Formulas of chemical reactions online. How to equalize a chemical equation: rules and algorithm

The main subject of comprehension in chemistry is the reactions between different chemical elements and substances. Great awareness of the validity of the interaction of substances and processes in chemical reactions makes it possible to manage them and apply them for their 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 connection, substitution, decomposition and exchange. Also among them it is allowed to distinguish redox, ionic, reversible and irreversible, exogenous, etc.

Instruction

1. Determine which substances interact with each other in your reaction. Write them down on the left side of the equation. For example, consider the chemical reaction between aluminum and sulfuric acid. Arrange the reagents on the left: Al + H2SO4 Next, put an "equal" sign, as in a mathematical equation. In chemistry, you can find an arrow pointing to the right, or two oppositely directed arrows, a “sign of reversibility.” As a result of the interaction of a metal with an acid, a salt and hydrogen are formed. Write the reaction products after the equal sign, on the right. Al + H2SO4 \u003d Al2 (SO4) 3 + H2 The reaction scheme is obtained.

2. In order to write a chemical equation, you need to find the exponents. On the left side of the previously obtained scheme, 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 composition of 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. In order to equalize the number of hydrogen elements, put the indicator 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, put a 2 in front of aluminum on the left side of the diagram. As a result, you will get the reaction equation for this scheme. 2Al + 3H2SO4 \u003d Al2 (SO4) 3 + 3H2

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

You will need

periodic system of chemical elements D.I. Mendeleev

Instruction

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

2. On the right side of the equation, the formula of the resulting one or more substances is written, 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. Later, writing 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 in the left and right parts of the equation remains identical.

4. In order to correctly arrange the indicators, you need to make out any of the substances that enter into the reaction. To do this, one of the elements is taken and the number of its atoms on the left and right is compared. If it is different, then it is necessary to find a multiple of the numbers denoting the number of atoms of a given substance in the left and right parts. After that, 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 any of its parts.

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

6. Later, after all the elements of the formula have been parsed, 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 the equations of chemical reactions, it is impossible to swap the left and right sides. Otherwise, a scheme of a completely different process will turn out.

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

How unsurprising nature is for a person: in winter it wraps the earth in a snowy duvet, in spring it reveals like popcorn flakes, all living things, 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 are standing Behind all these habitual 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 equalizing chemical equations is the knowledge of the law of conservation of the number of matter: 1) the number of matter before the reaction is equal to the number of matter 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 the chemical "example" you need to follow a few steps. Write down the equation reactions in general. For this, unknown indicators in front of the formulas of substances are denoted by the letters of the Latin alphabet (x, y, z, t, etc.). Let it be required to equalize the reaction of the combination of hydrogen and oxygen, as a result of which water will be obtained. Before the molecules of hydrogen, oxygen and water, put the Latin letters (x, y, z) - indicators.

2. For any element, on the basis of physical equilibrium, compose mathematical equations and obtain a system of equations. In this 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, otsel, x=z. For oxygen, take 2y on the left, because there is an index “2”, on the right - z, there is no index for tea, which means it is equal to one, which is usually not written. It turns out, 2y=z, and z=0.5y.

Note!
If a greater 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 frightened.

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

Tip 4: How to compose a redox reaction

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

Instruction

1. Depending not only on the reaction medium, 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 oxidation state it is a reducing agent. In order to make an acidic environment, sulfuric acid (H2SO4) is traditionally used, less often nitric acid (HNO3) and hydrochloric acid (HCl). If necessary, create an alkaline environment, use sodium hydroxide (NaOH) and potassium hydroxide (KOH). Let's take a 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, 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 turns 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. donates electrons; in an acidic medium, O2 is formed; in an alkaline medium, O2 + H2O. If H2O2 enters an environment with a strong oxidizing agent, it will itself be a reducing agent.

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

5. Let's give an example of the composition of the 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, sulfuric acid (H2SO4) shows us this. The reducing agent here is I (-1), it donates electrons, while 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 add indicators to your reactions!

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

Instruction

1. Any chemical reaction must be written correctly. One of the main requirements is that the number of atoms of the entire element of substances 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 record of the reaction must be equalized.

2. Let's look at a specific example. What happens when a gas burner is lit 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 heat up already cooked food.

3. For simplicity, assume that natural gas consists of only one of its components - methane, which has the formula CH4. Because how to compose and equalize this reaction?

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

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

6. Advance recording of this chemical reaction will be further: CH4 + O2 = CO2 + H2O.

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

8. You can 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 \u003d CO2 + 2H2O.

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

A reaction equation is a conditional record 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?, index 3 from aluminum (corresponding to its oxidation state in this compound) put near oxygen, and index 2 (oxidation state of oxygen) near aluminum. If the oxidation state is +1 or -1, then the index is not set. For example, you need to write down the formula for ammonium nitrate. Nitrate is the acid residue of nitric acid (-NO?, s.o. -1), ammonium (-NH?, s.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 which substances are reacting. Visible signs of reaction: gas evolution, color metamorphosis and precipitation. Quite often the 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 a soluble salt of sodium sulfate and water. The sodium ion is split off and combined with the acid residue, replacing the hydrogen. The reaction proceeds without external signs. Example 2: iodoform test С?H?OH + 4 J? + 6 NaOH?CHJ?? + 5 NaJ + HCOONa + 5 H?O The reaction proceeds in several stages. The final result is the precipitation of yellow iodoform crystals (good reaction to alcohols). Example 3: Zn + K?SO? ? The reaction is unthinkable, because in a series of metal stresses, zinc is later than potassium and cannot displace it from compounds.

3. The law of conservation of mass states that the mass of the reactants is equal to the mass of the formed substances. A competent record of a chemical reaction is half the furore. You need to set up indicators. Start equalizing with those compounds in the formulas of which there are large indices. K?Cr?O? + 14 HCl? 2CrCl? + 2 KCl + 3 Cl?? + 7 H?O its formula contains the largest index (7). Such accuracy in recording reactions is needed to calculate mass, volume, concentration, released energy, and other quantities. Be careful. Remember especially 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 reincarnation 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 in the course of a chemical reaction, the elements that make up the initial substances change their oxidation state. That is, they can accept other people's electrons and give 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 composition of the initial substances, and what are the oxidation states of these elements. Later, compare these figures with the oxidation states of the same elements on the right side of the reaction.

2. If the oxidation state has changed, this reaction is redox. If the oxidation states of all the elements remained the same, then no.

3. Here, for example, is the widely known good quality reaction for the detection of the sulfate ion SO4 ^2-. Its essence is that barium sulfate, which has the formula BaSO4, is virtually insoluble in water. When formed, it immediately precipitates 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 that is part of the initial substances has changed its oxidation state. Both on the left and on the right side of the chemical equation, barium has an oxidation state of +2, chlorine -1, sodium +1, sulfur +6, oxygen -2.

5. And here is the reaction Zn + 2HCl = ZnCl2 + H2. Is it redox? Elements of 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. And what are the oxidation states of these same elements in the right side of the reaction? In chlorine, it remained unshakable, that is, equal to -1. But for zinc it became equal to +2, and for hydrogen - 0 (from the fact that hydrogen was released in the form of a simple substance - gas). Therefore, this reaction is a redox reaction.

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The canonical equation of an ellipse is compiled from those considerations that the sum of the distances from any point of the ellipse to 2 of its foci is invariably continuous. By fixing this value and moving the point along the ellipse, it is possible to determine the equation of the ellipse.

You will need

Sheet of paper, ballpoint pen.

Instruction

1. Specify 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 draw the points F2 and F1. These points are the foci of the ellipse. The distance from the entire focus point to the origin must be the same value, c.

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

4. Determine the value 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 regarding, 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 ratio of the ellipse 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 brackets. After reducing the 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. Take out the square of the "X" variable.

7. Take the square of some quantity (say b) to 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 an ellipse, on the left side of which is the sum of the squares of the coordinates divided by the magnitudes of the axes, and on the left side is one.

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

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

Chemical reactions in nature and everyday life

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

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

Types of chemical reactions

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

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

Chemical reaction of the compound

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

Decomposition chemical reaction

It's simple, the decomposition reaction is the opposite of the compound reaction. It produces two or more substances from one substance. A simple example of a chemical decomposition reaction would be the decomposition of chalk, which produces quicklime and carbon dioxide from the chalk itself.

Chemical substitution reaction

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

Fe + CuSO 4 → FeSO 4 + Cu

Chemical exchange reaction

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

NaOH + HCl → NaCl + H 2 O

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

Signs of chemical reactions

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

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

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

How to determine the sign of a chemical reaction

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

The rate of a chemical reaction

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

Factors in the rate of a chemical reaction include:

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

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

Equilibrium of a chemical reaction

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

Conditions for the occurrence of chemical reactions

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

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

Thermal effect of a chemical reaction

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

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

Chemical reactions, video

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

During chemical reactions, other substances are obtained from one substance (not to be confused with nuclear reactions, in which one chemical element is converted into another).

Any chemical reaction is described by a chemical equation:

Reagents → Reaction products

The arrow indicates the direction of the reaction.

For example:

In this reaction, methane (CH 4) reacts with oxygen (O 2), resulting in the formation of carbon dioxide (CO 2) and water (H 2 O), or rather, water vapor. This is exactly the reaction that happens in your kitchen when you light a gas burner. The equation should be read like this: one molecule of methane gas reacts with two molecules of oxygen gas, resulting in one molecule of carbon dioxide and two molecules of water (steam).

The numbers in front of the components of a chemical reaction are called reaction coefficients.

Chemical reactions are endothermic(with energy absorption) and exothermic(with energy release). The combustion of methane is a typical example of an exothermic reaction.

There are several types of chemical reactions. The most common:

compound reactions;
decomposition reactions;
single substitution reactions;
double substitution reactions;
oxidation reactions;
redox reactions.

Connection reactions

In a compound reaction, at least two elements form one product:

2Na (t) + Cl 2 (g) → 2NaCl (t)- the formation of salt.

Attention should be paid to an essential nuance of compound reactions: depending on the conditions of the reaction or the proportions of the reactants that enter into the reaction, different products can be its result. For example, under normal conditions of combustion of coal, carbon dioxide is obtained:
C (t) + O 2 (g) → CO 2 (g)

If there is not enough oxygen, then deadly carbon monoxide is formed:
2C (t) + O 2 (g) → 2CO (g)

Decomposition reactions

These reactions are, as it were, opposite in essence to the reactions of the compound. As a result of the decomposition reaction, the substance decomposes into two (3, 4...) simpler elements (compounds):

2H 2 O (g) → 2H 2 (g) + O 2 (g)- water decomposition
2H 2 O 2 (g) → 2H 2 (g) O + O 2 (g)- decomposition of hydrogen peroxide

Single substitution reactions

As a result of single substitution reactions, the more active element replaces the less active element in the compound:

Zn (t) + CuSO 4 (solution) → ZnSO 4 (solution) + Cu (t)

The zinc in the copper sulfate solution displaces the less active copper, resulting in a zinc sulfate solution.

The degree of activity of metals in ascending order of activity:

The most active are alkali and alkaline earth metals.

The ionic equation for the above reaction will be:

Zn (t) + Cu 2+ + SO 4 2- → Zn 2+ + SO 4 2- + Cu (t)

The ionic bond CuSO 4, when dissolved in water, decomposes into a copper cation (charge 2+) and an anion sulfate (charge 2-). As a result of the substitution reaction, a zinc cation is formed (which has the same charge as the copper cation: 2-). Note that the sulfate anion is present on both sides of the equation, i.e., by all the rules of mathematics, it can be reduced. The result is an ion-molecular equation:

Zn (t) + Cu 2+ → Zn 2+ + Cu (t)

Double substitution reactions

In double substitution reactions, two electrons are already replaced. Such reactions are also called exchange reactions. These reactions take place in solution to form:

insoluble solid (precipitation reaction);
water (neutralization reactions).

Precipitation reactions

When mixing a solution of silver nitrate (salt) with a solution of sodium chloride, silver chloride is formed:

Molecular equation: KCl (solution) + AgNO 3 (p-p) → AgCl (t) + KNO 3 (p-p)

Ionic equation: K + + Cl - + Ag + + NO 3 - → AgCl (t) + K + + NO 3 -

Molecular-ionic equation: Cl - + Ag + → AgCl (t)

If the compound is soluble, it will be in solution in ionic form. If the compound is insoluble, it will precipitate, forming a solid.

Neutralization reactions

These are reactions between acids and bases, as a result of which water molecules are formed.

For example, the reaction of mixing a solution of sulfuric acid and a solution of sodium hydroxide (lye):

Molecular equation: H 2 SO 4 (p-p) + 2NaOH (p-p) → Na 2 SO 4 (p-p) + 2H 2 O (l)

Ionic equation: 2H + + SO 4 2- + 2Na + + 2OH - → 2Na + + SO 4 2- + 2H 2 O (l)

Molecular-ionic equation: 2H + + 2OH - → 2H 2 O (g) or H + + OH - → H 2 O (g)

Oxidation reactions

These are reactions of interaction of substances with gaseous oxygen in the air, in which, as a rule, a large amount of energy is released in the form of heat and light. A typical oxidation reaction is combustion. At the very beginning of this page, the reaction of the interaction of methane with oxygen is given:

CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (g)

Methane refers to hydrocarbons (compounds of carbon and hydrogen). When a hydrocarbon reacts with oxygen, a lot of heat energy is released.

Redox reactions

These are reactions in which electrons are exchanged between the atoms of the reactants. The reactions discussed above are also redox reactions:

2Na + Cl 2 → 2NaCl - compound reaction
CH 4 + 2O 2 → CO 2 + 2H 2 O - oxidation reaction
Zn + CuSO 4 → ZnSO 4 + Cu - single substitution reaction

The most detailed redox reactions with a large number of examples of solving equations by the electron balance method and the half-reaction method are described in the section

Scheme of a chemical reaction.

There are several ways to write chemical reactions. You familiarized yourself 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 in 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.

In the process of burning paper, oxygen is involved, which is contained in the air (Fig. 48, a).

Therefore, two substances are involved in the reaction. 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)

An outstanding French chemist, one of the founders of scientific chemistry. Academician of the Paris Academy of Sciences. Introduced quantitative (exact) 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), actually proposing a classification of chemical elements. Independently of M. V. Lomonosov, he discovered the law of conservation of the mass of substances in chemical reactions.

Rice. 49. Experience confirming the law of Lomonosov - Lavoisier: a - the beginning of the experiment; b - the end of the 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 the name is cuprum (P) oxide). Obviously, the mass of the reaction product must exceed the mass of copper.

Comment on the experience 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.

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

The law of conservation of mass of substances in 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, to regulate processes in chemical technology.

In order to explain the law, hypotheses are put forward, which are tested with the help of appropriate experiments. If one of the hypotheses is confirmed, a theory is created on its basis. 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 the atoms of chemical elements do not appear and 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's replace the arrows between the left and right sides with equal signs:

Such records are called chemical equations.

A chemical equation is a record 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 scheme contain the same number of hydrogen atoms, but a different number of oxygen atoms.

Let's turn this scheme into a chemical equation.

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

H 2 + O 2 -> H 2 O.

Now there are four Hydrogen atoms on the right. In order for the same number of Hydrogen atoms to be on the left side, we write the coefficient 2 in front of the hydrogen formula. We get the chemical equation:

2H 2 + O 2 \u003d 2H 2 0.

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

Perhaps one of you will write this equation: 4H 2 + 20 2 \u003d 4H 2 0. In it, the left and right sides contain the same number of atoms of each element, but all coefficients can be reduced by dividing by 2. This should be done.

This is interesting

The chemical equation has much in common with the mathematical one.

Below are various ways of recording the considered reaction.

Turn the reaction scheme Cu + O 2 -> CuO into a chemical equation.

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

On the left side of the scheme - I atom of Aluminum, and on the right - 2. Put a coefficient 2 in front of the metal formula:

There are three times more Sulfur atoms on the right than on the left. We write the coefficient 3 in front of the formula of the Sulfur compound on the left side:

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

Let us compare the number of oxygen atoms in both parts of the scheme. They are the same: 3 4 = 4 * 3. Let's replace the arrow with an equal sign:

conclusions

Chemical reactions are written using reaction schemes and chemical equations.

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

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

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105. What is the difference between a chemical equation and a reaction scheme?

106. Arrange the missing coefficients in the reaction records:

107. Turn the following reaction schemes into chemical equations:

108. Make the formulas of the reaction products and the corresponding chemical equations:

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

Bear in mind that boron and carbon are made up of atoms; fluorine, chlorine, hydrogen and oxygen - from diatomic molecules, and phosphorus (white) - from four-atomic molecules.

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

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

Popel P. P., Kriklya L. S., Chemistry: Pdruch. for 7 cells. zahalnosvit. navch. zakl. - K .: Exhibition Center "Academy", 2008. - 136 p.: il.

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

Basic Rules

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

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

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

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

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

Notation

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

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

Rice. 1. Precipitation.

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

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

Rice. 2. Examples of reaction conditions.

What are the equations

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

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

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

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

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

What have we learned?

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

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