. Redox reactions in inorganic chemistry include all substitution reactions and those decomposition and combination reactions in which at least one simple substance is involved. Reactions that occur without changing the oxidation states of the elements that form the reactants and reaction products include all exchange reactions.
According to the number and composition of reagents and products
Chemical reactions are classified by the nature of the process, that is, by the number and composition of reagents and products.
Compound reactions are chemical reactions as a result of which complex molecules are obtained from several simpler ones, for example:
4Li + O 2 = 2Li 2 O
Decomposition reactions are called chemical reactions as a result of which simple molecules are obtained from more complex ones, for example:
CaCO 3 = CaO + CO 2
Decomposition reactions can be considered as the reverse processes of combination.
Substitution reactions are chemical reactions 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 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 chemistry is broader than in inorganic chemistry. If in the molecule of the original 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.
- exchange (including neutralization).
Exchange reactions are chemical reactions that occur without changing the oxidation states of elements and lead to the exchange of constituent parts of the reactants, 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 chemical reactions occurring at a given temperature simultaneously in two opposite directions with comparable speeds. When writing equations for such reactions, the equal sign is replaced by 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 reactions that occur only in the forward direction, resulting in the formation of products that do not interact with each other. Irreversible reactions include chemical reactions that result in the formation of slightly dissociated compounds, the release of a large amount of energy, as well as those in which the final products leave the reaction sphere in gaseous form or in the form of a precipitate, for example:
HCl + NaOH = NaCl + H2O
2Ca + O2 = 2CaO
BaBr 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaBr
By thermal effect
Exothermic are called chemical reactions that occur with the release of heat. Symbol for the change in enthalpy (heat content) ΔH, and the thermal effect of the reaction Q. For exothermic reactions Q > 0, and ΔH< 0.
Endothermic are chemical reactions that involve the absorption of heat. For endothermic reactions Q< 0, а ΔH > 0.
Compounding reactions will generally be exothermic reactions and decomposition reactions will be endothermic. A rare exception is the reaction of nitrogen with oxygen - endothermic:
N2 + O2 → 2NO – Q
By phase
Homogeneous are called reactions occurring in a homogeneous medium (homogeneous substances in one phase, for example g-g, reactions in solutions).
Heterogeneous are reactions that occur in a heterogeneous medium, on the contact surface of reacting substances that are in different phases, for example, solid and gaseous, liquid and gaseous, in two immiscible liquids.
According to the use of catalyst
A catalyst is a substance that accelerates a chemical reaction.
Catalytic reactions occur only in the presence of a catalyst (including enzymatic ones).
Non-catalytic reactions go in the absence of a catalyst.
By type of severance
Homolytic and heterolytic reactions are distinguished based on the type of chemical bond cleavage in the starting molecule.
Homolytic are called reactions in which, as a result of breaking bonds, particles are formed that have an unpaired electron - free radicals.
Heterolytic are reactions that occur through the formation of ionic particles - cations and anions.
- homolytic (equal gap, each atom receives 1 electron)
- heterolytic (unequal gap - one gets a pair of electrons)
Radical(chain) are chemical reactions involving radicals, for example:
CH 4 + Cl 2 hv →CH 3 Cl + HCl
Ionic are chemical reactions that occur with the participation of ions, for example:
KCl + AgNO 3 = KNO 3 + AgCl↓
Electrophilic reactions are heterolytic reactions of organic compounds with electrophiles - particles that carry a whole or fractional positive charge. They are divided 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 reactions are heterolytic reactions of organic compounds with nucleophiles - particles that carry a whole or fractional negative charge. They are divided 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 given in the table:
When a compound reacts from several reacting substances of relatively simple composition, one substance of a more complex composition is obtained:
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.
Reactions of compounds of simple substances are always redox in nature. Compound reactions occurring between complex substances can occur without a change in valency:
CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2,
and also be classified as redox:
2FeCl 2 + Cl 2 = 2FeCl 3.
2. Decomposition reactions
Decomposition reactions lead to the formation of several compounds from one complex substance:
A = B + C + D.
The decomposition products of a complex substance can be both simple and complex substances.
Of the decomposition reactions that occur without changing the valence states, noteworthy is the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids:
|
CuSO 4 + 5H 2 O |
|
2H 2 O + 4NO 2 O + O 2 O. |
2AgNO3 = 2Ag + 2NO2 + O2, (NH4)2Cr2O7 = Cr2O3 + N2 + 4H2O.
Redox decomposition reactions are especially characteristic for nitric acid salts.
Decomposition reactions in organic chemistry are called cracking:
C 18 H 38 = C 9 H 18 + C 9 H 20,
or dehydrogenation
C4H10 = C4H6 + 2H2.
3. Substitution reactions
In substitution reactions, usually a simple substance reacts with a complex one, forming another simple substance and another complex one:
A + BC = AB + C.
These reactions overwhelmingly belong to redox reactions:
2Al + Fe 2 O 3 = 2Fe + Al 2 O 3,
Zn + 2HCl = ZnСl 2 + H 2,
2KBr + Cl 2 = 2KCl + Br 2,
2KlO 3 + l 2 = 2KlO 3 + Cl 2.
Examples of substitution reactions that are not accompanied by a change in the valence states of atoms are extremely few. It should be noted the reaction of silicon dioxide with salts of oxygen-containing acids, which correspond to gaseous or volatile anhydrides:
CaCO 3 + SiO 2 = CaSiO 3 + CO 2,
Ca 3 (PO 4) 2 + 3SiO 2 \u003d 3СаSiO 3 + P 2 O 5,
Sometimes these reactions are considered as exchange reactions:
CH 4 + Cl 2 = CH 3 Cl + HCl.
4. Exchange reactions
Exchange reactions are reactions between two compounds that exchange their constituents with each other:
AB + CD = AD + CB.
If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of the atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts:
ZnO + H 2 SO 4 = ZnSO 4 + H 2 O,
AgNO 3 + KBr = AgBr + KNO 3,
CrCl 3 + ZNaON = Cr(OH) 3 + ZNaCl.
A special case of these exchange reactions is the neutralization reaction:
HCl + KOH = KCl + H 2 O.
Typically, these reactions obey the laws of chemical equilibrium and proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous, volatile substance, precipitate or low-dissociating (for solutions) compound:
NaHCO 3 + HCl = NaCl + H 2 O + CO 2,
Ca(HCO 3) 2 + Ca(OH) 2 = 2CaCO 3 ↓ + 2H 2 O,
CH 3 COONa + H 3 PO 4 = CH 3 COOH + NaH 2 PO 4.
The chemical properties of substances are revealed in a variety of chemical reactions.
Transformations of substances accompanied by changes in their composition and (or) structure are called chemical reactions. The following definition is often found: chemical reaction is the process of converting starting substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations and diagrams containing the formulas of the starting substances and reaction products. In chemical equations, unlike diagrams, the number of atoms of each element is the same on the left and right sides, which reflects the law of conservation of mass.
On the left side of the equation the formulas of the starting substances (reagents) are written, on the right side - the substances obtained as a result of the chemical reaction (reaction products, final substances). The equal sign connecting the left and right sides indicates that the total number of atoms of the substances involved in the reaction remains constant. This is achieved by placing integer stoichiometric coefficients in front of the formulas, showing the quantitative relationships between the reactants and reaction products.
Chemical equations may contain additional information about the characteristics of the reaction. If a chemical reaction occurs under the influence of external influences (temperature, pressure, radiation, etc.), this is indicated by the appropriate symbol, usually above (or “below”) the equal sign.
A huge number of chemical reactions can be grouped into several types of reactions, which have very specific characteristics.
As classification characteristics the following can be selected:
1. The number and composition of starting substances and reaction products.
2. Physical state of the reagents and reaction products.
3. The number of phases in which the reaction participants are located.
4. The nature of the transferred particles.
5. Possibility of the reaction occurring in forward and reverse directions.
6. The sign of the thermal effect divides all reactions into: exothermic reactions occurring with exo-effect - release of energy in the form of heat (Q>0, ∆H<0):
C + O 2 = CO 2 + Q
And endothermic reactions occurring with the endo effect - the absorption of energy in the form of heat (Q<0, ∆H >0):
N 2 + O 2 = 2NO - Q.
Such reactions are referred to as thermochemical.
Let's take a closer look at each type of reaction.
Classification according to the number and composition of reagents and final substances
1. Compound reactions
When a compound reacts from several reacting substances of relatively simple composition, one substance of a more complex composition is obtained:
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.
Reactions of compounds of simple substances are always redox in nature. Compound reactions occurring between complex substances can occur without a change in valence:
CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2,
and also be classified as redox:
2FeCl 2 + Cl 2 = 2FeCl 3.
2. Decomposition reactions
Decomposition reactions lead to the formation of several compounds from one complex substance:
A = B + C + D.
The decomposition products of a complex substance can be both simple and complex substances.
Of the decomposition reactions that occur without changing the valence states, noteworthy is the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids:
| t o | ||
| 4HNO3 | = | 2H 2 O + 4NO 2 O + O 2 O. |
2AgNO3 = 2Ag + 2NO2 + O2,
(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O.
Redox decomposition reactions are especially characteristic for salts of nitric acid.
Decomposition reactions in organic chemistry are called cracking:
C 18 H 38 = C 9 H 18 + C 9 H 20,
or dehydrogenation
C4H10 = C4H6 + 2H2.
3. Substitution reactions
In substitution reactions, usually a simple substance reacts with a complex one, forming another simple substance and another complex one:
A + BC = AB + C.
These reactions overwhelmingly belong to redox reactions:
2Al + Fe 2 O 3 = 2Fe + Al 2 O 3,
Zn + 2HCl = ZnСl 2 + H 2,
2KBr + Cl 2 = 2KCl + Br 2,
2KlO 3 + l 2 = 2KlO 3 + Cl 2.
Examples of substitution reactions that are not accompanied by a change in the valence states of atoms are extremely few. It should be noted the reaction of silicon dioxide with salts of oxygen-containing acids, which correspond to gaseous or volatile anhydrides:
CaCO 3 + SiO 2 = CaSiO 3 + CO 2,
Ca 3 (PO 4) 2 + 3SiO 2 \u003d 3СаSiO 3 + P 2 O 5,
Sometimes these reactions are considered as exchange reactions:
CH 4 + Cl 2 = CH 3 Cl + HCl.
4. Exchange reactions
Exchange reactions are reactions between two compounds that exchange their constituents with each other:
AB + CD = AD + CB.
If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of the atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts:
ZnO + H 2 SO 4 = ZnSO 4 + H 2 O,
AgNO 3 + KBr = AgBr + KNO 3,
CrCl 3 + ZNaON = Cr(OH) 3 + ZNaCl.
A special case of these exchange reactions is neutralization reactions:
HCl + KOH = KCl + H 2 O.
Typically, these reactions obey the laws of chemical equilibrium and proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous, volatile substance, precipitate or low-dissociating (for solutions) compound:
NaHCO 3 + HCl = NaCl + H 2 O + CO 2,
Ca(HCO 3) 2 + Ca(OH) 2 = 2CaCO 3 ↓ + 2H 2 O,
CH 3 COONa + H 3 PO 4 = CH 3 COOH + NaH 2 PO 4.
5. Transfer reactions.
In transfer reactions, an atom or group of atoms moves from one structural unit to another:
AB + BC = A + B 2 C,
A 2 B + 2CB 2 = DIA 2 + DIA 3.
For example:
2AgCl + SnCl 2 = 2Ag + SnCl 4,
H 2 O + 2NO 2 = HNO 2 + HNO 3.
Classification of reactions according to phase characteristics
Depending on the state of aggregation of the reacting substances, the following reactions are distinguished:
1. Gas reactions
| H2+Cl2 | 2HCl. |
2. Reactions in solutions
NaOH(solution) + HCl(p-p) = NaCl(p-p) + H 2 O(l)
3. Reactions between solids
| t o | ||
| CaO(tv) + SiO 2 (tv) | = | CaSiO 3 (sol) |
Classification of reactions according to the number of phases.
A phase is understood as a set of homogeneous parts of a system with the same physical and chemical properties and separated from each other by an interface.
The whole variety of reactions from this point of view can be divided into two classes:
1. Homogeneous (single-phase) reactions. These include reactions occurring in the gas phase and a number of reactions occurring in solutions.
2. Heterogeneous (multiphase) reactions. These include reactions in which the reactants and reaction products are in different phases. For example:
gas-liquid-phase reactions
CO 2 (g) + NaOH(p-p) = NaHCO 3 (p-p).
gas-solid-phase reactions
CO 2 (g) + CaO (tv) = CaCO 3 (tv).
liquid-solid-phase reactions
Na 2 SO 4 (solution) + BaCl 3 (solution) = BaSO 4 (tv)↓ + 2NaCl (p-p).
liquid-gas-solid-phase reactions
Ca(HCO 3) 2 (solution) + H 2 SO 4 (solution) = CO 2 (r) + H 2 O (l) + CaSO 4 (sol)↓.
Classification of reactions according to the type of particles transferred
1. Protolytic reactions.
TO protolytic reactions include chemical processes, the essence of which is the transfer of a proton from one reacting substance to another.
This classification is based on the protolytic theory of acids and bases, according to which an acid is any substance that donates a proton, and a base is a substance that can accept a proton, for example:
Protolytic reactions include neutralization and hydrolysis reactions.
2. Redox reactions.
These include reactions in which reacting substances exchange electrons, thereby changing the oxidation states of the atoms of the elements that make up the reacting substances. For example:
Zn + 2H + → Zn 2 + + H 2,
FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O,
The vast majority of chemical reactions are redox reactions; they play an extremely important role.
3. Ligand exchange reactions.
These include reactions during which the transfer of an electron pair occurs with the formation of a covalent bond via a donor-acceptor mechanism. For example:
Cu(NO 3) 2 + 4NH 3 = (NO 3) 2,
Fe + 5CO = ,
Al(OH) 3 + NaOH = .
A characteristic feature of ligand exchange reactions is that the formation of new compounds, called complexes, occurs without changing the oxidation state.
4. Reactions of atomic-molecular exchange.
This type of reaction includes many of the substitution reactions studied in organic chemistry that occur via a radical, electrophilic or nucleophilic mechanism.
Reversible and irreversible chemical reactions
Reversible chemical processes are those whose products are capable of reacting with each other under the same conditions in which they were obtained to form the starting substances.
For reversible reactions, the equation is usually written as follows:
Two oppositely directed arrows indicate that, under the same conditions, both forward and reverse reactions occur simultaneously, for example:
CH 3 COOH + C 2 H 5 OH CH 3 COOC 2 H 5 + H 2 O.
Irreversible chemical processes are those whose products are not able to react with each other to form the starting substances. Examples of irreversible reactions include the decomposition of Berthollet salt when heated:
2КlО 3 → 2Кl + ЗО 2,
or oxidation of glucose by atmospheric oxygen:
C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O.
(photochemical reactions), electric current (electrode processes), ionizing radiation (radiation-chemical reactions), mechanical action (mechanochemical reactions), in low-temperature plasma (plasmochemical 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, and 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 bonds being broken and those 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 can change their external form or state of aggregation.
In 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 the elements involved, which leads to the formation of atoms of new elements.
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There are a large number of characteristics by which chemical reactions can be classified.
1. Based on the presence of phase boundaries, all chemical reactions are divided into homogeneous And heterogeneous
A chemical reaction occurring within one phase is called homogeneous chemical reaction . The chemical reaction occurring at the interface 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 one phase) and heterophasic (the starting substances and products form several phases). Homo- and heterophasicity of a reaction is not related to whether the reaction is homo- or heterogeneous. Therefore, four types of processes can be distinguished:
- Homogeneous reactions (homophasic) . In this type of reaction, 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 homophasic reactions . The components are within one phase, but the reaction occurs at the phase boundary, for example, on the surface of the catalyst. An example would be the hydrogenation of ethylene over a nickel catalyst:
- Homogeneous heterophasic reactions . The reactants and products in such a reaction exist within several phases, but the reaction occurs in one phase. This is how the oxidation of hydrocarbons in the liquid phase with gaseous oxygen can take place.
- Heterogeneous heterophasic reactions . In this case, the reactants are in different phase states, and the reaction products can also be in any phase state. The reaction process occurs at the phase boundary. An example is the reaction of carbonic acid salts (carbonates) with Bronsted acids:
2.By changing the oxidation states of the reactants
In this case, there is a distinction
- Redox reactions in which atoms of one element (oxidizing agent) are being restored , that is lower their oxidation state, and the atoms of another element (reducing agent) oxidize , 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 comporportionation reaction is the decomposition reaction of ammonium nitrate when heated. In this case, the oxidizing agent is nitrogen (+5) of the nitro group, and the reducing agent is nitrogen (-3) of the ammonium cation:
NH4NO3 → N2O + 2H2O (< 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 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 in reagents are broken, energy is released, which is mainly used to form new chemical bonds. In some reactions the energies of these processes are close, and in this case the overall thermal effect of the reaction approaches zero. In other cases we can distinguish:
- exothermic reactions that come with heat release,(positive thermal effect) for example, the above combustion of hydrogen
- endothermic reactions during which heat is absorbed(negative thermal effect) from the environment.
The thermal effect of a reaction (enthalpy of reaction, Δ r H), which is often very important, can be calculated using Hess's law if the enthalpies of formation of the 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 release, otherwise (Δ r H > 0) - absorption.
4.By the type of transformation of reacting particles
Chemical reactions are always accompanied by physical effects: absorption or release of energy, change in color of the reaction mixture, etc. It is by these physical effects that the progress of chemical reactions is often judged.
Compound reaction - a chemical reaction as a result of which only one new substance is formed from two or more starting substances. Both simple and complex substances can enter into such reactions.
Decomposition reaction -a chemical reaction that results in the formation of several new substances from one substance. Reactions of this type involve only complex compounds, 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 that 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 substances must be simple and the other complex.
Exchange reactions - a reaction in which two complex substances exchange their constituent parts
5. Based on the direction of occurrence, chemical reactions are divided into irreversible and reversible
Irreversible chemical reactions that proceed in only one direction are called from left to right"), as a result of which the starting substances are transformed into reaction products. Such chemical processes are said to proceed “to the end.” These include combustion reactions, and also reactions accompanied by the formation of poorly soluble or gaseous substances Reversible are chemical reactions that occur 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, they are distinguished straight( flows from left to right) and reverse(proceeds “from right to left”). Since during a reversible reaction the starting substances are simultaneously consumed and formed, they are not completely converted into reaction products. Therefore, reversible reactions are said to proceed “not completely.” As a result, a mixture of starting 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 sign or reversibility sign, sometimes along with the designation of the conditions of occurrence (temperature t, pressure p). Reactions of this type include many decomposition and combination reactions.
