Instructions

Consider an example of the formation of a sparingly soluble compound.

Na2SO4 + BaCl2 = BaSO4 + 2NaCl

Or an ionic version:

2Na+ +SO42- +Ba2++ 2Cl- = BaSO4 + 2Na+ + 2Cl-

When solving ionic equations, the following rules must be observed:

Identical ions from both parts are excluded;

It should be remembered that the sum of the electric charges on the left side of the equation must be equal to the sum of the electric charges on the right side of the equation.

Write ionic equations interactions between aqueous solutions of the following substances: a) HCl and NaOH; b) AgNO3 and NaCl; c) K2CO3 and H2SO4; d) CH3COOH and NaOH.

Solution. Write down the equations of interaction of these substances in molecular form:

a) HCl + NaOH = NaCl + H2O

b) AgNO3 + NaCl = AgCl + NaNO3

c) K2CO3 + H2SO4 = K2SO4 + CO2 + H2O

d) CH3COOH + NaOH = CH3COONa + H2O

Note that the interaction of these substances is possible, because the result is the binding of ions with the formation of either weak (H2O), or sparingly soluble substance (AgCl), or gas (CO2).

By excluding identical ions from the left and right sides of the equality (in the case of option a) - ions and , in case b) - sodium ions and -ions, in case c) - potassium ions and sulfate ions), d) - sodium ions, you get solving these ionic equations:

a) H+ + OH- = H2O

b) Ag+ + Cl- = AgCl

c) CO32- + 2H+ = CO2 + H2O

d) CH3COOH + OH- = CH3COO- + H2O

Quite often in independent and tests There are tasks that involve solving reaction equations. However, without some knowledge, skills and abilities, even the simplest chemical equations don't write.

Instructions

First of all, you need to study the basic organic and inorganic compounds. As a last resort, you can have a suitable cheat sheet in front of you that can help during the task. After training, the necessary knowledge and skills will still be stored in your memory.

The basic material is covering, as well as methods for obtaining each compound. They are usually presented in the form of general diagrams, for example: 1. + base = salt + water
2. acid oxide + base = salt + water
3. basic oxide + acid = salt + water
4. metal + (diluted) acid = salt + hydrogen
5. soluble salt + soluble salt = insoluble salt + soluble salt
6. soluble salt + = insoluble base + soluble salt
Having before your eyes a table of salt solubility, and, as well as cheat sheets, you can decide on them equations reactions. It is only important to have full list such schemes, as well as information about the formulas and names of various classes of organic and inorganic compounds.

After the equation itself is completed, it is necessary to check the correct spelling of the chemical formulas. Acids, salts and bases are easily checked using the solubility table, which shows the charges of the acidic residues and metal ions. It is important to remember that any one must be generally electrically neutral, that is, the number of positive charges must coincide with the number of negative ones. In this case, it is necessary to take into account the indices, which are multiplied by the corresponding charges.

If this stage has been passed and you are confident in the correctness of the spelling equations chemical reactions, then you can now safely set the coefficients. The chemical equation is represented by the conventional notation reactions using chemical symbols, indices and coefficients. At this stage of the task, you must adhere to the rules: The coefficient is placed in front of the chemical formula and applies to all elements that make up the substance.
The index is placed after chemical element slightly below, and refers only to the chemical element to the left of it.
If a group (for example, an acid residue or a hydroxyl group) is in brackets, then you need to understand that two adjacent indices (before and after the bracket) are multiplied.
When counting the atoms of a chemical element, the coefficient is multiplied (not added!) by the index.

Next, the amount of each chemical element is calculated so that the total number of elements included in the starting substances coincides with the number of atoms included in the compounds formed in the products reactions. By analyzing and applying the above rules, you can learn to solve equations reactions included in chains of substances.

DEFINITION

Chemical reaction are called transformations of substances in which a change in their composition and (or) structure occurs.

Most often, chemical reactions are understood as the process of converting starting substances (reagents) into final substances (products).

Chemical reactions are written using chemical equations containing the formulas of the starting substances and reaction products. According to the law of conservation of mass, the number of atoms of each element on the left and right sides of a chemical equation is the same. Typically, the formulas of the starting substances are written on the left side of the equation, and the formulas of the products on the right. The equality of the number of atoms of each element on the left and right sides of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.

Chemical equations may contain additional information about the characteristics of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or “below”) the equal sign.

All chemical reactions can be grouped into several classes, which have certain characteristics.

Classification of chemical reactions according to the number and composition of starting and resulting substances

According to this classification, chemical reactions are divided into reactions of connection, decomposition, substitution, and exchange.

As a result compound reactions from two or more (complex or simple) substances one new substance is formed. IN general view The equation for such a chemical reaction will look like this:

For example:

CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2

SO 3 + H 2 O = H 2 SO 4

2Mg + O 2 = 2MgO.

2FeCl 2 + Cl 2 = 2FeCl 3

The reactions of the compound are in most cases exothermic, i.e. proceed with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox reactions (ORR), i.e. occur with changes in the oxidation states of elements. It is impossible to say unambiguously whether the reaction of a compound between complex substances will be classified as ORR.

Reactions that result in the formation of several other new substances (complex or simple) from one complex substance are classified as decomposition reactions. In general, the equation for the chemical reaction of decomposition will look like this:

For example:

CaCO 3 CaO + CO 2 (1)

2H 2 O = 2H 2 + O 2 (2)

CuSO 4 × 5H 2 O = CuSO 4 + 5H 2 O (3)

Cu(OH) 2 = CuO + H 2 O (4)

H 2 SiO 3 = SiO 2 + H 2 O (5)

2SO 3 =2SO 2 + O 2 (6)

(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 +4H 2 O (7)

Most decomposition reactions occur when heated (1,4,5). Possible decomposition due to exposure electric current(2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) occurs without changing the oxidation states of the elements, i.e. these reactions are not related to ODD. ORR decomposition reactions include the decomposition of oxides, acids and salts formed by elements in higher degrees oxidation (6).

Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):

C 18 H 38 = C 9 H 18 + C 9 H 20 (8)

C 4 H 10 = C 4 H 6 + 2H 2 (9)

At substitution reactions a simple substance interacts with a complex substance, forming a new simple and a new complex substance. In general, the equation for a chemical substitution reaction will look like this:

For example:

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

Zn + 2HCl = ZnСl 2 + H 2 (2)

2KBr + Cl 2 = 2KCl + Br 2 (3)

2КlO 3 + l 2 = 2KlO 3 + Сl 2 (4)

CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (5)

Ca 3 (PO 4) 2 + 3SiO 2 = 3СаSiO 3 + P 2 O 5 (6)

CH 4 + Cl 2 = CH 3 Cl + HCl (7)

Most substitution reactions are redox (1 – 4, 7). Examples of decomposition reactions in which no change in oxidation states occurs are few (5, 6).

Exchange reactions are reactions that occur between complex substances in which they exchange their constituent parts. Typically this term is used for reactions involving ions found in aqueous solution. In general, the equation for a chemical exchange reaction will look like this:

AB + CD = AD + CB

For example:

CuO + 2HCl = CuCl 2 + H 2 O (1)

NaOH + HCl = NaCl + H 2 O (2)

NaHCO 3 + HCl = NaCl + H 2 O + CO 2 (3)

AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)

CrCl 3 + ZNaON = Cr(OH) 3 ↓+ ZNaCl (5)

Exchange reactions are not redox. Special case these exchange reactions are neutralization reactions (reactions between acids and 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 poorly 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 reagents and reaction products, all chemical reactions are divided into redox reactions (1, 2) and those occurring without changing the oxidation state (3, 4).

2Mg + CO 2 = 2MgO + C (1)

Mg 0 – 2e = Mg 2+ (reducing agent)

C 4+ + 4e = C 0 (oxidizing agent)

FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)

Fe 2+ -e = Fe 3+ (reducing agent)

N 5+ +3e = N 2+ (oxidizing agent)

AgNO 3 +HCl = AgCl ↓ + HNO 3 (3)

Ca(OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)

Classification of chemical reactions by thermal effect

Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conventionally divided into exothermic (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during a reaction is called the thermal effect of the reaction. If the equation indicates the amount of heat released or absorbed, then such equations are called thermochemical.

N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)

2Mg + O 2 = 2MgO + 602.5 kJ (2)

N 2 + O 2 = 2NO – 90.4 kJ (3)

Classification of chemical reactions according to the direction of the reaction

Based on the direction of the reaction, a distinction is made between reversible (chemical processes whose products are capable of reacting with each other under the same conditions in which they were obtained to form the starting substances) and irreversible (chemical processes whose products are not able to react with each other to form the 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 COOC 2 H 5 + H 2 O

Examples of irreversible reactions include the following reactions:

2КlО 3 → 2Кl + ЗО 2

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O

Evidence of the irreversibility of a reaction can be the release of a gaseous substance, a precipitate, or a poorly dissociating compound, most often water, as reaction products.

Classification of chemical reactions according to 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 the progress of a chemical reaction. Reactions that occur with the participation of catalysts are called catalytic. Some reactions cannot take place at all without the presence of a catalyst:

2H 2 O 2 = 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 = MeF 2 + H 2 O, where Me is a metal.

Examples of problem solving

EXAMPLE 1

You can often hear from decent-looking people about the health hazards of some product or product. Moreover, the main argument in favor of such a statement will be the phrase: “This is chemistry!” However, only those who clearly skipped classes in this subject at school can say this. The fact is that the human, and indeed any biological organism, itself consists of many organic and inorganic substances. At the same time, various processes continuously occurring inside it help maintain its viability. One of the main ones is the chemical decomposition reaction. Let's learn more about it and the features of its occurrence with organic and inorganic substances.

What kind of process is called a chemical reaction?

First of all, it is worth knowing the meaning of the concept of “chemical reaction”. This phrase means the transformation of one or more starting substances (called reagents) into others. In the process of such metamorphosis, the nuclei of the atoms of interacting compounds are not subject to change, but a redistribution of electrons occurs. Thus, after the transformation, new atomic compounds are formed at the output.

Chemical reactions have a qualitative difference from physical and nuclear ones.

• As a result of the former, the initial reagents never change their composition, although they are capable of forming mixtures or moving from one state of aggregation to another. In contrast, chemical processes are accompanied by the formation of new compounds with completely different properties.
• The result of the latter is changes in the isotopic composition and number of atoms. Thus, at the exit from some elements, others are formed. However, such deep metamorphoses are not typical. Because the changes that occur due to them do not affect the internal structure of atoms.

Conditions for chemical reactions

In many cases, for processes of this kind to proceed successfully, simply physical contact of the reagents with each other or their mixing is necessary. But often, for a chemical reaction to start, it needs catalysts. This role can be played by both various substances and certain external conditions.

• Impact of temperature. In order to start individual chemical processes, it is necessary to heat the reagents. For example, to begin the decomposition reaction of calcium carbonate, the temperature of this compound must be increased to 900-1200 °C.
• Electromagnetic waves. The most effective way to stimulate the course of any process is to expose the reagents to light waves. Such reactions are called “photochemical”. A classic example of such a reaction is photosynthesis.
• Ionizing radiation.
• Exposure to electric current.
• Various types of mechanical influence on reacting substances.

What types of chemical reactions exist

Classification of such processes is mainly made according to six criteria.

• Based on the presence of a phase separation boundary: homo-/heterogeneous reactions.
• By heat release/absorption: exothermic and endothermic processes.
• By the presence/absence of catalysts: catalytic and non-catalytic reactions.
• In the direction of flow: reversible and Depending on this category, the type of sign is between left and right sides chemical equation. With irreversible, these are two arrows directed in opposite directions, with reversible, there is only one, directed from left to right.
• By change in oxidation state. According to this principle, an oxidation-reduction reaction is distinguished.
• Decomposition (cleavage), combination, substitution and exchange are types of chemical processes similar to the metamorphosis of reagents.

(cleavage): what is it

This term refers to the process by which one thing is divided into two or more simple ones. In most cases, the catalyst for this is high temperature. For this reason, this process is also called a thermal decomposition reaction.

As an example, one of the classical methods for producing pure oxygen (O 2) in industry can be cited. This occurs as a result of heating KMnO 4 (better known to everyone under the common name “potassium permanganate”).

As a result of the splitting, not only oxygen is formed, but also potassium manganate (K 2 MnO 4), as well as manganese dioxide (MnO 2).

Decomposition reaction equation

Any chemical equation consists of two parts: left and right. In the first of them the reacting compounds are written, and in the second - the reaction products. An arrow pointing to the right is usually placed between them. Sometimes it is bilateral, if we're talking about about a reversible process. In some cases, it can be replaced with an equal sign (=).

The process under consideration, like other types of chemical processes, has its own formula. Schematically, the decomposition reaction equation looks like this: AB (t) → A+B.

It is worth remembering that the vast majority of such processes occur under the influence of heat. To communicate this, either a t or a triangle is often placed above or next to the arrow. However, sometimes instead of heat, various substances and radiation act as catalysts.

In the formula discussed above, AB is the original complex compound, A, B are new substances formed as a result of the decomposition reaction.

Practical examples of such a process are very common. This formula can be illustrated using the process equation described in the previous paragraph: 2KMnO 4 (t) → K 2 MnO 4 + MnO 2 + O 2 .

Types of decomposition reactions

Depending on the type of catalyst (which helps break down a complex substance into simpler ones), several types of decomposition are distinguished.

H2O splitting

Having understood the theory regarding the decomposition reaction, examples of its practical implementation are worth considering. Since H 2 O is one of the most accessible substances for chemical experiments today, it is worth starting with it.

This reaction of water decomposition is also called electrolysis and looks like this: 2H 2 O (electric current) → 2H 2 + O 2.
This equation is deciphered as follows: under the influence of electric current on water molecules, they split and form two gases - oxygen and hydrogen.

It is worth noting that this method is actively used on submarines to obtain oxygen. IN modern world it replaced the more expensive method of obtaining this vital substance from sodium peroxide (Na 2 O 2), by reacting it with carbon dioxide: Na 2 O 2 + CO 2 → Na 2 CO 3 + O 2.

In the future, the reaction of water decomposition could be of great importance for the future of the planet. Because in this way it is possible to produce not only oxygen, but also hydrogen, which is used as rocket fuel. Developments in this area have been going on for many years, but the main problem is the need to reduce the amount of energy spent on splitting water molecules.

H2O2 splitting

Among other examples of decomposition reactions, it is worth paying attention to the formation of water and oxygen from hydrogen peroxide (peroxide).

It looks like this: H 2 O 2 (t) → 2H 2 O + O 2.

This process is also thermal, since to begin it, it is necessary that the starting material be heated to a temperature of 150 °C.

It is for this reason that hydrogen peroxide (which most people use to treat wounds) does not turn into water when standing in home medicine cabinets.

However, it is worth remembering that the decomposition reaction of hydrogen peroxide can also occur at ordinary room temperature if the substance comes into contact with compounds such as caustic soda (NaOH) or manganese dioxide (MnO 2). Platinum (Pt) and cuprum (Cu) can also act as catalysts.

Thermal decomposition reaction of CaCO3

Another interesting example is the breakdown of calcium carbonate. This process can be written using the following equation: CaCO 3 (t) → CaO + CO 2.

The product of this reaction will be (calcium oxide) and carbon dioxide.

The process presented above is actively used in industry to produce carbon dioxide. Such reactions are carried out in specialized mines, since the breakdown of calcium carbonate occurs only at temperatures above 900 °C.

To characterize a certain chemical reaction, you must be able to create a record that will display the conditions for the chemical reaction, show which substances reacted and which were formed. To do this, chemical reaction schemes are used.

Chemical reaction diagram– a conditional record showing which substances react, what reaction products are formed, as well as the conditions for the reaction to occur

Let us consider, as an example, the reaction between coal and oxygen. Scheme this reaction is written as follows:

C + O2 → CO2.

Coal reacts with oxygen to form carbon dioxide

Carbon and oxygen- in this reaction there are reactants, and the resulting carbon dioxide is the product of the reaction. Sign " → " indicates the progress of the reaction. Often the conditions under which the reaction occurs are written above the arrow.

For example, the sign « t° → » indicates that the reaction occurs when heated. Sign "R →" denotes pressure, and the sign "hv →"– that the reaction occurs under the influence of light. Additional substances involved in the reaction may also be indicated above the arrow. For example, "O2 →".

If a gaseous substance is formed as a result of a chemical reaction, then in the reaction scheme, after the formula of this substance, write the sign “ → " If a precipitate is formed during the reaction, it is indicated by the sign “ → ».

For example, when chalk powder is heated (it contains a substance with the chemical formula CaCO3), two substances are formed: quicklime CaO and carbon dioxide.

СaCO3 t° → CaO + CO2.

In cases where both the reactants and reaction products, for example, are gases, the “” sign is not used. So, natural gas, mainly consists of methane CH4, when heated to 1500°C it turns into two other gases: hydrogen H2 and acetylene C2H2. The reaction scheme is written as follows:

CH4 t° → C2H2 + H2.

It is important not only to be able to draw up diagrams of chemical reactions, but also to understand what they mean. Let's consider another reaction scheme:

H2O electric current → H2 + O2

This diagram means that under the influence of electric current, water decomposes into two simple gaseous substances: hydrogen and oxygen. The diagram of a chemical reaction is a confirmation of the law of conservation of mass and shows that chemical elements do not disappear during a chemical reaction, but are only rearranged into new chemical compounds.

Chemical Reaction Equations

According to the law of conservation of mass, the initial mass of products is always equal to the mass of the resulting reactants. The number of atoms of elements before and after the reaction is always the same; the atoms only rearrange and form new substances.

Let's return to the reaction schemes recorded earlier:

СaCO3 t° → CaO + CO2; C + O2 CO2.

In these reaction schemes the sign “ → " can be replaced with the "=" sign, since it is clear that the number of atoms before and after the reactions is the same. The entries will look like this:

CaCO3 = CaO + CO2; C + O2 = CO2.

It is these records that are called equations of chemical reactions, that is, these are records of reaction schemes in which the number of atoms before and after the reaction is the same.

Chemical reaction equation– a conventional notation of a chemical reaction using chemical formulas, which corresponds to the law of conservation of mass of a substance

If we look at the other equation schemes given earlier, we can see that in At first glance, the law of conservation of mass does not hold true in them:

CH4 t° → C2H2 + H2.

It can be seen that on the left side of the diagram there is one carbon atom, and on the right there are two. There are equal numbers of hydrogen atoms and there are four of them on the left and right sides. Let's turn this diagram into an equation. For this it is necessary equalize number of carbon atoms. Chemical reactions are equalized using coefficients that are written before the formulas of substances.

Obviously, in order for the number of carbon atoms to become the same on the left and right, on the left side of the diagram, before the methane formula, it is necessary to put coefficient 2:

2CH4 t° → C2H2 + H2

It can be seen that there are now equal numbers of carbon atoms on the left and right, two each. But now the number of hydrogen atoms is not the same. On the left side of the equation their 2∙4 = 8. On the right side of the equation there are 4 hydrogen atoms (two of them in the acetylene molecule, and two more in the hydrogen molecule). If you put a coefficient in front of acetylene, the equality of carbon atoms will be violated. Let's put a factor of 3 in front of the hydrogen molecule:

2CH4 = C2H2 + 3H2

Now the number of carbon and hydrogen atoms on both sides of the equation is the same. The law of conservation of mass is fulfilled!

Let's look at another example. Reaction scheme Na + H2O → NaOH + H2 needs to be turned into an equation.

In this scheme, the number of hydrogen atoms is different. On the left side there are two, and on the right side - three atoms. Let's put a factor of 2 in front of NaOH.

Na + H2O → 2NaOH + H2

Then there will be four hydrogen atoms on the right side, therefore, coefficient 2 must be added before the water formula:

Na + 2H2O → 2NaOH + H2

Let's equalize the number of sodium atoms:

2Na + 2H2O = 2NaOH + H2

Now the number of all atoms before and after the reaction is the same.

Thus, we can conclude: To turn a chemical reaction diagram into a chemical reaction equation, it is necessary to equalize the number of all atoms that make up the reactants and reaction products using coefficients. Coefficients are placed before the formulas of substances.

Let's summarize the equations of chemical reactions

• A chemical reaction diagram is a conventional notation showing which substances react, what reaction products are formed, as well as the conditions for the reaction to occur
• In reaction schemes, symbols are used that indicate the peculiarities of their occurrence.
• The equation of a chemical reaction is a conventional representation of a chemical reaction using chemical formulas, which corresponds to the law of conservation of mass of a substance
• A chemical reaction diagram is converted into an equation by placing coefficients in front of the formulas of substances

Solving equations of chemical reactions causes difficulties for a considerable number of students high school largely due to the wide variety of elements involved in them and the ambiguity of their interaction. But since the main part of the general chemistry course at school examines the interaction of substances based on their reaction equations, students must necessarily fill gaps in this area and learn to solve chemical equations in order to avoid problems with the subject in the future.

The equation of a chemical reaction is a symbolic notation that displays the interacting chemical elements, their quantitative ratio and the substances resulting from the interaction. These equations reflect the essence of the interaction of substances from the point of view of atomic-molecular or electronic interaction.

1. At the very beginning of the school chemistry course, they are taught to solve equations based on the concept of valence of elements of the periodic table. Based on this simplification, let us consider the solution of a chemical equation using the example of the oxidation of aluminum with oxygen. Aluminum reacts with oxygen to form aluminum oxide. Having the specified initial data, we will draw up an equation diagram.

   Al + O 2 → AlO

   
   

   In this case, we have written down an approximate diagram of a chemical reaction, which only partially reflects its essence. The substances involved in the reaction are written on the left side of the diagram, and the result of their interaction is written on the right. Additionally, oxygen and other typical oxidizing agents are usually written to the right of metals and other reducing agents on both sides of the equation. The arrow shows the direction of the reaction.

2. In order for this compiled reaction scheme to acquire a complete form and comply with the law of conservation of mass of substances, it is necessary:
   • Place indices on the right side of the equation for the substance resulting from the interaction.
     
   • Level the amount of elements participating in the reaction with the amount of the resulting substance in accordance with the law of conservation of mass of substances.
3. Let's start by suspending the subscripts in the chemical formula of the finished substance. Indices are set in accordance with the valence of chemical elements. Valence is the ability of atoms to form compounds with other atoms due to the combination of their unpaired electrons, when some atoms give up their electrons, while others add them to themselves at an external energy level. It is generally accepted that the valency of a chemical element is determined by its group (column) in the periodic table. However, in practice, the interaction of chemical elements is much more complex and varied. For example, the oxygen atom has a valence of Ⅱ in all reactions, despite the fact that it is in the sixth group in the periodic table.
4. To help you navigate this diversity, we offer you the following small reference assistant that will help you determine the valence of a chemical element. Select the element you are interested in and you will see the possible values ​​of its valence. Rare valencies for the selected element are indicated in brackets.
5. Let's return to our example. Let us write down its valence on the right side of the reaction diagram above each element.

   For aluminum Al the valence will be equal to Ⅲ, and for the oxygen molecule O 2 the valence will be equal to Ⅱ. Find the least common multiple of these numbers. It will be equal to six. We divide the least common multiple by the valence of each element and get the indices. For aluminum, divide six by valence to obtain an index of 2, for oxygen 6/2 = 3. Chemical formula The aluminum oxide obtained as a result of the reaction will take the form Al 2 O 3.

   Al + O 2 → Al 2 O 3

6. After obtaining the correct formula of the finished substance, it is necessary to check and, in most cases, equalize the right and left parts of the diagram according to the law of conservation of mass, since the reaction products are formed from the same atoms that were originally part of the starting substances participating in the reaction.
7. Law of conservation of mass states that the number of atoms that entered into the reaction must be equal to the number of atoms resulting from the interaction. In our scheme, the interaction involves one aluminum atom and two oxygen atoms. As a result of the reaction, we obtain two aluminum atoms and three oxygen atoms. Obviously, the diagram must be leveled using coefficients for elements and matter in order for the law of conservation of mass to be observed.
8. Equalization is also performed by finding the least common multiple, which is located between the elements with the largest indices. In our example, this will be oxygen with an index on the right side equal to 3 and on the left side equal to 2. The least common multiple in this case will also be equal to 6. Now we divide the least common multiple by the value of the largest index on the left and right sides of the equation and get the following indices for oxygen.

   Al + 3∙O 2 → 2∙Al 2 O 3

9. Now all that remains is to equalize the aluminum on the right side. To do this, put a coefficient of 4 on the left side.

   4∙Al + 3∙O 2 = 2∙Al 2 O 3

10. After arranging the coefficients, the equation of a chemical reaction corresponds to the law of conservation of mass and an equal sign can be placed between its left and right sides. The coefficients placed in the equation indicate the number of molecules of substances participating in the reaction and resulting from it, or the ratio of these substances in moles.