Make reaction equations in chemistry. How to write equations for chemical reactions

Reactions between various types of chemical substances and elements are one of the main subjects of study in chemistry. To understand how to create a reaction equation and use them for your own purposes, you need a fairly deep understanding of all the patterns in the interaction of substances, as well as processes with chemical reactions.

Writing equations

One way to express a chemical reaction is a chemical equation. It records the formula of the starting substance and the product, coefficients that show how many molecules each substance has. All known chemical reactions are divided into four types: substitution, combination, exchange and decomposition. Among them are: redox, exogenous, ionic, reversible, irreversible, etc.

Learn more about how to write equations chemical reactions:

  1. It is necessary to determine the name of the substances that interact with each other in the reaction. We write them on the left side of our equation. As an example, consider the chemical reaction that formed between sulfuric acid and aluminum. We place the reagents on the left: H2SO4 + Al. Next we write the equal sign. In chemistry, you may come across an “arrow” sign that points to the right, or two arrows directed in the opposite direction, they mean “reversibility”. The result of the interaction of metal and acid is salt and hydrogen. Write the products obtained after the reaction after the equal sign, that is, on the right. H2SO4+Al= H2+ Al2(SO4)3. So, we can see the reaction scheme.
  2. To compose a chemical equation, you must find the coefficients. Let's return to the previous diagram. Let's look at its left side. Sulfuric acid contains hydrogen, oxygen and sulfur atoms in an approximate ratio of 2:4:1. On the right side there are 3 sulfur atoms and 12 oxygen atoms in the salt. Two hydrogen atoms are contained in a gas molecule. On the left side the ratio of these elements is 2:3:12
  3. To equalize the number of oxygen and sulfur atoms that are in the composition of aluminum (III) sulfate, it is necessary to place a coefficient of 3 in front of the acid on the left side of the equation. Now we have 6 hydrogen atoms on the left side. To equalize the number of elements of hydrogen, you need to put 3 in front of hydrogen on the right side of the equation.
  4. Now all that remains is to equalize the amount of aluminum. Since the salt contains two metal atoms, we set a coefficient of 2 on the left side in front of aluminum. As a result, we obtain the reaction equation for this scheme: 2Al+3H2SO4=Al2(SO4)3+3H2

Having understood the basic principles of how to create an equation for the reaction of chemical substances, in the future it will not be difficult to write down any reaction, even the most exotic one from the point of view of chemistry.

Chapter 2. Basics of chemical language and initial information about the classification of individual chemical substances (continued)

2.4. Schemes and equations of chemical reactions

In chemical reactions, some substances are transformed into others. Let us recall the well-known reaction of sulfur with oxygen. And it contains only substances ( starting materials or reagents) others are formed ( final substances or reaction products).

To record and transmit information about chemical reactions, they are used schemes And reaction equations.

A reaction diagram shows which substances react and which are formed as a result of the reaction. In both diagrams and reaction equations, substances are designated by their formulas.

The sulfur combustion scheme is written as follows: S 8 + O 2 SO 2.

This means that when sulfur reacts with oxygen, a chemical reaction occurs, resulting in the formation of sulfur dioxide (sulfur dioxide). All substances here are molecular, therefore, when writing the diagram, the molecular formulas of these substances were used. The same applies to the scheme of another reaction - the combustion reaction of white phosphorus:

P 4 + O 2 P 4 O 10 .

When heated to 900 o C calcium carbonate (chalk, limestone) a chemical reaction occurs: calcium carbonate is converted into calcium oxide (quicklime) and carbon dioxide (carbon dioxide) according to the following scheme:

CaCO 3 CaO + CO 2.

To indicate that the process occurs when heated, the diagram (and equation) is usually supplemented with the sign " t " , and the fact that carbon dioxide evaporates is indicated by an arrow pointing upward:

CaCO 3 CaO + CO 2.

Calcium carbonate and calcium oxide are non-molecular substances, so the diagram uses their simplest formulas, reflecting the composition of their formula units. For a molecular substance - carbon dioxide - a molecular formula is used.

Let's consider the reaction scheme that occurs when phosphorus pentachloride interacts with water: PCl 5 + H 2 OH 3 PO 4 + HCl.
The diagram shows that phosphoric acid and hydrogen chloride are formed.

Sometimes it is enough to convey information about a chemical reaction. brief outline this reaction, for example:

S 8 SO 2; P 4 P 4 O 10 ; CaCO 3 CaO.

Naturally, several different reactions may correspond to a short scheme.

For any chemical reaction, one of the most important laws of chemistry is true:
When chemical reactions occur, atoms do not appear, disappear, or transform into each other.

When writing equations of chemical reactions, in addition to formulas of substances, coefficients are used. As in algebra, the coefficient “1” in the equation of a chemical reaction is not put, but is implied. The reactions we have considered are described by the following equations:

1S 8 + 8O 2 = 8SO 2, or S 8 + 8O 2 = 8SO 2;
1P 4 + 5O 2 = 1P 4 O 10, or P 4 + 5O 2 = P 4 O 10;
1CaCO 3 = 1CaO + 1CO 2, or CaCO 3 = CaO + CO 2;
1PCl 5 + 4H 2 O = 1H 3 PO 4 + 5HCl, or PCI 5 + 4H 2 O = H 3 PO 4 + 5HCI.

The equal sign between the right and left sides of the equation means that the number of atoms of each element included in the composition of the starting substances, equals number of atoms of this element included in the reaction products.

The coefficients in the equation of a chemical reaction show the relationship between the number of reactants and the number of molecules formed (for non-molecular substances - the number of formula units) of the corresponding substances. So, for the reaction that occurs when phosphorus pentachloride interacts with water

and so on (6 proportions in total).Usually a single coefficient in a reaction equation has no meaning, but in some cases it can mean the number of molecules or formula units of a given substance.Examples of information provided by reaction schemes and equations.
1st example. The combustion reaction of methane in oxygen (or air):
CH 4 + O 2 CO 2 + H 2 O (scheme),
CH 4 + 2O 2 = CO 2 + 2H 2 O (equation).

The chemical reaction diagram shows that (1) the reaction between methane and oxygen produces carbon dioxide and water.

The reaction equation adds that (2) the number of methane molecules that reacted is related to the number of oxygen molecules that reacted as 1 to 2, and so on, that is:

Additionally, the equation shows that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

2nd example. Reduction of iron with hydrogen from its oxide:
Fe 2 O 3 + H 2 Fe + H 2 O (scheme),
Fe 2 O 3 + 3H 2 = 2Fe + 3H 2 O (equation).

The chemical reaction diagram shows that (1) when iron oxide (Fe 2 O 3) reacts with hydrogen (which occurs when heated), iron and water are formed.

The equation adds to this that (2) the number of formula units of iron oxide reacted is related to the number of hydrogen molecules reacted as 1 to 3, and so on. That is:

In addition, the equation shows that one formula unit of iron oxide reacts with three hydrogen molecules to form two iron atoms and three water molecules.

As you will learn later, reaction equations also give us other quantitative information.

Calcium carbonate – CaCO 3 . A colorless non-molecular substance, insoluble in water. Commonly known rocks such as marble and limestone are composed primarily of calcium carbonate. The chalk you use to write on the blackboard is also calcium carbonate: many marine organisms (radiolaria, etc.) build their shells from this substance; for long time At the bottom of the ocean, deposits of chalk are formed, which are huge layers of compressed shells of these organisms.
Calcium carbonate does not have the ability to melt; when heated, it decomposes. Rocks formed by calcium carbonate are used in construction as finishing materials, building stone, and for the production of quicklime (CaO). In metallurgy, calcium carbonate in the form of limestone is added to ore to better education slag.

REAGENTS, REACTION PRODUCTS, REACTION SCHEMES AND EQUATIONS, COEFFICIENTS IN REACTION EQUATIONS

1.Write down the equations corresponding to the following reaction schemes:
a) Na+ Cl 2 NaCl; b) CuO + Al Al 2 O 3 + Cu;
c) N 2 O N 2 + O 2; d)NaOH + H 2 SO 4 Na 2 SO 4 + H 2 O.
2. What information is conveyed by the reaction equations you have compiled (among the given substances, Cl 2 , N 2 O, N 2 , O 2 , H 2 SO 4 and H 2 O are molecular; the rest are non-molecular).

2.5. Initial information on the classification of pure chemicals

You have already become acquainted, to one degree or another, with about fifty individual (pure) chemicals. In total, science knows several million such substances. In order not to drown in this “sea” of substances, they need to be systematized and, first of all, classified - classified in more detail than we did in paragraph 1.4 (Fig. 1.3).
Substances differ from each other in their properties, and the properties of substances are determined by their composition and structure. Therefore, the most important characteristics by which substances are classified are composition, structure and properties.
Based on their composition, or more precisely, based on the number of elements they contain, substances are divided into simple and complex (you already know this). There are hundreds of thousands of times more complex substances than simple ones, so binary substances (binary compounds) are distinguished among them.

The scheme of this classification is shown in Figure 2.1.
The sign by which substances are further classified is their properties.
Let's start with simple substances.
Based on their physical properties, simple substances are divided into metals And nonmetals.
Characteristic physical properties metals:
1) high electrical conductivity (the ability to conduct electric current well),
2) high thermal conductivity (the ability to conduct heat well),
3) high ductility (ductility, bendability, elongation).

In addition, all metals have a “metallic” luster. But it should be remembered that not only metals, but also some non-metals, and even some complex substances have such a shine. Crystalline silicon, one of the polymorphic modifications of arsenic, shines, and tellurium, and these are all non-metals. Among the complex substances are pyrite FeS 2, chalcopyrite CuFeS 2 and some others.

The basis for systematizing chemical elements, simple substances and compounds is NATURAL SYSTEM OF CHEMICAL ELEMENTS, discovered in 1869 by the outstanding Russian chemist Dmitry Ivanovich Mendeleev (1834–1907) and named by him " periodic system". Improved by many generations of scientists, this system continues to be called “periodic,” although this is not entirely correct. Graphically, the system of chemical elements is expressed in the form of a table of elements (Fig. 2.2); You will become familiar with the structure of this table in detail by studying Chapter 6. For now, let’s see where the elements are located in the table elements that form nonmetals, and where – elements that form metals. It turns out that the elements that form nonmetals are grouped in the upper right corner of the table of elements. All other elements are elements that form metals. You will learn the reason for this by studying the structure of atoms and chemical bonds.

At room temperature, metals are solid substances (the exception is mercury, its melting point is 39 o C).
Unlike metals, nonmetals do not have any specific set of characteristic physical characteristics. Even their state of aggregation may be different. At room temperature, there are twelve gaseous simple substances (H 2, He, N 2, O 2, O 3, F 2, Ne, Cl 2, Ar, Kr, Xe, Rn), one liquid (Br 2), and more solid substances ten (B, C (diamond), C (graphite), Si, P 4, S 8, As, Se, Te, I 2, etc.). In their chemical properties, most metals are very different from most non-metals, but there is no sharp boundary between them.
Many simple substances can react with each other under certain conditions, for example:

2H 2 + O 2 = 2H 2 O; 2Na + Cl 2 = 2NaCl; 2Ca + O 2 = 2CaO.

As a result of such reactions, binary compounds are formed.

In principle, a binary compound can contain any elements (except helium and neon). But often one of these elements is oxygen, hydrogen or one of the halogens (fluorine, chlorine, bromine or iodine). Such substances are called oxygen compounds, hydrogen compounds or halides. Examples of binary compounds: CaO, Al 2 O 3, KH, HCl, AlI 3, CaC 2.

Examples oxygen compounds: H 2 O (water), H 2 O 2 (hydrogen peroxide), Na 2 O (sodium oxide), Na 2 O 2 (sodium peroxide), CO 2 (carbon dioxide), OF 2 (oxygen fluoride). Most oxygen compounds are oxides. You will learn later how oxides differ from other oxygen compounds.
Examples of oxides:
Li 2 O – lithium oxide, CO 2 – carbon dioxide, CaO – calcium oxide, SiO 2 – silicon dioxide, Al 2 O 3 – aluminum oxide, H 2 O – water,
MnO 2 – manganese dioxide, SO 3 – sulfur trioxide.

Examples of hydrogen compounds: NaH – sodium hydride, H 2 O – water, KH – potassium hydride, HCl – hydrogen chloride, CaH 2 – calcium hydride,
NH 3 – ammonia, BaH 2 – barium hydride, CH 4 – methane.

Examples of halides: CaF 2 - calcium fluoride, BF 3 - boron trifluoride, NaCl - sodium chloride, PCl 5 - phosphorus pentachloride, KBr - potassium bromide, HBr - hydrogen bromide, AlI 3 - aluminum iodide, HI - hydrogen iodide.
Examples of names of binary compounds are given in Table 6.

Table 6. Examples of names of binary compounds.

Please note that all of these names contain the suffix -id. Any binary compound can be called this method, except for binary compounds of elements that form metals ( intermetallic connections). At the same time, some binary compounds have their traditional names (water, ammonia, hydrogen chloride, methane and some others).

Among binary compounds on Earth, oxides are most often found. This is caused by the fact that every second atom in earth's crust(in the atmosphere, hydrosphere and lithosphere) – oxygen atom. And among oxides, the most common substance is water. One reason for this is that hydrogen is also one of the most abundant elements in the earth's crust.

Now - about more complex connections. Let the compound contain three elements. There are a lot of such connections. Which ones are the most important? Of course, oxygen-containing compounds. And above all, those that contain hydrogen. The importance of these compounds is also due to the fact that as a result of chemical reactions between oxides and water, just such substances are obtained, for example:

CaO + H 2 O = Ca(OH) 2; P 4 O 10 + 6H 2 O = 4H 3 PO 4 ;
Li 2 O + H 2 O = 2LiOH; SO 3 + H 2 O = H 2 SO 4.

The substances formed as a result of these reactions are called hydroxides. The name comes from the combination of the words “oxide hydrate,” that is, a compound of oxide and water.

There are many hydroxides, including those that are not formed by direct interaction of the oxide with water, for example: H 2 SiO 3, Al(OH) 3, Cu(OH) 2 and others. These substances are also called hydroxides because when heated they decompose into oxide and water.

In fact, almost all hydroxides decompose when heated, forming the corresponding oxide and water, for example:
Cu(OH) 2 = CuO + H 2 O at 100 o C;
Ca(OH) 2 = CaO +H 2 O at 500 o C;
H 2 SO 4 = SO 3 + H 2 O at 450 o C;
2Al(OH) 3 = Al 2 O 3 + 3H 2 O at 200 o C;
H 2 SiO 3 = SiO 2 + H 2 O below 100 o C.
But hydroxides, such as NaOH and KOH, do not decompose even when heated to 1500 o C.

Examples of the names of some hydroxides are given in Table 7.

Name

Name
NaOH Sodium hydroxide H2SO4 Sulfuric acid
CON Potassium hydroxide H2SO3 Sulfurous acid
Ca(OH)2 Calcium hydroxide HNO3 Nitric acid
Ba(OH)2 Barium hydroxide HNO2 Nitrous acid
Al(OH) 3 Aluminum hydroxide H3PO4 Phosphoric acid
Cu(OH)2 Copper hydroxide H2CO3 Carbonic acid
Zn(OH)2 Zinc hydroxide H2SiO3 Silicic acid

Please note that the left half of the table contains hydroxides of elements that form metals (the name begins with the word “hydroxide”), and the right half contains hydroxides of elements that form nonmetals (the name contains the word “acid”). The different forms of names are due to the fact that these hydroxides are very different in their chemical properties. For example, their solutions change the color of substances called indicators(more precisely, acidic-main indicators). Such indicator substances are dyes contained in blueberries, raspberries, black currants, red cabbage and even tea. In the laboratory, litmus (a natural dye), methyl orange, and phenolphthalein (both synthetic) are commonly used as indicators. Thus, litmus in solutions containing acids turns red, and in solutions containing soluble metal hydroxides ( alkalis) – in blue. The colors of other indicators are given in Appendix 3. Acids have a sour taste, but you should never try them, since most of them are poisonous or have a burning effect.

Of the hydroxides listed in Table 6, alkalis are NaOH, KOH and Ba(OH) 2. Slightly soluble Ca(OH) 2 also changes the color of the indicators. Of the acids listed in the same table, only silicic acid does not change the color of the indicators, in particular because, unlike other acids, it is insoluble in water.

Acids, as a rule, do not react with each other, but react with metal hydroxides, for example:
H 2 SO 4 + 2KOH = K 2 SO 4 + H 2 O;
2HNO 3 + Ba(OH) 2 = Ba(NO 3) 2 + 2H 2 O;
H 3 PO 4 + 3NaOH = Na 3 PO 4 + 3H 2 O.

In addition to water, the products of these reactions are salt- complex substances of another important class. As a result of the reaction, neither acid nor alkali remains in the solution, and the solution becomes neutral, therefore such reactions are called reactions neutralization.

Pay attention to the suffixes in the names of the salts given in Table 8.

Table 8. Salts and their names

Name

Name
K 2 SO 3 Sulf it potassium Na 2 CO 3 Carbon at sodium
CaSO4 Sulf at calcium MgCO 3 Carbon at magnesium
Al 2 (SO 4) 3 Sulf at aluminum K2SiO3 Silik at potassium
Ba(NO2)2 Nitre it barium K3PO4 Phosph at potassium
Ba(NO3)2 Nitre at barium Ca 3 (PO 4) 2 Phosph at calcium

Some hydroxides, out of all other hydroxides, react only with acids. Such hydroxides are called reasons. The same hydroxides that react with both acids and bases (alkalis) are called amphoteric hydroxides. The grounds correspond basic oxides, acids – acid oxides, and amphoteric hydroxides – amphoteric oxides. Examples of oxides with different chemical behavior are given in Table 9.

Table 9. Examples of basic, amphoteric and acid oxides, as well as their corresponding hydroxides.

Basic

Amphoteric

Acidic

Hydroxides

Hydroxides

Hydroxides

*)The idealized formula of the hydroxide is given
**)Exists only in aqueous solution

Salts are formed not only from the reactions of acids with bases, but also from the interaction of metals with acids:
Mg + H 2 SO 4 = MgSO 4 + H 2 O,
2Al + 6HCl = 2AlCl3 + 3H2,
as well as during the interaction of basic oxides with acidic oxides Li 2 O + CO 2 = Li 2 CO 3,
basic oxides with acids FeO + H 2 SO 4 = FeSO 4 + H 2 O
and acidic oxides with bases SO 2 + 2NaOH = Na 2 SO 3 + H 2 O.
Amphoteric oxides and hydroxides also undergo similar reactions.
Now let’s return to the familiar division of substances into molecular and non-molecular, that is, to their classification according to the type of structure. How molecular and non-molecular substances are distributed among the various classes of complex substances is shown in Table 10.

Table 10.Type of structure of some complex substances

Connection class

Molecular structure

Non-molecular structure

Basic and amphoteric oxides

Acidic oxides

CO 2, N 2 O 3, N 2 O 5, P 4 O 10, SO 2, SO 3

B 2 O 3, SiO 2, CrO 3

Basic and amphoteric hydroxides

Acidic hydroxides (acids)

H 3 BO 3, H 2 CO 3, HNO 2, HNO 3, H 3 PO 4, H 2 SO 3, H 2 SO 4

CH 4, NH 3, H 2 O, H 2 S, HF, HCl

Halides

BF 3, SiCl 4, CBr 4, C 2 I 6, NCl 3

NaF, KS1, CaBr 2, MgI 2, BaF 2

As you can see, chemicals– very different, different in composition, in their physical characteristics, different in chemical properties. But you cannot yet answer the questions of why this substance has such a composition, why it has such characteristics, why it reacts with these substances and how it reacts with them. Remember that the properties of a substance are determined by its composition and structure. Therefore, to answer these questions, you must first study how substances are structured, that is, the structure of matter.

Carbon dioxide - CO 2, or carbon dioxide. molecular substance, acid oxide. Despite the fact that its volume fraction in the earth's atmosphere is only 0.03 - 0.04%, carbon dioxide is one of the most essential components of air, and its role in our lives cannot be overestimated. It is directly involved in two important natural processes: respiration and photosynthesis. For example, in one hour an adult exhales about 20 liters of carbon dioxide. An increase in its content is detrimental to humans and animals: at a volume fraction of 0.2 - 0.15%, a person loses consciousness. Atmospheric CO 2 protects our planet from hypothermia, as it is able to retain thermal radiation, emanating from the surface of the Earth, but its excess can cause the so-called “greenhouse effect”. Solid CO 2 - "dry ice" - is used for cooling: for example, pieces of ice from an ice cream saleswoman are nothing more than "dry ice"

Calcium oxide – CaO, or quicklime (burnt) lime - the main oxide is white, hygroscopic (absorbs moisture). This substance reacts vigorously with water to form "slaked lime" - calcium hydroxide. This oxide is obtained by burning various rocks formed by calcium carbonate, hence the name “burnt lime”. Causes burns upon contact with skin. It is especially dangerous if it gets into the eyes.

Calcium hydroxide - Ca(OH) 2, or slaked lime, is a white base, slightly soluble in water. It is obtained by so-called quenching - adding water to calcium oxide. The reaction produces so much heat that the reaction mixture boils. Slaked lime is used in construction as a binding material and as a raw material for the production of white sand-lime brick, as well as in the production of mineral fertilizers.

NATURAL SYSTEM OF CHEMICAL ELEMENTS, METALS, NON-METALS, BINARY COMPOUNDS, OXYGEN COMPOUNDS, HYDROGEN COMPOUNDS, HALOGENIDES, HYDRIDES, OXIDES, HYDROXIDES, ACIDS, BASES, SALTS, ALKALI, AMPHOTERIC HYDROXIDES , INDICATORS, NEUTRALIZATION REACTION, BASIC OXIDES, ACID OXIDES, AMPHOTERIC OXIDES a) Fe(OH) 2; b) Pb(OH) 2; c) Fe(OH) 3; d) Cr(OH) 3.

7. Create reaction equations using the following reaction schemes:
Li 2 O + P 4 O 1 0 Li 3 PO 4 ; MnSO 4 + NaOH M n(OH) 2 + Na 2 SO 4 ;
Fe 3 O 4 + Al Al 2 O 3 + Fe; La 2 (SO 4) 3 + KOH La (OH) 3 + K 2 SO 4 ;
Fe 2 O 3 + Mg MgO + Fe; Ag NO 3 + NaO H Ag 2 O + NaNO 3 + H 2 O.
What classes do the starting and final substances of these reactions belong to?

1. Interaction of solutions of acids and bases with indicators.
2. Chemical properties acids and bases.
3. Chemical properties of metals.
4. Chemical properties of oxides.

Quite often, schoolchildren and students have to compose the so-called. ionic reaction equations. In particular, task 31, proposed at the Unified State Exam in Chemistry, is devoted to this topic. In this article we will discuss in detail the algorithm for writing short and complete ionic equations, and will analyze many examples of different levels of complexity.

Why are ionic equations needed?

Let me remind you that when many substances are dissolved in water (and not only in water!), a dissociation process occurs - the substances break up into ions. For example, HCl molecules in aquatic environment dissociate into hydrogen cations (H +, more precisely, H 3 O +) and chlorine anions (Cl -). Sodium bromide (NaBr) is found in an aqueous solution not in the form of molecules, but in the form of hydrated Na + and Br - ions (by the way, solid sodium bromide also contains ions).

When writing “ordinary” (molecular) equations, we do not take into account that it is not molecules that react, but ions. Here, for example, is what the equation for the reaction between hydrochloric acid and sodium hydroxide looks like:

HCl + NaOH = NaCl + H 2 O. (1)

Of course, this diagram does not describe the process entirely correctly. As we have already said, in an aqueous solution there are practically no HCl molecules, but there are H + and Cl - ions. The same is true with NaOH. It would be more correct to write the following:

H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O. (2)

This is it complete ionic equation. Instead of “virtual” molecules, we see particles that are actually present in the solution (cations and anions). We will not dwell on the question of why we wrote H 2 O in molecular form. This will be explained a little later. As you can see, there is nothing complicated: we replaced the molecules with ions that are formed during their dissociation.

However, even the complete ionic equation is not perfect. Indeed, take a closer look: both the left and right sides of equation (2) contain the same particles - Na + cations and Cl - anions. These ions do not change during the reaction. Why then are they needed at all? Let's remove them and get Brief ionic equation:

H + + OH - = H 2 O. (3)

As you can see, it all comes down to the interaction of H + and OH - ions with the formation of water (neutralization reaction).

All complete and brief ionic equations are written down. If we had solved problem 31 on the Unified State Exam in chemistry, we would have received the maximum score for it - 2 points.


So, again about the terminology:

  • HCl + NaOH = NaCl + H 2 O - molecular equation ("ordinary" equation, schematically reflecting the essence of the reaction);
  • H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O - complete ionic equation (real particles in solution are visible);
  • H + + OH - = H 2 O - a short ionic equation (we removed all the "garbage" - particles that do not participate in the process).

Algorithm for writing ionic equations

  1. Let's create a molecular equation for the reaction.
  2. All particles that dissociate in solution to a noticeable extent are written in the form of ions; substances that are not prone to dissociation are left “in the form of molecules.”
  3. We remove the so-called from the two parts of the equation. observer ions, i.e. particles that do not participate in the process.
  4. We check the coefficients and get the final answer - a short ionic equation.

Example 1. Write complete and short ionic equations describing the interaction of aqueous solutions of barium chloride and sodium sulfate.

Solution. We will act in accordance with the proposed algorithm. Let's first create a molecular equation. Barium chloride and sodium sulfate are two salts. Let's look at the section of the reference book "Properties of inorganic compounds". We see that salts can interact with each other if a precipitate is formed during the reaction. Let's check:

Exercise 2. Complete the equations for the following reactions:

  1. KOH + H2SO4 =
  2. H 3 PO 4 + Na 2 O=
  3. Ba(OH) 2 + CO 2 =
  4. NaOH + CuBr 2 =
  5. K 2 S + Hg(NO 3) 2 =
  6. Zn + FeCl 2 =

Exercise 3. Write the molecular equations for the reactions (in aqueous solution) between: a) sodium carbonate and nitric acid, b) nickel (II) chloride and sodium hydroxide, c) phosphoric acid and calcium hydroxide, d) silver nitrate and potassium chloride, e) phosphorus oxide (V) and potassium hydroxide.

I sincerely hope that you have no problems completing these three tasks. If this is not the case, you need to return to the topic "Chemical properties of the main classes inorganic compounds".

How to turn a molecular equation into a complete ionic equation

The fun begins. We must understand which substances should be written as ions and which should be left in “molecular form”. You will have to remember the following.

In the form of ions write:

  • soluble salts (I emphasize, only salts that are highly soluble in water);
  • alkalis (let me remind you that alkalis are bases that are soluble in water, but not NH 4 OH);
  • strong acids (H 2 SO 4, HNO 3, HCl, HBr, HI, HClO 4, HClO 3, H 2 SeO 4, ...).

As you can see, remembering this list is not at all difficult: it includes strong acids and bases and all soluble salts. By the way, especially vigilant young chemists who may be outraged by the fact that strong electrolytes(insoluble salts) are not included in this list, I can report the following: NOT including insoluble salts in this list does not at all deny the fact that they are strong electrolytes.

All other substances must be present in the ionic equations in the form of molecules. Those demanding readers who are not satisfied with the vague term “all other substances” and who, following the example of the hero of a famous film, demand “to make public full list"I give the following information.

In the form of molecules write:

  • all insoluble salts;
  • all weak bases (including insoluble hydroxides, NH 4 OH and similar substances);
  • All weak acids(H 2 CO 3, HNO 2, H 2 S, H 2 SiO 3, HCN, HClO, almost all organic acids...);
  • in general, all weak electrolytes (including water!!!);
  • oxides (all types);
  • all gaseous compounds (in particular, H 2, CO 2, SO 2, H 2 S, CO);
  • simple substances (metals and non-metals);
  • almost everything organic compounds(exception is water-soluble salts of organic acids).

Phew, looks like I haven't forgotten anything! Although it’s easier, in my opinion, to remember list No. 1. Of the fundamentally important things in list No. 2, I’ll once again mention water.


Let's train!

Example 2. Write a complete ionic equation describing the interaction of copper (II) hydroxide and hydrochloric acid.

Solution. Let's start, naturally, with the molecular equation. Copper(II) hydroxide is an insoluble base. All insoluble bases react with strong acids to form salt and water:

Cu(OH) 2 + 2HCl = CuCl 2 + 2H 2 O.

Now let’s find out which substances should be written down as ions and which ones as molecules. The lists above will help us. Copper(II) hydroxide is an insoluble base (see solubility table), a weak electrolyte. Insoluble bases written in molecular form. HCl is a strong acid; in solution it almost completely dissociates into ions. CuCl 2 - soluble salt. We write it in ionic form. Water - only in the form of molecules! We get the complete ionic equation:

Сu(OH) 2 + 2H + + 2Cl - = Cu 2+ + 2Cl - + 2H 2 O.

Example 3. Write a complete ionic equation for the reaction of carbon dioxide with an aqueous solution of NaOH.

Solution. Carbon dioxide is a typical acidic oxide, NaOH is an alkali. When acidic oxides interact with aqueous solutions of alkalis, salt and water are formed. Let’s create a molecular equation for the reaction (don’t forget about the coefficients, by the way):

CO 2 + 2NaOH = Na 2 CO 3 + H 2 O.

CO 2 - oxide, gaseous compound; maintaining molecular shape. NaOH - strong base (alkali); We write it in the form of ions. Na 2 CO 3 - soluble salt; we write in the form of ions. Water is a weak electrolyte and practically does not dissociate; leave in molecular form. We get the following:

CO 2 + 2Na + + 2OH - = Na 2+ + CO 3 2- + H 2 O.

Example 4. Sodium sulfide in aqueous solution reacts with zinc chloride to form a precipitate. Write a complete ionic equation for this reaction.

Solution. Sodium sulfide and zinc chloride are salts. When these salts interact, a precipitate of zinc sulfide precipitates:

Na 2 S + ZnCl 2 = ZnS↓ + 2NaCl.

I will immediately write down the complete ionic equation, and you will analyze it yourself:

2Na + + S 2- + Zn 2+ + 2Cl - = ZnS↓ + 2Na + + 2Cl - .

I offer you several tasks for independent work and a small test.

Exercise 4. Write molecular and complete ionic equations for the following reactions:

  1. NaOH + HNO3 =
  2. H2SO4 + MgO =
  3. Ca(NO 3) 2 + Na 3 PO 4 =
  4. CoBr 2 + Ca(OH) 2 =

Exercise 5. Write complete ionic equations describing the interaction of: a) nitric oxide (V) with an aqueous solution of barium hydroxide, b) a solution of cesium hydroxide with hydroiodic acid, c) aqueous solutions of copper sulfate and potassium sulfide, d) calcium hydroxide and aqueous solution iron(III) nitrate.

Let's talk about how to create a chemical equation, because they are the main elements of this discipline. Thanks to a deep understanding of all the patterns of interactions and substances, you can control them and apply them in various fields of activity.

Theoretical features

Drawing up chemical equations is an important and responsible stage, considered in the eighth grade. secondary schools. What should precede this stage? Before the teacher tells his students how to create a chemical equation, it is important to introduce schoolchildren to the term “valence” and teach them to determine this value for metals and non-metals using the periodic table of elements.

Compilation of binary formulas by valency

In order to understand how to create a chemical equation by valency, you first need to learn how to create formulas for compounds consisting of two elements using valence. We propose an algorithm that will help cope with the task. For example, you need to create a formula for sodium oxide.

First, it is important to take into account that the chemical element that is mentioned last in the name should be in first place in the formula. In our case, sodium will be written first in the formula, oxygen second. Let us recall that oxides are binary compounds in which the last (second) element must be oxygen with an oxidation state of -2 (valency 2). Next, using the periodic table, it is necessary to determine the valence of each of the two elements. To do this we use certain rules.

Since sodium is a metal that is located in main subgroup 1 group, its valence is a constant value, it is equal to I.

Oxygen is a non-metal, since it is the last one in the oxide; to determine its valence, we subtract 6 from eight (the number of groups) (the group in which oxygen is located), we obtain that the valency of oxygen is II.

Between certain valences we find the least common multiple, then divide it by the valency of each of the elements to obtain their indices. We write down the finished formula Na 2 O.

Instructions for composing an equation

Now let's talk in more detail about how to write a chemical equation. First, let's look at the theoretical aspects, then move on to specific examples. So, composing chemical equations presupposes a certain procedure.

  • 1st stage. After reading the proposed task, you need to determine which chemicals should be present on the left side of the equation. A “+” sign is placed between the original components.
  • 2nd stage. After the equal sign, you need to create a formula for the reaction product. When performing such actions, you will need the algorithm for composing formulas for binary compounds, which we discussed above.
  • 3rd stage. Checking the number of atoms of each element before and after chemical interaction, if necessary, we put additional coefficients in front of the formulas.

Example of a combustion reaction

Let's try to figure out how to create a chemical equation for the combustion of magnesium using an algorithm. On the left side of the equation we write the sum of magnesium and oxygen. Do not forget that oxygen is a diatomic molecule, so it must be indexed 2. After the equals sign, we compose the formula for the product obtained after the reaction. It will be in which magnesium is written first, and oxygen is written second in the formula. Next, using the table of chemical elements, we determine the valencies. Magnesium, located in group 2 (main subgroup), has constant valency II, for oxygen, by subtracting 8 - 6 we also get valency II.

The process record will look like: Mg+O 2 =MgO.

In order for the equation to comply with the law of conservation of mass of substances, it is necessary to arrange the coefficients. First, we check the amount of oxygen before the reaction, after the process is completed. Since there were 2 oxygen atoms, but only one was formed, a coefficient of 2 must be added on the right side before the magnesium oxide formula. Next, we count the number of magnesium atoms before and after the process. As a result of the interaction, 2 magnesium was obtained, therefore, on the left side in front of the simple substance magnesium, a coefficient of 2 is also required.

The final type of reaction: 2Mg+O 2 =2MgO.

Example of a substitution reaction

Any chemistry abstract contains a description different types interactions.

Unlike a compound, in a substitution there will be two substances on both the left and right sides of the equation. Let's say we need to write the reaction of interaction between zinc and We use the standard writing algorithm. First, on the left side we write zinc and hydrochloric acid through the sum, and on the right side we write the formulas for the resulting reaction products. Since zinc is located before hydrogen in the electrochemical voltage series of metals, in this process it displaces molecular hydrogen from the acid and forms zinc chloride. As a result, we get the following entry: Zn+HCL=ZnCl 2 +H 2.

Now we move on to equalizing the number of atoms of each element. Since there was one atom on the left side of chlorine, and after the interaction there were two, it is necessary to put a factor of 2 in front of the formula of hydrochloric acid.

As a result, we obtain a ready-made reaction equation corresponding to the law of conservation of mass of substances: Zn+2HCL=ZnCl 2 +H 2 .

Conclusion

A typical chemistry note necessarily contains several chemical transformations. Not a single section of this science is limited to a simple verbal description of transformations, processes of dissolution, evaporation; everything is necessarily confirmed by equations. The specificity of chemistry lies in the fact that all processes that occur between different inorganic or organic substances, can be described using coefficients and indices.

How else does chemistry differ from other sciences? Chemical equations help not only to describe the transformations that occur, but also to carry out quantitative calculations based on them, thanks to which it is possible to carry out laboratory and industrial production of various substances.

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