How to solve chemical equations step by step. Chemical reactions. Equations of chemical reactions. Rigorous determination of the oxidation rate

Class: 8

Presentation for the lesson

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The purpose of the lesson: help students to form knowledge about the chemical equation as a conditional notation chemical reaction using chemical formulas.

Tasks:

Educational:

systematize previously studied material;
to teach the ability to write equations of chemical reactions.

Educational:

develop communication skills (work in pairs, the ability to listen and hear).

Developing:

develop educational and organizational skills aimed at fulfilling the task;
develop analytical thinking skills.

Lesson type: combined.

Equipment: computer, multimedia projector, screen, evaluation sheets, reflection card, “set of chemical symbols”, notebook with printed base, reagents: sodium hydroxide, iron(III) chloride, spirit lamp, holder, matches, drawing paper sheet, multi-colored chemical symbols.

Lesson presentation (Appendix 3)

Lesson structure.

I. Organizing time.
II. Updating knowledge and skills.
III. Motivation and goal setting.
IV. Learning new material:
4.1 combustion reaction of aluminum in oxygen;
4.2 decomposition reaction of iron (III) hydroxide;
4.3 algorithm for placing coefficients;
4.4 minute of relaxation;
4.5 arrange the coefficients;
V. Consolidation of acquired knowledge.
VI. Summing up the lesson and grading.
VII. Homework.
VIII. Final word from the teacher.

During the classes

The chemical nature of a complex particle
determined by the nature of the elementary
components,
their number and
chemical structure.
D.I. Mendeleev

Teacher. Hello guys. Sit down.
Please note: there is a notebook with a printed basis on your table (Appendix 2), in which you will work today, and an evaluation sheet, in which you will record your achievements, sign it.

Updating knowledge and skills.

Teacher. We got acquainted with physical and chemical phenomena, chemical reactions and signs of their occurrence. We studied the law of conservation of mass of substances.
Let's test your knowledge. I suggest that you open your notebooks with a printed base and complete task 1. You are given 5 minutes to complete the task.

Test on the topic “Physical and chemical phenomena. The law of conservation of mass of substances.

1. How do chemical reactions differ from physical phenomena?

Change in the form, state of aggregation of matter.
The formation of new substances.
Location change.

2. What are the signs of a chemical reaction?

Precipitation, color change, gas evolution.

Magnetization, evaporation, oscillation.

Growth and development, movement, reproduction.

3. In accordance with what law are the equations of chemical reactions compiled?

The law of the constancy of the composition of matter.
The law of conservation of mass of matter.
Periodic law.
The law of dynamics.
The law of universal gravitation.

4. The law of conservation of mass of matter discovered:

DI. Mendeleev.
C. Darwin.
M.V. Lomonosov.
I. Newton.
A.I. Butlerov.

5. The chemical equation is called:

Conditional notation of a chemical reaction.

A conditional record of the composition of a substance.

Recording the conditions of a chemical problem.

Teacher. You've done the job. I suggest you check it out. Swap notebooks and check each other. Attention to the screen. For each correct answer - 1 point. Record the total score on the score sheet.

Motivation and goal setting.

Teacher. Using this knowledge, today we will compose the equations of chemical reactions, revealing the problem “Is the law of conservation of mass of substances the basis for compiling the equations of chemical reactions”

Learning new material.

Teacher. We are used to thinking that an equation is a mathematical example where there is an unknown, and this unknown needs to be calculated. But in chemical equations, there is usually nothing unknown: everything is simply written in them with formulas: what substances enter into the reaction and what are obtained during this reaction. Let's see the experience.

(The reaction of sulfur and iron compounds.) Appendix 3

Teacher. From the point of view of the mass of substances, the reaction equation for the combination of iron and sulfur is understood as follows

Iron + sulfur → iron (II) sulfide (task 2 tpo)

But in chemistry words are reflected by chemical signs. Write this equation in chemical symbols.

Fe + S → FeS

(One student writes on the blackboard, the rest in the TVET.)

Teacher. Now read.
Learners. An iron molecule interacts with a sulfur molecule, one molecule of iron (II) sulfide is obtained.
Teacher. In this reaction, we see that the amount of starting materials is equal to the amount of substances in the reaction product.
It must always be remembered that when drawing up reaction equations, not a single atom should be lost or unexpectedly appear. Therefore, sometimes, having written all the formulas in the reaction equation, you have to equalize the number of atoms in each part of the equation - to arrange the coefficients. Let's see another experience

(Combustion of aluminum in oxygen.) Appendix 4

Teacher. Let's write the chemical reaction equation (task 3 in TPO)

Al + O 2 → Al +3 O -2

To write down the formula of the oxide correctly, remember that

Learners. Oxygen in oxides has an oxidation state of -2, aluminum is a chemical element with a constant oxidation state of +3. LCM = 6

Al + O 2 → Al 2 O 3

Teacher. We see that 1 aluminum atom enters the reaction, two aluminum atoms are formed. Two oxygen atoms enter, three oxygen atoms are formed.
Simple and beautiful, but disrespectful to the law of conservation of mass of substances - it is different before and after the reaction.
Therefore, we need to arrange the coefficients in this chemical reaction equation. To do this, we find the LCM for oxygen.

Learners. LCM = 6

Teacher. Before the formulas for oxygen and aluminum oxide, we set the coefficients so that the number of oxygen atoms on the left and right is 6.

Al + 3 O 2 → 2 Al 2 O 3

Teacher. Now we get that as a result of the reaction four aluminum atoms are formed. Therefore, before the aluminum atom on the left side we put the coefficient 4

Al + 3O 2 → 2Al 2 O 3

Once again, we count all the atoms before and after the reaction. We put it equal.

4Al + 3O 2 _ = 2 Al 2 O 3

Teacher. Consider another example

(The teacher demonstrates an experiment on the decomposition of iron (III) hydroxide.)

Fe(OH) 3 → Fe 2 O 3 + H 2 O

Teacher. Let's set up the coefficients. 1 iron atom enters the reaction, two iron atoms are formed. Therefore, before the formula of iron hydroxide (3) we put the coefficient 2.

Fe(OH) 3 → Fe 2 O 3 + H 2 O

Teacher. We get that 6 hydrogen atoms (2x3) enter into the reaction, 2 hydrogen atoms are formed.

Learners. LCM =6. 6/2 \u003d 3. Therefore, we set the coefficient 3 for the water formula

2Fe(OH) 3 → Fe 2 O 3 + 3 H 2 O

Teacher. We count oxygen.

Learners. Left - 2x3 = 6; right – 3+3 = 6

Learners. The number of oxygen atoms involved in the reaction is equal to the number of oxygen atoms formed during the reaction. You can set equal.

2Fe(OH) 3 = Fe 2 O 3 +3 H 2 O

Teacher. Now let's summarize everything that was said earlier and get acquainted with the algorithm for arranging the coefficients in the equations of chemical reactions.

Count the number of atoms of each element on the right and left sides of the chemical reaction equation.

Determine which element has a changing number of atoms, find the LCM.

Divide the LCM into indices - get the coefficients. Put them before the formulas.

Count the number of atoms, repeat if necessary.

The last thing to check is the number of oxygen atoms.

Teacher. You've worked hard and you're probably tired. I suggest you relax, close your eyes and remember some pleasant moments of life. Each of you is different. Now open your eyes and make circular movements with them, first clockwise, then counterclockwise. Now intensively move your eyes horizontally: right - left, and vertically: up - down.
And now we will activate mental activity and massage the earlobes.

Teacher. We continue to work.
In notebooks with a printed base, we will complete task 5. You will work in pairs. You need to place the coefficients in the equations of chemical reactions. You have 10 minutes to complete the task.

P + Cl 2 →PCl 5
Na + S → Na 2 S
HCl + Mg → MgCl 2 + H 2
N 2 + H 2 → NH 3
H 2 O → H 2 + O 2

Teacher. Let's check the execution of the task ( the teacher asks and displays the correct answers on the slide). For each correctly set coefficient - 1 point.
You have completed the task. Well done!

Teacher. Now let's get back to our problem.
Guys, what do you think, is the law of conservation of mass of substances the basis for compiling equations of chemical reactions.

Learners. Yes, during the lesson we proved that the law of conservation of mass of substances is the basis for compiling equations of chemical reactions.

Consolidation of knowledge.

Teacher. We have covered all the key issues. Now let's do a little test to see how well you've mastered the topic. You must answer it only “yes” or “no”. You have 3 minutes to work.

Statements.

In the reaction Ca + Cl 2 → CaCl 2 coefficients are not needed.(Yes)
In the reaction Zn + HCl → ZnCl 2 + H 2, the coefficient of zinc is 2. (No)
In the reaction Ca + O 2 → CaO, the coefficient of calcium oxide is 2.(Yes)
In the CH 4 → C + H 2 reaction, the coefficients are not needed.(No)
In the reaction CuO + H 2 → Cu + H 2 O, the coefficient for copper is 2. (No)
In the reaction C + O 2 → CO, the coefficient 2 must be set for both carbon monoxide (II) and carbon. (Yes)
In the reaction CuCl 2 + Fe → Cu + FeCl 2, the coefficients are not needed.(Yes)

Teacher. Let's check the work. For each correct answer - 1 point.

Summary of the lesson.

Teacher. You did a good job. Now calculate the total number of points scored for the lesson and rate yourself according to the rating that you see on the screen. Give me the score sheets to put your grade in the journal.

Homework.

Teacher. Our lesson came to an end, during which we were able to prove that the law of conservation of mass of substances is the basis for compiling reaction equations, and learned how to write chemical reaction equations. And, as a final point, write down homework

§ 27, ex. 1 - for those who received a rating of "3"
ex. 2 - for those who received a rating of "4"
ex. 3 - for those who received a rating“5”

Final word from the teacher.

Teacher. I thank you for the lesson. But before you leave the office, pay attention to the table (the teacher points to a sheet of drawing paper with a table and multi-colored chemical signs). You see chemical signs in different colors. Each color symbolizes your mood .. I suggest you make your own table chemical elements(it will differ from D.I. Mendeleev's PSHE) - a table of the mood of the lesson. To do this, you must go to the music sheet, take one chemical element, according to the characteristic that you see on the screen, and attach it to the cell of the table. I will do it first, showing you my comfort from working with you.

F I felt comfortable at the lesson, I got an answer to all my questions.

F In the lesson, I reached the goal halfway.
F I was bored at the lesson, I did not learn anything new.

Chemical reactions, their properties, types, flow conditions, etc., are one of the cornerstones of an interesting science called chemistry. Let's try to figure out what a chemical reaction is and what its role is. So, a chemical reaction in chemistry is considered to be the transformation of one or more substances into other substances. At the same time, their nuclei do not change (unlike nuclear reactions), but there is a redistribution of electrons and nuclei, and, of course, new chemical elements appear.

Chemical reactions in nature and everyday life

You and I are surrounded by chemical reactions, moreover, we ourselves regularly carry them out by various household activities, when, for example, we light a match. Especially a lot of chemical reactions themselves without suspecting (and maybe suspecting) cooks do when they prepare food.

Of course, also in natural conditions many chemical reactions take place: the eruption of a volcano, foliage and trees, but what can I say, almost any biological process can be attributed to examples of chemical reactions.

Types of chemical reactions

All chemical reactions can be divided into simple and complex. Simple chemical reactions, in turn, are divided into:

compound reactions,
decomposition reactions,
substitution reactions,
exchange reactions.

Chemical reaction of the compound

According to the very apt definition of the great chemist D. I. Mendeleev, the reaction of a compound takes place when “one of their two substances occurs.” An example of a chemical reaction of a compound can be the heating of iron and sulfur powders, in which iron sulfide is formed from them - Fe + S = FeS. Another striking example of this reaction is the combustion of simple substances, such as sulfur or in air (perhaps, such a reaction can also be called a thermal chemical reaction).

Decomposition chemical reaction

It's simple, the decomposition reaction is the opposite of the compound reaction. It produces two or more substances from one substance. A simple example The chemical reaction of decomposition can be the reaction of decomposition of chalk, during which quicklime and carbon dioxide are formed from the actual chalk.

Chemical substitution reaction

The substitution reaction is carried out when a simple substance interacts with a complex one. Let's give an example of a chemical substitution reaction: if you lower a steel nail into a solution with blue vitriol, then in the course of this simple chemical experiment we will get iron sulphate (iron will displace copper from salt). The equation for such a chemical reaction would look like this:

Fe + CuSO 4 → FeSO 4 + Cu

Chemical exchange reaction

Exchange reactions take place exclusively between complex chemicals, during which they change their parts. A lot of these reactions take place in various solutions. Neutralization of acid by bile good example chemical exchange reaction.

NaOH + HCl → NaCl + H 2 O

This is the chemical equation of this reaction, in which a hydrogen ion from the HCl compound is exchanged with a sodium ion from the NaOH compound. The consequence of this chemical reaction is the formation of a salt solution.

Signs of chemical reactions

According to the signs of the occurrence of chemical reactions, one can judge whether a chemical reaction between the reagents has passed or not. Here are examples of signs of chemical reactions:

Color change (light iron, for example, in humid air is covered with a brown coating, as a result of a chemical reaction between iron and iron).
Precipitation (if carbon dioxide is suddenly passed through a lime solution, we will get a precipitation of a white insoluble precipitate of calcium carbonate).
Gas release (if you drop citric acid on baking soda, you will get carbon dioxide release).
The formation of weakly dissociated substances (all reactions resulting in the formation of water).
The glow of the solution (an example here is the reactions that occur with a solution of luminol, which emits light during chemical reactions).

In general, it is difficult to distinguish which signs of chemical reactions are the main ones; different substances and different reactions have their own signs.

How to determine the sign of a chemical reaction

You can determine the sign of a chemical reaction visually (with a change in color, glow), or by the results of this very reaction.

The rate of a chemical reaction

The rate of a chemical reaction is usually understood as the change in the amount of one of the reactants per unit of time. Moreover, the rate of a chemical reaction is always a positive value. In 1865, the chemist N. N. Beketov formulated the law of mass action, which states that "the rate of a chemical reaction at any given time is proportional to the concentrations of reagents raised to powers equal to their stoichiometric coefficients."

Factors in the rate of a chemical reaction include:

the nature of the reactants
the presence of a catalyst
temperature,
the surface area of the reactants.

All of them have the most direct influence on the rate of a chemical reaction.

Equilibrium of a chemical reaction

This state is called chemical equilibrium. chemical system, at which several chemical reactions take place and the rates in each pair of forward and reverse reactions are equal to each other. Thus, the equilibrium constant of a chemical reaction is singled out - this is the value that determines for a given chemical reaction the ratio between the thermodynamic activities of the starting substances and products in a state of chemical equilibrium. Knowing the equilibrium constant, you can determine the direction of a chemical reaction.

Conditions for the occurrence of chemical reactions

To initiate chemical reactions, it is necessary to create the appropriate conditions for this:

bringing substances into close contact.
heating substances to a certain temperature (the temperature of the chemical reaction must be appropriate).

Thermal effect of a chemical reaction

This is the name given to the change in the internal energy of the system as a result of the occurrence of a chemical reaction and the transformation of the starting materials (reactants) into reaction products in quantities corresponding to the chemical reaction equation under the following conditions:

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

Chemical reactions, video

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

Chemistry is the science of substances, their properties and transformations. .
That is, if nothing happens to the substances around us, then this does not apply to chemistry. But what does "nothing happens" mean? If a thunderstorm suddenly caught us in the field, and we all got wet, as they say, “to the skin”, then is this not a transformation: after all, the clothes were dry, but became wet.

If, for example, you take an iron nail, process it with a file, and then assemble iron filings (Fe) , then this is also not a transformation: there was a nail - it became powder. But if after that to assemble the device and hold obtaining oxygen (O 2): heat up potassium permanganate(KMpo 4) and collect oxygen in a test tube, and then place these iron filings heated “to red” in it, then they will flare up with a bright flame and, after combustion, will turn into a brown powder. And this is also a transformation. So where is the chemistry? Despite the fact that in these examples the shape (iron nail) and the state of clothing (dry, wet) change, these are not transformations. The fact is that the nail itself, as it was a substance (iron), remained so, despite its different form, and our clothes soaked up the water from the rain, and then it evaporated into the atmosphere. The water itself has not changed. So what are transformations in terms of chemistry?

From the point of view of chemistry, transformations are such phenomena that are accompanied by a change in the composition of a substance. Let's take the same nail as an example. It does not matter what form it took after being filed, but after being collected from it iron filings placed in an atmosphere of oxygen - it turned into iron oxide(Fe 2 O 3 ) . So, has something really changed? Yes, it has. There was a nail substance, but under the influence of oxygen a new substance was formed - element oxide gland. molecular equation this transformation can be represented by the following chemical symbols:

4Fe + 3O 2 = 2Fe 2 O 3 (1)

For a person uninitiated in chemistry, questions immediately arise. What is the "molecular equation", what is Fe? Why are there numbers "4", "3", "2"? What are the small numbers "2" and "3" in the formula Fe 2 O 3? This means that the time has come to sort things out in order.

Signs of chemical elements.

Despite the fact that they begin to study chemistry in the 8th grade, and some even earlier, many people know the great Russian chemist D. I. Mendeleev. And of course, his famous "Periodic Table of Chemical Elements". Otherwise, more simply, it is called the "Mendeleev's Table".

In this table, in the appropriate order, the elements are located. To date, about 120 of them are known. The names of many elements have been known to us for a long time. These are: iron, aluminum, oxygen, carbon, gold, silicon. Previously, we used these words without hesitation, identifying them with objects: an iron bolt, aluminum wire, oxygen in the atmosphere, a golden ring, etc. etc. But in fact, all these substances (bolt, wire, ring) consist of their respective elements. The whole paradox is that the element cannot be touched, picked up. How so? They are in the periodic table, but you can’t take them! Yes exactly. A chemical element is an abstract (that is, abstract) concept, and is used in chemistry, however, as in other sciences, for calculations, drawing up equations, and solving problems. Each element differs from the other in that it is characterized by its own electronic configuration of an atom. The number of protons in the nucleus of an atom is equal to the number of electrons in its orbitals. For example, hydrogen is element #1. Its atom consists of 1 proton and 1 electron. Helium is element number 2. Its atom consists of 2 protons and 2 electrons. Lithium is element number 3. Its atom consists of 3 protons and 3 electrons. Darmstadtium - element number 110. Its atom consists of 110 protons and 110 electrons.

Each element is denoted by a certain symbol, Latin letters, and has a certain reading in translation from Latin. For example, hydrogen has the symbol "N", read as "hydrogenium" or "ash". Silicon has the symbol "Si" read as "silicium". Mercury has a symbol "Hg" and is read as "hydrargyrum". And so on. All these designations can be found in any chemistry textbook for the 8th grade. For us now, the main thing is to understand that when compiling chemical equations, it is necessary to operate with the indicated symbols of the elements.

Simple and complex substances.

Denoting various substances with single symbols of chemical elements (Hg mercury, Fe iron, Cu copper, Zn zinc, Al aluminum) we essentially denote simple substances, that is, substances consisting of atoms of the same type (containing the same number of protons and neutrons in an atom). For example, if iron and sulfur substances interact, then the equation will take the following form:

Fe + S = FeS (2)

Simple substances include metals (Ba, K, Na, Mg, Ag), as well as non-metals (S, P, Si, Cl 2, N 2, O 2, H 2). And you should pay attention
special attention to the fact that all metals are denoted by single symbols: K, Ba, Ca, Al, V, Mg, etc., and non-metals - either by simple symbols: C, S, P or may have different indices that indicate their molecular structure: H 2 , Cl 2 , O 2 , J 2 , P 4 , S 8 . In the future, this will be very great importance when writing equations. It is not at all difficult to guess that complex substances are substances formed from atoms. different kind, For example,

1). Oxides:
aluminium oxide Al 2 O 3,

sodium oxide Na 2 O
copper oxide CuO,
zinc oxide ZnO
titanium oxide Ti2O3,
carbon monoxide or carbon monoxide (+2) CO
sulfur oxide (+6) SO 3

2). Reasons:
iron hydroxide(+3) Fe (OH) 3,
copper hydroxide Cu(OH)2,
potassium hydroxide or potassium alkali KOH,
sodium hydroxide NaOH.

3). Acids:
hydrochloric acid HCl
sulfurous acid H2SO3,
Nitric acid HNO3

4). Salts:
sodium thiosulfate Na 2 S 2 O 3,
sodium sulfate or Glauber's salt Na 2 SO 4,
calcium carbonate or limestone CaCO 3,
copper chloride CuCl 2

5). organic matter:
sodium acetate CH 3 COOHa,
methane CH 4,
acetylene C 2 H 2,
glucose C 6 H 12 O 6

Finally, after we have clarified the structure of various substances, we can begin to write chemical equations.

Chemical equation.

The word “equation” itself is derived from the word “equalize”, i.e. divide something into equal parts. In mathematics, equations are almost the very essence of this science. For example, you can give such a simple equation in which the left and right sides will be equal to "2":

40: (9 + 11) = (50 x 2): (80 - 30);

And in chemical equations, the same principle: the left and right sides of the equation must correspond to the same number of atoms, the elements participating in them. Or, if given ionic equation, then in it number of particles must also meet this requirement. A chemical equation is a conditional record of a chemical reaction using chemical formulas and mathematical signs. A chemical equation inherently reflects a particular chemical reaction, that is, the process of interaction of substances, during which new substances arise. For example, it is necessary write a molecular equation reactions that take part barium chloride BaCl 2 and sulfuric acid H 2 SO 4. As a result of this reaction, an insoluble precipitate is formed - barium sulfate BaSO 4 and hydrochloric acid Hcl:

ВаСl 2 + H 2 SO 4 = BaSO 4 + 2НCl (3)

First of all, it is necessary to understand that the large number “2” in front of the HCl substance is called the coefficient, and the small numbers “2”, “4” under the formulas ВаСl 2, H 2 SO 4, BaSO 4 are called indices. Both the coefficients and indices in chemical equations play the role of factors, not terms. In order to correctly write a chemical equation, it is necessary arrange the coefficients in the reaction equation. Now let's start counting the atoms of the elements on the left and right sides of the equation. On the left side of the equation: the substance BaCl 2 contains 1 barium atom (Ba), 2 chlorine atoms (Cl). In the substance H 2 SO 4: 2 hydrogen atoms (H), 1 sulfur atom (S) and 4 oxygen atoms (O). On the right side of the equation: in the BaSO 4 substance there is 1 barium atom (Ba) 1 sulfur atom (S) and 4 oxygen atoms (O), in the HCl substance: 1 hydrogen atom (H) and 1 chlorine atom (Cl). Whence it follows that on the right side of the equation the number of hydrogen and chlorine atoms is half that on the left side. Therefore, before the HCl formula on the right side of the equation, it is necessary to put the coefficient "2". If we now add the number of atoms of the elements involved in this reaction, both on the left and on the right, we get the following balance:

In both parts of the equation, the number of atoms of the elements participating in the reaction are equal, therefore it is correct.

Chemical equation and chemical reactions

As we have already found out, chemical equations are a reflection of chemical reactions. Chemical reactions are such phenomena in the process of which the transformation of one substance into another occurs. Among their diversity, two main types can be distinguished:

1). Connection reactions
2). decomposition reactions.

The overwhelming majority of chemical reactions belong to addition reactions, since changes in its composition can rarely occur with a single substance if it is not subjected to external influences (dissolution, heating, light). Nothing characterizes a chemical phenomenon, or reaction, as much as the changes that occur when two or more substances interact. Such phenomena can occur spontaneously and be accompanied by an increase or decrease in temperature, light effects, color changes, sedimentation, release of gaseous products, noise.

For clarity, we present several equations that reflect the processes of compound reactions, during which we obtain sodium chloride(NaCl), zinc chloride(ZnCl 2), silver chloride precipitate(AgCl), aluminum chloride(AlCl 3)

Cl 2 + 2Nа = 2NaCl (4)

CuCl 2 + Zn \u003d ZnCl 2 + Cu (5)

AgNO 3 + KCl \u003d AgCl + 2KNO 3 (6)

3HCl + Al(OH) 3 \u003d AlCl 3 + 3H 2 O (7)

Among the reactions of the compound, the following should be especially noted : substitution (5), exchange (6), and how special case exchange reactions - reaction neutralization (7).

Substitution reactions include those in which atoms of a simple substance replace the atoms of one of the elements in a complex substance. In example (5), zinc atoms replace copper atoms from the CuCl 2 solution, while zinc passes into the soluble ZnCl 2 salt, and copper is released from the solution in the metallic state.

Exchange reactions are reactions in which two compounds exchange their constituent parts. In the case of reaction (6), the soluble salts of AgNO 3 and KCl, when both solutions are drained, form an insoluble precipitate of the AgCl salt. At the same time, they exchange their constituent parts - cations and anions. Potassium cations K + are attached to NO 3 anions, and silver cations Ag + - to Cl - anions.

A special, particular case of exchange reactions is the neutralization reaction. Neutralization reactions are reactions in which acids react with bases to form salt and water. In example (7), hydrochloric acid HCl reacts with base Al(OH) 3 to form AlCl 3 salt and water. In this case, aluminum cations Al 3+ from the base are exchanged with Cl anions - from the acid. As a result, it happens hydrochloric acid neutralization.

Decomposition reactions include those in which two or more new simple or complex substances, but of a simpler composition, are formed from one complex one. As reactions, one can cite those in the process of which 1) decompose. potassium nitrate(KNO 3) with the formation of potassium nitrite (KNO 2) and oxygen (O 2); 2). Potassium permanganate(KMnO 4): potassium manganate is formed (K 2 MnO 4), manganese oxide(MnO 2) and oxygen (O 2); 3). calcium carbonate or marble; in the process are formed carbonicgas(CO 2) and calcium oxide(Cao)

2KNO 3 \u003d 2KNO 2 + O 2 (8)
2KMnO 4 \u003d K 2 MnO 4 + MnO 2 + O 2 (9)
CaCO 3 \u003d CaO + CO 2 (10)

In reaction (8), one complex and one simple substance is formed from a complex substance. In reaction (9) there are two complex and one simple. In reaction (10) there are two complex substances, but simpler in composition

All classes of complex substances undergo decomposition:

1). Oxides: silver oxide 2Ag 2 O = 4Ag + O 2 (11)

2). Hydroxides: iron hydroxide 2Fe(OH) 3 = Fe 2 O 3 + 3H 2 O (12)

3). Acids: sulfuric acid H 2 SO 4 \u003d SO 3 + H 2 O (13)

4). Salts: calcium carbonate CaCO 3 \u003d CaO + CO 2 (14)

5). organic matter: alcoholic fermentation of glucose

C 6 H 12 O 6 \u003d 2C 2 H 5 OH + 2CO 2 (15)

According to another classification, all chemical reactions can be divided into two types: reactions that take place with the release of heat, they are called exothermic, and reactions that go with the absorption of heat - endothermic. The criterion for such processes is thermal effect of the reaction. As a rule, exothermic reactions include oxidation reactions, i.e. interactions with oxygen methane combustion:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + Q (16)

and to endothermic reactions - decomposition reactions, already given above (11) - (15). The Q sign at the end of the equation indicates whether heat is released during the reaction (+Q) or absorbed (-Q):

CaCO 3 \u003d CaO + CO 2 - Q (17)

You can also consider all chemical reactions according to the type of change in the degree of oxidation of the elements involved in their transformations. For example, in reaction (17), the elements participating in it do not change their oxidation states:

Ca +2 C +4 O 3 -2 \u003d Ca +2 O -2 + C +4 O 2 -2 (18)

And in reaction (16), the elements change their oxidation states:

2Mg 0 + O 2 0 \u003d 2Mg +2 O -2

These types of reactions are redox . They will be considered separately. To formulate equations for reactions of this type, it is necessary to use half-reaction method and apply electronic balance equation.

After bringing various types of chemical reactions, you can proceed to the principle of compiling chemical equations, in other words, the selection of coefficients in their left and right parts.

Mechanisms for compiling chemical equations.

Whatever type this or that chemical reaction belongs to, its record (chemical equation) must correspond to the condition of equality of the number of atoms before the reaction and after the reaction.

There are equations (17) that do not require adjustment, i.e. placement of coefficients. But in most cases, as in examples (3), (7), (15), it is necessary to take actions aimed at equalizing the left and right parts of the equation. What principles should be followed in such cases? Is there any system in the selection of coefficients? There is, and not one. These systems include:

1). Selection of coefficients according to given formulas.

2). Compilation according to the valencies of the reactants.

3). Compilation according to the oxidation states of the reactants.

In the first case, it is assumed that we know the formulas of the reactants both before and after the reaction. For example, given the following equation:

N 2 + O 2 →N 2 O 3 (19)

It is generally accepted that until the equality between the atoms of the elements before and after the reaction is established, the equal sign (=) is not put in the equation, but is replaced by an arrow (→). Now let's get down to the actual balancing. On the left side of the equation there are 2 nitrogen atoms (N 2) and two oxygen atoms (O 2), and on the right side there are two nitrogen atoms (N 2) and three oxygen atoms (O 3). It is not necessary to equalize it by the number of nitrogen atoms, but by oxygen it is necessary to achieve equality, since two atoms participated before the reaction, and after the reaction there were three atoms. Let's make the following diagram:

before reaction after reaction
O 2 O 3

Let's define the smallest multiple between the given numbers of atoms, it will be "6".

O 2 O 3
\ 6 /

Divide this number on the left side of the oxygen equation by "2". We get the number "3", put it in the equation to be solved:

N 2 + 3O 2 →N 2 O 3

We also divide the number "6" for the right side of the equation by "3". We get the number "2", just put it in the equation to be solved:

N 2 + 3O 2 → 2N 2 O 3

The number of oxygen atoms in both the left and right parts of the equation became equal, respectively, 6 atoms:

But the number of nitrogen atoms in both sides of the equation will not match:

On the left side there are two atoms, on the right side there are four atoms. Therefore, in order to achieve equality, it is necessary to double the amount of nitrogen on the left side of the equation, putting the coefficient "2":

Thus, the equality for nitrogen is observed and, in general, the equation will take the form:

2N 2 + 3O 2 → 2N 2 O 3

Now in the equation, instead of an arrow, you can put an equal sign:

2N 2 + 3O 2 \u003d 2N 2 O 3 (20)

Let's take another example. The following reaction equation is given:

P + Cl 2 → PCl 5

On the left side of the equation there is 1 phosphorus atom (P) and two chlorine atoms (Cl 2), and on the right side there is one phosphorus atom (P) and five oxygen atoms (Cl 5). It is not necessary to equalize it by the number of phosphorus atoms, but for chlorine it is necessary to achieve equality, since two atoms participated before the reaction, and after the reaction there were five atoms. Let's make the following diagram:

before reaction after reaction
Cl 2 Cl 5

Let's define the smallest multiple between the given numbers of atoms, it will be "10".

Cl 2 Cl 5
\ 10 /

Divide this number on the left side of the equation for chlorine by "2". We get the number "5", put it in the equation to be solved:

Р + 5Cl 2 → РCl 5

We also divide the number "10" for the right side of the equation by "5". We get the number "2", just put it in the equation to be solved:

Р + 5Cl 2 → 2РCl 5

The number of chlorine atoms in both the left and right parts of the equation became equal, respectively, 10 atoms:

But the number of phosphorus atoms in both sides of the equation will not match:

Therefore, in order to achieve equality, it is necessary to double the amount of phosphorus on the left side of the equation, putting the coefficient "2":

Thus, the equality for phosphorus is observed and, in general, the equation will take the form:

2Р + 5Cl 2 = 2РCl 5 (21)

When writing equations by valency must be given definition of valence and set values for the most famous elements. Valency is one of the previously used concepts, currently in a number of school programs not used. But with its help it is easier to explain the principles of compiling equations of chemical reactions. By valency is meant number chemical bonds, which one or another atom can form with another, or other atoms . Valence has no sign (+ or -) and is indicated by Roman numerals, usually above the symbols of chemical elements, for example:

Where do these values come from? How to apply them in the preparation of chemical equations? The numerical values of the valencies of the elements coincide with their group number of the Periodic system of chemical elements of D. I. Mendeleev (Table 1).

For other elements valency values may have other values, but never greater than the number of the group in which they are located. Moreover, for even numbers of groups (IV and VI), the valences of elements take only even values, and for odd ones, they can have both even and odd values (Table.2).

Of course, there are exceptions to the valency values for some elements, but in each specific case, these points are usually specified. Now consider general principle compiling chemical equations for given valences for certain elements. More often this method acceptable in the case of compiling equations of chemical reactions of the combination of simple substances, for example, when interacting with oxygen ( oxidation reactions). Suppose you want to display the oxidation reaction aluminum. But recall that metals are denoted by single atoms (Al), and non-metals that are in a gaseous state - with indices "2" - (O 2). First, we write the general scheme of the reaction:

Al + O 2 → AlO

At this stage, it is not yet known what the correct spelling should be for alumina. And it is precisely at this stage that knowledge of the valencies of the elements will come to our aid. For aluminum and oxygen, we put them above the proposed formula for this oxide:

III II
Al O

After that, "cross"-on-"cross" these symbols of the elements will put the corresponding indices below:

III II
Al 2 O 3

Composition of a chemical compound Al 2 O 3 determined. The further scheme of the reaction equation will take the form:

Al + O 2 → Al 2 O 3

It remains only to equalize the left and right parts of it. We proceed in the same way as in the case of formulating equation (19). We equalize the number of oxygen atoms, resorting to finding the smallest multiple:

before reaction after reaction

O 2 O 3
\ 6 /

Divide this number on the left side of the oxygen equation by "2". We get the number "3", put it in the equation to be solved. We also divide the number "6" for the right side of the equation by "3". We get the number "2", just put it in the equation to be solved:

Al + 3O 2 → 2Al 2 O 3

In order to achieve equality for aluminum, it is necessary to adjust its amount on the left side of the equation by setting the coefficient "4":

4Al + 3O 2 → 2Al 2 O 3

Thus, the equality for aluminum and oxygen is observed and, in general, the equation will take the final form:

4Al + 3O 2 \u003d 2Al 2 O 3 (22)

Using the valency method, it is possible to predict which substance is formed in the course of a chemical reaction, what its formula will look like. Suppose nitrogen and hydrogen with the corresponding valences III and I entered into the reaction of the compound. Let's write the general reaction scheme:

N 2 + H 2 → NH

For nitrogen and hydrogen, we put down the valencies over the proposed formula of this compound:

As before, "cross"-on-"cross" for these element symbols, we put the corresponding indices below:

III I
N H 3

The further scheme of the reaction equation will take the form:

N 2 + H 2 → NH 3

Equalizing in the already known way, through the smallest multiple for hydrogen, equal to "6", we obtain the desired coefficients, and the equation as a whole:

N 2 + 3H 2 \u003d 2NH 3 (23)

When compiling equations for oxidation states reacting substances, it must be recalled that the degree of oxidation of an element is the number of electrons received or given away in the process of a chemical reaction. The oxidation state in compounds basically, numerically coincides with the values of the element's valences. But they differ in sign. For example, for hydrogen, the valence is I, and the oxidation state is (+1) or (-1). For oxygen, the valence is II, and the oxidation state is (-2). For nitrogen, the valencies are I, II, III, IV, V, and the oxidation states are (-3), (+1), (+2), (+3), (+4), (+5), etc. . The oxidation states of the elements most commonly used in equations are shown in Table 3.

In the case of compound reactions, the principle of compiling equations in terms of oxidation states is the same as in compiling in terms of valencies. For example, let's give the reaction equation for the oxidation of chlorine with oxygen, in which chlorine forms a compound with an oxidation state of +7. Let's write the proposed equation:

Cl 2 + O 2 → ClO

We put the oxidation states of the corresponding atoms over the proposed ClO compound:

As in the previous cases, we establish that the desired compound formula will take the form:

7 -2
Cl 2 O 7

The reaction equation will take the following form:

Cl 2 + O 2 → Cl 2 O 7

Equalizing for oxygen, finding the smallest multiple between two and seven, equal to "14", we finally establish the equality:

2Cl 2 + 7O 2 \u003d 2Cl 2 O 7 (24)

A slightly different method must be used with oxidation states when compiling exchange, neutralization, and substitution reactions. In some cases, it is difficult to find out: what compounds are formed during the interaction of complex substances?

How do you know what happens in a reaction?

Indeed, how do you know: what reaction products can arise in the course of a particular reaction? For example, what is formed when barium nitrate and potassium sulfate react?

Ba (NO 3) 2 + K 2 SO 4 →?

Maybe VAC 2 (NO 3) 2 + SO 4? Or Ba + NO 3 SO 4 + K 2? Or something else? Of course, during this reaction, compounds are formed: BaSO 4 and KNO 3. And how is this known? And how to write formulas of substances? Let's start with what is most often overlooked: the very concept of "exchange reaction". This means that in these reactions, the substances change with each other in constituent parts. Since the exchange reactions are mostly carried out between bases, acids or salts, the parts with which they will change are metal cations (Na +, Mg 2+, Al 3+, Ca 2+, Cr 3+), H + ions or OH -, anions - acid residues, (Cl -, NO 3 2-, SO 3 2-, SO 4 2-, CO 3 2-, PO 4 3-). In general, the exchange reaction can be given in the following notation:

Kt1An1 + Kt2An1 = Kt1An2 + Kt2An1 (25)

Where Kt1 and Kt2 are the metal cations (1) and (2), and An1 and An2 are the anions (1) and (2) corresponding to them. In this case, it must be taken into account that in compounds before and after the reaction, cations are always established in the first place, and anions in the second. Therefore, if it reacts potassium chloride And silver nitrate, both in solution

KCl + AgNO 3 →

then in the process of it substances KNO 3 and AgCl are formed and the corresponding equation will take the form:

KCl + AgNO 3 \u003d KNO 3 + AgCl (26)

In neutralization reactions, protons from acids (H +) will combine with hydroxyl anions (OH -) to form water (H 2 O):

HCl + KOH \u003d KCl + H 2 O (27)

The oxidation states of metal cations and the charges of anions of acid residues are indicated in the table of the solubility of substances (acids, salts and bases in water). Metal cations are shown horizontally, and anions of acid residues are shown vertically.

Based on this, when compiling the equation for the exchange reaction, it is first necessary to establish the oxidation states of the particles receiving in this chemical process in its left part. For example, you need to write an equation for the interaction between calcium chloride and sodium carbonate. Let's draw up the initial scheme for this reaction:

CaCl + NaCO 3 →

Ca 2+ Cl - + Na + CO 3 2- →

Having performed the already known “cross”-to-“cross” action, we determine the real formulas of the starting substances:

CaCl 2 + Na 2 CO 3 →

Based on the principle of exchange of cations and anions (25), we establish the preliminary formulas of the substances formed during the reaction:

CaCl 2 + Na 2 CO 3 → CaCO 3 + NaCl

We put down the corresponding charges over their cations and anions:

Ca 2+ CO 3 2- + Na + Cl -

Substance formulas are written correctly, in accordance with the charges of cations and anions. Let's compose complete equation, equalizing its left and right parts in terms of sodium and chlorine:

CaCl 2 + Na 2 CO 3 \u003d CaCO 3 + 2NaCl (28)

As another example, here is the equation for the neutralization reaction between barium hydroxide and phosphoric acid:

VaON + NPO 4 →

We put the corresponding charges over cations and anions:

Ba 2+ OH - + H + RO 4 3- →

Let's define the real formulas of the starting substances:

Va (OH) 2 + H 3 RO 4 →

Based on the principle of exchange of cations and anions (25), we establish the preliminary formulas of the substances formed during the reaction, taking into account that in the exchange reaction, one of the substances must necessarily be water:

Ba (OH) 2 + H 3 RO 4 → Ba 2+ RO 4 3- + H 2 O

Let's determine the correct record of the formula of the salt formed during the reaction:

Ba (OH) 2 + H 3 RO 4 → Ba 3 (RO 4) 2 + H 2 O

Equate the left side of the equation for barium:

3VA (OH) 2 + H 3 RO 4 → Ba 3 (RO 4) 2 + H 2 O

Since on the right side of the equation the residue of phosphoric acid is taken twice, (PO 4) 2, then on the left it is also necessary to double its amount:

3VA (OH) 2 + 2H 3 RO 4 → Ba 3 (RO 4) 2 + H 2 O

It remains to match the number of hydrogen and oxygen atoms on the right side of the water. Since the total number of hydrogen atoms on the left is 12, on the right it must also correspond to twelve, therefore, before the water formula, it is necessary put a coefficient"6" (since there are already 2 hydrogen atoms in the water molecule). For oxygen, equality is also observed: on the left 14 and on the right 14. So, the equation has the correct form of writing:

3Ва (ОН) 2 + 2Н 3 РО 4 → Ва 3 (РО 4) 2 + 6Н 2 O (29)

Possibility of chemical reactions

The world is made up of a great variety of substances. The number of variants of chemical reactions between them is also incalculable. But can we, having written this or that equation on paper, assert that a chemical reaction will correspond to it? There is a misconception that if the right arrange odds in the equation, then it will be feasible in practice. For example, if we take sulfuric acid solution and drop into it zinc, then we can observe the process of hydrogen evolution:

Zn + H 2 SO 4 \u003d ZnSO 4 + H 2 (30)

But if copper is lowered into the same solution, then the process of gas evolution will not be observed. The reaction is not feasible.

Cu + H 2 SO 4 ≠

If concentrated sulfuric acid is taken, it will react with copper:

Cu + 2H 2 SO 4 \u003d CuSO 4 + SO 2 + 2H 2 O (31)

In reaction (23) between nitrogen and hydrogen gases, thermodynamic balance, those. how many molecules ammonia NH 3 is formed per unit time, the same number of them will decompose back into nitrogen and hydrogen. Shift in chemical equilibrium can be achieved by increasing the pressure and decreasing the temperature

N 2 + 3H 2 \u003d 2NH 3

If you take potassium hydroxide solution and pour on it sodium sulfate solution, then no changes will be observed, the reaction will not be feasible:

KOH + Na 2 SO 4 ≠

Sodium chloride solution when interacting with bromine, it will not form bromine, despite the fact that this reaction can be attributed to a substitution reaction:

NaCl + Br 2 ≠

What are the reasons for such discrepancies? The fact is that it is not enough just to correctly define compound formulas, it is necessary to know the specifics of the interaction of metals with acids, to skillfully use the table of solubility of substances, to know the rules of substitution in the series of activity of metals and halogens. This article outlines only the most basic principles of how arrange the coefficients in the reaction equations, How write molecular equations, How determine the composition of a chemical compound.

Chemistry, as a science, is extremely diverse and multifaceted. This article reflects only a small part of the processes taking place in the real world. Types, thermochemical equations, electrolysis, processes organic synthesis and many many others. But more on that in future articles.

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In order to figure out how to equalize a chemical equation, you first need to know the purpose of this science.

Definition

Chemistry studies substances, their properties, and transformations. If there is no change in color, precipitation, release of a gaseous substance, then no chemical interaction occurs.

For example, when filing an iron nail with a file, the metal simply turns into powder. In this case, no chemical reaction occurs.

The calcination of potassium permanganate is accompanied by the formation of manganese oxide (4), the release of oxygen, that is, an interaction is observed. In this case, a completely natural question arises about how to properly equalize chemical equations. We will analyze all the nuances associated with such a procedure.

Specificity of chemical transformations

Any phenomena that are accompanied by a change in the qualitative and quantitative composition of substances are chemical transformations. In molecular form, the process of iron combustion in the atmosphere can be expressed using signs and symbols.

The method of placing the coefficients

How to equalize coefficients in chemical equations? In the course of chemistry high school understands the method of electronic balance. Let's consider the process in more detail. To begin with, in the initial reaction, it is necessary to arrange the oxidation states of each chemical element.

There are certain rules by which they can be determined for each element. In simple substances, the oxidation states will be zero. In binary compounds, the first element has a positive value, corresponding to the highest valency. For the latter, this parameter is determined by subtracting the group number from eight and has a minus sign. Formulas consisting of three elements have their own nuances for calculating oxidation states.

For the first and last element, the order is similar to the definition in binary compounds, and an equation is made to calculate the central element. The sum of all indicators must be equal to zero, based on this, the indicator for the middle element of the formula is calculated.

Let's continue the conversation about how to equalize chemical equations using the electron balance method. After the oxidation states are set, it is possible to determine those ions or substances that have changed their value during the chemical interaction.

The plus and minus signs indicate the number of electrons that were accepted (given away) in the process of chemical interaction. Between the numbers obtained, find the least common multiple.

When dividing it into received and given electrons, coefficients are obtained. How to balance a chemical equation? The figures obtained in the balance sheet must be placed in front of the corresponding formulas. A prerequisite is to check the number of each element in the left and right parts. If the coefficients are placed correctly, their number should be the same.

The law of conservation of mass of substances

Arguing over how to equalize a chemical equation, it is necessary to use this law. Given that the mass of those substances that entered into a chemical reaction is equal to the mass of the resulting products, it becomes possible to set coefficients in front of the formulas. For example, how to equalize a chemical equation if simple substances calcium and oxygen interact, and after the process is completed, an oxide is obtained?

To cope with the task, it must be taken into account that oxygen is a diatomic molecule with a covalent non-polar bond, so its formula is written in the following form - O2. On the right side, when compiling calcium oxide (CaO), the valencies of each element are taken into account.

First you need to check the amount of oxygen in each part of the equation, as it is different. According to the law of conservation of mass of substances, a factor of 2 must be put in front of the product formula. Next, calcium is checked. In order for it to be equalized, we put a factor of 2 in front of the original substance. As a result, we get the record:

2Ca+O2=2CaO.

Analysis of the reaction by the electronic balance method

How to equalize chemical equations? Examples of RIAs will help answer this question. Suppose that it is necessary to place the coefficients in the proposed scheme using the electronic balance method:

CuO + H2=Cu + H2O.

To begin with, for each of the elements in the initial substances and interaction products, we will place the values of the oxidation states. We get the following form of the equation:

Cu(+2)O(-2)+H2(0)=Cu(0)+H2(+)O(-2).

The indicators have changed for copper and hydrogen. It is on their basis that we will draw up an electronic balance:

Cu(+2)+2e=Cu(0) 1 reducing agent, oxidation;
H2(0)-2e=2H(+) 1 oxidizing agent, reduction.

Based on the coefficients obtained in the electronic balance, we obtain the following record of the proposed chemical equation:

CuO+H2=Cu+H2O.

Let's take another example that involves setting coefficients:

H2+O2=H2O.

In order to equalize this scheme on the basis of the law of conservation of substances, it is necessary to start with oxygen. Considering that a diatomic molecule entered into the reaction, it is necessary to put a factor of 2 before the formula of the interaction product.

2H2+O2=2H2O.

Conclusion

Based on the electronic balance, you can place the coefficients in any chemical equations. Graduates of the ninth and eleventh grades educational institutions choosing an exam in chemistry, in one of the tasks final tests offer similar assignments.

A record of a chemical interaction that reflects quantitative and qualitative information about a reaction is called an equation of chemical reactions. The reaction is written in chemical and mathematical symbols.

Basic Rules

Chemical reactions involve the transformation of some substances (reagents) into others (reaction products). This is due to the interaction of the outer electron shells of substances. As a result, new compounds are formed from the initial compounds.

To express the course of a chemical reaction graphically, certain rules for compiling and writing chemical equations are used.

On the left side, the initial substances are written that interact with each other, i.e. summed up. When one substance is decomposed, its formula is written down. On the right side, the substances obtained during the chemical reaction are recorded. Examples of written equations with legend:

CuSO 4 + 2NaOH → Cu(OH) 2 ↓+ Na 2 SO 4;
CaCO 3 \u003d CaO + CO 2;
2Na 2 O 2 + 2CO 2 → 2Na 2 CO 3 + O 2;
CH 3 COONa + H 2 SO 4 (conc.) → CH 3 COOH + NaHSO 4;
2NaOH + Si + H 2 O → Na 2 SiO 3 + H 2.

The coefficients in front of chemical formulas show the number of molecules of a substance. The unit is not set, but implied. For example, the equation Ba + 2H 2 O → Ba (OH) 2 + H 2 shows that from one molecule of barium and two molecules of water, one molecule of barium hydroxide and hydrogen is obtained. If you count the amount of hydrogen, then both on the right and on the left you get four atoms.

Notation

To draw up equations of chemical reactions, it is necessary to know certain notations that show how the reaction proceeds. The following signs are used in chemical equations:

→ - irreversible, direct reaction (goes in one direction);
⇄ or ↔ - the reaction is reversible (flows in both directions);
- gas is released;
↓ - precipitation occurs;
hv - lighting;
t° - temperature (the number of degrees can be indicated);
Q - heat;
E (tv.) - solid matter;
E (gas) or E (g) - a gaseous substance;
E(conc.) - concentrated substance;
E (water) - water solution substances.

Rice. 1. Precipitation.

Instead of an arrow (→), an equal sign (=) can be put, showing compliance with the law of conservation of matter: both on the left and on the right, the number of atoms of substances is the same. When solving equations, an arrow is placed first. After calculating the coefficients and the equations of the right and left parts, a line is drawn under the arrow.

Reaction conditions (temperature, lighting) are indicated above the reaction progress sign (→,⇄). The formulas of catalysts are also signed at the top.

Rice. 2. Examples of reaction conditions.

What are the equations

Chemical equations are classified according to different criteria. The main methods of classification are presented in the table.

*sign*	*Reactions*	*Description*	*Example*
By changing the amount of reagents and final substances	Substitutions	From a simple and complex substance, new simple and complex substances are formed	2Na + 2H 2 O → 2NaOH + H 2
	Connections	Several substances form a new substance	C + O 2 \u003d CO 2
	expansions	Several substances are formed from one substance	2Fe(OH) 3 → Fe 2 O 3 + 3H 2 O
	Ion exchange	Exchange of constituents (ions)	Na 2 CO 3 + H 2 SO 4 → Na 2 SO 4 + CO 2 + H 2 O
By heat release	exothermic	Heat generation	C + 2H 2 = CH 4 + Q
By heat release	Endothermic	Heat absorption	N 2 + O 2 → 2NO - Q
By type of energy impact	Electrochemical	The action of electric current
	Photochemical	action of light
	Thermochemical	High temperature effect
By state of aggregation	homogeneous	same state	CuCl 2 + Na 2 S → 2NaCl + CuS↓
By state of aggregation	Heterogeneous	Miscellaneous condition	4H 2 O (l) + 3Fe (t) → Fe 3 O 4 + 4H 2

There is a concept of chemical equilibrium inherent only in reversible reactions. This is a state in which the rates of the forward and reverse reactions, as well as the concentrations of substances, are equal. This state is characterized by a chemical equilibrium constant.

With external influence of temperature, pressure, light, the reaction can shift towards a decrease or increase in the concentration of a certain substance. The dependence of the equilibrium constant on temperature is expressed using the isobar and isochore equations. The isotherm equation reflects the dependence of the energy and the equilibrium constant. These equations show the direction of the reaction.

Rice. 3. Equations of isobar, isochore and isotherm.

What have we learned?

In the 8th grade chemistry lesson, the topic of equations of chemical reactions was considered. Drawing up and writing equations reflects the course of a chemical reaction. There are certain notations that show the state of substances and the conditions for the reaction. There are several types of chemical reactions according to different criteria: by the amount of substance, state of aggregation, energy absorption, energy impact.