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

Basic Rules

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

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

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

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

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

Designations

To draw up equations for chemical reactions, you need to know certain notations that show how the reaction proceeds. The following symbols are used in chemical equations:

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

Rice. 1. Precipitation.

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

Reaction conditions (temperature, lighting) are indicated above the reaction sign (→,⇄). Catalyst formulas are also written at the top.

Rice. 2. Examples of reaction conditions.

What are the equations?

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

*Sign*	*Reactions*	*Description*	*Example*
By changing the amount of reagents and final substances	Substitutions	New simple and complex substances are formed from simple and complex substances	2Na +2H 2 O → 2NaOH + H 2
	Connections	Several substances form a new substance	C + O 2 = CO 2
	Decompositions	Several substances are formed from one substance	2Fe(OH) 3 → Fe 2 O 3 + 3H 2 O
	Ion exchange	Exchange of constituents (ions)	Na 2 CO 3 + H 2 SO 4 → Na 2 SO 4 + CO 2 + H 2 O
By heat release	Exothermic	Heat release	C + 2H 2 = CH 4 + Q
By heat release	Endothermic	Heat absorption	N 2 + O 2 → 2NO – Q
By type of energy impact	Electrochemical	Action of electric current
	Photochemical	Action of light
	Thermochemical	Effect of high temperature
By state of aggregation	Homogeneous	Same condition	CuCl 2 + Na 2 S → 2NaCl + CuS↓
By state of aggregation	Heterogeneous	Various condition	4H 2 O (l) + 3Fe (s) → Fe 3 O 4 + 4H 2

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

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

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

What have we learned?

In the 8th grade chemistry lesson, the topic of equations of chemical reactions was discussed. Compiling and writing equations reflects the progress of a chemical reaction. There are certain symbols showing the state of substances and the conditions under which the reaction occurs. There are several types of chemical reactions based on different characteristics: the amount of substance, state of aggregation, energy absorption, energy impact.

Test on the topic

Evaluation of the report

Average rating: 4.3. Total ratings received: 520.

Chemical reactions These are chemical interactions of substances. Depicting reactions using chemical formulas and mathematical symbols are called chemical equation.

During chemical reactions, new substances are formed from the atoms of the reacting substances, and the number of atoms of each element before the reaction is equal to the number of atoms of these elements after the reaction, i.e. on the left and right sides of the equation the number of atoms of all elements must be the same − law of conservation of mass of substances .

Let's create an equation for the reaction of dissolving aluminum hydroxide in excess sulfuric acid. Reaction scheme:

To compose the reaction equation in the reaction scheme, it is necessary to select coefficients. The selection of coefficients usually begins with the formula of the substance containing the largest number of atoms of the elements, regardless of where the substance is located - to the right or to the left of the equal sign. Let's equalize the number of aluminum atoms:

2 Al(OH) 3 + H 2 SO 4 → Al 2 (SO 4) 3 + H 2 O.

Let's equalize the number of sulfur atoms:

2 Al(OH)3+ 3 H 2 SO 4 → Al 2 (SO 4) 3 + H 2 O.

Let's equalize the number of hydrogen atoms:

2 Al(OH)3+ 3 H 2 SO 4 → Al 2 (SO 4) 3 + 6 H2O.

Let's count the number of oxygen atoms on the left and right sides of the reaction equation (check the correctness of the selection of coefficients).

The reaction equation by stages is written in order to show the sequence in the selection of coefficients. In practice, only one scheme is written, which is converted into a reaction equation by selecting coefficients.

Classification of chemical reactions

Chemical reactions are classified according to the following criteria:

1. based on changes in number and composition starting materials and reaction products are divided into the following types (or groups) of reactions:

− compound reactions;

− decomposition reactions;

− substitution reactions;

− exchange reactions.

2 . According to reversibility, reactions are divided into:

− irreversible reactions;

− reversible reactions.

3. According to the thermal effect, reactions are divided into:

− exothermic reactions;

− endothermic reactions.

4. according to the change in the oxidation states of atoms of elements during a chemical reaction, they are divided into:

− reactions without changing oxidation states;

− reactions with changes in oxidation states (or redox).

Let's look at these types of chemical reactions.

1. Classification based on changes in the number and composition of starting substances and reaction products.

Compound reactions– these are reactions as a result of which one new substance is formed from two or more substances, for example:

2H 2 +O 2 → 2H 2 O,

SO 3 + H 2 O → H 2 SO 4,

2Cu + O 2 2CuO,

CaO + H 2 O → Ca(OH) 2,

4NO 2 + O 2 + 2H 2 O → 4HNO 3.

Decomposition reactions– these are reactions as a result of which two or more new substances are formed from one complex substance, for example:

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

Zn(OH) 2 ZnO + H 2 O,

2KNO 3 → 2KNO 2 + O 2,

CaCO 3 CaO + CO 2,

2AgNO 3 2Ag + 2NO 2 + O 2,

4KClO 3 3KClO 4 + KCl.

Substitution reactions- these are reactions between simple and complex substances, as a result of which atoms of a simple substance replace atoms of a complex substance (when drawing up equations for reactions of this type need to remember about the rules of substitution and use Appendix B1), for example:

Fe + CuSO 4 → Cu + FeSO 4,

Zn + 2HCl → ZnCl 2 + H 2,

Cl 2 + 2KI → I 2 + 2KCl,

Ca + 2H 2 O → Ca(OH) 2 + H 2.

Exchange reactions- These are reactions between two complex substances, as a result of which the two substances exchange their ions, forming two new substances. Exchange reactions occur if, as a result of the exchange of ions, poorly soluble substances (precipitates), gaseous substances or soluble, slightly dissociating substances (weak electrolytes) are formed, for example:

BaCl 2 + Na 2 SO 4 → BaSO 4 ↓ + 2NaCl,

CaCO 3 + 2HCl → CaCl 2 + CO 2 + H 2 O,

HCl + NaOH → NaCl + H 2 O,

(neutralization reaction).

When writing ionic exchange reaction equations, weak electrolytes, sparingly soluble and gaseous substances are written in undissociated form (in the form of molecules).

2. Classification based on reversibility

Chemical reactions based on reversibility are divided into reversible and irreversible.

Reversible chemical reactions- these are chemical reactions that simultaneously occur in two mutually opposite directions, forward and reverse, for example: 2SO 2 + O 2 ↔ 2SO 3,

N 2 + 3H 2 ↔ 2NH 3,

H 2 + I 2 ↔ 2HI.

Irreversible chemical reactions- these are chemical reactions that proceed in one direction and end with the complete transformation of the initial reacting substances into final substances (the resulting products leave the reaction sphere - they precipitate, are released in the form of a gas, slightly dissociated compounds are formed, or the reaction is accompanied by a large release of energy), for example :

H 2 SO 4 + 2NaOH → Na 2 SO 4 + 2H 2 O,

AgNO 3 + NaBr → AgBr↓ + NaNO 3,

Cu + 4HNO 3 → Cu(NO 3) 2 + 2NO 2 +2H 2 O.

3. Classification according to the thermal effect of the reaction

According to the thermal effect (Q or ∆H; ∆H is the change in enthalpy (thermal effect of the reaction)), chemical reactions are divided into exothermic and endothermic.

Exothermic chemical reactions (∆H< 0) – these are chemical reactions that occur with the release of heat (energy), the heat content of the system decreases, for example: Fe + S → FeS, ∆Н = − 96 kJ,

C + O 2 → CO 2, ∆H = − 394 kJ.

Endothermic chemical reactions (∆H > 0)– these are chemical reactions that occur with the absorption of heat (energy), the heat content of the system increases, for example: 2Hg → 2Hg + O 2, ∆H = + 18 kJ,

CaCO 3 → CaO + CO 3, ∆H = + 1200 kJ.

Many compound reactions are exothermic. Many decomposition reactions are endothermic reactions.

4. Classification based on changes in the oxidation states of atoms of elements of reacting substances.

Chemical reactions, based on changes in the oxidation states of atoms of elements in molecules during a chemical reaction, are divided into two groups:

1. reactions that occur without changing the oxidation states of atoms of elements, for example: .

2. reactions that occur with a change in the oxidation states of atoms of elements (redox reactions), for example:

Compound reactions involving simple substances, as well as substitution reactions are redox reactions.

Decomposition reactions compounds of complex substances can occur both without changing the oxidation states of the elements, and with a change in the oxidation states of the atoms of the elements.

Exchange reactions always occur without changing oxidation states (Table 2).

Table 2 - Examples of reactions various types, occurring with and without changes in oxidation states

Reactions	No change in oxidation state	Redox
Connections	CaO + H 2 O → Ca(OH) 2 Na 2 O + SO 3 → Na 2 SO 4
Decompositions	t 0 (CuOH) 2 CO 3 2CuO +CO 2 +H 2 O t 0 Cu(OH) 2 CuO + H 2 O
Substitutions	No
Exchange	BaCl 2 + Na 2 SO 4 →BaSO 4 ↓ + 2NaCl CuO + 2HNO 3 → Cu(NO 3) 2 + H 2 O	No

The classification of chemical reactions has great importance in chemistry. It helps to generalize, systematize knowledge about reactions and establish patterns of their occurrence.

Each chemical reaction can be characterized by several characteristics, for example: reaction, ∆Н = − 92 kJ

has the following characteristics:

This is the reaction of 1) a compound;

2) exothermic;

3) reversible;

4) redox.

Questions and tasks for self-control

1) What volume will it take: a) 1 g of hydrogen; b) 32 g of oxygen; c) 14 g of nitrogen under normal conditions?

2) Calculate the mass in grams under normal conditions:

a) 1 liter of nitrogen; b) 8 l CO 2; c) 1 m 3 oxygen.

3) How much space will they take? 9.03 × 10 23 chlorine molecules under normal conditions?

4) How many molecules are there? in 16 g of oxygen?

5) How many moles of sulfuric acid(H 2 SO 4) is contained in 196 g of it?

6) How many moles of sodium carbonate(Na 2 CO 3) contained in 53 g of it?

7) How many moles of sodium hydroxide(NaOH) contained in 160 g of it?

8) Determine the oxidation state of chlorine in the following compounds:

NaClO, NaClO 2, NaClO 4, CaCl 2, Cl 2 O 7, KClO 3, HCl.

9) Determine the oxidation state of phosphorus in the following compounds:

H 3 PO 4, PH 3, KH 2 PO 4, K 2 HPO 4, HPO 3, H 4 P 2 O 7.

10) Determine the oxidation state of manganese in the following compounds:

MnO, Mn(OH) 4, KMnO 4, K 2 MnO 4, K 2 MnO 3.

11) What types of chemical reactions do you know? Give examples.

12) What reaction: connection, decomposition, substitution or exchange occurs during the formation of water:

a) as a result of the combustion of hydrogen in air;

b) as a result of the interaction of hydrogen with copper (II) oxide;

c) as a result of heating iron (III) hydroxide;

d) when potassium bicarbonate reacts with potassium hydroxide.

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.

Instructions

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

Reaction is called the reincarnation of some chemical substances to others. And the formula for writing them with the help of special symbols is the equation for this reaction. Exist different types chemical interactions, but the rule for writing their formulas is identical.

You will need

periodic table chemical elements DI. Mendeleev

Instructions

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.

Video on the topic

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 there 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.

Instructions

1. In order to equalize a chemical “example” you need to perform several steps. Write down the equation reactions to everything general view. 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 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.

Instructions

1. Depending not only on the reaction environment, but also on the degree of oxidation, the substance behaves differently. The substance in its highest degree oxidation is invariably an oxidizing agent; in a lower 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 alkaline environment 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 CrO4(-2) is formed yellow color.

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.

Video on the topic

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.

Instructions

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 take a look 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 are used. chemical properties connections.

Instructions

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.

Instructions

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 equal to 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.

Video on the topic

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.

Instructions

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 ordinate axis, thus forming Cartesian system coordinates, 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 magnitudes of the vectors F1M and F2M into the basic relation of the ellipse and square both sides of the equation, moving one of them in advance square roots to the right side of the equation and opening the brackets. 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. So you got canonical equation an 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.

Class: 8

Presentation for the lesson

Back forward

Attention! Slide previews are for informational purposes only and may not represent all the features of the presentation. If you are interested this work, please download the full version.

The purpose of the lesson: help students develop knowledge of a chemical equation as a conditional recording of a chemical reaction using chemical formulas.

Tasks:

Educational:

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

Educational:

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

Educational:

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

Lesson type: combined.

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

Lesson presentation (Appendix 3)

Lesson structure.

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

During the classes

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

Teacher. Hello guys. Sit down.
Please note: you have a printed notebook on your desk. (Appendix 2), in which you will work today, and a score sheet in which you will record your achievements, sign it.

Updating knowledge and skills.

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

Test on the topic “Physical and chemical phenomena. Law of conservation of mass of substances.”

1. How do chemical reactions differ from physical phenomena?

Changing shape state of aggregation substances.
Formation of new substances.
Change of location.

2. What are the signs of a chemical reaction?

Precipitate formation, color change, gas evolution.

Magnetization, evaporation, vibration.

Growth and development, movement, reproduction.

3. In accordance with what law are equations of chemical reactions drawn up?

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

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

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

5. A chemical equation is called:

Conventional notation of a chemical reaction.

Conventional notation of the composition of a substance.

Recording the conditions of a chemical problem.

Teacher. You've done the job. I suggest you check it out. Exchange notebooks and check each other. Attention to the screen. For each correct answer - 1 point. Enter the total number of points on the evaluation sheets.

Motivation and goal setting.

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

Learning new material.

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

(Reaction of sulfur and iron compound.) Appendix 3

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

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

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

Fe + S → FeS

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

Teacher. Now read it.
Students. An iron molecule interacts with a sulfur molecule to produce one molecule of iron (II) sulfide.
Teacher. In this reaction, we see that the amount of starting substances is equal to the amount of substances in the reaction product.
We must always remember that when composing reaction equations, not a single atom should be lost or unexpectedly appear. Therefore, sometimes, having written all the formulas in the reaction equation, you have to equalize the number of atoms in each part of the equation - set the coefficients. Let's see another experiment

(Combustion of aluminum in oxygen.) Appendix 4

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

Al + O 2 → Al +3 O -2

To write the oxide formula correctly, remember that

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

Al + O 2 → Al 2 O 3

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

Students. LCM = 6

Teacher. We put coefficients in front of the formulas for oxygen and aluminum oxide so that the number of oxygen atoms on the left and right is equal to 6.

Al + 3 O 2 → 2 Al 2 O 3

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

Al + 3O 2 → 2Al 2 O 3

Let us once again count all the atoms before and after the reaction. We bet equal.

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

Teacher. Let's look at another example

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

Fe(OH) 3 → Fe 2 O 3 + H 2 O

Teacher. Let's arrange the coefficients. One iron atom reacts and two iron atoms are formed. Therefore, before the formula of iron hydroxide (3) we put a coefficient of 2.

Fe(OH) 3 → Fe 2 O 3 + H 2 O

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

Students. NOC =6. 6/2 = 3. Therefore, we set the coefficient of 3 for the water formula

2Fe(OH) 3 → Fe 2 O 3 + 3 H 2 O

Teacher. We count oxygen.

Students. Left – 2x3 =6; on the right – 3+3 = 6

Students. The number of oxygen atoms that entered into the reaction is equal to the number of oxygen atoms formed during the reaction. You can bet equally.

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

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

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

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

Divide the NOC into indices to obtain coefficients. Place them before the formulas.

Recalculate the number of atoms and repeat the action if necessary.

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

Teacher. You've worked hard and you're probably tired. I suggest you relax, close your eyes and remember some pleasant moments in life. They are different for each of you. Now open your eyes and make circular movements with them, first clockwise, then counterclockwise. Now move your eyes intensively horizontally: right - left, and vertically: up - down.
Now let's activate mental activity and massage your earlobes.

Teacher. We continue to work.
In printed notebooks we will complete task 5. You will work in pairs. You need to place the coefficients in the equations of chemical reactions. You are given 10 minutes to complete the task.

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

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

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

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

Consolidation of knowledge.

Teacher. We have studied all the main issues. Now let's do a short test that will allow you to see how you have mastered the topic. You should only answer “yes” or “no”. You have 3 minutes to work.

Statements.

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

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

Lesson summary.

Teacher. You did a good job. Now calculate the total number of points scored for the lesson and give yourself a grade according to the rating that you see on the screen. Give me your evaluation sheets so that you can enter your grade into the journal.

Homework.

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

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

Final words from the teacher.

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

F I felt comfortable in the lesson, I received answers to all my questions.

F I achieved half the goal in the lesson.
F I was bored in class, I didn’t learn anything new.

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 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 figured out the structure various substances, you can begin to compile 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 how special case exchange reactions - 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. Such 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, currently in a number of school programs not used. But with its help it is easier to explain the principles of drawing up equations of chemical reactions. Valence is understood as number chemical bonds, which one or another 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 elements coincide with their group number Periodic table 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 general principle compiling chemical equations based on given valences for certain elements. More often this method 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 which correct writing should be 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 proposed 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. Chemical equilibrium shift 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.

blog.site, when copying material in full or in part, a link to the original source is required.