Lesson 2
Classification of chemical reactions in inorganic chemistry
Chemical reactions are classified according to various criteria.
According to the number of starting substances and reaction products
Decomposition - a reaction in which two or more simple or complex substances are formed from one compound
2KMnO 4 → K 2 MnO 4 + MnO 2 + O 2
Compound- a reaction in which two or more simple or complex substances are formed into one more complex
NH 3 + HCl → NH 4 Cl
substitution- a reaction that occurs between simple and complex substances, in which the atoms of a simple substance are replaced by atoms of one of the elements in a complex substance.
Fe + CuCl 2 → Cu + FeCl 2
Exchange a reaction in which two compounds exchange their constituents
Al 2 O 3 + 3H 2 SO 4 → Al 2 (SO 4) 3 + 3H 2 O
One of the exchange reactions neutralization It is a reaction between an acid and a base that produces salt and water.
NaOH + HCl → NaCl + H 2 O
By thermal effect
Reactions that release heat are called exothermic reactions.
C + O 2 → CO 2 + Q
2) Reactions that proceed with the absorption of heat are called endothermic reactions.
N 2 + O 2 → 2NO - Q
On the basis of reversibility
reversible Reactions that take place under the same conditions in two mutually opposite directions.
Reactions that proceed in only one direction and end with the complete transformation of the starting materials into the final ones are called irreversible in this case, a gas, a precipitate, or a low-dissociating substance, water, should be released.
BaCl 2 + H 2 SO 4 → BaSO 4 ↓ + 2HCl
Na 2 CO 3 + 2HCl → 2NaCl + CO 2 + H 2 O
Redox reactions- reactions occurring with a change in the degree of oxidation.
Ca + 4HNO 3 → Ca(NO 3) 2 + 2NO 2 + 2H 2 O
And reactions that occur without changing the oxidation state.
HNO 3 + KOH → KNO 3 + H 2 O
5.Homogeneous reactions, if the starting materials and reaction products are in the same state of aggregation. And heterogeneous reactions, if the reaction products and the starting materials are in different states of aggregation.
For example: ammonia synthesis.
Redox reactions.
There are two processes:
Oxidation- this is the return of electrons, as a result, the degree of oxidation increases. An atom is a molecule or ion that donates an electron is called reducing agent.
Mg 0 - 2e → Mg +2
Recovery - the process of adding electrons, as a result, the degree of oxidation decreases. Atom A molecule or ion that accepts an electron is called oxidizing agent.
S 0 +2e → S -2
O 2 0 +4e → 2O -2
In redox reactions, the rule must be observed electronic balance(the number of attached electrons should be equal to the number of given ones, there should not be free electrons). Also, it must be observed atomic balance(the number of like atoms on the left side should be equal to the number of atoms on the right side)
The rule of writing redox reactions.
Write a reaction equation
Set the oxidation state
Find elements whose oxidation state changes
Write them out in pairs.
Find an oxidizing agent and a reducing agent
Write the process of oxidation or reduction
Equalize the electrons using the electronic balance rule (find the i.c.) by placing the coefficients
Write a summary equation
Put the coefficients in the chemical reaction equation
KClO 3 → KClO 4 + KCl; N 2 + H 2 → NH 3; H 2 S + O 2 → SO 2 + H 2 O; Al + O 2 \u003d Al 2 O 3;
Сu + HNO 3 → Cu(NO 3) 2 + NO + H 2 O; KClO 3 → KCl + O 2; P + N 2 O \u003d N 2 + P 2 O 5;
NO 2 + H 2 O \u003d HNO 3 + NO
. The rate of chemical reactions. Dependence of the rate of chemical reactions on the concentration, temperature and nature of the reactants.
Chemical reactions proceed at different rates. Science is engaged in the study of the rate of a chemical reaction, as well as the identification of its dependence on the conditions of the process - chemical kinetics.
υ of a homogeneous reaction is determined by the change in the amount of substance per unit volume:
υ \u003d Δ n / Δt ∙ V
where Δ n is the change in the number of moles of one of the substances (most often the initial, but may also be the reaction product), (mol);
V - volume of gas or solution (l)
Since Δ n / V = ΔC (change in concentration), then
υ \u003d Δ C / Δt (mol / l ∙ s)
υ of a heterogeneous reaction is determined by the change in the amount of a substance per unit of time per unit of the contact surface of the substances.
υ \u003d Δ n / Δt ∙ S
where Δ n is the change in the amount of a substance (reagent or product), (mol);
Δt is the time interval (s, min);
S - surface area of contact of substances (cm 2, m 2)
Why are the rates of different reactions not the same?
In order for a chemical reaction to start, the molecules of the reactants must collide. But not every collision results in a chemical reaction. In order for a collision to lead to a chemical reaction, the molecules must have a sufficiently high energy. Particles that collide with each other to undergo a chemical reaction are called active. They have an excess energy compared to the average energy of most particles - the activation energy E Act . There are much fewer active particles in a substance than with an average energy, therefore, in order to start many reactions, the system must be supplied with some energy (a flash of light, heating, mechanical shock).
Energy barrier (value E Act) of different reactions is different, the lower it is, the easier and faster the reaction proceeds.
2. Factors affecting υ(number of particle collisions and their efficiency).
1) The nature of the reactants: their composition, structure => activation energy
▪ the less E Act, the more υ;
2) Temperature: at t for every 10 0 C, υ 2-4 times (van't Hoff rule).
υ 2 = υ 1 ∙ γ Δt/10
Task 1. The rate of a certain reaction at 0 0 C is 1 mol/l ∙ h, the temperature coefficient of the reaction is 3. What will be the rate of this reaction at 30 0 C?
υ 2 \u003d υ 1 ∙ γ Δt / 10
υ 2 \u003d 1 ∙ 3 30-0 / 10 \u003d 3 3 \u003d 27 mol / l ∙ h
3) Concentration: the more, the more often collisions and υ occur. At a constant temperature for the reaction mA + nB = C according to the law of mass action:
υ \u003d k ∙ С A m ∙ C B n
where k is the rate constant;
С – concentration (mol/l)
Law of acting masses:
The rate of a chemical reaction is proportional to the product of the concentrations of the reactants, taken in powers equal to their coefficients in the reaction equation.
Task 2. The reaction proceeds according to the equation A + 2B → C. How many times and how will the reaction rate change with an increase in the concentration of substance B by 3 times?
Solution: υ = k ∙ C A m ∙ C B n
υ \u003d k ∙ C A ∙ C B 2
υ 1 = k ∙ a ∙ in 2
υ 2 \u003d k ∙ a ∙ 3 in 2
υ 1 / υ 2 \u003d a ∙ in 2 / a ∙ 9 in 2 \u003d 1/9
Answer: increase by 9 times
For gaseous substances, the reaction rate depends on the pressure
The more pressure, the higher the speed.
4) Catalysts Substances that change the mechanism of a reaction E Act => υ .
▪ Catalysts remain unchanged at the end of the reaction
▪ Enzymes are biological catalysts, proteins by nature.
▪ Inhibitors - substances that ↓ υ
1. During the course of the reaction, the concentration of reagents:
1) increases
2) does not change
3) decreases
4) don't know
2. When the reaction proceeds, the concentration of products:
1) increases
2) does not change
3) decreases
4) don't know
3. For a homogeneous reaction A + B → ... with a simultaneous increase in the molar concentration of the starting substances by 3 times, the reaction rate increases:
1) 2 times
2) 3 times
4) 9 times
4. The reaction rate H 2 + J 2 → 2HJ will decrease by 16 times with a simultaneous decrease in the molar concentrations of the reagents:
1) 2 times
2) 4 times
5. The reaction rate of CO 2 + H 2 → CO + H 2 O increases with an increase in molar concentrations by 3 times (CO 2) and 2 times (H 2) increases:
1) 2 times
2) 3 times
4) 6 times
6. The reaction rate C (T) + O 2 → CO 2 with V-const and an increase in the amount of reagents by 4 times increases:
1) 4 times
4) 32 times
10. The reaction rate A + B → ... will increase with:
1) lowering the concentration of A
2) an increase in the concentration of B
3) cooling
4) pressure reduction
7. The reaction rate of Fe + H 2 SO 4 → FeSO 4 + H 2 is higher when using:
1) iron powder, not shavings
2) Iron chips, not powder
3) concentrated H 2 SO 4, not dilute H 2 SO 4
4) don't know
8
. The reaction rate 2H 2 O 2 2H 2 O + O 2 will be higher if you use:
1) 3% H 2 O 2 solution and catalyst
2) 30% H 2 O 2 solution and catalyst
3) 3% H 2 O 2 solution (without catalyst)
4) 30% H 2 O 2 solution (without catalyst)
chemical balance. Factors affecting the shifting balance. Le Chatelier's principle.
Chemical reactions can be divided according to their direction
▪ irreversible reactions proceed in only one direction (ion exchange reactions with , ↓, MDS, combustion, and some others.)
For example, AgNO 3 + HCl → AgCl↓ + HNO 3
▪ Reversible reactions under the same conditions flow in opposite directions (↔).
For example, N 2 + 3H 2 ↔ 2NH 3
The state of a reversible reaction, in which υ → = υ ← called chemical balance.
In order for the reaction in chemical industries to take place as completely as possible, it is necessary to shift the balance towards the product. In order to determine how one or another factor will change the equilibrium in the system, use Le Chatelier's principle(1844):
Le Chatelier's principle: If an external influence is exerted on a system in equilibrium (change t, p, C), then the equilibrium will shift in the direction that will weaken this impact.
The balance shifts:
1) at C react →,
at C prod ← ;
2) at p (for gases) - in the direction of decreasing volume,
at ↓ p - in the direction of increasing V;
if the reaction proceeds without changing the number of molecules of gaseous substances, then the pressure does not affect the equilibrium in this system.
3) at t - towards the endothermic reaction (- Q),
at ↓ t - towards the exothermic reaction (+ Q).
Task 3. How should the concentrations of substances, pressure and temperature of a homogeneous system PCl 5 ↔ PCl 3 + Cl 2 – Q be changed in order to shift the equilibrium towards decomposition of PCl 5 (→)
↓ C (PCl 3) and C (Cl 2)
Task 4. How to shift the chemical equilibrium of the reaction 2CO + O 2 ↔ 2CO 2 + Q at
a) an increase in temperature;
b) pressure increase
1. The method that shifts the equilibrium of the reaction 2CuO (T) + CO Cu 2 O (T) + CO 2 to the right (→) is:
1) increase in carbon monoxide concentration
2) increase in carbon dioxide concentration
3) decrease in the concentration of shallow oxide (I)
4) decrease in the concentration of copper oxide (II)
2. In a homogeneous reaction 4HCl + O 2 2Cl 2 + 2H 2 O, with increasing pressure, the equilibrium will shift:
2) right
3) will not move
4) don't know
8. When heated, the equilibrium of the reaction N 2 + O 2 2NO - Q:
1) move to the right
2) move to the left
3) will not move
4) don't know
9. Upon cooling, the equilibrium of the reaction H 2 + S H 2 S + Q:
1) move to the left
2) move to the right
3) will not move
4) don't know
Classification of chemical reactions in inorganic and organic chemistry
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Chemical reactions can be classified according to the following criteria:
1. According to the number and composition of the initial and resulting substances2. According to the degree of oxidation
3. According to the reversibility of the process
4. By thermal effect
5. By the presence of a catalyst
6. According to the state of aggregation
1. According to the degree of oxidation. Redox reactions. These are reactions in which one element donates an electron and another accepts.
Na + O 2 \u003d 2Na 2 O
4Na - 1e = Na 4 reducer
O 2 + 2x2e \u003d 2O 1 oxidizer
2. According to the number and composition of the initial formed substances:
A) Combination reactions (from two simple substances one complex is formed)
B) Decomposition reactions (two or more simple ones are formed from one complex substance)
C) Exchange reactions (reactions between complex substances as a result of which it exchanges its constituent parts)
D) Substitution reactions (reactions between complex and simple substances, as a result of which one of the atoms in a complex substance is replaced by a simple substance)
3. According to the thermal effect:
A) Exothermic reactions (Reactions proceed with the release of heat)
SO 2 + O 2 \u003d 2SO 3 + Q
B) Endothermic reactions (Reactions go with the absorption of heat)
C 4 H 10 \u003d C 4 H 8 + H 2 - Q
4. By reversibility, reactions are divided into reversible and irreversible
(Under certain conditions, reactions proceed in opposite directions)
5. According to the presence of a catalyst, reactions are divided into catalytic and non-catalytic.
6. According to the state of aggregation, the reactions are divided into homogeneous and heterogeneous.
Homogeneous - reacting and forming substances are in the same state of aggregation
Cl 2 + H 2 \u003d 2HCl
Heterogeneous - reacting and formed substances are in different states of aggregation
2C 2 H 2 + 5O 2 \u003d 4CO 2 + 2H 2 O + Q
Diene hydrocarbons, their structure, properties, production and practical significance.
Alcodienes are acyclic hydrocarbons in the molecule of which, in addition to single bonds, there are two double bonds between carbon atoms and which correspond to the general formula C n H 2 n -2
According to the arrangement of double bonds, three types of alkodienes are distinguished:
1. Alkodienes cumulated by arrangement of double bonds
CH 2 \u003d C \u003d CH 2- propadiene
2. Alcodienes with conjugated double bonds
CH 2 \u003d CH - CH \u003d CH 2- butadiene 1.3
3. Alcodienes with isolated arrangement of double bonds
CH 2 \u003d CH - CH 2 - CH \u003d CH 2-pentadiene 1,4
physical properties.
Propadiene and butadiene 1,3 are gaseous substances, alkodienes with isolated bonds are liquids, higher dienes are solids.
Chemical properties.
Alcodienes are characterized by addition reactions:
1. Halogenation reaction (the addition of halogens is due to double bonds)
CH 2 \u003d CH - CH \u003d CH 2 + Br 2 \u003d CH 2 Br \u003d CHBr - CH \u003d CH 2- 3,4 dibromobutene - 1
2. Hydrogenation reaction (hydrogen addition)
CH 2 \u003d CH - CH \u003d CH 2 + H 2 \u003d CH 3 - CH 2 - CH \u003d CH 2– butene-1
3. Polymerization reaction (combination of many monomer molecules into a polymer molecule).
CH 2 \u003d CH - CH \u003d CH 2 \u003d (-CH 2 - CH \u003d CH - CH 2 -) n- synthetic butadiene rubber
Receipt.
In our country, the production of butadiene began in 1932. The method of obtaining it from ethyl alcohol was developed by Academician S.V. Lebedev
But a more promising method for obtaining butadiene is the dehydrogenation of butane contained in petroleum gases. For this purpose, butane is passed over a heated catalyst.
Application.
Diene hydrocarbons are mainly used for the synthesis of rubbers.
CH 2 \u003d CH - CH \u003d CH 3 - 1.3 butadiene (butadiene rubber)
Synthetic rubbers are formed as a result of the polymerization reaction of the corresponding monomers.
Ticket number 4
General methods for obtaining metals. The practical significance of electrolysis.
Metals in nature are found mainly in the form of compounds, only metals located in the electrochemical series of voltages after hydrogen are found in the free form.
Obtaining metals from ores (compounds) is the task of metallurgy. There are the following methods for obtaining metals: pyrometalurgy, hydrometalurgy and electrometallurgy.
1. Pyrometallurgy- this is the recovery of metals from ores with the help of carbon, carbon monoxide (II), CO and hydrogen, at high temperature
2ZnO + C → 2Zn + CO 2
Fe2O 3 + 3CO → 2Fe + 3CO 2
CuO + H 2 →Cu + H 2 O
If a metal is used as a reducing agent, then this method is called metallothermy.
Cr 2 O 3 + 2Al → Al 2 O 3 + 2Cr
2. Hydrometallurgy is the reduction of metals from salts in solution. The process takes place in two stages: a natural compound is dissolved in a metal suitable for obtaining a salt of a given metal.
CuO + H 2 SO 4 → CuSO 4 + H 2 O
The metal is displaced from the solution by a more active metal.
CuSO 4 + Fe→FeSO 4 + Cu
3. Electrometallurgy- this is the reduction of metals in the process of electrolysis of solutions or melts of compounds.
Electrolysis- This is a redox process that occurs on the electrodes of the passage of electric current through an electrolyte solution or melt.
2NaCl ↔ 2Na + Cl 2
2Na + 2e → 2Na
2Cl – 2e→Cl 2
Application of electrolysis
Electrolysis of solutions and melts of substances is used in industry:1. For obtaining metals (alkali metals - Aluminum)
2. For the production of hydrogen, halogens and alkalis
3. For cleaning metals (refining)
4. To protect metals from corrosion
5. Obtaining metal copies and records
Chemical reactions should be distinguished from nuclear reactions. As a result of chemical reactions, the total number of atoms of each chemical element and its isotopic composition do not change. Nuclear reactions are another matter - the processes of transformation of atomic nuclei as a result of their interaction with other nuclei or elementary particles, for example, the transformation of aluminum into magnesium:
27 13 Al + 1 1 H \u003d 24 12 Mg + 4 2 He
The classification of chemical reactions is multifaceted, that is, it can be based on various signs. But under any of these signs, reactions both between inorganic and between organic substances can be attributed.
Consider the classification of chemical reactions according to various criteria.
I. According to the number and composition of the reactants
Reactions that take place without changing the composition of substances.
In not organic chemistry such reactions include the processes of obtaining allotropic modifications of one chemical element, for example:
C (graphite) ↔ C (diamond)
S (rhombic) ↔ S (monoclinic)
R (white) ↔ R (red)
Sn (white tin) ↔ Sn (grey tin)
3O 2 (oxygen) ↔ 2O 3 (ozone)In organic chemistry, this type of reactions can include isomerization reactions that occur without changing not only the qualitative, but also the quantitative composition of the molecules of substances, for example:
1. Isomerization of alkanes.
The reaction of isomerization of alkanes is of great practical importance, since hydrocarbons of the isostructure have a lower ability to detonate.
2. Isomerization of alkenes.
3. Isomerization of alkynes (reaction of A. E. Favorsky).
CH 3 - CH 2 - C \u003d - CH ↔ CH 3 - C \u003d - C- CH 3
ethylacetylene dimethylacetylene
4. Isomerization of haloalkanes (A. E. Favorsky, 1907).
5. Isomerization of ammonium cyanite upon heating.
For the first time, urea was synthesized by F. Wehler in 1828 by isomerization of ammonium cyanate when heated.
Reactions that go with a change in the composition of a substance
There are four types of such reactions: compounds, decompositions, substitutions and exchanges.
1. Connection reactions are such reactions in which one complex substance is formed from two or more substances
In inorganic chemistry, the whole variety of compound reactions can be considered, for example, using the example of reactions for obtaining sulfuric acid from sulfur:
1. Obtaining sulfur oxide (IV):
S + O 2 \u003d SO - one complex substance is formed from two simple substances.
2. Obtaining sulfur oxide (VI):
SO 2 + 0 2 → 2SO 3 - one complex substance is formed from a simple and complex substance.
3. Obtaining sulfuric acid:
SO 3 + H 2 O \u003d H 2 SO 4 - one complex is formed from two complex substances.
An example of a compound reaction in which one complex substance is formed from more than two starting materials is the final stage in the production of nitric acid:
4NO 2 + O 2 + 2H 2 O \u003d 4HNO 3
In organic chemistry, compound reactions are commonly referred to as "addition reactions". The whole variety of such reactions can be considered on the example of a block of reactions characterizing the properties of unsaturated substances, for example, ethylene:
1. Hydrogenation reaction - hydrogen addition:
CH 2 \u003d CH 2 + H 2 → H 3 -CH 3
ethene → ethane
2. Hydration reaction - addition of water.
3. Polymerization reaction.
2. Decomposition reactions are such reactions in which several new substances are formed from one complex substance.
In inorganic chemistry, the whole variety of such reactions can be considered in the block of reactions for obtaining oxygen by laboratory methods:
1. Decomposition of mercury (II) oxide - two simple ones are formed from one complex substance.
2. Decomposition of potassium nitrate - from one complex substance, one simple and one complex are formed.
3. Decomposition of potassium permanganate - from one complex substance, two complex and one simple are formed, that is, three new substances.
In organic chemistry, decomposition reactions can be considered on the block of reactions for the production of ethylene in the laboratory and in industry:
1. The reaction of dehydration (water splitting) of ethanol:
C 2 H 5 OH → CH 2 \u003d CH 2 + H 2 O
2. Dehydrogenation reaction (hydrogen splitting) of ethane:
CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2
or CH 3 -CH 3 → 2C + ZH 2
3. Cracking reaction (splitting) of propane:
CH 3 -CH 2 -CH 3 → CH 2 \u003d CH 2 + CH 4
3. Substitution reactions are such reactions as a result of which the atoms of a simple substance replace the atoms of an element in a complex substance.
In inorganic chemistry, an example of such processes is a block of reactions that characterize the properties of, for example, metals:
1. Interaction of alkali or alkaline earth metals with water:
2Na + 2H 2 O \u003d 2NaOH + H 2
2. Interaction of metals with acids in solution:
Zn + 2HCl = ZnCl 2 + H 2
3. Interaction of metals with salts in solution:
Fe + CuSO 4 = FeSO 4 + Cu
4. Metalthermy:
2Al + Cr 2 O 3 → Al 2 O 3 + 2Cr
The subject of study of organic chemistry is not simple substances, but only compounds. Therefore, as an example of a substitution reaction, we give the most characteristic property of saturated compounds, in particular methane, the ability of its hydrogen atoms to be replaced by halogen atoms. Another example is the bromination of an aromatic compound (benzene, toluene, aniline).
C 6 H 6 + Br 2 → C 6 H 5 Br + HBr
benzene → bromobenzene
Let us pay attention to the peculiarity of the substitution reaction in organic substances: as a result of such reactions, not a simple and complex substance is formed, as in inorganic chemistry, but two complex substances.
In organic chemistry, substitution reactions also include some reactions between two complex substances, for example, the nitration of benzene. It is formally an exchange reaction. The fact that this is a substitution reaction becomes clear only when considering its mechanism.
4. Exchange reactions are such reactions in which two complex substances exchange their constituent parts
These reactions characterize the properties of electrolytes and proceed in solutions according to the Berthollet rule, that is, only if a precipitate, gas, or a low-dissociating substance (for example, H 2 O) is formed as a result.
In inorganic chemistry, this can be a block of reactions characterizing, for example, the properties of alkalis:
1. Neutralization reaction that goes with the formation of salt and water.
2. The reaction between alkali and salt, which goes with the formation of gas.
3. The reaction between alkali and salt, which goes with the formation of a precipitate:
СuSO 4 + 2KOH \u003d Cu (OH) 2 + K 2 SO 4
or in ionic form:
Cu 2+ + 2OH - \u003d Cu (OH) 2
In organic chemistry, one can consider a block of reactions characterizing, for example, the properties of acetic acid:
1. The reaction proceeding with the formation of a weak electrolyte - H 2 O:
CH 3 COOH + NaOH → Na (CH3COO) + H 2 O
2. The reaction that goes with the formation of gas:
2CH 3 COOH + CaCO 3 → 2CH 3 COO + Ca 2+ + CO 2 + H 2 O
3. The reaction proceeding with the formation of a precipitate:
2CH 3 COOH + K 2 SO 3 → 2K (CH 3 COO) + H 2 SO 3
2CH 3 COOH + SiO → 2CH 3 COO + H 2 SiO 3
II. By changing the oxidation states of chemical elements that form substances
On this basis, the following reactions are distinguished:
1. Reactions that occur with a change in the oxidation states of elements, or redox reactions.
These include many reactions, including all substitution reactions, as well as those reactions of combination and decomposition in which at least one simple substance participates, for example:
1. Mg 0 + H + 2 SO 4 \u003d Mg + 2 SO 4 + H 2
2. 2Mg 0 + O 0 2 = Mg +2 O -2
Complex redox reactions are compiled using the electron balance method.
2KMn +7 O 4 + 16HCl - \u003d 2KCl - + 2Mn +2 Cl - 2 + 5Cl 0 2 + 8H 2 O
In organic chemistry, the properties of aldehydes can serve as a striking example of redox reactions.
1. They are reduced to the corresponding alcohols:
Aldecides are oxidized to the corresponding acids:
2. Reactions that take place without changing the oxidation states of chemical elements.
These include, for example, all ion exchange reactions, as well as many compound reactions, many decomposition reactions, esterification reactions:
HCOOH + CHgOH = HSOCH 3 + H 2 O
III. By thermal effect
According to the thermal effect, the reactions are divided into exothermic and endothermic.
1. Exothermic reactions proceed with the release of energy.
These include almost all compound reactions. A rare exception is the endothermic reactions of the synthesis of nitric oxide (II) from nitrogen and oxygen and the reaction of gaseous hydrogen with solid iodine.
Exothermic reactions that proceed with the release of light are referred to as combustion reactions. The hydrogenation of ethylene is an example of an exothermic reaction. It runs at room temperature.
2. Endothermic reactions proceed with the absorption of energy.
Obviously, almost all decomposition reactions will apply to them, for example:
1. Calcination of limestone
2. Butane cracking
The amount of energy released or absorbed as a result of the reaction is called the thermal effect of the reaction, and the equation of a chemical reaction indicating this effect is called the thermochemical equation:
H 2 (g) + C 12 (g) \u003d 2HC 1 (g) + 92.3 kJ
N 2 (g) + O 2 (g) \u003d 2NO (g) - 90.4 kJ
IV. According to the state of aggregation of reacting substances (phase composition)
According to the state of aggregation of the reacting substances, there are:
1. Heterogeneous reactions - reactions in which the reactants and reaction products are in different states of aggregation (in different phases).
2. Homogeneous reactions - reactions in which the reactants and reaction products are in the same state of aggregation (in one phase).
V. According to the participation of the catalyst
According to the participation of the catalyst, there are:
1. Non-catalytic reactions that take place without the participation of a catalyst.
2. Catalytic reactions taking place with the participation of a catalyst. Since all biochemical reactions occurring in the cells of living organisms proceed with the participation of special biological catalysts of a protein nature - enzymes, they all belong to catalytic or, more precisely, enzymatic ones. It should be noted that more than 70% of chemical industries use catalysts.
VI. Towards
By direction there are:
1. Irreversible reactions proceed under given conditions in only one direction. These include all exchange reactions accompanied by the formation of a precipitate, gas or a low-dissociating substance (water) and all combustion reactions.
2. Reversible reactions under these conditions proceed simultaneously in two opposite directions. Most of these reactions are.
In organic chemistry, the sign of reversibility is reflected in the names - antonyms of processes:
Hydrogenation - dehydrogenation,
Hydration - dehydration,
Polymerization - depolymerization.
All esterification reactions are reversible (the opposite process, as you know, is called hydrolysis) and hydrolysis of proteins, esters, carbohydrates, polynucleotides. The reversibility of these processes underlies the most important property of a living organism - metabolism.
VII. According to the mechanism of flow, there are:
1. Radical reactions take place between the radicals and molecules formed during the reaction.
As you already know, in all reactions, old chemical bonds are broken and new chemical bonds are formed. The method of breaking the bond in the molecules of the starting substance determines the mechanism (path) of the reaction. If the substance is formed by a covalent bond, then there can be two ways to break this bond: hemolytic and heterolytic. For example, for the molecules of Cl 2 , CH 4 , etc., a hemolytic rupture of bonds is realized, it will lead to the formation of particles with unpaired electrons, that is, free radicals.
Radicals are most often formed when bonds are broken in which shared electron pairs are distributed approximately equally between atoms (non-polar covalent bond), but many polar bonds can also be broken in a similar way, in particular when the reaction takes place in the gas phase and under the action of light. , as, for example, in the case of the processes discussed above - the interaction of C 12 and CH 4 - . Radicals are highly reactive, as they tend to complete their electron layer by taking an electron from another atom or molecule. For example, when a chlorine radical collides with a hydrogen molecule, it breaks the shared electron pair that binds the hydrogen atoms and forms a covalent bond with one of the hydrogen atoms. The second hydrogen atom, becoming a radical, forms a common electron pair with the unpaired electron of the chlorine atom from the collapsing Cl 2 molecule, resulting in a chlorine radical that attacks a new hydrogen molecule, etc.
Reactions, which are a chain of successive transformations, are called chain reactions. For the development of the theory of chain reactions, two outstanding chemists - our compatriot N. N. Semenov and the Englishman S. A. Hinshelwood were awarded the Nobel Prize.
The substitution reaction between chlorine and methane proceeds similarly:
Most of the combustion reactions of organic and inorganic substances, the synthesis of water, ammonia, the polymerization of ethylene, vinyl chloride, etc. proceed according to the radical mechanism.
2. Ionic reactions take place between ions already present or formed during the reaction.
Typical ionic reactions are interactions between electrolytes in solution. Ions are formed not only during the dissociation of electrolytes in solutions, but also under the action of electrical discharges, heating or radiation. γ-rays, for example, convert water and methane molecules into molecular ions.
According to another ionic mechanism, reactions of addition of hydrogen halides, hydrogen, halogens to alkenes, oxidation and dehydration of alcohols, replacement of alcohol hydroxyl by halogen occur; reactions characterizing the properties of aldehydes and acids. Ions in this case are formed by heterolytic breaking of covalent polar bonds.
VIII. According to the type of energy
initiating the reaction, there are:
1. Photochemical reactions. They are initiated by light energy. In addition to the above photochemical processes of HCl synthesis or the reaction of methane with chlorine, they include the production of ozone in the troposphere as a secondary atmospheric pollutant. In this case, nitric oxide (IV) acts as the primary one, which forms oxygen radicals under the action of light. These radicals interact with oxygen molecules, resulting in ozone.
The formation of ozone goes on as long as there is enough light, since NO can interact with oxygen molecules to form the same NO 2 . The accumulation of ozone and other secondary air pollutants can lead to photochemical smog.
This type of reaction includes critical process, which takes place in plant cells, is photosynthesis, the name of which speaks for itself.
2. Radiation reactions. They are initiated by high-energy radiation - x-rays, nuclear radiation (γ-rays, a-particles - He 2+, etc.). With the help of radiation reactions, very fast radiopolymerization, radiolysis (radiation decomposition), etc. are carried out.
For example, instead of a two-stage production of phenol from benzene, it can be obtained by the interaction of benzene with water under the action of radiation. In this case, radicals [OH] and [H] are formed from water molecules, with which benzene reacts to form phenol:
C 6 H 6 + 2 [OH] → C 6 H 5 OH + H 2 O
Rubber vulcanization can be carried out without sulfur using radiovulcanization, and the resulting rubber will be no worse than traditional rubber.
3. Electrochemical reactions. They are initiated by an electric current. In addition to the electrolysis reactions well known to you, we also indicate the reactions of electrosynthesis, for example, the reactions of the industrial production of inorganic oxidants
4. Thermochemical reactions. They are initiated by thermal energy. These include all endothermic reactions and many exothermic reactions that require an initial supply of heat, that is, the initiation of the process.
The above classification of chemical reactions is reflected in the diagram.
The classification of chemical reactions, like all other classifications, is conditional. Scientists agreed to divide the reactions into certain types according to the signs they identified. But most chemical transformations can be attributed to different types. For example, let's characterize the ammonia synthesis process.
This is a compound reaction, redox, exothermic, reversible, catalytic, heterogeneous (more precisely, heterogeneous catalytic), proceeding with a decrease in pressure in the system. To successfully manage the process, all of the above information must be taken into account. A specific chemical reaction is always multi-qualitative, it is characterized by different features.
DEFINITION
Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.
Most often, chemical reactions are understood as the process of transformation of initial substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations containing the formulas of the starting materials and reaction products. According to the law of conservation of mass, the number of atoms of each element in the left and right sides of the chemical equation is the same. Usually, the formulas of the starting substances are written on the left side of the equation, and the formulas of the products are written on the right. The equality of the number of atoms of each element in the left and right parts of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.
Chemical equations may contain additional information about the features of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or "under") the equals sign.
All chemical reactions can be grouped into several classes, which have certain characteristics.
Classification of chemical reactions according to the number and composition of the initial and resulting substances
According to this classification, chemical reactions are divided into reactions of combination, decomposition, substitution, exchange.
As a result compound reactions from two or more (complex or simple) substances, one new substance is formed. In general, the equation for such a chemical reaction will look like this:
For example:
CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2
SO 3 + H 2 O \u003d H 2 SO 4
2Mg + O 2 \u003d 2MgO.
2FeCl 2 + Cl 2 = 2FeCl 3
Combination reactions are in most cases exothermic, i.e. flow with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox (ORD), i.e. occur with a change in the oxidation states of the elements. It is impossible to say unequivocally whether the reaction of a compound between complex substances can be attributed to OVR.
Reactions in which several other new substances (complex or simple) are formed from one complex substance are classified as decomposition reactions. In general, the equation for a chemical decomposition reaction will look like this:
For example:
CaCO 3 CaO + CO 2 (1)
2H 2 O \u003d 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O \u003d CuSO 4 + 5H 2 O (3)
Cu (OH) 2 \u003d CuO + H 2 O (4)
H 2 SiO 3 \u003d SiO 2 + H 2 O (5)
2SO 3 \u003d 2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O (7)
Most decomposition reactions proceed with heating (1,4,5). Decomposition by electric current is 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 apply to OVR. OVR 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 \u003d C 9 H 18 + C 9 H 20 (8)
C 4 H 10 \u003d C 4 H 6 + 2H 2 (9)
At substitution reactions a simple substance interacts with a complex one, forming a new simple and a new complex substance. In general, the equation for a chemical substitution reaction will look like this:
For example:
2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3 (1)
Zn + 2HCl = ZnCl 2 + H 2 (2)
2KBr + Cl 2 \u003d 2KCl + Br 2 (3)
2KSlO 3 + l 2 = 2KlO 3 + Cl 2 (4)
CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2 (5)
Ca 3 (RO 4) 2 + ZSiO 2 = ZCaSiO 3 + P 2 O 5 (6)
CH 4 + Cl 2 = CH 3 Cl + Hcl (7)
Substitution reactions are mostly redox reactions (1 - 4, 7). Examples of decomposition reactions in which there is no change in oxidation states are few (5, 6).
Exchange reactions called the reactions that occur between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions located in aqueous solution. In general, the equation for a chemical exchange reaction will look like this:
AB + CD = AD + CB
For example:
CuO + 2HCl \u003d CuCl 2 + H 2 O (1)
NaOH + HCl \u003d NaCl + H 2 O (2)
NaHCO 3 + HCl \u003d NaCl + H 2 O + CO 2 (3)
AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)
CrCl 3 + ZNaOH = Cr(OH) 3 ↓+ ZNaCl (5)
Exchange reactions are not redox. A special case of these exchange reactions is neutralization reactions (reactions of interaction of acids with alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5) or a low-dissociating compound, most often water (1, 2).
Classification of chemical reactions according to changes in oxidation states
Depending on the change in the oxidation states of the elements that make up the reactants and reaction products, all chemical reactions are divided into redox (1, 2) and those occurring without changing the oxidation state (3, 4).
2Mg + CO 2 \u003d 2MgO + C (1)
Mg 0 - 2e \u003d Mg 2+ (reductant)
C 4+ + 4e \u003d C 0 (oxidizing agent)
FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)
Fe 2+ -e \u003d Fe 3+ (reductant)
N 5+ + 3e \u003d N 2+ (oxidizing agent)
AgNO 3 + HCl \u003d AgCl ↓ + HNO 3 (3)
Ca(OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)
Classification of chemical reactions by thermal effect
Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conditionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during a reaction is called the heat of the reaction. If the equation indicates the amount of released or absorbed heat, then such equations are called thermochemical.
N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)
2Mg + O 2 \u003d 2MgO + 602.5 kJ (2)
N 2 + O 2 \u003d 2NO - 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
According to the direction of the reaction, there are reversible (chemical processes whose products are able to react with each other under the same conditions in which they are obtained, with the formation of starting substances) and irreversible (chemical processes, the products of which are not able to react with each other with the formation of starting substances ).
For reversible reactions, the equation in general form is usually written as follows:
A + B ↔ AB
For example:
CH 3 COOH + C 2 H 5 OH ↔ H 3 COOS 2 H 5 + H 2 O
Examples of irreversible reactions are the following reactions:
2KSlO 3 → 2KSl + ZO 2
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
Evidence of the irreversibility of the reaction can serve as the reaction products of a gaseous substance, a precipitate or a low-dissociating compound, most often water.
Classification of chemical reactions by the presence of a catalyst
From this point of view, catalytic and non-catalytic reactions are distinguished.
A catalyst is a substance that speeds up a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:
2H 2 O 2 \u003d 2H 2 O + O 2 (MnO 2 catalyst)
Often, one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):
MeO + 2HF \u003d MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1
The classification of chemical reactions in inorganic and organic chemistry is carried out on the basis of various classifying features, details of which are given in the table below.
By changing the oxidation state of elements
The first sign of classification is by changing the degree of oxidation of the elements that form the reactants and products.
a) redox
b) without changing the oxidation state
redox called reactions accompanied by a change in the oxidation states of the chemical elements that make up the reagents. Redox in inorganic chemistry includes all substitution reactions and those decomposition and compound reactions in which at least one simple substance is involved. Reactions that proceed 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 according to the nature of the process, i.e., according to the number and composition of reactants and products.
Connection reactions called chemical reactions, as a result of which complex molecules are obtained from several simpler ones, for example:
4Li + O 2 = 2Li 2 ODecomposition reactions called chemical reactions, as a result of which simple molecules are obtained from more complex ones, for example:
CaCO 3 \u003d CaO + CO 2Decomposition reactions can be viewed as processes inverse to 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 \u003d FeCl 2 + H 2 Their distinguishing feature is the interaction of a simple substance with a complex one. Such reactions exist in organic chemistry.
However, the concept of "substitution" in organics 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 call chemical reactions that occur 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
Run in the opposite direction if possible.
If possible, proceed in the opposite direction - reversible and irreversible.
reversible called chemical reactions occurring at a given temperature simultaneously in two opposite directions with commensurate speeds. When writing the equations of 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 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 slightly dissociated compounds, a large amount of energy is released, 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 + O 2 \u003d 2CaO
BaBr 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaBr
By thermal effect
exothermic are chemical reactions that release heat. The symbol for the change in enthalpy (heat content) is ΔH, and the thermal effect of the reaction is Q. For exothermic reactions, Q > 0, and ΔH< 0.
endothermic called chemical reactions that take place with the absorption of heat. For endothermic reactions Q< 0, а ΔH > 0.
Coupling 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 called reactions occurring in a homogeneous medium (homogeneous substances, in one phase, for example, g-g, reactions in solutions).
heterogeneous called reactions that occur in an inhomogeneous medium, on the contact surface of the reacting substances that are in different phases, for example, solid and gaseous, liquid and gaseous, in two immiscible liquids.
By using a catalyst
A catalyst is a substance that speeds up a chemical reaction.
catalytic reactions proceed only in the presence of a catalyst (including enzymatic ones).
Non-catalytic reactions run in the absence of a catalyst.
By type of rupture
According to the type of chemical bond breaking in the initial molecule, homolytic and heterolytic reactions are distinguished.
homolytic called reactions in which, as a result of breaking bonds, particles are formed that have an unpaired electron - free radicals.
Heterolytic called reactions that proceed 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) chemical reactions involving radicals are called, for example:
CH 4 + Cl 2 hv → CH 3 Cl + HCl
Ionic called chemical reactions that take place with the participation of ions, for example:
KCl + AgNO 3 \u003d KNO 3 + AgCl ↓
Electrophilic refers to heterolytic reactions of organic compounds with electrophiles - particles that carry a whole or fractional positive charge. They are divided into reactions of electrophilic substitution and electrophilic addition, for example:
C 6 H 6 + Cl 2 FeCl3 → C 6 H 5 Cl + HCl
H 2 C \u003d CH 2 + Br 2 → BrCH 2 -CH 2 Br
Nucleophilic refers to heterolytic reactions of organic compounds with nucleophiles - particles that carry an integer or fractional negative charge. They are subdivided 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:
