Conditions for the occurrence of chemical reactions and the theory of elementary interactions. Conditions for the spontaneous occurrence of a chemical reaction The condition for a chemical reaction to occur can be

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 proceed, 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.

Chemical reaction rate is the change in the amount of a reactant or reaction product per unit time per unit volume (for a homogeneous reaction) or per unit interface surface (for a heterogeneous reaction).

Law of mass action: 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 the reaction rate on concentration.

Solids are 0

Molecularity of the reaction is the minimum number of molecules involved in an elementary chemical process. Based on 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-degree increase in temperature, the rate constant of a homogeneous elementary reaction increases two to four times

Active collision theory(TAC), there are three conditions necessary for a reaction to occur:

Molecules must collide. This is an important condition, but it is not sufficient, since a collision does not necessarily cause a reaction.

Molecules must have the necessary energy (activation energy).

The molecules must be correctly oriented relative to each other.

Activation energy- the minimum amount of energy that is required to 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.

The 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 and is not included in the final products.

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 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.

Chemical equilibrium conditions

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 that is in equilibrium, then the equilibrium will shift towards 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 starting substances into reaction products.

ΔG<0 К р >1 From pr > From out

ΔG>0 K p<1 С пр <С исх

§ 1 Signs of chemical reactions

In chemical reactions, starting substances are transformed into other substances that have 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 the substance.

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 has turned black.

Add 1-2 ml of acetic acid solution to baking soda powder. We observe the appearance of gas bubbles and the disappearance of soda.

Add 3-4 ml of copper chloride solution to the sodium hydroxide solution. In this case, the blue transparent solution will turn into a bright blue precipitate.

Add 1-2 drops of iodine solution to 2 ml of starch solution. And the translucent white liquid will become an 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 reaction:

Precipitation;

Color change;

Gas release;

Odor appears;

The release or absorption of energy in the form of heat, electricity, or light.

For example, if you bring a lighted splinter to a mixture of hydrogen and oxygen or pass an electric discharge through this mixture, a deafening explosion will occur, and a new substance will form on the walls of the vessel - water. A reaction occurred in 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 begin, the molecules of alcohol and oxygen must come into contact. But if we open the cap of the alcohol lamp, then when the starting substances - alcohol and oxygen - come into contact, no reaction occurs. Let's bring a lit match. The alcohol on the wick of the alcohol lamp heats up and ignites, and a combustion reaction begins. The condition necessary for the reaction to occur here is initial heating.

Pour a 3% solution of hydrogen peroxide into a test tube. If we leave the test tube open, the hydrogen peroxide will slowly begin to decompose into water and oxygen. In this case, the reaction rate will be so low that we will not see any signs of gas evolution. Add some black manganese (IV) oxide powder. We observe rapid gas release. This is oxygen that was formed during the decomposition reaction 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.

It is obvious that for the occurrence and course of chemical reactions certain conditions are necessary, namely:

Contact of starting substances (reagents),

Heating them to a certain temperature,

Application 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.

Dmitry Ivanovich Mendeleev pointed out that the most important feature of all chemical reactions is the change in energy during their occurrence.

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.

Reactions that occur with the absorption of heat are called endothermic.

The amount of heat released or absorbed is called the thermal effect of a 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 branch of chemistry that studies the thermal effects of chemical reactions is called thermochemistry. The first studies of thermochemical phenomena belonged to the scientist Nikolai Nikolaevich Beketov.

The value of the thermal effect is referred to 1 mole of the substance and expressed in kilojoules (kJ).

Most chemical processes occurring in nature, laboratory and industry are exothermic. These include all reactions of combustion, oxidation, combinations of metals with other elements, and others.

However, there are also endothermic processes, for example the decomposition of water under the influence of electric current.

The thermal effects of chemical reactions vary widely from 4 to 500 kJ/mol. The thermal effect is most significant during combustion reactions.

Let's try to explain the essence of the ongoing transformations of substances and what happens to the atoms of the reacting substances. According to the atomic-molecular theory, all substances consist of atoms connected to each other into molecules or other particles. During the reaction process, the starting substances (reagents) are destroyed and new substances (reaction products) are formed. Thus, all reactions come down 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:

1. NOT. Kuznetsova. Chemistry. 8th grade. Textbook for general education institutions. – M. Ventana-Graf, 2012.