What are harsh conditions in chemical reactions? Signs and conditions for chemical reactions

In industry, 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 presented decomposition reactions water and sodium bicarbonate. Shown is a device for passing direct electric current through water. The cathode and anode are metal plates immersed in water and connected to a source of electric current. Due to the fact that pure water practically does not conduct electric current, 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 turns out to be twice as large as in the tube where oxygen is collected (its presence can be verified with the help of a smoldering splinter). The model diagram demonstrates the reaction of water decomposition. Chemical (covalent) bonds between atoms in water molecules are destroyed, and molecules of hydrogen and oxygen are formed from the released atoms.

Model diagram connection reactions metallic iron and molecular sulfur S 8 shows that as a result of rearrangement of atoms during the reaction, iron sulfide is formed. In this case, the chemical bonds in the iron crystal (metallic bond) and the sulfur molecule (covalent bond) are destroyed, and the released atoms are combined to form ionic bonds to form a salt crystal.

Another reaction of the compound is the slaking of lime with CaO with water to form calcium hydroxide. At the same time, the burnt (quicklime) lime begins to heat up and loose slaked lime powder 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. The more active metal displaces the less active metal from the solution of its salt.

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 progress of the neutralization reaction is monitored using the phenolphthalein indicator (the crimson color disappears).

Table 12

Types of chemical reactions

AIR. OXYGEN. COMBUSTION

Oxygen is the most abundant chemical element on Earth. Its content in the earth's crust and hydrosphere is presented in Table 2 "Occurrence of chemical elements." Oxygen accounts for approximately half (47%) of the mass of the lithosphere. It is the predominant chemical element of 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 contains 20.9% free oxygen by volume. Air is a complex mixture of gases. Dry air consists of 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) (dianitrogen oxide, nitrogen hemioxide - N 2 O), ozone, sulfur dioxide, carbon monoxide, xenon, nitric oxide ( IV) (nitrogen dioxide – NO 2).

The composition of air was determined by the French chemist Antoine Laurent Lavoisier at the end of the 18th century (Table 13). He proved the oxygen content in the air and called it “life air.” To do this, he heated mercury on a stove in a glass retort, the thin part of which was placed under a glass cap placed in a water bath. The air under the hood turned out to be closed. When heated, mercury combined with oxygen, turning into red mercuric oxide. The “air” remaining in the glass bell after heating the mercury did not contain oxygen. The mouse, placed under the hood, was suffocating. Having calcined the mercury oxide, Lavoisier again isolated oxygen from it and again obtained pure mercury.

The oxygen content in the atmosphere began to increase noticeably 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 = C 6 H 12 O 6 + 6O 2.

The resulting glucose becomes insoluble in water starch, which 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, when visible light (red and violet) is absorbed, electrons transition from one energy state to another. Only a small portion of solar energy (0.03%) reaching the Earth's surface is consumed for photosynthesis.

All carbon dioxide on Earth goes through the photosynthesis cycle on average in 300 years, oxygen in 2000 years, and ocean water in 2 million years. Currently, a constant oxygen content has been established in the atmosphere. It is almost completely spent on respiration, combustion and decay of organic substances.

Oxygen is one of the most active substances. Processes involving oxygen are called oxidation reactions. These include combustion, breathing, rotting and many others. The table shows the combustion of oil, which occurs with the release of heat and light.

Combustion reactions can bring not only benefits, but also harm. Combustion can be stopped by cutting off the access of air (oxidizer) to the burning object using 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, located in a glass ampoule at the top of the fire extinguisher, a foam of carbon dioxide is formed. To activate the fire extinguisher, turn it over and hit the floor with a metal pin. In this case, the ampoule with sulfuric acid breaks and the carbon dioxide formed as a result of the reaction of the acid with sodium bicarbonate foams the liquid and throws it out of the fire extinguisher in a strong stream. Foamy liquid and carbon dioxide, enveloping a burning object, pushes away the air and extinguishes the flame.

Throughout our lives, we constantly encounter 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 constantly occur in our body. The energy that is released during chemical reactions is constantly used in everyday life, in production, and when launching spaceships. Many of the materials from which the things around us are made are not taken from nature in a ready-made form, but are made using chemical reactions. In everyday life, it doesn’t make much sense for us to figure out what happened. But when studying physics and chemistry at a sufficient level, you cannot do without this knowledge. How to distinguish physical phenomena from chemical ones? Are there any signs that can help to do this?

During chemical reactions, new substances are formed from some substances, different from the original ones. By the disappearance of signs of the former and the appearance of signs of the latter, as well as by the release or absorption of energy, we conclude that a chemical reaction has occurred.

If you heat a copper plate, a black coating appears on its surface; When carbon dioxide is blown through lime water, a white precipitate forms; when wood burns, drops of water appear on the cold walls of the vessel; when magnesium burns, a white powder is obtained.

It turns out that signs of a chemical reaction are changes in color, smell, formation of sediment, and the appearance of gas.

When considering chemical reactions, it is necessary to pay attention not only to how they occur, but also to the conditions that must be met for the reaction to begin and proceed.

So, what conditions must be met for a chemical reaction to begin?

To do this, first of all, it is necessary to bring the reacting substances into contact (combine, mix them). The more crushed the substances are, the larger the surface of their contact, the faster and more active the reaction between them occurs. For example, lump sugar is difficult to set on fire, but crushed and sprayed in the air it burns in a matter of seconds, forming a kind of explosion.

With the help of dissolution, we can crush a substance into tiny particles. Sometimes preliminary dissolution of the starting substances facilitates the chemical reaction between the substances.

In some cases, the contact of substances, for example, iron with moist air, is enough for a reaction to occur. But more often than not, the contact of substances alone is not enough for this: some other conditions must be met.

Thus, copper does not react with air oxygen at low temperatures of about 20˚-25˚С. To cause a reaction between copper and oxygen, it is necessary to use heat.

Heating affects the occurrence of chemical reactions in different ways. Some reactions require continuous heating. When heating stops, the chemical reaction stops. For example, constant heat is required 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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I. Signs and conditions for chemical reactions

You already know many substances, have observed their transformations and the transformations accompanying these transformations. signs.

The most main feature A chemical reaction is the formation of new substances. But this can also be judged by some external signs of the reactions occurring.

External signs of chemical reactions occurring:

• precipitation
• color change
• gas evolution
• appearance of 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 starting substances (reagents)
• heating to a certain temperature
• the use of substances that accelerate chemical reactions (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 during their occurrence.

Each substance stores a certain amount of energy. We encounter this property of substances already at breakfast, lunch or dinner, since 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 quite high!) body temperature.

The release or absorption of heat during 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 manifest themselves either in the release or absorption of heat.

Reactions that occur with the release of heat are called exothermic (from the Greek “exo” - out).

Reactions that occur 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 measurement result is expressed in kilojoules (kJ) for one MOLE of reactant or (less commonly) for one mole of reaction product. The amount of heat released or absorbed during a chemical reaction is called thermal effect of reaction(Q).

Exothermic reaction:

Starting substances → reaction products + Q kJ

Endothermic reaction:

Starting substances → reaction products - Q kJ

The 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.

Let's say you know the work (in kJ) that will have to be spent to deliver a rocket with cargo from the surface of the Earth 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 launch the rocket into orbit. In the combustion chambers of a rocket, this heat is converted into the kinetic energy of molecules of hot gas (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 occur. In the energy sector, thermal energy production is calculated using the heat of combustion of fuel.

Dietitians use the thermal effects of food oxidation in the body to create proper diets not only for patients, but also for healthy people - athletes, workers in various professions. Traditionally, calculations here use not joules, but other energy units - calories (1 cal = 4.1868 J). The energy content of food is referred to any mass of food products: 1 g, 100 g, or even 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 area of ​​chemistry that deals with the study of thermal effects and chemical reactions is called thermochemistry.

Equations of chemical reactions in which the thermal effect is indicated are called thermochemical.

In Chapter 5.2 we learned about the basic principles of chemical reactions. They constitute the theory of elementary interactions.

§ 5.3.1 Theory of elementary interactions

Listed below main provisions TEV answer the question:

What is necessary for chemical reactions to occur?

1. A chemical reaction is initiated by active reagent particles other than saturated molecules: radicals, ions, coordinatively unsaturated compounds. The reactivity of the starting substances is determined by the presence of these active particles in their composition.

Chemistry identifies three main factors influencing a chemical reaction:

• temperature;
• catalyst (if needed);
• nature of the reacting substances.

Of these, the most important is the last one. It is the nature of a substance that determines its ability to form certain active particles. And incentives only help this process to happen.

2. Active particles are in thermodynamic equilibrium with the original saturated molecules.

3. Active particles interact with the original molecules via a chain mechanism.

4. The interaction between the active particle and the reagent molecule occurs in three stages: association, electronic isomerization and dissociation.

At the first stage of a chemical reaction, the association stage, the active particle attaches to a saturated molecule of another reagent using chemical bonds that are weaker than covalent ones. An associate can be formed using van der Waals, hydrogen, donor-acceptor and dynamic bonds.

At the second stage of the chemical reaction - the stage of electronic isomerization - the most important process occurs - the transformation of a strong covalent bond in the initial reagent molecule into a weaker one: hydrogen, donor-acceptor, dynamic, or even van der Waals.

5. The third stage of interaction between the active particle and the reagent molecule - dissociation of the isomerized associate with the formation of the final reaction product - is the limiting and slowest stage of the entire process.

The great “cunning” of the chemical nature of substances

It is this stage that determines the total energy costs for the entire three-stage process of the chemical reaction. And here lies the great “cunning” of the chemical nature of substances. The most energy-consuming process - breaking the covalent bond in the reagent - occurred easily and gracefully, almost unnoticeably in time compared to the third, limiting stage of the reaction. In our example, the bond in a hydrogen molecule with an energy of 430 kJ/mol was so easily and naturally transformed into a van der Waals bond with an energy of 20 kJ/mol. And all the energy consumption of the reaction was reduced to breaking this weak van der Waals bond. This is why the energy costs required to break a covalent bond chemically are significantly less than the costs of thermal destruction of this bond.

Thus, the theory of elementary interactions gives a strict physical meaning to the concept of “activation energy”. This is the energy required to break the corresponding chemical bond in an associate, the formation of which precedes the production of the final product of a chemical reaction.

We once again emphasize the unity of the chemical nature of the substance. It can react only in one case: when an active particle appears. And temperature, catalyst and other factors, despite all their physical differences, play the same role: the initiator.

1. Indicate whether the phenomena depicted in the pictures are physical or chemical.

2. Match.

Examples of chemical reactions:
I. interaction of marble with hydrochloric acid;
II. interaction of iron with sulfur;
III. hydrogen peroxide decomposition;
IV. interaction of carbon dioxide with lime water.

Conditions for chemical reactions to occur:
a) contact of substances;
b) heating;
c) use of a catalyst.

Answer: I - a; II - a, b; III - in; IV - a.

3. Fill out diagram 2.

4. "Crossword - in reverse." All the words in the crossword have already been entered. Define each word as precisely as possible.

"Key word" is the first chemical reaction with which man became acquainted.

1. One of the four states of matter.
2. Formation of a solid in solution during a chemical reaction.
3. The position of two or more bodies, objects, substances.
4. A portable or mobile device for extinguishing fires.
5. The process is characterized by an increase in temperature.
6. A chemical substance that accelerates a reaction, but is not part of the reaction products.
7. The impact of objects on each other.