How to write a reaction equation. Chemical reaction scheme

For solutions chemical equations, it is enough to remember the basics of mathematics, what is on the left is equal to what is on the right. For example, 2 + 1 = 3.

Only chemical signs and the valence of the elements is taken into account.

Н + Сl = HCl - conditionally before hydrogen 1, before chlorine 1 and as a result we have 1 hydrogen and 1 chlorine.

NaOH + H2SO4 = Na2SO4 + H2O. Sodium on the right side is 1, and on the left side is 2, which means we set the coefficient 2.

2NaOH + H2SO4 = Na2SO4 + H2O. Hydrogen on the left side is 4, and on the right side is 2, we set the coefficient 2.

2NaOH + H2SO4 = Na2SO4 + 2H2O. Sulfur on the right side is 1 and on the left 1 molecule. Oxygen on the left side is 8 and on the right side 8. The left and right sides are equal, so the equation is solved. The rest are solved by analogy.

In order to learn how to equalize chemical equations, you first need to highlight the main points and use the correct algorithm.

Key points

It is not difficult to build the logic of the process. To do this, we highlight the following stages:

1. Determination of the type of reagents (all reagents are organic, all reagents are inorganic, organic and inorganic reagents in one reaction)
2. Type definition chemical reaction(reaction with a change in the oxidation states of the components or not)
3. Isolation of a test atom or group of atoms

Examples of

1. All components are inorganic, without changing the oxidation state, the test atom will be oxygen - O (it was not affected by any interactions:

NaOH + HCl = NaCl + H2O

Let's count the number of atoms of each element on the right and left sides and make sure that there is no need to arrange the coefficients (by default, the absence of a coefficient is a coefficient equal to 1)

NaOH + H2SO4 = Na 2 SO4 + H2O

In this case, on the right side of the equation we see 2 sodium atoms, which means on the left side of the equation we need to substitute a factor of 2 in front of the compound containing sodium:

2 NaOH + H2SO4 = Na 2 SO4 + H2O

We check for oxygen - O: on the left side 2O from NaOH and 4 from sulfate ion SO4, and in the right side 4 from SO4 and 1 in water. Add 2 in front of the water:

2 NaOH + H2SO4 = Na 2 SO4 + 2 H2O

1. All components are organic, without changing the oxidation state:

HOOC-COOH + CH3OH = CH3OOC-COOCH3 + H2O (the reaction is possible under certain conditions)

In this case, we see that on the right side there are 2 groups of CH3 atoms, and on the left there is only one. Add to left side coefficient 2 in front of CH3OH, check for oxygen and add 2 in front of water

HOOC-COOH + 2CH3OH = CH3OOC-COOCH3 + 2H2O

1. Organic and inorganic components without changing oxidation states:

CH3NH2 + H2SO4 = (CH3NH2) 2 ∙ SO4

In this reaction, a check atom is optional. On the left side there is 1 molecule of methylamine CH3NH2, and on the right side 2. So you need a factor of 2 in front of methylamine.

2CH3NH2 + H2SO4 = (CH3NH2) 2 ∙ SO4

1. Organic component, inorganic, change in oxidation state.

СuO + C2H5OH = Cu + CH3COOH + Н2O

In this case, it is necessary to draw up an electronic balance, and it is better to convert the formulas of organic substances into gross. Oxygen will be the checking atom - from its quantity it is clear that the coefficients are not required, the electronic balance confirms

CuO + C2H6O = Cu + C2H4O2

2C +2 - 2e = 2C0

C3H8 + O2 = CO2 + H2O

Here O cannot be a test, since it itself changes the oxidation state. We check according to N.

O2 0 + 2 * 2 e = 2O-2 (we are talking about oxygen from CO2)

3C (-8/3) - 20e = 3C +4 (conditional fractional oxidation states are used in organic redox reactions)

It can be seen from the electronic balance that 5 times more oxygen is required for the oxidation of carbon. We put 5 in front of O2, also from the electronic balance, m should put 3 in front of C from CO2, check for H, and put 4 in front of water

C3H8 + 5O2 = 3CO2 + 4H2O

1. Inorganic compounds, changes in oxidation states.

Na2SO3 + KMnO4 + H2SO4 = Na2SO4 + K2SO4 + H2O + MnO2

Hydrogen in water and acid residues SO4 2- from sulfuric acid will be the test ones.

S + 4 (from SO3 2-) - 2e = S +6 (from Na2SO4)

Mn + 7 + 3e = Mn + 4

Thus, you need to put 3 in front of Na2SO3 and Na2SO4, 2 in front of KMnO4 and MNO2.

3Na2SO3 + 2KMnO4 + H2SO4 = 3Na2SO4 + K2SO4 + H2O + 2MnO2

A record of a chemical interaction that reflects quantitative and qualitative information about a reaction is called the equation of chemical reactions. The reaction is recorded 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 connections are formed from the initial connections.

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

On the left side, initial substances are written that interact with each other, i.e. are summed up. When one substance decomposes, its formula is written down. In the right part, the substances obtained in the course of a chemical reaction are recorded. 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.

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

Designations

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

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

Rice. 1. Precipitation of sediment.

Instead of an arrow (→), an equal sign (=) can be put, showing the observance of 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 first put. After calculating the coefficients and the equations of the right and left sides, a line is drawn under the arrow.

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

Rice. 2. Examples of reaction conditions.

What are the equations

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

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 concentration of substances, are equal. This state is characterized by a constant of chemical equilibrium.

Under 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 energy and equilibrium constants. These equations show the direction of the reaction.

Rice. 3. Equations of isobars, isochores and isotherms.

What have we learned?

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

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A chemical equation can be called the visualization of a chemical reaction using the signs of mathematics and chemical formulas... Such an action is a reflection of any reaction in the process of which new substances appear.

Chemical tasks: types

A chemical equation is a sequence of chemical reactions. They are based on the law of conservation of mass of any substance. There are only two types of reactions:

• Compounds - these include (there is a replacement of atoms complex elements atoms of simple reagents), exchange (replacement by constituent parts of two complex substances), neutralization (reaction of acids with bases, formation of salt and water).
• Decomposition - the formation of two or more complex or simple substances from one complex, but their composition is simpler.

Chemical reactions can also be divided into types: exothermic (occurring with the release of heat) and endothermic (absorption of heat).

This question worries many students. Here are some simple tips on how to learn how to solve chemical equations:

• Desire to understand and master. You cannot deviate from your goal.
• Theoretical knowledge. Without them, it is impossible to compose even an elementary compound formula.
• Correctness of recording chemical task- even the slightest mistake in the condition will nullify all your efforts in solving it.

It is advisable that the process of solving chemical equations itself was fun for you. Then the chemical equations (how to solve them and what points you need to remember, we will analyze in this article) will cease to be problematic for you.

Tasks that are solved using the equations of chemical reactions

These tasks include:

• Finding the mass of a component based on a given mass of another reagent.
• Tasks on the combination "mass-mole".
• Calculations for the "volume-mol" combination.
• Examples using the term "excess".
• Calculations using reagents, one of which is not devoid of impurities.
• Tasks for the decay of the result of the reaction and for production losses.
• Formula search tasks.
• Tasks in which reagents are provided as solutions.
• Problems containing mixes.

Each of these types of tasks includes several subtypes, which are usually considered in detail even at the first school lessons chemistry.

Chemical equations: how to solve

There is an algorithm that helps to cope with almost any task from this difficult science. To understand how to correctly solve chemical equations, you need to adhere to a certain pattern:

• When writing down the reaction equation, do not forget to place the coefficients.
• Determining the way in which unknown data can be found.
• Correctness of application in the selected formula of proportions or the use of the concept of "amount of substance".
• Pay attention to units of measurement.

At the end, it is important to check the problem. In the process of solving, you could make an elementary mistake that influenced the result of the solution.

Basic rules for drawing up chemical equations

If you adhere to the correct sequence, then the question of what are chemical equations, how to solve them, will not worry you:

• Formulas of substances that react (reagents) are written on the left side of the equation.
• The formulas of substances that are formed as a result of the reaction are written already in the right side of the equation.

The composition of the reaction equation is based on the law of conservation of the mass of substances. Therefore, both sides of the equation must be equal, that is, with the same number of atoms. This can be achieved provided that the coefficients are correctly placed in front of the formulas of the substances.

Arrangement of coefficients in a chemical equation

The algorithm for placing the coefficients is as follows:

• Counting on the left and right side of the equation for the atoms of each element.
• Determination of the changing number of atoms in an element. You also need to find N.O.K.
• The coefficients are obtained by dividing N.O.K. to indexes. Be sure to put these numbers in front of the formulas.
• The next step is to recalculate the number of atoms. Sometimes it becomes necessary to repeat the action.

Equalization of parts of a chemical reaction occurs using coefficients. The indices are calculated through valence.

For the successful compilation and solution of chemical equations, it is necessary to take into account physical properties substances such as volume, density, mass. You also need to know the state of the reacting system (concentration, temperature, pressure), understand the units of measurement of these quantities.

To understand the question of what chemical equations are, how to solve them, it is necessary to use the basic laws and concepts of this science. To successfully calculate such problems, you must also remember or master the skills of mathematical operations, be able to perform actions with numbers. We hope this tips will make it easier for you to deal with chemical equations.

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

Theoretical features

The compilation of chemical equations is an important and crucial stage, considered in the eighth grade general education schools... What should precede this stage? Before the teacher tells his pupils about how to make a chemical equation, it is important to acquaint schoolchildren with the term "valence", to teach them to determine this value for metals and non-metals, using the periodic table of elements.

Drawing up binary formulas for valence

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

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

Since sodium is a metal that is located in the main subgroup of group 1, its valency is unchanged, it is I.

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

We find the least common multiple between certain valencies, then divide it by the valence of each of the elements, and obtain their indices. We write the finished formula Na 2 O.

Equation Instructions

Now let's talk in more detail about how to make a chemical equation. First, we will consider the theoretical points, then move on to specific examples. So, drawing up chemical equations involves a certain course of action.

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

Combustion reaction example

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

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

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

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

An example of a substitution reaction

Any chemistry synopsis contains a description different types interactions.

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

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

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

Conclusion

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

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