Write down the equation of the chemical reaction. How to arrange coefficients in chemical equations
Let's talk about how to write a chemical equation, because they are the main elements of this discipline. Thanks to a deep awareness of all the patterns of interactions and substances, you can control them, apply them in various fields of activity.
Theoretical Features
Compilation of chemical equations is an important and crucial stage, considered in the eighth grade of secondary schools. What should precede this stage? Before the teacher tells his pupils how to write a chemical equation, it is important to introduce schoolchildren to the term "valency", to teach them to determine this value for metals and non-metals using the periodic table of elements.
Compilation of binary formulas by valence
In order to understand how to write a chemical equation in terms of valence, you first need to learn how to formulate compounds consisting of two elements using valency. We propose an algorithm that will help to cope with the task. For example, you need to write a formula for sodium oxide.
First, it is important to consider that the chemical element that is mentioned last in the name should be in the first place in the formula. In our case, sodium will be written first in the formula, oxygen second. Recall that binary compounds are called oxides, in which the last (second) element must necessarily be oxygen with an oxidation state of -2 (valency 2). Further, according to the periodic table, it is necessary to determine the valencies of each of the two elements. To do this, we use certain rules.
Since sodium is a metal that is located in the main subgroup of group 1, its valence is a constant value, it is equal to I.
Oxygen is a non-metal, since it is the last 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 valence of oxygen is II.
Between certain valences, we find the least common multiple, then divide it by the valency of each of the elements, we get their indices. We write down the finished formula Na 2 O.
Instructions for compiling an equation
Now let's talk more about how to write a chemical equation. First, let's look at the theoretical points, then move on to specific examples. So, the compilation of chemical equations involves a certain procedure.
- 1st stage. After reading the proposed task, it is necessary to determine which chemicals should be present on the left side of the equation. A "+" sign is placed between the original components.
- 2nd stage. After the equal sign, it is necessary to draw up a formula for the reaction product. When performing such actions, an algorithm for compiling formulas for binary compounds, which we discussed above, will be required.
- 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 make a chemical equation for the combustion of magnesium using an algorithm. On the left side of the equation, we write through the sum of magnesium and oxygen. Do not forget that oxygen is a diatomic molecule, so it must have an index of 2. After the equal sign, we draw up a formula for the product obtained after the reaction. They will be in which magnesium is written first, and we put oxygen second in the formula. Further, according to the table of chemical elements, we determine the valencies. Magnesium, which is in group 2 (the main subgroup), has a constant valency II, for oxygen, by subtracting 8 - 6, we also obtain valence 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, you must add a factor of 2. Next, we count the number of magnesium atoms before and after the process. As a result of the interaction, 2 magnesium was obtained, therefore, on the left side, a coefficient of 2 is also required in front of a simple substance magnesium.
The final form of the reaction: 2Mg + O 2 \u003d 2MgO.
An example of a substitution reaction
Any abstract in chemistry contains a description of different types of interactions.
Unlike a compound, a substitution will have two substances on both the left and right sides of the equation. Suppose you need to write the interaction reaction between zinc and We use the standard writing algorithm. First, on the left side we write zinc and hydrochloric acid through the sum, on the right side we draw up the formulas of the resulting reaction products. Since in the electrochemical series of voltages of metals, zinc is located before hydrogen, in this process it displaces molecular hydrogen from the acid, forming zinc chloride. As a result, we get the following entry: Zn+HCL=ZnCl 2 +H 2 .
Now we turn 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, a factor of 2 must be put in front of the hydrochloric acid formula.
As a result, we obtain a ready-made reaction equation corresponding to the law of conservation of mass of substances: Zn + 2HCL = ZnCl 2 +H 2.
Conclusion
A typical chemistry abstract necessarily contains several chemical transformations. Not a single section of this science is limited to a simple verbal description of transformations, processes of dissolution, evaporation, everything is necessarily confirmed by equations. The specificity of chemistry lies in the fact that all the processes that occur between different inorganic or organic substances can be described using coefficients, indices.
How is chemistry different from other sciences? Chemical equations help not only to describe the ongoing transformations, but also to carry out quantitative calculations on them, thanks to which it is possible to carry out laboratory and industrial production of various substances.
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 kind of 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 do 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.
?
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.
Lesson content lesson summary and support frame lesson presentation interactive technologies accelerating teaching methods Practice quizzes, testing online tasks and exercises homework workshops and trainings questions for class discussions Illustrations video and audio materials photos, pictures graphics, tables, schemes comics, parables, sayings, crossword puzzles, anecdotes, jokes, quotes Add-ons abstracts cheat sheets chips for inquisitive articles (MAN) literature main and additional glossary of terms Improving textbooks and lessons correcting errors in the textbook replacing obsolete knowledge with new ones Only for teachers calendar plans training programs methodological recommendationschemical reactions These are chemical interactions of substances. Representation of reactions using chemical formulas and mathematical signs is called chemical equation.
In chemical reactions, new substances are formed from the atoms of the substances that have entered into the reaction, 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 .
Let's make an equation for the dissolution of aluminum hydroxide in excess sulfuric acid. Reaction scheme:
To compile the reaction equation in the reaction scheme, it is necessary to select the coefficients. The selection of coefficients usually begins with the formula of the substance containing the largest number of element atoms, regardless of where the substance is located - to the right or left of the equal sign. Equalize the number of aluminum atoms:
2 Al(OH) 3 + H 2 SO 4 → Al 2 (SO 4) 3 + H 2 O.
Equalize the number of sulfur atoms:
2 Al(OH) 3 + 3 H 2 SO 4 → Al 2 (SO 4) 3 + H 2 O.
Equalize the number of hydrogen atoms:
2 Al(OH) 3 + 3 H 2 SO 4 → Al 2 (SO 4) 3 + 6 H2O.
Let us calculate the number of oxygen atoms in the left and right parts of the reaction equation (we will check the correctness of the selection of the coefficients).
The reaction equation for the stages is written in order to show the consistency in the selection of coefficients. In practice, only one scheme is written, which, by selecting the coefficients, is converted into a reaction equation.
Classification of chemical reactions
Chemical reactions are classified according to the following criteria:
1. according to the change in the number and composition of the starting substances and reaction products, they are divided into the following types (or groups) of reactions:
− compound reactions;
− decomposition reactions;
− substitution reactions;
- exchange reactions.
2 . according to the reversibility of the reaction are divided into:
− irreversible reactions;
− reversible reactions.
3. according to the thermal effect, the reactions are divided into:
− exothermic reactions;
− endothermic reactions.
4. according to the change in the oxidation states of the atoms of the elements in the course of a chemical reaction, they are divided into:
− reactions without changing the oxidation states;
- reactions with a change in the degree of oxidation (or redox).
Consider these types of chemical reactions.
1. Classification on the basis of changes in the number and composition of the starting substances and reaction products.
Connection reactions- these are reactions in 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 in 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 the atoms of a simple substance replace the atoms of a complex substance (when compiling equations for reactions of this type need to remember about substitution rules 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 two substances exchange their ions, forming two new substances. Exchange reactions occur if, as a result of ion exchange, sparingly 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 equations for exchange reactions, weak electrolytes, sparingly soluble and gaseous substances are written in an undissociated form (in the form of molecules).
2. Classification based on reversibility
Chemical reactions on the basis of reversibility are divided into reversible and irreversible.
Reversible chemical reactions- these are chemical reactions that simultaneously proceed in two mutually opposite directions, in 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 reactants into final substances (the resulting products leave the reaction sphere - they precipitate in the form of a precipitate, are released in the form of a gas, poorly 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 by heat of reaction
According to the thermal effect (Q or ∆Н; ∆Н is the change in enthalpy (heat effect of the reaction)) chemical reactions are divided into exothermic and endothermic.
Exothermic chemical reactions (∆N< 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, ∆Н = + 18 kJ,
CaCO 3 → CaO + CO 3, ∆Н = + 1200 kJ.
Exothermic reactions are many compound reactions. Many decomposition reactions are endothermic reactions.
4. Classification on the basis of changes in the oxidation states of the atoms of the elements of the reacting substances.
Chemical reactions on the basis of changes in the oxidation states of atoms of elements in molecules during a chemical reaction are divided into two groups:
1. reactions that proceed without changing the oxidation states of the atoms of the elements, for example: .
2. reactions that occur with a change in the oxidation states of the atoms of elements (redox reactions), for example:
Connection reactions with the participation of 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 of 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 | |
| expansions | 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 is of great importance in chemistry. It helps to generalize, systematize knowledge about reactions and establish patterns of their course.
Each chemical reaction can be characterized by several features, for example: reaction, ∆Н = - 92 kJ
has the following characteristics:
this is the reaction of 1) compounds;
2) exothermic;
3) reversible;
4) redox.
Questions and tasks for self-control
1) How much 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 will they take 9.03 × 10 23 chlorine molecules under normal conditions?
4) How many molecules are contained 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) is contained in 53 g of it?
7) How many moles of sodium hydroxide(NaOH) is 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: compounds, 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) in the interaction of potassium bicarbonate with potassium hydroxide.
DEFINITION
Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.
Most often, chemical reactions are understood as the process of transformation of initial substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations containing the formulas of the starting materials and reaction products. According to the law of conservation of mass, the number of atoms of each element in the left and right sides of the chemical equation is the same. Usually, the formulas of the starting substances are written on the left side of the equation, and the formulas of the products are written on the right. The equality of the number of atoms of each element in the left and right parts of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.
Chemical equations may contain additional information about the features of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or "under") the equals sign.
All chemical reactions can be grouped into several classes, which have certain characteristics.
Classification of chemical reactions according to the number and composition of the initial and resulting substances
According to this classification, chemical reactions are divided into reactions of combination, decomposition, substitution, exchange.
As a result compound reactions from two or more (complex or simple) substances, one new substance is formed. In general, the equation for such a chemical reaction will look like this:
For example:
CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2
SO 3 + H 2 O \u003d H 2 SO 4
2Mg + O 2 \u003d 2MgO.
2FeCl 2 + Cl 2 = 2FeCl 3
Combination reactions are in most cases exothermic, i.e. flow with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox (ORD), i.e. occur with a change in the oxidation states of the elements. It is impossible to say unequivocally whether the reaction of a compound between complex substances can be attributed to OVR.
Reactions in which several other new substances (complex or simple) are formed from one complex substance are classified as decomposition reactions. In general, the equation for a chemical decomposition reaction will look like this:
For example:
CaCO 3 CaO + CO 2 (1)
2H 2 O \u003d 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O \u003d CuSO 4 + 5H 2 O (3)
Cu (OH) 2 \u003d CuO + H 2 O (4)
H 2 SiO 3 \u003d SiO 2 + H 2 O (5)
2SO 3 \u003d 2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O (7)
Most decomposition reactions proceed with heating (1,4,5). Decomposition by electric current is possible (2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) proceeds without changing the oxidation states of the elements, i.e. these reactions do not apply to OVR. OVR decomposition reactions include the decomposition of oxides, acids and salts formed by elements in higher oxidation states (6).
Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):
C 18 H 38 \u003d C 9 H 18 + C 9 H 20 (8)
C 4 H 10 \u003d C 4 H 6 + 2H 2 (9)
At substitution reactions a simple substance interacts with a complex one, forming a new simple and a new complex substance. In general, the equation for a chemical substitution reaction will look like this:
For example:
2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3 (1)
Zn + 2HCl = ZnCl 2 + H 2 (2)
2KBr + Cl 2 \u003d 2KCl + Br 2 (3)
2KSlO 3 + l 2 = 2KlO 3 + Cl 2 (4)
CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2 (5)
Ca 3 (RO 4) 2 + ZSiO 2 = ZCaSiO 3 + P 2 O 5 (6)
CH 4 + Cl 2 = CH 3 Cl + Hcl (7)
Substitution reactions are mostly redox reactions (1 - 4, 7). Examples of decomposition reactions in which there is no change in oxidation states are few (5, 6).
Exchange reactions called the reactions that occur between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions in aqueous solution. In general, the equation for a chemical exchange reaction will look like this:
AB + CD = AD + CB
For example:
CuO + 2HCl \u003d CuCl 2 + H 2 O (1)
NaOH + HCl \u003d NaCl + H 2 O (2)
NaHCO 3 + HCl \u003d NaCl + H 2 O + CO 2 (3)
AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)
CrCl 3 + ZNaOH = Cr(OH) 3 ↓+ ZNaCl (5)
Exchange reactions are not redox. A special case of these exchange reactions is neutralization reactions (reactions of interaction of acids with alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5) or a low-dissociating compound, most often water (1, 2).
Classification of chemical reactions according to changes in oxidation states
Depending on the change in the oxidation states of the elements that make up the reactants and reaction products, all chemical reactions are divided into redox (1, 2) and those occurring without changing the oxidation state (3, 4).
2Mg + CO 2 \u003d 2MgO + C (1)
Mg 0 - 2e \u003d Mg 2+ (reductant)
C 4+ + 4e \u003d C 0 (oxidizing agent)
FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)
Fe 2+ -e \u003d Fe 3+ (reductant)
N 5+ + 3e \u003d N 2+ (oxidizing agent)
AgNO 3 + HCl \u003d AgCl ↓ + HNO 3 (3)
Ca(OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)
Classification of chemical reactions by thermal effect
Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conditionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during a reaction is called the heat of the reaction. If the equation indicates the amount of released or absorbed heat, then such equations are called thermochemical.
N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)
2Mg + O 2 \u003d 2MgO + 602.5 kJ (2)
N 2 + O 2 \u003d 2NO - 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
According to the direction of the reaction, there are reversible (chemical processes whose products are able to react with each other under the same conditions in which they are obtained, with the formation of starting substances) and irreversible (chemical processes, the products of which are not able to react with each other with the formation of starting substances ).
For reversible reactions, the equation in general form is usually written as follows:
A + B ↔ AB
For example:
CH 3 COOH + C 2 H 5 OH ↔ H 3 COOS 2 H 5 + H 2 O
Examples of irreversible reactions are the following reactions:
2KSlO 3 → 2KSl + ZO 2
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
Evidence of the irreversibility of the reaction can serve as the reaction products of a gaseous substance, a precipitate or a low-dissociating compound, most often water.
Classification of chemical reactions by the presence of a catalyst
From this point of view, catalytic and non-catalytic reactions are distinguished.
A catalyst is a substance that speeds up a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:
2H 2 O 2 \u003d 2H 2 O + O 2 (MnO 2 catalyst)
Often, one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):
MeO + 2HF \u003d MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1
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