§ 1 Features chemical reactions
In chemical reactions, the initial substances are converted into other substances with different properties. This can be judged by the external signs of chemical reactions: the formation of a gaseous or insoluble substance, the release or absorption of energy, a change in the color of a substance.
We heat a piece of copper wire in the flame of an alcohol lamp. We will see that the part of the wire that was in the flame turned black.
Pour 1-2 ml of solution acetic acid to baking soda powder. We observe the appearance of gas bubbles and the disappearance of soda.
Pour 3-4 ml of copper chloride solution to a solution of caustic soda. In this case, the blue transparent solution will turn into a bright blue precipitate.
To 2 ml of starch solution add 1-2 drops of iodine solution. And the translucent white liquid will become opaque dark blue.
The most important sign of a chemical reaction is the formation of new substances.
But this can also be judged by some external signs of the course of reactions:
precipitation;
Color change;
Gas release;
The appearance of an odor;
The release or absorption of energy in the form of heat, electricity, or light.
For example, if a lighted splinter is brought to a mixture of hydrogen and oxygen or an electric discharge is passed through this mixture, a deafening explosion will occur, and a new substance, water, will form on the walls of the vessel. There was a reaction of the formation of water molecules from hydrogen and oxygen atoms with the release of heat.
On the contrary, the decomposition of water into oxygen and hydrogen requires electrical energy.
§ 2 Conditions for the occurrence of a chemical reaction
However, certain conditions are necessary for a chemical reaction to occur.
Consider the combustion reaction of ethyl alcohol.
It occurs when alcohol interacts with oxygen in the air; for the reaction to start, the contact of alcohol and oxygen molecules is necessary. But if we open the cap of the spirit lamp, then when the initial substances - alcohol and oxygen come into contact, the reaction does not occur. Let's bring a lit match. The alcohol on the wick of the spirit lamp heats up and lights up, the combustion reaction begins. The condition necessary for the occurrence of the reaction here is the initial heating.
Pour a 3% solution of hydrogen peroxide into a test tube. If we leave the test tube open, then hydrogen peroxide will slowly decompose into water and oxygen. In this case, the reaction rate will be so low that we will not see signs of gas evolution. Let's add some black manganese (IV) oxide powder. We observe a rapid release of gas. This is oxygen, which was formed during the decomposition of hydrogen peroxide.
A necessary condition for the start of this reaction was the addition of a substance that does not participate in the reaction, but accelerates it.
This substance is called a catalyst.
Obviously, for the occurrence and course of chemical reactions, certain conditions are necessary, namely:
Contact of starting substances (reagents),
heating them up to a certain temperature,
The use of catalysts.
§ 3 Features of chemical reactions
A characteristic feature of chemical reactions is that they are often accompanied by the absorption or release of energy.
Dmitri Ivanovich Mendeleev pointed out that the most important feature of all chemical reactions is the change in energy during their course.
The release or absorption of heat in the process of chemical reactions is due to the fact that energy is spent on the process of destruction of some substances (destruction of bonds between atoms and molecules) and is released during the formation of other substances (formation of bonds between atoms and molecules).
Energy changes are manifested either in the release or absorption of heat. Reactions that release heat are called exothermic.
Reactions that absorb heat are called endothermic.
The amount of heat released or absorbed is called the heat of the reaction.
The thermal effect is usually denoted by the Latin letter Q and the corresponding sign: +Q for exothermic reactions and -Q for endothermic reactions.
The field of chemistry that studies the thermal effects of chemical reactions is called thermochemistry. The first studies of thermochemical phenomena belong to the scientist Nikolai Nikolaevich Beketov.
The value of the thermal effect is related to 1 mol of a substance and is expressed in kilojoules (kJ).
Most of the chemical processes carried out in nature, laboratory and industry are exothermic. These include all reactions of combustion, oxidation, compounds of metals with other elements, and others.
However, there are also endothermic processes, for example, the decomposition of water under the action of an electric current.
The thermal effects of chemical reactions vary widely from 4 to 500 kJ/mol. The thermal effect is most significant in combustion reactions.
Let's try to explain what is the essence of the ongoing transformations of substances and what happens to the atoms of the reacting substances. According to the atomic-molecular doctrine, all substances are composed of atoms connected to each other into molecules or other particles. During the reaction, the destruction of the initial substances (reagents) and the formation of new substances (reaction products) occur. Thus, all reactions are reduced to the formation of new substances from the atoms that make up the original substances.
Therefore, the essence of a chemical reaction is the rearrangement of atoms, as a result of which new molecules (or other forms of matter) are obtained from molecules (or other particles).
List of used literature:
- NOT. Kuznetsova. Chemistry. 8th grade. Tutorial for educational institutions. – M. Ventana-Graf, 2012.
In industry, such conditions are selected so that the necessary reactions are carried out, and harmful ones are slowed down.
TYPES OF CHEMICAL REACTIONS
Table 12 shows the main types of chemical reactions according to the number of particles involved in them. Drawings and equations of reactions often described in textbooks are given. decomposition, connections, substitution and exchange.
At the top of the table are decomposition reactions water and sodium bicarbonate. A device for passing a direct electric current through water is shown. The cathode and anode are metal plates immersed in water and connected to an electric current source. Due to the fact that pure water practically does not electricity, a small amount of soda (Na 2 CO 3) or sulfuric acid (H 2 SO 4) is added to it. When current passes through both electrodes, gas bubbles are released. In the tube where hydrogen is collected, the volume is twice as large as in the tube where oxygen is collected (you can verify its presence with the help of a smoldering splinter). The model scheme demonstrates the reaction of water decomposition. Chemical (covalent) bonds between atoms in water molecules are destroyed, and hydrogen and oxygen molecules are formed from the released atoms.
Model scheme compound reactions metallic iron and molecular sulfur S 8 shows that as a result of the rearrangement of atoms during the reaction, iron sulfide is formed. At the same time, they are destroyed chemical bonds in an iron crystal ( metallic bond) and a sulfur molecule ( covalent bond), and the released atoms combine to form ionic bonds into a salt crystal.
Another reaction of the compound is the slaking of lime CaO with water to form calcium hydroxide. At the same time, burnt (quicklime) lime begins to warm up and a loose powder of slaked lime is formed.
To substitution reactions refers to the interaction of a metal with an acid or salt. When a sufficiently active metal is immersed in a strong (but not nitric) acid, hydrogen bubbles are released. More active metal displaces the less active from its salt solution.
typical exchange reactions is a neutralization reaction and a reaction between solutions of two salts. The figure shows the preparation of barium sulfate precipitate. The course of the neutralization reaction is monitored using the phenolphthalein indicator (crimson color disappears).
Table 12
Types of chemical reactions
AIR. OXYGEN. COMBUSTION
Oxygen is the most common chemical element on the ground. Its content in earth's crust and hydrosphere is presented in table 2 "The prevalence of chemical elements". Oxygen accounts for approximately half (47%) of the mass of the lithosphere. It is the predominant chemical element in the hydrosphere. In the earth's crust, oxygen is present only in bound form (oxides, salts). The hydrosphere is also represented mainly by bound oxygen (part of the molecular oxygen is dissolved in water).
The atmosphere of free oxygen contains 20.9% by volume. Air is a complex mixture of gases. Dry air is 99.9% nitrogen (78.1%), oxygen (20.9%) and argon (0.9%). The content of these gases in the air is almost constant. The composition of dry atmospheric air also includes carbon dioxide, neon, helium, methane, krypton, hydrogen, nitric oxide (I) (diazot oxide, nitrogen hemioxide - N 2 O), ozone, sulfur dioxide, carbon monoxide, xenon, nitrogen oxide ( IV) (nitrogen dioxide - NO 2).
The composition of air was determined by the French chemist Antoine Laurent Lavoisier in late XVIII century (table 13). He proved the content of oxygen in the air, and called it "vital air". To do this, he heated mercury on a furnace in a glass retort, the thin part of which was placed under a glass cap, lowered into a water bath. The air under the cap turned out to be closed. When heated, mercury combined with oxygen, turning into red mercury oxide. The "air" remaining in the glass cap after heating the mercury contained no oxygen. The mouse, placed under the cap, suffocated. Having calcined mercury oxide, Lavoisier again isolated oxygen from it and again received pure mercury.
The oxygen content in the atmosphere began to noticeably increase about 2 billion years ago. As a result of the reaction photosynthesis a certain volume of carbon dioxide was absorbed and the same volume of oxygen was released. The figure in the table schematically shows the formation of oxygen during photosynthesis. During photosynthesis in the leaves of green plants containing chlorophyll, when solar energy is absorbed, water and carbon dioxide are converted into carbohydrates(sugar) and oxygen. The reaction of the formation of glucose and oxygen in green plants can be written as follows:
6H 2 O + 6CO 2 \u003d C 6 H 12 O 6 + 6O 2.
The resulting glucose becomes insoluble in water. starch that accumulates in plants.
Table 13
Air. Oxygen. Combustion
Photosynthesis is a complex chemical process that includes several stages: the absorption and transport of solar energy, the use of sunlight energy to initiate photochemical redox reactions, the reduction of carbon dioxide and the formation of carbohydrates.
sunlight is electromagnetic radiation of different wavelengths. In the chlorophyll molecule upon absorption visible light(red and violet) there are transitions of electrons from one energy state to another. Photosynthesis consumes only a small part of the solar energy (0.03%) reaching the Earth's surface.
All carbon dioxide available on Earth goes through the cycle of photosynthesis in an average of 300 years, oxygen - in 2000 years, ocean water - in 2 million years. At present, a constant oxygen content has been established in the atmosphere. It is almost completely spent on respiration, combustion and decay of organic matter.
Oxygen is one of the most active substances. Processes involving oxygen are called oxidation reactions. These include combustion, breathing, decay and many others. The table shows the combustion of oil, which goes with the release of heat and light.
Combustion reactions can bring not only benefits, but also harm. Combustion can be stopped by stopping the air (oxidizer) from reaching the burning object with foam, sand, or a blanket.
Foam fire extinguishers are filled with a concentrated solution of baking soda. When it comes into contact with concentrated sulfuric acid, which is in a glass ampoule at the top of the fire extinguisher, carbon dioxide foam is formed. To activate the fire extinguisher, turn over and hit the floor with a metal pin. In this case, the ampoule with sulfuric acid breaks and the resulting reaction of the acid with sodium bicarbonate carbon dioxide foams the liquid and throws it out of the fire extinguisher with a strong jet. Foamy liquid and carbon dioxide, enveloping the burning object, push the air and extinguish the flame.
The rate of a chemical reaction is the change in the amount of a reactant or reaction product per unit time in a unit volume (for homogeneous reaction) or per unit interface (for a heterogeneous reaction).
Law of acting masses: dependence of the reaction rate on the concentration of reactants. The higher the concentration, the greater the number of molecules contained in the volume. Consequently, the number of collisions increases, which leads to an increase in the speed of the process.
Kinetic equation– dependence of reaction rate on concentration.
Solids are 0
Reaction molecularity is the minimum number of molecules involved in an elementary chemical process. By molecularity, elementary chemical reactions are divided into molecular (A →) and bimolecular (A + B →); trimolecular reactions are extremely rare.
General reaction order is the sum of the exponents of the degrees of concentration in the kinetic equation.
Reaction rate constant- coefficient of proportionality in the kinetic equation.
Van't Hoff's rule: For every 10 degrees increase in temperature, the rate constant of a homogeneous elementary reaction increases two to four times.
Theory of active collisions(TAC), there are three conditions necessary for a reaction to occur:
The molecules must collide. This is an important condition, but it is not sufficient, since a reaction will not necessarily occur during a collision.
Molecules must have the necessary energy (activation energy).
The molecules must be correctly oriented relative to each other.
Activation energy is the minimum amount of energy that must be supplied to the system for a reaction to occur.
Arrhenius equation establishes the dependence of the rate constant of a chemical reaction on temperature
A - characterizes the frequency of collisions of reacting molecules
R is the universal gas constant.
Influence of catalysts on the reaction rate.
A catalyst is a substance that changes the rate of a chemical reaction but is not consumed in the reaction itself. final products Excluded.
In this case, the change in the reaction rate occurs due to a change in the activation energy, and the catalyst with the reagents forms an activated complex.
Catalysis - a chemical phenomenon, the essence of which is to change the rates of chemical reactions under the action of certain substances (they are called catalysts).
Heterogeneous catalysis - the reactant and the catalyst are in different phases - gaseous and solid.
Homogeneous catalysis - the reactants (reagents) and the catalyst are in the same phase - for example, both are gases or both are dissolved in some solvent.
Terms chemical equilibrium
the state of chemical equilibrium is maintained as long as the reaction conditions remain unchanged: concentration, temperature and pressure.
Le Chatelier's principle: if any external influence is exerted on a system in equilibrium, then the equilibrium will shift in the direction of the reaction that this action will weaken.
Equilibrium constant - this is a measure of the completeness of the reaction, the greater the value of the equilibrium constant, the higher the degree of conversion of the starting materials into reaction products.
|
K p \u003d C pr \ C ref |
ΔG<0 К р >1 C pr > C ref
ΔG>0 K p<1 С пр <С исх
I. Signs and conditions for the occurrence of chemical reactions
You already know many substances, have observed their transformations and the accompanying transformations. signs.
by the most main feature chemical reaction is the formation of new substances. But this can also be judged by some external signs of the course of reactions.
External signs of chemical reactions:
- precipitation
- color change
- outgassing
- the appearance of an odor
- absorption and release of energy (heat, electricity, light)
It's obvious that For the occurrence and course of chemical reactions, certain conditions are necessary:
- contact of initial substances (reagents)
- heating to a certain temperature
- the use of substances that speed up a chemical reaction (catalysts)
II. Thermal effect of a chemical reaction
DI. Mendeleev pointed out: the most important feature of all chemical reactions is the change in energy in the process of their occurrence.
Every substance has a certain amount of energy stored in it. We encounter this property of substances already at breakfast, lunch or dinner, as food allows our body to use the energy of a wide variety of chemical compounds contained in food. In the body, this energy is converted into movement, work, and is used to maintain a constant (and rather high!) body temperature.
The release or absorption of heat in the process of chemical reactions is due to the fact that energy is spent on the process of destruction of some substances (destruction of bonds between atoms and molecules) and is released during the formation of other substances (formation of bonds between atoms and molecules).
Energy changes are manifested either in the release or absorption of heat.
Reactions that release heat are called exothermic (from the Greek "exo" - out).
Reactions that take place with the absorption of energy are calledendothermic (from the Latin "endo" - inside).
Most often, energy is released or absorbed in the form of heat (less often, in the form of light or mechanical energy). This heat can be measured. The result of the measurement is expressed in kilojoules (kJ) for one MOL of the reactant or (more rarely) for the mole of the reaction product. The amount of heat released or absorbed in a chemical reaction is called the thermal effect of the reaction(Q).
Exothermic reaction:
Starting materials → reaction products + Q kJ
Endothermic reaction:
Starting materials → reaction products - Q kJ
Thermal effects of chemical reactions are needed for many technical calculations. Imagine yourself for a moment as a designer of a powerful rocket capable of launching spaceships and other payloads into orbit.
Suppose you know the work (in kJ) that will have to be spent to deliver a rocket with a load from the Earth's surface to orbit, you also know the work to overcome air resistance and other energy costs during the flight. How to calculate the required supply of hydrogen and oxygen, which (in a liquefied state) are used in this rocket as fuel and oxidizer?
Without the help of the thermal effect of the reaction of the formation of water from hydrogen and oxygen, this is difficult to do. After all, the thermal effect is the very energy that should put the rocket into orbit. In the combustion chambers of the rocket, this heat is converted into the kinetic energy of hot gas molecules (steam), which escapes from the nozzles and creates jet thrust.
In the chemical industry, thermal effects are needed to calculate the amount of heat to heat reactors in which endothermic reactions take place. In the energy sector, using the heat of combustion of fuel, the generation of thermal energy is calculated.
Dietitians use the thermal effects of food oxidation in the body to formulate proper diets not only for patients, but also for healthy people - athletes, workers of various professions. Traditionally, for calculations, not joules are used here, but other energy units - calories (1 cal = 4.1868 J). The energy content of food refers to some mass of food products: to 1 g, to 100 g, or even to the standard packaging of the product. For example, on the label of a jar of condensed milk, you can read the following inscription: "calorie content 320 kcal / 100 g."
The branch of chemistry concerned with the study of thermal effects and chemical reactions is called thermochemistry.
The equations of chemical reactions in which the thermal effect is indicated are called thermochemical.
Throughout life, we are constantly confronted with physical and chemical phenomena. Natural physical phenomena are so familiar to us that we have not attached much importance to them for a long time. Chemical reactions are constantly taking place in our body. The energy that is released during chemical reactions is constantly used in everyday life, in production, and when launching spacecraft. Many of the materials from which the things around us are made are not taken in nature in finished form, but are made using chemical reactions. In everyday life, it does not make much sense for us to understand what happened. But when studying physics and chemistry at a sufficient level, this knowledge is indispensable. How to distinguish physical phenomena from chemical ones? Are there any signs that can help to do this?
In chemical reactions, new substances are formed from some substances, which are different from the original ones. By the disappearance of the signs of the first and the appearance of signs of the second, as well as by the release or absorption of energy, we conclude that a chemical reaction has occurred.
If a copper plate is calcined, a black coating appears on its surface; blowing carbon dioxide through lime water produces a white precipitate; when wood burns, drops of water appear on the cold walls of the vessel; when magnesium is burned, a white powder is obtained.
It turns out that the signs of chemical reactions are a change in color, smell, the formation of a precipitate, the appearance of a gas.
When considering chemical reactions, it is necessary to pay attention not only to how they proceed, but also to the conditions that must be met for the reaction to start and proceed.
So, what conditions must be met in order for a chemical reaction to begin?
For this, first of all, it is necessary to bring the reacting substances into contact (combine, mix them). The more crushed the substances, the larger the surface of their contact, the faster and more actively the reaction between them proceeds. For example, lump sugar is difficult to ignite, but crushed and sprayed in the air, it burns out in a matter of fractions of a second, forming a kind of explosion.
With the help of dissolution, we can break the substance into tiny particles. Sometimes the preliminary dissolution of the starting substances facilitates the chemical reaction between the substances.
In some cases, the contact of substances, such as iron with moist air, is enough for a reaction to occur. But more often than not, one contact of substances is not enough for this: some other conditions must be met.
So, copper does not react with atmospheric oxygen at a low temperature of about 20˚-25˚С. To cause the reaction of the combination of copper with oxygen, it is necessary to resort to heating.
Heating affects the occurrence of chemical reactions in different ways. Some reactions require continuous heating. Heating stops - the chemical reaction stops. For example, constant heating is necessary to decompose sugar.
In other cases, heating is required only for the reaction to occur, it gives an impetus, and then the reaction proceeds without heating. For example, we observe such heating during the combustion of magnesium, wood and other combustible substances.
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