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

Compiling chemical equations is an important and responsible stage, considered in the eighth grade of 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 valence, 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 the main subgroup of group 1, 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 valency, 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. We check 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 given an index of 2. After the equal 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, which is in group 2 (the main subgroup), has a constant valency II; for oxygen, by subtracting 8 - 6 we also get valence 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 summary contains a description of different types of 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.

Let's talk about how to write an equation for a chemical reaction. It is this question that mainly causes serious difficulties for schoolchildren. Some cannot understand the algorithm for composing product formulas, others incorrectly place the coefficients in the equation. Considering that all quantitative calculations are carried out using equations, it is important to understand the algorithm of actions. Let's try to figure out how to write equations for chemical reactions.

Drawing up formulas for valence

In order to correctly record processes occurring between different substances, you need to learn how to write formulas. Binary compounds are composed taking into account the valences of each element. For example, for metals of the main subgroups it corresponds to the group number. When compiling the final formula, the smallest multiple is determined between these indicators, then indices are placed.

What is the equation

It is understood as a symbolic record that displays interacting chemical elements, their quantitative relationships, as well as those substances that are obtained as a result of the process. One of the tasks offered to ninth grade students at the final certification in chemistry has the following wording: “Make up reaction equations that characterize the chemical properties of the proposed class of substances.” In order to cope with the task, students must master the algorithm of actions.

Action algorithm

For example, you need to write the process of calcium combustion using symbols, coefficients, and indices. Let's talk about how to create an equation for a chemical reaction using the order of operations. On the left side of the equation, we write through “+” the signs of the substances that participate in this interaction. Since combustion occurs with the participation of oxygen in the air, which is a diatomic molecule, we write its formula as O2.

Following the equal sign, we form the composition of the reaction product using the rules for arranging valency:

2Ca + O2 = 2CaO.

Continuing the conversation about how to create an equation for a chemical reaction, we note the need to use the law of constancy of composition, as well as maintaining the composition of substances. They allow you to carry out the equalization process and place the missing coefficients in the equation. This process is one of the simplest examples of interactions occurring in inorganic chemistry.

Important Aspects

In order to understand how to write an equation for a chemical reaction, we note some theoretical issues related to this topic. The law of conservation of mass of substances, formulated by M.V. Lomonosov, explains the possibility of arranging coefficients. Since the number of atoms of each element remains the same before and after the interaction, mathematical calculations can be carried out.

When equalizing the left and right sides of the equation, the least common multiple is used, similar to how the compound formula is compiled taking into account the valences of each element.

Redox interactions

After schoolchildren have worked out the algorithm of actions, they will be able to create an equation of reactions that characterize the chemical properties of simple substances. Now we can move on to analyzing more complex interactions, for example those occurring with changes in oxidation states of elements:

Fe + CuSO4 = FeSO4 + Cu.

There are certain rules according to which oxidation states are assigned in simple and complex substances. For example, in diatomic molecules this indicator is zero; in complex compounds the sum of all oxidation states should also be equal to zero. When compiling an electronic balance, the atoms or ions that give up electrons (reducing agent) and accept them (oxidizing agent) are determined.

Between these indicators the smallest multiple is determined, as well as the coefficients. The final stage of analysis of redox interactions is the placement of coefficients in the scheme.

Ionic equations

One of the important issues that is discussed in the school chemistry course is the interaction between solutions. For example, the following task is given: “Make an equation for the chemical reaction of ion exchange between barium chloride and sodium sulfate.” It involves writing the molecular, complete, abbreviated ionic equation. To consider the interaction at the ionic level, it is necessary to indicate the solubility table for each starting substance and reaction product. For example:

BaCl2 + Na2SO4 = 2NaCl + BaSO4

Substances that do not dissolve into ions are written in molecular form. The ion exchange reaction occurs completely in three cases:

• sediment formation;
• gas release;
• obtaining a slightly dissociable substance, for example water.

If a substance has a stereochemical coefficient, it is taken into account when writing the complete ionic equation. After the complete ionic equation has been written, the reduction of those ions that were not bound in solution is carried out. The final result of any task involving the consideration of the process occurring between solutions of complex substances will be the recording of an abbreviated ionic reaction.

Conclusion

Chemical equations make it possible to explain with the help of symbols, indices, and coefficients those processes that are observed between substances. Depending on the exact process taking place, there are certain subtleties in writing the equation. The general algorithm for composing reactions, discussed above, is based on valence, the law of conservation of mass of substances, and constancy of composition.

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 were all wet, as they say, “to the skin,” then isn’t this a transformation: after all, the clothes were dry, but they became wet.

If, for example, you take an iron nail, file it, and then assemble iron filings (Fe) , then isn’t this also a transformation: there was a nail - it became powder. But if you then assemble the device and carry out obtaining oxygen (O 2): heat up potassium permanganate(KMpO 4) and collect oxygen in a test tube, and then place these red-hot iron filings into 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 condition of the clothing (dry, wet) change, these are not transformations. The fact is that the nail itself was a substance (iron), and remained so, despite its different shape, and our clothes absorbed the water from the rain and then evaporated it into the atmosphere. The water itself has not changed. So what are transformations from a chemical point of view?

From a chemical point of view, transformations are those phenomena that are accompanied by a change in the composition of a substance. Let's take the same nail as an example. It doesn’t matter what shape it took after being filed, but after the pieces collected from it iron filings placed in an oxygen atmosphere - it turned into iron oxide(Fe 2 O 3 ) . So, something has changed after all? Yes, it has changed. There was a substance called a nail, 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 someone uninitiated in chemistry, questions immediately arise. What is "molecular equation", what is Fe? Why are the numbers “4”, “3”, “2”? What are the little numbers “2” and “3” in the formula Fe 2 O 3? This means it’s time to sort everything out in order.

Signs of chemical elements.

Despite the fact that chemistry begins to be studied 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 “Periodical Table”.

In this table, the elements are arranged in the appropriate order. 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 thinking, identifying them with objects: an iron bolt, an aluminum wire, oxygen in the atmosphere, a gold ring, etc. etc. But in fact, all these substances (bolt, wire, ring) consist of their corresponding elements. The whole paradox is that the element cannot be touched or 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, as well as in other sciences, for calculations, drawing up equations, and solving problems. Each element differs from the other in that it has its own characteristic 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 No. 1. Its atom consists of 1 proton and 1 electron. Helium is element #2. Its atom consists of 2 protons and 2 electrons. Lithium is element #3. Its atom consists of 3 protons and 3 electrons. Darmstadtium – element No. 110. Its atom consists of 110 protons and 110 electrons.

Each element is designated by a certain symbol, Latin letters, and has a certain reading translated 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 notations can be found in any 8th grade chemistry textbook. The main thing for us now is to understand that when composing 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 the substances iron and sulfur interact, then the equation will take the following writing 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). Moreover, one should pay attention
special attention to the fact that all metals are designated by single symbols: K, Ba, Ca, Al, V, Mg, etc., and non-metals are either 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 important when composing equations. It is not at all difficult to guess that complex substances are substances formed from atoms of different types, for example,

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

sodium oxide Na2O,
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 alkali potassium 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 Na2SO4,
calcium carbonate or limestone CaCO 3,
copper chloride CuCl2

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

Finally, after we have figured out 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 constitute almost the very essence of this science. For example, you can give 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 numbers of atoms and elements participating in them. Or, if an ionic equation is given, then in it number of particles must also meet this requirement. A chemical equation is a conventional representation of a chemical reaction using chemical formulas and mathematical symbols. A chemical equation inherently reflects one or another 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 in which they 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:

BaCl 2 + H 2 SO 4 = BaSO 4 + 2HCl (3)

First of all, it is necessary to understand that the large number “2” standing in front of the substance HCl is called a coefficient, and the small numbers “2”, “4” under the formulas BaCl 2, H 2 SO 4, BaSO 4 are called indices. Both coefficients and indices in chemical equations act as multipliers, not summands. To write a chemical equation correctly, you need assign 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). It follows that on the right side of the equation the number of hydrogen and chlorine atoms is half as much as 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 up the numbers of atoms of the elements participating in this reaction, both on the left and on the right, we obtain the following balance:

In both sides of the equation, the numbers of atoms of the elements participating in the reaction are equal, therefore it is composed correctly.

Chemical equation and chemical reactions

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

1). Compound reactions
2). Decomposition reactions.

The overwhelming majority of chemical reactions belong to addition reactions, since changes in its composition can rarely occur with an individual substance if it is not exposed to external influences (dissolution, heating, exposure to light). Nothing characterizes a chemical phenomenon or reaction better than the changes that occur during the interaction of two or more substances. Such phenomena can occur spontaneously and be accompanied by an increase or decrease in temperature, light effects, color changes, sediment formation, release of gaseous products, and noise.

For clarity, we present several equations reflecting the processes of compound reactions, during which we obtain sodium chloride(NaCl), zinc chloride(ZnCl2), silver chloride precipitate(AgCl), aluminum chloride(AlCl 3)

Cl 2 + 2Nа = 2NaCl (4)

CuCl 2 + Zn = ZnCl 2 + Cu (5)

AgNO 3 + KCl = AgCl + 2KNO 3 (6)

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

Among the reactions of the compound, special mention should be made of the following: : substitution (5), exchange (6), and as a special case of an exchange reaction - the reaction neutralization (7).

Substitution reactions include those in which atoms of a simple substance replace 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 salt ZnCl 2, and copper is released from the solution in the metallic state.

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

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

Decomposition reactions include those in which two or more new simple or complex substances, but of a simpler composition, are formed from one complex substance. Examples of reactions include those in the process of which 1) decomposes. potassium nitrate(KNO 3) with the formation of potassium nitrite (KNO 2) and oxygen (O 2); 2). Potassium permanganate(KMnO 4): potassium manganate (K 2 MnO 4) is formed, manganese oxide(MnO 2) and oxygen (O 2); 3). Calcium carbonate or marble; in the process are formed carbonicgas(CO2) and calcium oxide(CaO)

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

In reaction (8), one complex and one simple substance are 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 are subject to 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 = SO 3 + H 2 O (13)

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

5). Organic matter: alcoholic fermentation of glucose

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

According to another classification, all chemical reactions can be divided into two types: reactions that release heat are called exothermic, and reactions that occur 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. interaction with oxygen, for example methane combustion:

CH 4 + 2O 2 = 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 (+Q) or absorbed (-Q) during the reaction:

CaCO 3 = 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 = 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 = 2Mg +2 O -2

Reactions of this type are redox . They will be considered separately. To compose equations for reactions of this type, you must use half-reaction method and apply electronic balance equation.

After presenting the various types of chemical reactions, you can proceed to the principle of composing chemical equations, or, in other words, selecting coefficients on the left and right sides.

Mechanisms for composing chemical equations.

Whatever type a chemical reaction belongs to, its recording (chemical equation) must correspond to the condition that the number of atoms before and after the reaction is equal.

There are equations (17) that do not require equalization, 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 sides of the equation. What principles should be followed in such cases? Is there any system for selecting odds? There is, and not only one. These systems include:

1). Selection of coefficients according to given formulas.

2). Compilation by valences of reacting substances.

3). Arrangement of reacting substances according to oxidation states.

In the first case, it is assumed that we know the formulas of the reacting substances 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 equality is established between the atoms of the elements before and after the reaction, the equal sign (=) is not placed in the equation, but is replaced by an arrow (→). Now let's get down to the actual adjustment. 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). There is no need to equalize it in terms of the number of nitrogen atoms, but in terms of oxygen it is necessary to achieve equality, since before the reaction there were two atoms involved, and after the reaction there were three atoms. Let's make the following diagram:

before reaction after reaction
O 2 O 3

Let's determine the smallest multiple between the given numbers of atoms, it will be “6”.

O 2 O 3
\ 6 /

Let's divide this number on the left side of the oxygen equation by “2”. We get the number “3” and put it into 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”, and also put it in the equation to be solved:

N 2 + 3O 2 → 2N 2 O 3

The numbers of oxygen atoms on both the left and right sides of the equation became equal, respectively, 6 atoms each:

But the number of nitrogen atoms on both sides of the equation will not correspond to each other:

The left one has two atoms, the right one has four atoms. Therefore, in order to achieve equality, it is necessary to double the amount of nitrogen on the left side of the equation, setting the coefficient to “2”:

Thus, equality in nitrogen is observed and, in general, the equation takes the form:

2N 2 + 3О 2 → 2N 2 О 3

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

2N 2 + 3О 2 = 2N 2 О 3 (20)

Let's give 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). There is no need to equalize it in terms of the number of phosphorus atoms, but in terms of chlorine it is necessary to achieve equality, since before the reaction there were two atoms involved, and after the reaction there were five atoms. Let's make the following diagram:

before reaction after reaction
Cl 2 Cl 5

Let's determine 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 chlorine equation by “2”. Let’s get the number “5” and put it into the equation to be solved:

P + 5Cl 2 → PCl 5

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

P + 5Cl 2 → 2РCl 5

The numbers of chlorine atoms on both the left and right sides of the equation became equal, respectively, 10 atoms each:

But the number of phosphorus atoms on both sides of the equation will not correspond to each other:

Therefore, in order to achieve equality, it is necessary to double the amount of phosphorus on the left side of the equation by setting the coefficient “2”:

Thus, equality in phosphorus is observed and, in general, the equation takes the form:

2Р + 5Cl 2 = 2РCl 5 (21)

When composing equations by valencies must be given valency determination and set values ​​for the most famous elements. Valence is one of the previously used concepts, but is currently not used in a number of school programs. But with its help it is easier to explain the principles of drawing up equations of chemical reactions. Valence is understood as the number of chemical bonds that an atom can form with another or other atoms . Valency does not have a 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 use them when writing chemical equations? The numerical values ​​of the valences of the elements coincide with their group number of the Periodic Table of Chemical Elements by D.I. Mendeleev (Table 1).

For other elements valence values may have other values, but never greater than the number of the group in which they are located. Moreover, for even group numbers (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 valence values ​​for some elements, but in each specific case these points are usually specified. Now let's consider the general principle of composing chemical equations based on given valences for certain elements. Most often, this method is acceptable in the case of drawing up equations of chemical reactions of compounds of simple substances, for example, when interacting with oxygen ( oxidation reactions). Let's say you need to display an oxidation reaction aluminum. But let us recall that metals are designated by single atoms (Al), and non-metals in the gaseous state are designated by the indices “2” - (O 2). First, let's write the general reaction scheme:

Al + О 2 →AlО

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

III II
Al O

After that, “cross”-on-“cross” for these element symbols we will put the corresponding indices at the bottom:

III II
Al 2 O 3

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

Al+ O 2 →Al 2 O 3

All that remains is to equalize its left and right parts. Let us proceed in the same way as in the case of composing equation (19). Let's equalize the numbers of oxygen atoms by finding the smallest multiple:

before reaction after reaction

O 2 O 3
\ 6 /

Let's divide this number on the left side of the oxygen equation by “2”. Let’s get the number “3” and put it into the equation being solved. We also divide the number “6” for the right side of the equation by “3”. We get the number “2”, and also put it in the equation to be solved:

Al + 3O 2 → 2Al 2 O 3

To achieve equality in aluminum, it is necessary to adjust its quantity on the left side of the equation by setting the coefficient to “4”:

4Al + 3O 2 → 2Al 2 O 3

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

4Al + 3O 2 = 2Al 2 O 3 (22)

Using the valence method, you can predict what substance is formed during a chemical reaction and what its formula will look like. Let’s assume that the compound reacted with nitrogen and hydrogen with the corresponding valences III and I. Let’s write the general reaction scheme:

N 2 + N 2 → NH

For nitrogen and hydrogen, let’s put the valencies above the expected formula of this compound:

As before, “cross”-on-“cross” for these element symbols, let’s put the corresponding indices below:

III I
NH 3

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

N 2 + N 2 → NH 3

Equating in a well-known way, through the smallest multiple for hydrogen equal to “6”, we obtain the required coefficients and the equation as a whole:

N 2 + 3H 2 = 2NH 3 (23)

When composing equations according to oxidation states reactants, it is necessary to recall that the oxidation state of a particular element is the number of electrons accepted or given up during a chemical reaction. Oxidation state in compounds Basically, it numerically coincides with the valence values ​​of the element. 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 valences 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 often used in equations are given in Table 3.

In the case of compound reactions, the principle of compiling equations by oxidation states is the same as when compiling by valences. For example, let us give the equation for the oxidation of chlorine with oxygen, in which chlorine forms a compound with an oxidation state of +7. Let's write down the expected equation:

Cl 2 + O 2 → ClO

Let us place the oxidation states of the corresponding atoms over the proposed compound ClO:

As in previous cases, we establish that the required 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

Equating for oxygen, finding the smallest multiple between two and seven, equal to “14,” we ultimately establish the equality:

2Cl 2 + 7O 2 = 2Cl 2 O 7 (24)

A slightly different method must be used with oxidation states when composing 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 to find out: what will happen in the reaction process?

Indeed, how do you know what reaction products may arise during a particular reaction? For example, what is formed when barium nitrate and potassium sulfate react?

Ba(NO 3) 2 + K 2 SO 4 → ?

Maybe BaK 2 (NO 3) 2 + SO 4? Or Ba + NO 3 SO 4 + K 2? Or something else? Of course, during this reaction the following compounds are formed: BaSO 4 and KNO 3. How is this known? And how to write the formulas of substances correctly? Let's start with what is most often overlooked: the very concept of “exchange reaction.” This means that in these reactions substances change their constituent parts with each other. Since exchange reactions are mostly carried out between bases, acids or salts, the parts with which they will be exchanged 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 metal cations (1) and (2), and An1 and An2 are their corresponding anions (1) and (2). In this case, it is necessary to take into account that in compounds before and after the reaction, cations are always installed in first place, and anions are in second place. Therefore, if the reaction occurs potassium chloride And silver nitrate, both in dissolved state

KCl + AgNO 3 →

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

KCl + AgNO 3 =KNO 3 + AgCl (26)

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

HCl + KOH = KCl + H 2 O (27)

The oxidation states of metal cations and the charges of anions of acidic residues are indicated in the table of solubility of substances (acids, salts and bases in water). The horizontal line shows metal cations, and the vertical line shows the anions of acid residues.

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

CaCl + NaCO 3 →

Ca 2+ Cl - + Na + CO 3 2- →

Having performed the already known “cross”-on-“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 will establish preliminary formulas for the substances formed during the reaction:

CaCl 2 + Na 2 CO 3 → CaCO 3 + NaCl

Let us place the corresponding charges above their cations and anions:

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

Substance formulas written correctly, in accordance with the charges of cations and anions. Let's create a complete equation, equalizing its left and right sides for sodium and chlorine:

CaCl 2 + Na 2 CO 3 = CaCO 3 + 2NaCl (28)

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

VaON + NPO 4 →

Let us place the corresponding charges over the cations and anions:

Ba 2+ OH - + H + PO 4 3- →

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

Ba(OH) 2 + H 3 PO 4 →

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

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

Let us determine the correct notation for the formula of the salt formed during the reaction:

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

Let's equalize the left side of the equation for barium:

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

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

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

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

3Ba (OH) 2 + 2H 3 PO 4 → Ba 3 (PO 4) 2 + 6H 2 O (29)

Possibility of chemical reactions

The world consists 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, say that a chemical reaction will correspond to it? There is a misconception that if it is correct set the odds in the equation, then it will be feasible in practice. For example, if we take sulfuric acid solution and put it in it zinc, then you can observe the process of hydrogen evolution:

Zn+ H 2 SO 4 = ZnSO 4 + H 2 (30)

But if copper is dropped 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 = CuSO 4 + SO 2 + 2H 2 O (31)

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

N 2 + 3H 2 = 2NH 3

If you take potassium hydroxide solution and pour it on him 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 classified as a substitution reaction:

NaCl + Br 2 ≠

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

Chemistry, as a science, is extremely diverse and multifaceted. The above article reflects only a small part of the processes occurring in the real world. Types, thermochemical equations, electrolysis, processes of organic synthesis and much, much more. But more on that in future articles.

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The main subject of comprehension in chemistry is the reactions between different chemical elements and substances. A greater awareness of the validity of the interaction of substances and processes in chemical reactions makes it possible to manage them and use them for one’s own purposes. A chemical equation is a method of expressing a chemical reaction, in which the formulas of the initial substances and products are written, indicators showing the number of molecules of any substance. Chemical reactions are divided into reactions of combination, substitution, decomposition and exchange. Also among them it is possible to distinguish redox, ionic, reversible and irreversible, exogenous, etc.

Instruction

1. Determine which substances interact with each other in your reaction. Write them on the left side of the equation. For example, consider the chemical reaction between aluminum and sulfuric acid. Place the reagents on the left: Al + H2SO4 Next, put the equal sign, as in a mathematical equation. In chemistry, you may come across an arrow pointing to the right, or two oppositely directed arrows, a “reversibility sign.” As a result of the interaction of a metal with an acid, salt and hydrogen are formed. Write the reaction products after the equal sign, on the right. Al + H2SO4 = Al2 (SO4) 3 + H2 The result is a reaction scheme.

2. To create a chemical equation, you need to find the exponents. On the left side of the previously obtained diagram, sulfuric acid contains hydrogen, sulfur and oxygen atoms in a ratio of 2:1:4, on the right side there are 3 sulfur atoms and 12 oxygen atoms in the salt and 2 hydrogen atoms in the H2 gas molecule. On the left side the ratio of these 3 elements is 2:3:12.

3. In order to equalize the number of sulfur and oxygen atoms in the composition of aluminum(III) sulfate, put the indicator 3 on the left side of the equation in front of the acid. Now there are six hydrogen atoms on the left side. To equalize the number of elements of hydrogen, place the exponent 3 in front of it on the right side. Now the ratio of atoms in both parts is 2:1:6.

4. It remains to equalize the number of aluminum. Because the salt contains two metal atoms, place the exponent 2 in front of aluminum on the left side of the diagram. As a result, you will get the reaction equation for this diagram. 2Al+3H2SO4=Al2(SO4)3+3H2

A reaction is the transformation of one chemical substance into another. And the formula for writing them with the help of special symbols is the equation for this reaction. There are different types of chemical interactions, but the rule for writing their formulas is identical.

You will need

• periodic table of chemical elements D.I. Mendeleev

Instruction

1. On the left side of the equation are written the initial substances that react. They are called reagents. The recording is made with the help of special symbols that denote each substance. A plus sign is placed between the reagent substances.

2. On the right side of the equation is written the formula of the resulting one or more substances, which are called reaction products. Instead of an equal sign, an arrow is placed between the left and right sides of the equation, which indicates the direction of the reaction.

3. After recording the formulas of the reactants and reaction products, you need to arrange the indicators of the reaction equation. This is done so that, according to the law of conservation of mass of matter, the number of atoms of the same element on the left and right sides of the equation remains identical.

4. In order to correctly set the indicators, you need to look at each of the substances that react. To do this, take one of the elements and compare the number of its atoms on the left and right. If it is different, then it is necessary to find a number that is a multiple of the numbers indicating the number of atoms of a given substance in the left and right parts. After this, this number is divided by the number of atoms of the substance in the corresponding part of the equation, and an indicator is obtained for each of its parts.

5. Since the indicator is placed before the formula and refers to each substance included in it, the next step will be to compare the data obtained with the number of another substance included in the formula. This is carried out according to the same scheme as with the first element and taking into account the existing indicator for each formula.

6. After all the elements of the formula have been sorted out, a final check of the correspondence of the left and right parts is carried out. Then the reaction equation can be considered complete.

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Note!
In equations of chemical reactions, it is impossible to interchange the left and right sides. In the opposite case, the result will be a diagram of a completely different process.

Helpful advice
The number of atoms of both individual reagent substances and substances included in the reaction products is determined using the periodic system of chemical elements by D.I. Mendeleev

How unsurprising nature is for humans: in winter it envelops the earth in a blanket of snow, in spring it reveals all living things like popcorn flakes, in summer it rages with a riot of colors, in autumn it sets plants on fire with red fire... And only if you think about it and look closely, you can see what they stand behind all these so familiar changes are difficult physical processes and CHEMICAL REACTIONS. And in order to study all living things, you need to be able to solve chemical equations. The main requirement when balancing chemical equations is knowledge of the law of conservation of the number of substances: 1) the number of substances before the reaction is equal to the number of substances after the reaction; 2) the total number of substances before the reaction is equal to the total number of substances after the reaction.

Instruction

1. In order to equalize a chemical “example” you need to perform several steps. Write down the equation reactions in general. To do this, indicate unknown indicators in front of the formulas of substances with letters of the Latin alphabet (x, y, z, t, etc.). Let the reaction of combining hydrogen and oxygen be equalized, resulting in water. Before the molecules of hydrogen, oxygen and water, put Latin letters (x, y, z) - indicators.

2. For each element, based on physical equilibrium, compose mathematical equations and obtain a system of equations. In the above example, for hydrogen on the left, take 2x, because it has the index “2”, on the right – 2z, tea, it also has the index “2”. It turns out 2x=2z, hence x=z. For oxygen on the left take 2y, because there is an index “2”, on the right – z, there is no index, which means it is equal to one, which is usually not written. It turns out that 2y=z, and z=0.5y.

Note!
If a larger number of chemical elements are involved in the equation, then the task does not become more complicated, but increases in volume, which should not be alarmed.

Helpful advice
It is also possible to equalize reactions using probability theory, using the valences of chemical elements.

Tip 4: How to write a redox reaction

Redox reactions are reactions involving changes in oxidation states. It often happens that initial substances are given and it is necessary to write the products of their interaction. Occasionally, the same substance can produce different final products in different environments.

Instruction

1. Depending not only on the reaction environment, but also on the degree of oxidation, the substance behaves differently. A substance in its highest oxidation state is invariably an oxidizing agent, and in its lowest state it is a reducing agent. To create an acidic environment, sulfuric acid (H2SO4) is traditionally used, and less commonly, nitric acid (HNO3) and hydrochloric acid (HCl). If necessary, create an alkaline environment using sodium hydroxide (NaOH) and potassium hydroxide (KOH). Next, let's look at some examples of substances.

2. MnO4(-1) ion. In an acidic environment it turns into Mn(+2), a colorless solution. If the medium is neutral, then MnO2 is formed and a brown precipitate forms. In an alkaline medium we obtain MnO4(+2), a green solution.

3. Hydrogen peroxide (H2O2). If it is an oxidizing agent, i.e. accepts electrons, then in neutral and alkaline media it is converted according to the scheme: H2O2 + 2e = 2OH(-1). In an acidic environment we get: H2O2 + 2H(+1) + 2e = 2H2O. Provided that hydrogen peroxide is a reducing agent, i.e. gives up electrons, O2 is formed in an acidic environment, and O2 + H2O in an alkaline environment. If H2O2 enters an environment with a strong oxidizing agent, it itself will be a reducing agent.

4. The Cr2O7 ion is an oxidizing agent; in an acidic environment it turns into 2Cr(+3), which are green. From the Cr(+3) ion in the presence of hydroxide ions, i.e. in an alkaline environment, yellow CrO4(-2) is formed.

5. Let's give an example of composing a reaction. KI + KMnO4 + H2SO4 - In this reaction, Mn is in its highest oxidation state, that is, it is an oxidizing agent, accepting electrons. The environment is acidic, as sulfuric acid (H2SO4) shows us. The reducing agent here is I(-1), it donates electrons, thereby increasing its oxidation state. We write down the reaction products: KI + KMnO4 + H2SO4 – MnSO4 + I2 + K2SO4 + H2O. We arrange the indicators using the electronic equilibrium method or the half-reaction method, we get: 10KI + 2KMnO4 + 8H2SO4 = 2MnSO4 + 5I2 + 6K2SO4 + 8H2O.

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Note!
Don't forget to place indicators in reactions!

Chemical reactions are the interaction of substances, accompanied by a change in their composition. In other words, the substances that enter into the reaction do not correspond to the substances resulting from the reaction. A person encounters similar interactions every hour, every minute. Tea, the processes occurring in his body (respiration, protein synthesis, digestion, etc.) are also chemical reactions.

Instruction

1. Any chemical reaction must be written down correctly. One of the main requirements is that the number of atoms of the entire element of the substances located on the left side of the reaction (they are called “initial substances”) corresponds to the number of atoms of the same element in the substances on the right side (they are called “reaction products”). In other words, the recording of the reaction must be equalized.

2. Let's look at a specific example. What happens when you turn on a gas burner in the kitchen? Natural gas reacts with oxygen in the air. This oxidation reaction is so exothermic, that is, accompanied by the release of heat, that a flame appears. With the support of which you either cook food or reheat already cooked food.

3. To make it easier, assume that natural gas consists of only one component - methane, which has the formula CH4. Because how to compose and equalize this reaction?

4. When carbon-containing fuel is burned, that is, when carbon is oxidized with oxygen, carbon dioxide is formed. You know its formula: CO2. What is formed when the hydrogen contained in methane is oxidized with oxygen? Of course, water in the form of steam. Even the most distant person from chemistry knows its formula by heart: H2O.

5. It turns out that on the left side of the reaction, write down the initial substances: CH4 + O2. On the right side, accordingly, there will be the reaction products: CO2 + H2O.

6. The advance notation for this chemical reaction is: CH4 + O2 = CO2 + H2O.

7. Equalize the above reaction, that is, achieve the fulfillment of the basic rule: the number of atoms of the entire element in the left and right sides of the chemical reaction must be identical.

8. You see that the number of carbon atoms is the same, but the number of oxygen and hydrogen atoms is different. There are 4 hydrogen atoms on the left side, and only 2 on the right side. Therefore, put the indicator 2 in front of the water formula. Get: CH4 + O2 = CO2 + 2H2O.

9. The carbon and hydrogen atoms are equalized, now it remains to do the same with oxygen. On the left side there are 2 oxygen atoms, and on the right - 4. By placing the indicator 2 in front of the oxygen molecule, you get the final record of the methane oxidation reaction: CH4 + 2O2 = CO2 + 2H2O.

A reaction equation is a conventional notation of a chemical process in which some substances are converted into others with a change in properties. To record chemical reactions, formulas of substances and skills about the chemical properties of compounds are used.

Instruction

1. Write the formulas correctly according to their names. Let's say, aluminum oxide Al?O?, put index 3 from aluminum (corresponding to its oxidation state in this compound) near oxygen, and index 2 (oxidation state of oxygen) near aluminum. If the oxidation state is +1 or -1, then the index is not given. For example, you need to write down the formula for ammonium nitrate. Nitrate is an acidic residue of nitric acid (-NO?, d.o. -1), ammonium (-NH?, d.o. +1). So the formula for ammonium nitrate is NH? NO?. Occasionally, the oxidation state is indicated in the name of the compound. Sulfur oxide (VI) – SO?, silicon oxide (II) SiO. Some primitive substances (gases) are written with index 2: Cl?, J?, F?, O?, H? etc.

2. You need to know what substances react. Visible signs of the reaction: gas evolution, color metamorphosis and precipitation. Very often reactions pass without visible changes. Example 1: neutralization reaction H?SO? + 2 NaOH ? Na?SO? + 2 H?O Sodium hydroxide reacts with sulfuric acid to form the soluble salt sodium sulfate and water. The sodium ion is split off and combines with the acidic residue, replacing the hydrogen. The reaction takes place without external signs. Example 2: iodoform test C?H?OH + 4 J? + 6 NaOH?CHJ?? + 5 NaJ + HCOONa + 5 H?OThe reaction occurs in several stages. The final result is the precipitation of yellow iodoform crystals (a good reaction to alcohols). Example 3: Zn + K?SO? ? The reaction is unthinkable, because In the series of metal stresses, zinc ranks later than potassium and cannot displace it from compounds.

3. The law of conservation of mass states: the mass of substances that react is equal to the mass of the substances formed. A competent recording of a chemical reaction is half the success. We need to set the indicators. Start equalizing with those compounds whose formulas contain large indices. K?Cr?O? + 14 HCl ? 2 CrCl? + 2 KCl + 3 Cl?? + 7 H?O Start arranging indicators with potassium dichromate, because its formula contains the largest index (7). Such accuracy in recording reactions is needed to calculate mass, volume, concentration, energy released and other quantities. Be careful. Remember the most common formulas of acids and bases, as well as acid residues.

Tip 7: How to Determine Redox Equations

A chemical reaction is a process of transformation of substances that occurs with a change in their composition. Those substances that enter into the reaction are called initial, and those that are formed as a result of this process are called products. It happens that during a chemical reaction, the elements that make up the initial substances change their oxidation state. That is, they can accept someone else's electrons and give away their own. In both cases, their charge changes. Such reactions are called redox reactions.

Instruction

1. Write down the exact equation for the chemical reaction you are considering. Look at what elements are included in the initial substances and what are the oxidation states of these elements. Later, compare these indicators with the oxidation states of the same elements on the right side of the reaction.

2. If the oxidation state has changed, the reaction is redox. If the oxidation states of all elements remain the same - no.

3. Here, let's say, is the widely known high-quality reaction for identifying the sulfate ion SO4 ^2-. Its essence is that barium sulfate, which has the formula BaSO4, is virtually insoluble in water. When formed, it instantly falls out in the form of a dense, heavy white precipitate. Write down some equation for a similar reaction, say, BaCl2 + Na2SO4 = BaSO4 + 2NaCl.

4. It turns out that from the reaction you see that in addition to the precipitate of barium sulfate, sodium chloride was formed. Is this reaction a redox reaction? No, it is not, because not a single element included in the initial substances has changed its oxidation state. On both the left and right sides of the chemical equation, barium has an oxidation state of +2, chlorine -1, sodium +1, sulfur +6, oxygen -2.

5. But the reaction is Zn + 2HCl = ZnCl2 + H2. Is it redox? Elements of the initial substances: zinc (Zn), hydrogen (H) and chlorine (Cl). See what their oxidation states are? For zinc it is 0, as in any simple substance, for hydrogen it is +1, for chlorine it is -1. What are the oxidation states of these same elements on the right side of the reaction? For chlorine it remained unshakable, that is, equal to -1. But for zinc it became equal to +2, and for hydrogen – 0 (due to the fact that hydrogen was released in the form of a simple substance - a gas). Consequently, this reaction is redox.

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The canonical equation of an ellipse is compiled from the considerations that the sum of the distances from any point of the ellipse to its two foci is invariably continuous. By fixing this value and moving the point along the ellipse, you can determine the equation of the ellipse.

You will need

• A sheet of paper, a ballpoint pen.

Instruction

1. Define two fixed points F1 and F2 on the plane. Let the distance between the points be equal to some fixed value F1F2 = 2s.

2. Draw a straight line on a piece of paper, which is the coordinate line of the abscissa axis, and depict points F2 and F1. These points represent the foci of the ellipse. The distance from the entire focal point to the origin must be the same value, equal to c.

3. Draw the y-axis, thus forming a Cartesian coordinate system, and write the basic equation defining the ellipse: F1M + F2M = 2a. Point M denotes the current point of the ellipse.

4. Determine the size of the segments F1M and F2M using the Pythagorean theorem. Keep in mind that point M has current coordinates (x,y) relative to the origin, and relative to, say, point F1, point M has coordinates (x+c, y), that is, the “x” coordinate acquires a shift. Thus, in the expression of the Pythagorean theorem, one of the terms must be equal to the square of the value (x+c) or the value (x-c).

5. Substitute the expressions for the moduli of the vectors F1M and F2M into the basic ellipse relation and square both sides of the equation, moving one of the square roots to the right side of the equation in advance and opening the parentheses. After reducing identical terms, divide the resulting ratio by 4a and again raise to the second power.

6. Give similar terms and collect terms with the same factor of the square of the “x” variable. Bring out the square of the “X” variable.

7. Let the square of some quantity (say b) be the difference between the squares of a and c and divide the resulting expression by the square of this new quantity. Thus, you have obtained the canonical equation of the ellipse, on the left side of which is the sum of the squares of the coordinates divided by the axes, and on the left side is unity.

Helpful advice
In order to check the completion of the task, you can use the law of conservation of mass.

Chemical reaction diagram.

There are several ways to record chemical reactions. You became familiar with the “verbal” reaction scheme in § 13.

Here's another example:

sulfur + oxygen -> sulfur dioxide.

Lomonosov and Lavoisier discovered the law of conservation of mass of substances during a chemical reaction. It is formulated like this:

Let's explain why masses ash and calcined copper are different from the masses of paper and copper before it is heated.

Oxygen contained in the air takes part in the combustion process of paper (Fig. 48, a).

Therefore, two substances react. In addition to ash, carbon dioxide and water (in the form of steam) are formed, which enter the air and dissipate.

Rice. 48. Reactions of paper (a) and copper (b) with oxygen

Antoine-Laurent Lavoisier (1743-1794)

Outstanding French chemist, one of the founders of scientific chemistry. Academician of the Paris Academy of Sciences. He introduced quantitative (precise) research methods into chemistry. He experimentally determined the composition of air and proved that combustion is a reaction of a substance with oxygen, and water is a combination of Hydrogen with Oxygen (1774-1777).

Compiled the first table of simple substances (1789), essentially proposing a classification of chemical elements. Independently of M.V. Lomonosov, he discovered the law of conservation of mass of substances in chemical reactions.

Rice. 49. An experiment confirming the Lomonosov-Lavoisier law: a - the beginning of the experiment; b - end of experiment

Their mass exceeds the mass of oxygen. Therefore, the mass of ash is less than the mass of paper.

When copper is heated, air oxygen “combines” with it (Fig. 48, b). The metal turns into a black substance (its formula is CuO, and its name is cuprum(P) oxide). Obviously, the mass of the reaction product must exceed the mass of copper.

Comment on the experiment shown in Figure 49 and draw a conclusion.

Law as a form of scientific knowledge.

The discovery of laws in chemistry, physics, and other sciences occurs after scientists conduct many experiments and analyze the results obtained.

A law is a generalization of objective, human-independent connections between phenomena, properties, etc.

The law of conservation of mass of substances during a chemical reaction is the most important law of chemistry. It applies to all transformations of substances that occur both in the laboratory and in nature.

Chemical laws make it possible to predict the properties of substances and the course of chemical reactions, and regulate processes in chemical technology.

In order to explain the law, hypotheses are put forward, which are tested using appropriate experiments. If one of the hypotheses is confirmed, a theory is created based on it. In high school, you will become familiar with several theories that chemists have developed.

The total mass of substances during a chemical reaction does not change because atoms of chemical elements do not appear or disappear during the reaction, but only their rearrangement occurs. In other words,
the number of atoms of each element before the reaction is equal to the number of its atoms after the reaction. This is indicated by the reaction schemes given at the beginning of the paragraph. Let us replace the arrows between the left and right parts with equal signs:

Such records are called chemical equations.

A chemical equation is a recording of a chemical reaction using the formulas of reactants and products, which is consistent with the law of conservation of mass of substances.

There are many reaction schemes that do not correspond to the Lomonosov-Lavoisier law.

For example, the reaction scheme for the formation of water:

H 2 + O 2 -> H 2 O.

Both parts of the diagram contain the same number of Hydrogen atoms, but a different number of Oxygen atoms.

Let's turn this diagram into a chemical equation.

In order for there to be 2 Oxygen atoms on the right side, we put a coefficient of 2 in front of the water formula:

H 2 + O 2 -> H 2 O.

Now there are four Hydrogen atoms on the right. So that the same number of Hydrogen atoms is on the left side, we write the coefficient 2 in front of the hydrogen formula. We obtain the chemical equation:

2H 2 + O 2 = 2H 2 0.

Thus, to turn a reaction scheme into a chemical equation, you need to select coefficients for each substance (if necessary), write them in front of the chemical formulas and replace the arrow with an equal sign.

Perhaps some of you will make the following equation: 4H 2 + 20 2 = 4H 2 0. In it, the left and right sides contain the same numbers of atoms of each element, but all coefficients can be reduced by dividing by 2. This is what should be done.

This is interesting

A chemical equation has much in common with a mathematical one.

Below are different ways to write the reaction discussed.

Convert the reaction diagram Cu + O 2 -> CuO into a chemical equation.

Let's do a more difficult task: turn the reaction scheme into a chemical equation

On the left side of the diagram is Aluminum atom I, and on the right side is Aluminum atom 2. Let us put a coefficient of 2 in front of the metal formula:

There are three times more Sulfur atoms on the right than on the left. Let us write coefficient 3 on the left side before the formula of the Sulfur compound:

Now on the left side the number of Hydrogen atoms is 3 2 = 6, and on the right - only 2. In order for there to be 6 of them on the right, we put the coefficient 3 (6: 2 = 3) in front of the hydrogen formula:

Let's compare the number of Oxygen atoms in both parts of the diagram. They are the same: 3 4 = 4 * 3. Replace the arrow with an equal sign:

conclusions

Chemical reactions are written using reaction diagrams and chemical equations.

The reaction scheme contains the formulas of the reactants and products, and the chemical equation also contains coefficients.

The chemical equation is consistent with the Lomonosov-Lavoisier law of conservation of mass of substances:

the mass of substances that entered into a chemical reaction is equal to the mass of substances formed as a result of the reaction.

Atoms of chemical elements do not appear or disappear during reactions, but only their rearrangement occurs.

?
105. How does a chemical equation differ from a reaction scheme?

106. Place the missing coefficients in the reaction records:

107. Convert the following reaction schemes into chemical equations:

108. Make up formulas for reaction products and corresponding chemical equations:

109. Instead of dots, write down the formulas of simple substances and make up chemical equations:

Consider that boron and carbon are made up of atoms; fluorine, chlorine, hydrogen and oxygen are from diatomic molecules, and phosphorus (white) is from tetraatomic molecules.

110. Comment on the reaction schemes and turn them into chemical equations:

111. What mass of quicklime was formed during long-term calcination of 25 g of chalk, if it is known that 11 g of carbon dioxide was released?

Popel P. P., Kryklya L. S., Chemistry: Pidruch. for 7 cells. zahalnosvit. navch. zakl. - K.: VC "Academy", 2008. - 136 p.: ill.

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