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 activities.
Theoretical features
Drawing up chemical equations is an important and crucial stage, considered in the eighth grade general education schools. What should come before this stage? Before the teacher tells his pupils how to make a chemical equation, it is important to introduce the schoolchildren to the term "valence", to teach them to determine this value for metals and non-metals using the periodic table of elements.
Compilation of binary formulas by valency
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. Let's look at the theoretical points first, then move on to concrete 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 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, 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 different types 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, before the formula of hydrochloric acid you need to set a factor of 2.
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 with all the processes that occur between different inorganic or organic matter, 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.
Chemical reactions, their properties, types, conditions, etc., are one of the cornerstones of 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 of chemical reactions themselves without suspecting (and maybe suspecting) cooks do when they prepare 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 we lower a steel nail into a solution with copper sulphate, then in the course of this simple chemical experiment we will get iron sulphate (iron will displace copper from salt). The equation for such a chemical reaction would look like this:
Fe + CuSO 4 → FeSO 4 + Cu
Chemical exchange reaction
Exchange reactions take place exclusively between complex chemicals, during which they change their parts. A lot of these reactions take place in various solutions. The neutralization of acid by bile is a good example of a chemical exchange reaction.
NaOH + HCl → NaCl + H 2 O
This is the chemical equation of this reaction, in which a hydrogen ion from the HCl compound is exchanged with a sodium ion from the NaOH compound. The consequence of this chemical reaction is the formation of a salt solution.
Signs of chemical reactions
According to the signs of the occurrence of chemical reactions, one can judge whether a chemical reaction between the reagents has passed or not. Here are examples of signs of chemical reactions:
- Color change (light iron, for example, in humid air is covered with a brown coating, as a result of a chemical reaction between iron and iron).
- Precipitation (if carbon dioxide is suddenly passed through a lime solution, we will get a precipitation of a white insoluble precipitate of calcium carbonate).
- Gas release (if you drop citric acid on baking soda, you will get carbon dioxide release).
- The formation of weakly dissociated substances (all reactions resulting in the formation of water).
- The glow of the solution (an example here is the reactions that occur with a solution of luminol, which emits light during chemical reactions).
In general, it is difficult to distinguish which signs of chemical reactions are the main ones; different substances and different reactions have their own signs.
How to determine the sign of a chemical reaction
You can determine the sign of a chemical reaction visually (with a change in color, glow), or by the results of this very reaction.
The rate of a chemical reaction
The rate of a chemical reaction is usually understood as the change in the amount of one of the reactants per unit of time. Moreover, the rate of a chemical reaction is always a positive value. In 1865, the chemist N. N. Beketov formulated the law of mass action, which states that "the rate of a chemical reaction at any given time is proportional to the concentrations of reagents raised to powers equal to their stoichiometric coefficients."
Factors in the rate of a chemical reaction include:
- the nature of the reactants
- the presence of a catalyst
- temperature,
- the surface area of the reactants.
All of them have the most direct influence on the rate of a chemical reaction.
Equilibrium of a chemical reaction
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 the state 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 what they call change. internal energy systems as a result of a chemical reaction and the transformation of the starting substances (reactants) into reaction products in amounts 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 chemical equation understand the difference between ratios 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 at the bottom right of 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 the 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 - 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 are made up of atoms, when compiling chemical equations, we will use the rule: the number of atoms of each chemical element starting substances should 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 record of the 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.
The reaction equation in chemistry is called the record chemical process using chemical formulas and mathematical symbols.
Such a record is a scheme of a chemical reaction. When the “=” sign appears, it is called an “equation”. Let's try to solve it.
In contact with
An example of parsing simple reactions
Calcium has one atom, since the coefficient is not worth it. The index is also not written here, which means it is one. On the right side of the equation, Ca is also one. We don't need to work on calcium.
Video: Coefficients in the equations of chemical reactions.
We look at the next element - oxygen. Index 2 indicates that there are 2 oxygen ions. There are no indices on the right side, that is, one particle of oxygen, and on the left - 2 particles. What are we doing? No additional indexes or fixes in chemical formula cannot be entered, as it is written correctly.
The coefficients are what is written before the smallest part. They have the right to change. For convenience, we do not rewrite the formula itself. On the right side, we multiply one by 2 to get 2 oxygen ions there as well.
After we set the coefficient, we got 2 calcium atoms. There is only one on the left side. So now we have to put 2 in front of calcium.
Now let's check the result. If the number of element atoms is equal on both sides, then we can put an "equal" sign.
Another good example: two hydrogens on the left, and after the arrow we also have two hydrogens.
- Two oxygens before the arrow, and after the arrow there are no indices, which means one.
- More on the left, less on the right.
- We put a factor of 2 in front of the water.
We multiplied the whole formula by 2, and now we have changed the amount of hydrogen. We multiply the index by the coefficient, and it turns out 4. And on the left side there are two hydrogen atoms. And to get 4, we have to multiply hydrogen by two.
Video: Arrangement of coefficients in a chemical equation
Here is the case when the element in one and the other formula is on the one hand, up to the arrow.
One sulfur ion on the left and one sulfur ion on the right. Two particles of oxygen, plus two more particles of oxygen. So there are 4 oxygens on the left side. On the right is 3 oxygen. That is, on the one hand, an even number of atoms is obtained, and on the other, an odd number. If we multiply an odd number by 2, we get an even number. We bring it to an even value first. To do this, multiply by two the entire formula after the arrow. After multiplication, we get six oxygen ions, and even 2 sulfur atoms. On the left, we have one microparticle of sulfur. Now let's equalize it. We put equations on the left in front of gray 2.
Called.
Complex reactions
This example is more complex, as there are more elements of matter.
This is called a neutralization reaction. What needs to be equalized here first of all:
- On the left side is one sodium atom.
- On the right side, the index says that there are 2 sodium.
The conclusion suggests itself that it is necessary to multiply the entire formula by two.
Video: Compilation of equations of chemical reactions
Now let's see how much sulfur. One on the left and right side. Pay attention to oxygen. On the left side we have 6 oxygen atoms. On the other hand - 5. Less on the right, more on the left. An odd number must be brought to an even value. To do this, we multiply the water formula by 2, that is, we make 2 from one oxygen atom.
Now on the right side there are already 6 oxygen atoms. There are also 6 atoms on the left side. Checking hydrogen. Two hydrogen atoms and 2 more hydrogen atoms. That is, there will be four hydrogen atoms on the left side. And on the other side also four hydrogen atoms. All elements are balanced. We put an "equal" sign.
Video: Chemical equations. How to write chemical equations.
Next example.
Here the example is interesting in that parentheses have appeared. They say that if the factor is outside the bracket, then every element in the brackets is multiplied by it. You need to start with nitrogen, since it is less than oxygen and hydrogen. On the left, there is one nitrogen, and on the right, taking into account the brackets, there are two.
There are two hydrogen atoms on the right, but four are needed. We get out of the situation by simply multiplying the water by two, resulting in four hydrogens. Great, hydrogen equalized. There is oxygen left. Before the reaction, there are 8 atoms, after - also 8.
Great, all the elements are equal, we can put "equal".
Last example.
Next up is barium. It is leveled, it is not necessary to touch it. Before the reaction, there are two chlorine, after it - only one. What needs to be done? Put 2 in front of chlorine after the reaction.
Video: Balancing chemical equations.
Now, due to the coefficient that has just been set, after the reaction, two sodium were obtained, and before the reaction, also two. Great, everything else is balanced.
Reactions can also be equalized using the electronic balance method. This method has a number of rules by which it can be implemented. The next step is to arrange the oxidation states of all elements in each substance in order to understand where the oxidation occurred and where the reduction took place.
Write down the chemical equation. As an example, consider the following reaction:
- C 3 H 8 + O 2 –> H 2 O + CO 2
- This reaction describes the combustion of propane (C 3 H 8) in the presence of oxygen to form water and carbon dioxide (carbon dioxide).
Write down the number of atoms of each element. Do this for both sides of the equation. Notice the subscripts next to each element to determine the total number of atoms. Write down the symbol for each element in the equation and note the corresponding number of atoms.
- For example, on the right side of the equation under consideration, as a result of addition, we get 3 oxygen atoms.
- On the left side we have 3 carbon atoms (C 3), 8 hydrogen atoms (H 8) and 2 oxygen atoms (O 2).
- On the right side we have 1 carbon atom (C), 2 hydrogen atoms (H 2) and 3 oxygen atoms (O + O 2).
Leave hydrogen and oxygen for later, as they are part of several compounds on the left and right side. Hydrogen and oxygen are part of several molecules, so it's best to balance them last.
- Before balancing hydrogen and oxygen, you will have to count the atoms again, as additional factors may be needed to balance other elements.
Start with the least frequently occurring element. If you need to balance several elements, choose one that is part of one molecule of reactants and one molecule of reaction products. So the first thing to do is to balance the carbon.
For balance, add a factor before the single carbon atom. Place a factor in front of the single carbon on the right side of the equation to balance it with the 3 carbons on the left side.
- C 3 H 8 + O 2 –> H 2 O + 3 CO 2
- The factor 3 in front of the carbon on the right side of the equation indicates that there are three carbon atoms, which correspond to the three carbon atoms included in the propane molecule on the left side.
- In a chemical equation, you can change the coefficients in front of atoms and molecules, but the subscripts must remain unchanged.
Then balance the hydrogen atoms. After you equalized the number of carbon atoms on the left and right side, hydrogen and oxygen remained unbalanced. The left side of the equation contains 8 hydrogen atoms, the same number should be on the right side. Achieve this with a ratio.
- C 3 H 8 + O 2 –> 4 H 2 O + 3CO 2
- We've added a factor of 4 on the right side because the subscript shows we already have two hydrogens.
- If you multiply the factor 4 by the subscript 2, you get 8.
- As a result, 10 oxygen atoms are obtained on the right side: 3x2=6 atoms in three 3CO 2 molecules and four more atoms in four water molecules.
