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, in chemistry, a chemical reaction 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 chemical reactions themselves without suspecting it (and maybe suspecting) cooks do when they cook food.
Of course, also in natural conditions many chemical reactions take place: 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 The chemical reaction of decomposition can be the reaction of decomposition of chalk, during which quicklime and carbon dioxide are formed from the actual chalk.
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 you lower a steel nail into a solution with blue vitriol, 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. Neutralization of acid by bile good example 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
This state is called chemical equilibrium. chemical system, at which several chemical reactions take place and the rates in each pair of forward and reverse reactions are equal to each other. 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.
Definition
chemical equation is a conditional record of a chemical reaction using chemical formulas and coefficients.
In order to correctly place the coefficients in a chemical equation, one should understand the difference between coefficients and indices.
Definition
Coefficient- shows the number of molecules and is represented by a large number in front of molecular formula substances. Index- shows the number of atoms of an element in one molecule of a substance, is depicted to the right below the symbol of the element.
To calculate the total number of atoms, you need to multiply the number of molecules by the number of atoms of the element in one molecule. For example, the record of three molecules of sulfuric acid (the gross formula) is shown on the right, and a variant of the structural record is shown below. So, one molecule of sulfuric acid consists of three of three elements and in total contains (2 + 1 + 4) \u003d 7 atoms: 2 hydrogen atoms, one sulfur atom and four oxygen atoms. In three molecules it will be three times more atoms, that is, 3*2=6 hydrogen atoms, 3*1=3 sulfur atoms and 3*4=12 oxygen atoms. This is clearly seen from structural formula below.
To understand the logic of equalizing chemical reactions, try to practice at home with self-made models of atoms and molecules: prepare balls of different colors (gray, red and black) from plasticine. Try to carry out the methane combustion reaction, the scheme of which is shown below.
When modeling, it will be obvious that the number of atoms (homemade plasticine balls) of each element (color) does not change during the reaction. That is, the number of carbon atoms before and after the transformation remains unchanged and is equal to one (one black ball). Two oxygen molecules on the left side of the equation consist of 4 atoms, on the right side of the equation two oxygen atoms are contained in carbon dioxide ($CO_2$) and two atoms are in two water molecules, that is, there are also 4 oxygen atoms on the right.
Law of acting masses
When compiling reaction equations, it is necessary to use the law of conservation of the mass of substances (the law of mass action or LMA), discovered by M.V. Lomonosov and A. Lavoisier.
Law of acting masses: the mass of the substances that entered into the reaction is equal to the mass of the substances resulting from it.
Since substances consist of atoms, when compiling chemical equations, we will use the rule: the number of atoms of each chemical element of the starting substances must be equal to the number of atoms in the reaction products. In a chemical reaction, the number of interacting atoms remains unchanged, only their rearrangement occurs with the destruction of the starting substances
Algorithm for compiling reaction equations.
Consider the algorithm for compiling chemical equations using the example of the interaction of simple substances: metals and non-metals with each other. Let phosphorus and oxygen interact (combustion reaction).
1. Write down the starting substances (reagents) side by side, put a "+" sign between them (here we will take into account the fact that oxygen is a diatomic molecule), and after them an arrow - as an equal sign.
$P+O_2 \rightarrow$
2. We write down the formula of the reaction product after the arrow:
$P+O_2\rightarrow P_2O_5$
3. It can be seen from the diagram that oxygen is 2 atoms on the left, 5 on the right, and in accordance with the law of conservation of mass of substances, the number of atoms of a given chemical element should be the same. To equalize their number, we find the least common multiple. For 2 and 5, this will be the number 10. Divide the least common multiple by the number of atoms in the formulas. 10:2=5, 10:5=2, these will be the coefficients that are placed respectively in front of oxygen $O_2$ and phosphorus oxide (V) $P_2O_5$.
$P+5O_2\rightarrow 2P_2O_5$
oxygen on the left and on the right became 10 (5 2=10, 2 5=10)
4. The coefficient refers to the entire formula and is placed in front of it. After it was placed on the right, there were 2 2 = 4 atoms of phosphorus. And on the left 1 (coefficient 1 is not set). So we put a coefficient 4 in front of phosphorus.
$4P + 5O_2\rightarrow 2P_2O_5$
This is the final entry. chemical equation.
It reads: four pe plus five o-two equals two pe-two o-five.
Let's analyze the algorithm for putting down the coefficients on another example:
$KNO_3 = KNO_2 + O_2$
When potassium nitrate decomposes, potassium nitrite and oxygen are formed.
There is one potassium atom on the left side of the equation, and one on the right side. The number of nitrogen atoms on the left and right is the same and equal to one. But the number of oxygen atoms is different: on the left - 3, on the right - 4. In such cases, you can resort to doubling, that is, put the coefficient \u003d 2 in front of potassium nitrate.
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 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.
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 - the most important law 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.
?
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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Why ionic equations are needed
Let me remind you that when many substances are dissolved in water (and not only in water!) A process of dissociation occurs - substances break up into ions. For example, HCl molecules in an aqueous medium dissociate into hydrogen cations (H + , more precisely, H 3 O +) and chlorine anions (Cl -). Sodium bromide (NaBr) is in an aqueous solution not in the form of molecules, but in the form of hydrated Na + and Br - ions (by the way, ions are also present in solid sodium bromide).
When writing the "ordinary" (molecular) equations, we do not take into account that not molecules enter into the reaction, but ions. For example, here is the equation for the reaction between hydrochloric acid and sodium hydroxide:
HCl + NaOH = NaCl + H 2 O. (1)
Of course, this diagram does not quite correctly describe the process. As we have already said, there are practically no HCl molecules in an aqueous solution, but there are H + and Cl - ions. The same is true for NaOH. It would be better to write the following:
H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O. (2)
That's what it is complete ionic equation. Instead of "virtual" molecules, we see particles that are actually present in the solution (cations and anions). We will not dwell on the question why we have written H 2 O in molecular form. This will be explained a little later. As you can see, there is nothing complicated: we have replaced the molecules with ions, which are formed during their dissociation.
However, even the complete ionic equation is not perfect. Indeed, take a closer look: both in the left and in the right parts of equation (2) there are identical particles - Na + cations and Cl - anions. These ions do not change during the reaction. Why then are they needed at all? Let's remove them and get short ionic equation:
H + + OH - = H 2 O. (3)
As you can see, it all comes down to the interaction of H + and OH - ions with the formation of water (neutralization reaction).
All complete and short ionic equations are written down. If we solved problem 31 at the exam in chemistry, we would get the maximum mark for it - 2 points.
So, once again about the terminology:
- HCl + NaOH = NaCl + H 2 O - molecular equation ("usual" equation, schematically reflecting the essence of the reaction);
- H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O - complete ionic equation (real particles in solution are visible);
- H + + OH - = H 2 O - a short ionic equation (we removed all the "garbage" - particles that do not participate in the process).
Algorithm for writing ionic equations
- We compose the molecular equation of the reaction.
- All particles that dissociate in solution to a noticeable degree are written as ions; substances that are not prone to dissociation, we leave "in the form of molecules."
- We remove from the two parts of the equation the so-called. observer ions, i.e., particles that do not participate in the process.
- We check the coefficients and get the final answer - a short ionic equation.
Example 1. Write a complete and short ionic equation describing the interaction of aqueous solutions of barium chloride and sodium sulfate.
Solution. We will act in accordance with the proposed algorithm. Let's set up the molecular equation first. Barium chloride and sodium sulfate are two salts. Let's look at the section of the reference book "Properties of inorganic compounds". We see that salts can interact with each other if a precipitate forms during the reaction. Let's check:
Exercise 2. Complete the equations for the following reactions:
- KOH + H 2 SO 4 \u003d
- H 3 PO 4 + Na 2 O \u003d
- Ba(OH) 2 + CO 2 =
- NaOH + CuBr 2 =
- K 2 S + Hg (NO 3) 2 \u003d
- Zn + FeCl 2 =
Exercise 3. Write the molecular equations for the reactions (in aqueous solution) between: a) sodium carbonate and nitric acid, b) nickel (II) chloride and sodium hydroxide, c) orthophosphoric acid and calcium hydroxide, d) silver nitrate and potassium chloride, e) phosphorus oxide (V) and potassium hydroxide.
I sincerely hope that you had no problems completing these three tasks. If this is not the case, you need to return to the topic " Chemical properties main classes inorganic compounds".
How to turn a molecular equation into a complete ionic equation
The most interesting begins. We must understand which substances should be written as ions and which should be left in "molecular form". You have to remember the following.
In the form of ions write:
- soluble salts (I emphasize that only salts are highly soluble in water);
- alkalis (let me remind you that water-soluble bases are called alkalis, but not NH 4 OH);
- strong acids (H 2 SO 4 , HNO 3 , HCl, HBr, HI, HClO 4 , HClO 3 , H 2 SeO 4 , ...).
As you can see, this list is easy to remember: it includes strong acids and bases and all soluble salts. By the way, to especially vigilant young chemists who may be outraged by the fact that strong electrolytes (insoluble salts) are not included in this list, I can tell you the following: NOT including insoluble salts in this list does not at all reject the fact that they are strong electrolytes.
All other substances must be present in the ionic equations in the form of molecules. Those demanding readers who are not satisfied with the vague term "all other substances", and who, following the example of the hero famous movie, require "announce full list I give the following information.
In the form of molecules, write:
- all insoluble salts;
- all weak bases (including insoluble hydroxides, NH 4 OH and similar substances);
- all weak acids (H 2 CO 3 , HNO 2 , H 2 S, H 2 SiO 3 , HCN, HClO, almost all organic acids ...);
- in general, all weak electrolytes (including water!!!);
- oxides (all types);
- all gaseous compounds (in particular H 2 , CO 2 , SO 2 , H 2 S, CO);
- simple substances (metals and non-metals);
- almost all organic compounds(exception - water-soluble salts of organic acids).
Phew, I don't think I forgot anything! Although it is easier, in my opinion, to remember list No. 1. Of the fundamentally important in list No. 2, I will once again note the water.
Let's train!
Example 2. Make a complete ionic equation describing the interaction of copper (II) hydroxide and hydrochloric acid.
Solution. Let's start, of course, with the molecular equation. Copper (II) hydroxide is an insoluble base. All insoluble bases react with strong acids to form a salt and water:
Cu(OH) 2 + 2HCl = CuCl 2 + 2H 2 O.
And now we find out which substances to write in the form of ions, and which - in the form of molecules. The lists above will help us. Copper (II) hydroxide is an insoluble base (see solubility table), a weak electrolyte. Insoluble bases are written in molecular form. HCl is a strong acid, in solution it almost completely dissociates into ions. CuCl 2 - soluble salt. We write in ionic form. Water - only in the form of molecules! We get the full ionic equation:
Cu (OH) 2 + 2H + + 2Cl - \u003d Cu 2+ + 2Cl - + 2H 2 O.
Example 3. Write a complete ionic equation for the reaction of carbon dioxide with an aqueous solution of NaOH.
Solution. Carbon dioxide is a typical acidic oxide, NaOH is an alkali. When acidic oxides interact with aqueous solutions of alkalis, salt and water are formed. We compose the molecular reaction equation (do not forget, by the way, about the coefficients):
CO 2 + 2NaOH \u003d Na 2 CO 3 + H 2 O.
CO 2 - oxide, gaseous compound; keep the molecular shape. NaOH - strong base (alkali); written in the form of ions. Na 2 CO 3 - soluble salt; write in the form of ions. Water is a weak electrolyte, practically does not dissociate; leave it in molecular form. We get the following:
CO 2 + 2Na + + 2OH - \u003d Na 2+ + CO 3 2- + H 2 O.
Example 4. Sodium sulfide in aqueous solution reacts with zinc chloride to form a precipitate. Write the complete ionic equation for this reaction.
Solution. Sodium sulfide and zinc chloride are salts. When these salts interact, zinc sulfide precipitates:
Na 2 S + ZnCl 2 \u003d ZnS ↓ + 2NaCl.
I will immediately write down the full ionic equation, and you will analyze it yourself:
2Na + + S 2- + Zn 2+ + 2Cl - = ZnS↓ + 2Na + + 2Cl - .
Here are some tasks for you to independent work and a little test.
Exercise 4. Write the molecular and full ionic equations for the following reactions:
- NaOH + HNO3 =
- H 2 SO 4 + MgO =
- Ca(NO 3) 2 + Na 3 PO 4 =
- CoBr 2 + Ca(OH) 2 =
Exercise 5. Write complete ionic equations describing the interaction of: a) nitric oxide (V) with an aqueous solution of barium hydroxide, b) a solution of cesium hydroxide with hydroiodic acid, c) aqueous solutions of copper sulfate and potassium sulfide, d) calcium hydroxide and an aqueous solution of iron nitrate ( III).
Story
Title page of Tyrocinium Chymicum.
At first there was no concept of chemical equations, the basic chemical laws were not yet known, but already in ancient times, in the alchemical period of the development of chemistry, they began to designate chemical elements with symbols.
With the further development of chemistry, ideas about the symbolism of chemical elements changed, knowledge about their compounds expanded. With the discovery of many chemical phenomena, it became necessary to move from their verbal description to a more convenient mathematical notation using chemical formulas. Jean Beguin was the first to propose the use of chemical equations in 1615 in the first chemistry textbook Tyrocinium Chymicum ("Principles of Chemistry").
Late XVIII - early XIX centuries - the formation of the laws of stoichiometry. At the origins of these studies was the German scientist I. V. Richter. In his student years on him great impression produced the words of his teacher, the philosopher I. Kant, that in certain areas of the natural sciences there is as much true science as there is mathematics in it. Richter devoted his dissertation to the use of mathematics in chemistry. Not being essentially a chemist, Richter introduced the first quantitative equations of chemical reactions, began to use the term stoichiometry.
Compilation rules
On the left side of the equation, write down the formulas (formula) of the substances that have entered into the reaction, connecting them with a plus sign. On the right side of the equation, write down the formulas (formula) of the formed substances, also connected by a plus sign. An arrow is placed between the parts of the equation. Then they find odds- the numbers in front of the formulas of substances so that the number of atoms of the same elements in the left and right parts of the equation is equal.
To compile the equations of chemical reactions, in addition to knowing the formulas of the reactants and reaction products, it is necessary to choose the right coefficients. This can be done using simple rules:
1. Before the formula of a simple substance, a fractional coefficient can be written, which shows the amount of the substance of the reacting and formed substances.
2. If there is a salt formula in the reaction scheme, then first equalize the number of ions that form the salt.
3. If the substances involved in the reaction contain hydrogen and oxygen, then the hydrogen atoms are equalized in the penultimate turn, and the oxygen atoms - in the last.
4. If there are several salt formulas in the reaction scheme, then it is necessary to start equalizing with the ions that are part of the salt containing a larger number of them.
Symbols in chemical equations
The following symbols are used to indicate different types of reactions:
Arrangement of coefficients in equations
The law of conservation of mass states that the amount of substance of each element before the reaction is equal to the amount of substance of each element after the reaction. Thus, the left and right sides of a chemical equation must have the same number of atoms of one or another element. The chemical equation must be electrically neutral, that is, the sum of the charges on the left and right sides of the equation must add up to zero. One way to equalize the number of atoms in a chemical equation is to select coefficients by trial and error. For more difficult cases a system of linear algebraic equations should be used. As a rule, chemical equations are written with the smallest integer coefficients. If there is no coefficient before the chemical formula, it is assumed that it equal to one. Checking the material balance, that is, the number of atoms on the left and right sides, can be as follows: a coefficient 1 is placed before the most complex chemical formula. Next, the coefficients are placed in front of the formulas in such a way that the number of atoms of each of the elements on the left and right sides of the equation is equal to . If one of the coefficients is fractional, then all coefficients should be multiplied by the number in the denominator of the fractional coefficient. If the coefficient is 1 before the formula, then it is omitted. An example, the arrangement of coefficients in the chemical reaction of methane combustion:
1CH 4 + O 2 CO 2 + H 2 O
The number of carbon atoms on the left and right sides is the same. The next element to balance is hydrogen. There are 4 hydrogen atoms on the left, 2 on the right, to equalize the number of hydrogen atoms, put a factor of 2 in front of water, as a result:
1CH 4 + O 2 CO 2 + 2H 2 O
Checking the correct placement of the coefficients in any chemical equation is carried out by counting the number of oxygen atoms, if the number of oxygen atoms is the same on the left and right sides, then the coefficients are placed correctly.
1CH 4 + 2O 2 CO 2 + 2H 2 O
Before the CH 4 and CO 2 molecules, the coefficient 1 is omitted.
Redox reactions
Redox reactions (ORRs) are counter-parallel chemical reactions that occur with a change in the oxidation states of the atoms that make up the reactants, realized by the redistribution of electrons between the oxidizing atom and the reducing atom.
In the process of a redox reaction, the reducing agent gives up electrons, that is, it is oxidized; The oxidizing agent gains electrons, that is, it is reduced. Moreover, any redox reaction is a unity of two opposite transformations - oxidation and reduction, occurring simultaneously and without separation of one from the other.
Oxidation is the process of donating electrons, with an increase in the degree of oxidation. When a substance is oxidized, as a result of the return of electrons, its oxidation state increases. The atoms of the oxidized substance are called electron donors, and the atoms of the oxidizing agent are called electron acceptors. The oxidizing agent, accepting electrons, acquires reducing properties, turning into a conjugated reducing agent.
Recovery is the process of attaching electrons to an atom of a substance, while its oxidation state decreases. During reduction, atoms or ions gain electrons. In this case, the oxidation state of the element decreases. The reducing agent, donating electrons, acquires oxidizing properties, turning into a conjugated oxidizing agent.
When drawing up an equation for a redox reaction, it is necessary to determine the reducing agent, oxidizing agent, and the number of given and received electrons. As a rule, the coefficients are selected using either the electron balance method or the electron-ion balance method (sometimes the latter is called the half-reaction method).
Selection of coefficients by the method of electronic balance.
IN simple equations coefficients are selected element by element in accordance with the formula of the final product. In more complex equations of redox reactions, the coefficients are selected using the electronic balance method:
1. Write down the reaction scheme (the formula of the reactants and products), and then find the elements that increase and decrease their oxidation states, and write them out separately;
2. Compose equations for half-reactions of reduction and oxidation, observing the laws of conservation of the number of atoms and charge in each half-reaction;
3. Additional factors are selected to equalize the half-reactions so that the charge conservation law is fulfilled for the reaction as a whole, for which the number of elements accepted in the reduction half-reactions is made equal to the number given elements in the oxidation half-reaction;
4. Put down (according to the factors found) stoichiometric coefficients in the reaction scheme (coefficient 1 is omitted);
5. Equalize the number of atoms of those elements that do not change their oxidation state during the course of the reaction (if there are two such elements, then it is enough to equalize the number of atoms of one of them, and check the second one). Get the chemical reaction equations;
6. They check for an element that has not changed its oxidation state (most often it is oxygen).
Arrangement of coefficients in ionic equations
Ionic equations are chemical equations in which electrolytes are written as dissociated ions. Ionic equations are used to write substitution reactions and exchange reactions in aqueous solutions. Example, exchange reaction, interaction of calcium chloride and silver nitrate with the formation of a precipitate of silver chloride:
CaCl 2 (l) + 2AgNO 3 (l) Ca(NO 3) 2 (l) + 2AgCl (tv)
full ionic equation:
Ca 2+ + 2Cl - + 2Ag + + 2NO 3 - Ca 2+ + 2NO 3 - + 2AgCl (solid)
reduced ionic equation:
2Cl − (l) + 2Ag + (l) 2AgCl(s)
ionic equation:
Ag + + Cl − AgCl(s)
Ions Ca 2+ and NO 3 - remain in solution, therefore they are not participants in the chemical reaction. In neutralization reactions, the ionic reaction equation is as follows:
H + + OH - H 2 O
There are several neutralization reactions that produce another low dissociating substance besides water. An example is the reaction of barium hydroxide with phosphoric acid, as water-insoluble barium phosphate is formed.
Literature
- Levitsky M. The language of chemists // Chemistry and life. - 2000. -№1. - P.50-52.
- Kudryavtsev A.A. Writing Chemical Equations - 4th Edition, Revised. and additional, 1968 - 359s.
- Berg L.G. Gromakov S.D. Zoroatskaya I.V. Averko-Antonovich I.N. Methods for selecting coefficients in chemical equations - Kazan: Publishing House of Kazan University, 1959.- 148 p.
- Leenson I.A. Even or odd - M .: Chemistry, 1987. - 176s.
- Chemistry, 8th grade textbook. ARC Publishing. 2003.
- Chemistry, 8th grade textbook. Drofa Publishing. 2009.
- Chemistry, 8th grade textbook. Publishing house "Mektep" Almaty. 2012.
- Chemistry, 9th grade textbook. Publishing house "Enlightenment" 2008.
see also
Links
- // Encyclopedic Dictionary of Brockhaus and Efron: In 86 volumes (82 volumes and 4 additional). - St. Petersburg. , 1890-1907.
