The mechanism of chemical reactions when burning.

Publication date 10.02.2013 20:58

The burning is called the oxidation reaction flowing at high speed, which is accompanied by heat release in large quantities and, as a rule, a bright glow that we call the flame. The combustion process studies physical chemistry, in which the burning is made to relate all exothermic processes that have a self-esteem reaction. Such self-esteem may occur due to temperature increase (i.e., have a thermal mechanism) or accumulation of active particles (have a diffusion nature).

The combustion reaction has a visual peculiarity - the presence of a high-temperature region (flame), limited spatially, where, most of the transformation of the source substances (fuel) in combustion products occurs. This process is accompanied by the emission of a large amount of thermal energy. To start the reaction (flame appearance), it is necessary to spend some amount of energy on the ignition, then the process is spontaneously. Its speed depends on the chemical properties of the substances involved in the reaction, as well as from gas-dynamic processes during combustion. The combustion reaction has certain characteristics, the most important of which are the calorific value of the mixture and the temperature (called adiabatic), which theoretically could be achieved with full combustion without taking into account heat loss.

According to the aggregate state of the oxidant and the combustion process, the combustion process can be attributed to one of the three types. The combustion reaction can be:

Homogeneous, if fuel and oxidizing agent (pre-mixed) are in a gaseous state,

Heterogeneous, in which solid or liquid fuel enters into interaction with the gaseous oxidizing agent,

The reaction of burning powder and explosives.

Homogenic burning is the simplest, has a constant speed, depending on the composition and molecular thermal conductivity of the mixture, temperature and pressure.

Heterogeneous burning is most common both in nature and in artificial conditions. Its speed depends on the specific conditions of the combustion process and on the physical characteristics of the ingredients. In liquid combustion on the combustion rate, the rate of evaporation has a large influence, solid - the speed of gasification. For example, when combustion of coal, the process forms two stages. On the first of them (in the case of relatively slow heating), the volatile components of the substance (coal) are distinguished, the coke residue is tramored.

The combustion of gases (for example, the burning of ethane) has its own characteristics. In the gas medium, the flame can spread to an extensive distance. It can move in a gas at a subsonic rate, with this property inherent not only by a gas medium, but also a fine mixture of liquid and solid combustible particles mixed with an oxidizing agent. To ensure sustainable burning in such cases, a special design of the furnace device is required.

The consequences that cause a combustion reaction in a gas environment are two species. The first is the turbulization of the gas stream, leading to a sharp increase in the speed of the process. The arising acoustic perturbations of the flow can lead to the next step - the origin of the shock wave leading to the detonation of the mixture. The combustion transition in the detonation stage depends not only on its own properties of gas, but also from the size of the system and distribution parameters.

Fuel combustion is used in the technique and industry. The main task is to achieve maximum completeness of the combustion (i.e., the optimization of heat generation) for the specified gap. The combustion is used, for example, in highlights - methods for the development of various minerals are based on the use of a combustible process. But in certain natural and geological conditions, the combustion phenomenon can be a factor carrying serious danger. A real danger, for example, represents the process of self-burning peat, leading to the emergence of endogenous fires.

Page 1.

Chemical combustion reactions begin after the creation of the initial focus of the flame in the prepared fuel-air mixture. In the piston DVS, it is created either by electrical spark, or due to the heating of the fuel assembly to such a temperature at which many initial foci of flame occur spontaneously arise in the volume of the mixture.

Chemical combustion reaction occurs not under all conditions collisions of combustible gas molecules with oxygen molecules.

If the chemical combustion reactions are not autocatalytic, the reason for the spread of the flame can only be heat transfer from the combustion products of the unburned mixture. This type of flame spread is called thermal. This, of course, does not exclude the fact that the diffusion of reactants and the reaction products occurs at the same time, so that the composition of the reacting mixture in the reaction zone differs from the composition of the initial mixture. But in this case, the diffusion is not the reason for the spread of the flame, but only a concomitant factor. In particular, this also applies to chain reactions with unbranched chains. Diffusion of free atoms and radicals, unless they are in thermodynamic equilibrium or in quasistationary concentrations, there can be no cause of the spread of a flame that remains thermal. The role of diffusion is fully taken into account in the correct thermal theory of flame distribution, as will be shown in the next section.

If the chemical combustion reactions are not autocatalytic, the reason for the spread of the flame can only be the transfer of heat from the combustion products of the uncomfortable place. This type of flame spread is called thermal. This, of course, does not exclude the fact that the diffusion of reactants and the reaction products occurs at the same time, so that the composition of the reacting mixture in the reaction zone differs from the composition of the initial mixture. But in this case, the diffusion is not the reason for the spread of the flame, but only a concomitant factor. In particular, this also applies to chain reactions with unbranched chains. Diffusion of free atoms and radicals, unless they are in thermodynamic equilibrium or in quasistationary concentrations, there can be no cause of the spread of a flame that remains thermal. The role of diffusion is fully taken into account in the correct thermal theory of flame distribution, as will be shown in the next section.

The speed of chemical gas combustion reactions with air in the burners is very large. These reactions at high temperatures leak over the thousandths of seconds. The duration of the combustion of the gas-air mixture flow is determined by the continuous supply of fresh portions of gas and air, which are burned as a result of the rapid occurrence of oxidation reactions under the action of heat flux.

The speed of chemical gas combustion reactions with air in the burners is very large. These reactions at high speakers proceed for thousands of seconds of a second. The duration of the combustion of the gas-air mixture flow is determined by the continuous supply of fresh portions of gas and air, which are burned as a result of the rapid occurrence of oxidation reactions under the action of heat flux.

The quantitative ratios of chemical combustion reactions can be obtained with known molecular weights of I substances and densities P C / 22 4 gases under normal physical conditions.

The mechanism of inhibition of chemical combustion reactions is not sufficiently studied. However, studies conducted in recent years make it possible to make some ideas about the nature of the impact of flames inhibitors.

Suppose that the chemical burning reaction proceeds completely and the reaction products are H20 water pairs, C02 carbon dioxide or with a lack of oxygen carbon monoxide CO. For stoichiometric hydrogen-oxygen (rattling) combustible mixture by dividing the heat of the formation of water vapor 58 kcal / mol to the heat capacity of 8 cal / mol-hail we obtain the combustion temperature of 7250 degrees. For the case of complete combustion of solid carbon in oxygen (ST 02C02 94 kcal / mol), we obtain the combustion temperature is more than 11,750 K. The temperatures of the same order are also obtained for other hydrocarbon fuels. The fantastically high combustion temperatures given here belong to the plasma state of the substance, they are not actually carried out; The combustion temperatures of oxygen mixtures lie within 3000 - 4000 K.

Since heating and chemical combustion reaction of the mixture proceeds very quickly, the main factor limiting the duration of the combustion process is the time spent on the mixing of gas and air.

Schemes for organizing combustion of combustible gases. Combustion. A - kinetic, b - diffusion, in - mixed.

Since the rate of chemical combustion reactions at high lesion temperatures is incommensurable above the mixture formation rate, then almost the gas combustion rate is always equal to the gas mixing rate with air. This circumstance makes it easy to adjust the gas burning rate in the widest limits. The mixed combustion method of combustible gases is intermediate between kinetic and diffusion.

Therefore, the equation of the balance of the chemical reaction of combustion of candles under certain conditions is indeed the first attempt to introduce the amount of heat in the description of the chemical reaction.

In the preparation of the equations of chemical combustion reactions in the air in the air, they are added as follows: the fuel and the air is written in burning in the left side, after the equality sign, the resulting reaction products are written. For example, it is necessary to draw up the equation of the reaction of the combustion of methane in the air. First, the left part of the reaction equation is recorded: the chemical formula of methane plus the chemical formulas that are part of the air.

Combustion - A complex physico-chemical process, the basis of which is the chemical reaction of the oxidation-reducing type, leading to the redistribution of valence electrons between atoms of interacting molecules.

Examples of burning reactions

methane: CH 4 + 2O 2 \u003d CO 2 + 2N 2 O;

acetylene: C 2 H 2 + 2.5O 2 \u003d 2 + 2 + N 2 O;

sodium: 2NA + CL 2 \u003d 2NACL;

hydrogen: H 2 + CL 2 \u003d 2NCL, 2N 2 + O 2 \u003d 2N 2 O;

total: C 6 H 2 (NO 2) 3 CH 3 \u003d 2.5H 2 O + 3.5CO + 3.5C + 1.5N 2.

The essence of oxidation is the return to the oxidizing substance of valence electrons to the oxidizing agent, which, taking electrons, is restored, the essence of the recovery is the addition of the reducing substance of the reducing agent electrons, which, giving the electrons, is oxidized. As a result of the transfer of electrons, the structure of the outer (valence) electron level of the atom changes. Each at the same time goes to the most sustainable condition in these conditions.

In chemical processes, electrons can completely move from the electronic shell of atoms of one substance (element) into the sheath of the atoms of the other.

Thus, when combustion of metallic sodium in chlorine, sodium atoms are given by one electron of chlorine atoms. At the same time, eight electrons (stable structure), and an atom who lost one electron turns into a positively charged ion turns out at the outer electron level of the sodium atom. At the chlorine atom, which received one electron, the external level is filled with eight electrons, and the atom turns into a negatively charged ion. As a result of the action of Coulomb electrostatic forces, the rapprochement of differently charged ions occurs and sodium chloride molecule (ion connection) is formed:

2mg + o 2 \u003d 2mg 2+ O 2-.

Thus, the combustion of magnesium (oxidation) is accompanied by the transition of its electrons to oxygen. In other processes, the electrons of the external shells of two different atoms come as it were for general use, thereby tightening atoms of molecules ( covalent or atomic Communication):

.

And finally, one atom can give in general use of its own pair of electrons (molecular communication):

.

Conclusions from the provisions of the modern theory of reduction oxidation:

1. The essence of oxidation lies in the loss of electrons by atoms or ions of an oxidizing substance, and the essence of the reduction is in the addition of electrons to atoms or ions of the reducing substance. The process in which the substance loses electrons is called oxidation, and the addition of electrons - restoration.

2. The oxidation of any substance cannot occur without simultaneously restoring another substance. For example, with magnesium burning in oxygen or air, magnesium oxidation occurs and at the same time - the reduction of oxygen. With full combustion, products are formed incapable of further burning (CO 2, H 2 O, HCL, etc.), with incomplete - the resulting products are capable of further burning (CO, H 2 S, HCN, NH 3, Aldehydes and T .d.). Scheme: Alcohol - aldehyde - acid.

I. Burning and slow oxidation

The burning is the first chemical reaction with which a person met. Fire ... Is it possible to introduce our existence without fire? He entered our life, became inseparable from her. Without fire, a person does not cook food, steel, without it impossible traffic of transport. The fire became our other and ally, a symbol of glorious cases, good achievements, the memory of the last.

Memorial of Glory in Syktyvkar

Flame, fire, as one of the manifestations of the burning reaction, has its monumental reflection. Bright example -memorial of Glory in Syktyvkar.

Once every four years there is an event in the world, accompanied by the transfer of the "living" fire. In terms of respect for the founders, the Olympiad fire is delivered from Greece. By tradition, one of the outstanding athletes delivers this torch to the main arena of the Olympiad.

Files are folded, legends. In the old days, people thought that small lizards live in fire - spirits of fire. And there were those who considered the fire the deity and built in his honor the temples. Hundreds of years burned in these temples, not fading, lamps dedicated to the fire of fire. The worship of fire was a consequence of ignorance by people of the combustion process.

Olympic fire

M.V. Lomonosov said: "The study of the nature of fire and without chemistry is not at all impossible."

Combustion - oxidation reaction leaking at quite high speed, accompanied by heat and light release.

Schematically, this oxidation process can be expressed as follows:

Reactions leaking with heat release are called exothermic (from Greek. "Exo" - outward).

When burning there is intensive oxidation, there is a fire in the process of burning, therefore, such oxidation takes place very quickly.If a The reaction rate will be quite large? An explosion may occur. So the mixtures of combustible substances with air or oxygen are exploded. Unfortunately, there are cases of explosions of air mixtures with methane, hydrogen, gasoline pairs, ether, flour and sugar dust, etc., leading to destruction and even human victims.

For the occurrence of the grit, go:

• fuel substance
• oxidizer (oxygen)
• the heating Fuel substanceto ignition temperature

The ignition temperature in each substance is different.

While the ether can ignite from the hot wire, in order to set fire to firewood, you need to heat them up to several hundred degrees. The inflammation temperature of substances is different. Sulfur and wood flammable at a temperature of about 270 ° C, coal - about 350 ° C, and white phosphorus is about 40 ° C.

However, not any oxidation must necessarily be accompanied by the appearance of light.

There is a significant number of oxidation cases that we cannot call burning processes, for they flow so slowly that they remain invisible to our senses. Only after a certain, often very long time we can catch oxidation products. So, for example, it is the case with very slow oxidation (rust) of metals

or in the processes of rotting.

Of course, the heat is distinguished during slow oxidation, but this isolation due to the duration of the process proceeds slowly. However, a piece of wood will burn quickly or exposed to slow oxidation in air for many years, still - in both cases, the same amount of heat is separated.

Slow oxidation - This is the process of slow interaction of oxygen substances with slow heat output (energy).

Examples of the interaction of substances with oxygen without highlight: rotation of manure, leaves, oils, oxidation of metals (iron nozzles with long-term use become thinner and less), respiration of aerobic beings, i.e. breathing oxygen, is accompanied by the release of heat, the formation of carbon dioxide and water.

We will get acquainted with the characteristic of combustion processes and slow oxidation given in the table.

Characteristics of combustion and slow oxidation processes

Signs of reaction	Process
	Combustion	Slow oxidation
Education of new substances	Yes (oxides)	Yes (oxides)
Selection of heat	Yes	Yes
Warm selection rate	Big	Small (goes slowly)
The appearance of light	Yes	Not

IN sip : combustion reactions and slow oxidation are exothermic reactions that differ in the speed of these processes.

II. The thermal effect of the chemical reaction.

Each substance is aware of a certain amount of energy. With this property of substances, we are already facing breakfast, lunch or dinner, as foodstuffs allow our body to use the energy of a wide variety of chemical compounds contained in food. In the body, this energy is transformed into motion, work, goes to maintain constant (and rather high!) Temperature of the body.

Any chemical reaction is accompanied by excretion or energy absorption. Most often, the energy is released or absorbed in the form of heat (less often - in the form of light or mechanical energy). This warmth can be measured. The measurement result is expressed in kilodzhoules (KJ) for one praying reagent or (less often) for praying the reaction product. The amount of heat released or absorbing during a chemical reaction is called thermal effect reaction (Q). For example, the thermal effect of a hydrogen combustion reaction in oxygen can be expressed by any of the two equations:

2 H 2 (g) + o 2 (g) \u003d 2 H 2 O (g) + 572 kJ

2 H 2 (g) + o 2 (g) \u003d 2 h 2 o (g) + q

This reaction equation is calledthermochemicals. Here symbol "+ Q." It means that when the hydrogen burning is highlighted. This heat is called thermal effect reaction. In thermochemical equations, aggregate states often indicate.

Reactions flowing with energy is called exothermic(from Latin "Exo" - outward). For example, burning methane:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + Q

The reactions of energy absorption occurring are called endothermic(from Latin "Endo" - inside). An example is the formation of carbon oxide (II) CO and hydrogen H 2 from coal and water, which occurs only when heated.

C + H 2 O \u003d CO + H 2 - Q

The thermal effects of chemical reactions are needed for many technical calculations.

The thermal effects of chemical reactions are needed for many technical calculations. Imagine yourself for a minute of a powerful rocket designer capable of bringing space ships and other useful loads into orbit (Fig.).

Fig. The most powerful Russian rocket "Energy" in the world before the start on the Baikonur cosmodrome. Engines one of its steps operate on liquefied gases - hydrogen and oxygen.

Suppose you know the work (in KJ), which will have to spend time to deliver the rocket with a cargo from the ground to the orbit, it is also known to work on overcoming air resistance and other energy costs during flight. How to calculate the required supply of hydrogen and oxygen, which (in liquefied state) are used in this rocket as fuel and oxidizing agent?

Without the help of the thermal effect of the reaction of water formation from hydrogen and oxygen, it is difficult to make it difficult. After all, the thermal effect is that the most energy that should withdraw the rocket in orbit. In the combustion chambers of the rocket, this heat turns into the kinetic energy of the chipped gas molecules (steam), which breaks out of the nozzles and creates a reactive traction.

In the chemical industry, thermal effects are needed to calculate the amount of heat to heat the reactors in which endothermic reactions are coming. In the energy sector with the help of heat combustion of fuel, the production of thermal energy is calculated.

Doctors nutritionists use the thermal effects of food oxidation in the body to compile the correct nutritional diet not only for patients, but also for healthy people - athletes, workers of various professions. By tradition, not Jouley, but other energy units - calories (1 cal \u003d 4,1868 J). The energy content of food refers to any mass of food products: to 1 g, to 100 g or even standard packaging of the product. For example, on a label of jars with condensed milk, you can read this inscription: "Caloric content of 320 kcal / 100 g".

№2. Puzzle "non-repeating letters."

To solve this puzzle carefully review each line. Choose from them never repetitive letters. If you do this correctly, you can make a proverb about the rules for handling fire.

ADDITIONALLY:

Topic 3. Chemical Basics of burning.

3.1. Chemistry of combustion reactions.

As you have already understood, the combustion is a fast checkered reaction, accompanied by heat release and the glow (fluid). Typically, this is an exothermic oxidative reaction of the combination of a throat substance with an oxidizing agent - air oxygen.

Combustible substances There may be gases, and liquids, and solid te-la. It is H 2, CO, sulfur, phosphorus, metals, with M H n (hydrocarbons in the form of gases, liquids and solids, i.e. organic substances. Natural UG-leopodors, for example, are natural gas, oil, coal). In principle, all substances capable of oxidation can be combustible.

Oxidifiers Serve: oxygen, ozone, halogens (F, cl, br, j), nitrogen oxide (NO 2), ammonium nitrate (NH 4 NO 3) and other metals with oxidizers can also be CO 2, H 2 O, N 2 .

In some cases, combustion occurs in the reactions of decomposition of the substances obtained in endothermic processes. For example, when decaying acetylene:

C 2 H 2 \u003d 2C + H 2.

Exothermic Reactions are reactions passing with heat release.

Endothermic Reactions are reactions passing with heat absorption.

For example:

2N 2 + O 2 \u003d 2N 2 O + Q - Exothermic reaction,

2N 2 O + Q \u003d 2N 2 + O 2 - endothermic reaction,

where: Q is thermal energy.

Thus, endothermic reactions can only flow with external thermal energy, i.e. When heated.

In chemical reactions, according to the law of maintaining mass of substances to the reaction, equals weight of substances formed after the reaction. When the chemical equations are equalized stoichiometric Compositions.

For example, in the reaction

CH 4 + 2O 2 \u003d CO 2 + 2N 2

we have 1 mole CH 4 + 2 praying o 2 \u003d 1 mol CO 2 + 2 praying H 2 O.

The number of moles in front of the formulas of substances is called stoichiometry coefficients.

Considering the concept of "molar volume", "molar concentration", "partial pressure", we obtain that 1 mole 4 with 2 miles o 2, or 1/3 \u003d 33.3% 2, or 1/3 \u003d 33.3% CH 4 and 2/3 moles should be mixed to fully respond methane. 3 \u003d 66.7% O 2. This composition is called stoichiometric.

If we consider burning CH 4 in the air, i.e. In the mixture of 21% o 2 + 79% N 2 or 2 + 79 / 21N 2 or 2 + 3.76N 2, the reaction will be recorded as follows:

CH 4 + 2O 2 + 2 × 3.76N 2 \u003d CO 2 + 2N 2 O + 2 × 3.76N 2.

1 mole CH 4 +2 mole o 2 +7.52 mole N 2 \u003d 10.52 praying mixture O 2, N 2 and CH 4.

Then the stoichiometric composition of the mixture will be:

(1 / 10.52) * 100% \u003d 9.5% CH 4; (2 / 10.52) * 100% \u003d 19.0% O 2;

(7.52 / 10.52) * 100% \u003d 71.5% N 2.

So in the most combustible mixture instead of 100% (CH 4 + O 2) in the reaction with oxygen will be 24% (CH 4 + O 2) in the reaction with air, i.e. The heat is highlighted significantly less.

The same picture will be able to mix arbitrary, non-stoichiometric compositions.

For example, in the reaction 2CH 4 + 2O 2 \u003d CO 2 + 2N 2 O + CH 4 1 mole CH 4 does not react.

In the reaction CH 4 + 4O 2 \u003d CO 2 + 2N 2 O + 2O 2 2 Praying 2 does not participate in the reaction, but play the role of ballast, requiring some kind of heat for their heating.

Thus, if we compare the reactions of methane burning in oxygen and air or in excess CH 4 and O 2, it is clear that the amount of heat released in the first reaction will be greater than in the rest, as in them:

Less concentrations of reacting substances in a common mixture;

We will take part of the heat to heat the ballast: nitrogen, oxygen or methane.

Let us ask questions:

What energy can stand out when reactions?

The amount of heat depends on, i.e. The thermal effect of

How much do you need to add thermal energy to flow

endothermal reaction?

For this, the concept of heat-containing substance has been introduced.

3.2. The containing substances.

Where did the heat in the reaction of methane burning? So it was hidden in CH 4 and O 2 molecules, and now he was released.

Let us give an example of a simpler reaction:

2N 2 + O 2 \u003d 2N 2 O + Q

So the energy level of the stoichiometric mixture of hydrogen with oxygen was higher than that of the product of H 2 O and "excess" the energy you are free from the substance.

With the reverse reaction of the electrolysis of water, i.e. The decomposition of water with the help of electrical energy, there is a redistribution of atoms in the mole-coole of water with the formation of hydrogen and oxygen. In this case, the heat generation H 2 and 2 rises.

Thus, each substance in its formation receives or from-gives a certain energy, and the measure of thermal energy, accumulated by the thing in its formation, is called heat-containing or entalpy.

In contrast to chemistry, in the chemical thermodynamics of the heat of the substance, the substance is not denoted by the symbol Q, but the symbol of DN with the sign (+), if the heat is absorbed by the chemical compound, and with the sign (-), if the heat is released during the reaction, that is, "leaves" from Systems.

The standard heat of formation 1 praying substance at a pressure of 101.3 kPa and a temperature of 298 to is denoted.

In reference books there are warmth of the formation of compounds from pro-stable substances.

For example:

From 2 \u003d - 393,5 kJ / mol

N 2 O GAZ \u003d - 241.8 KJ / MOL

But in the substances formed under endothermic processes, for example, acetylene C 2 H 2 \u003d +226.8 kJ / mol, when the hydrogen-type atom is formed by the reaction H 2 \u003d H + + N + \u003d + 217.9 KJ / mol.

For pure substances consisting of one chemical element in a mustache form (H 2, O 2, C, NA, etc.), DN is conditionally adopted equal to zero.

However, if we discuss the macroscopic properties of substances, then we allocate several forms of energy: kinetic, potential, chemical, whim, electrical, thermal, nuclear energy and mechanical work. And if we consider the question at the molecular level, these forms of energy can be explained on the basis of only two forms of the kinetic energy of the movement and the potential energy of resting atoms and molecules.

With chemical reactions, only molecules are changed. Atoms remain unchanged. Molecule energy - This is the binding energy of its atoms, the prisoner in the molecule. It is determined by the attraction of atoms to each other. In addition, there is a potential energy of attraction of molecules to each other. In the gases, it is small, in liquids more and even more in solids.

Each atom has energy, part of which is associated with electrons, and part with the kernel. Electrons have the kinetic energy of rotation in the circle of the core and the potential electrical energy of attraction to each other and repulsion from each other.

The sum of these molecular energy forms is the heat generation of the molecule.

If the heat generation of 6.02 × 10 23 molecules of substance, then we will get the molar heat generation of this substance.

Why the heat-containing of one-element substances (molecules of one element) is taken over zero, can be explained as follows.

DN of the chemical element, that is, the energy of its formation is associated with internal studies. Nuclear energy is associated with the forces of interaction of internal particles and the transformation of one chemical element in another under nuclear reactions. For example, uranium decay reaction:

or easier: U + N®BA + KR + 3N.

where: n. ’ O. - neutron particle with weight 1 and zero charge.

The uranium captures the neutron, as a result of which it is split (disintegrating) into two new elements - barium and crypton - with the formation of 3 x neutrons, and nuclear energy is distinguished.

It should be said that large changes in energy are associated with nuclear reactions than with chemical reactions. Thus, uranium decay energy is 4.5 × 10 9 kcal / mol × uranium. This is 10 million times larger than when combustion of one praying coal.

In chemical reactions, atoms do not change, but molecules are changed. Therefore, the energy of atomic formation by chemists is not taken into account, and the DN of single-element gas molecules and atoms of pure substances are taken equal to zero.

The reduced reaction of the decay of uranium is a classic example of a chain reaction. The theory of the chain mechanism of the burning reaction we will consider later. But where the neutron comes from and what makes it react with uranium - this is due to the so-called activation energy that Ras-looking a little later.

3.3. The thermal effect of the reaction.

The fact that a certain amount of energy is concluded in each individual substance, serves as an explanation of the thermal effects of chemical reactions.

According to the GESS law: the thermal effect of the chemical reaction depends only on the nature of the initial and final products and does not depend on the number of industrial reactions of transition from one state to another.

Corollary 1. This Law: The thermal effect of the chemical reaction is equal to the difference between the sum of the heat of the formation of the final products and the sum of the heat of the formation of the source substances, taking into account the coefficients in the form-lax of these substances in the reaction equation.

For example, in the reaction 2N 2 + O 2 \u003d 2N 2 O ± DN.

; ; .

As a result, the general response equation will look like this:

2N 2 + O 2 \u003d 2N 2 O - 582 kJ / mol.

And if DN is familiar (-), then the reaction is exothermic.

Corollary 2.. According to the law of Lavoisier-Laplace, the thermal effect of the decomposition of the chemical compound is equal and the opposite of the thermal effect of its formation.

Then the water decomposition reaction will be:

2N 2 O \u003d 2N 2 + O 2 +582 kJ / mol, i.e. This reaction is endothermic.

An example of a more complex reaction:

CH 4 + 2O 2 \u003d CO 2 + 2N 2 O.

Then the reaction will be recorded like this:

CH 4 + 2O 2 \u003d CO 2 + 2N 2 O - 742.3 kJ / mol, then the reaction is exothermic-skye.

3.4. Kinetic basics of gas reactions.

Under the law of the active masses, the reaction rate at a constant temperature is proportional to the concentration of reacting substances or, as a govo-rip, "active masses".

Chemical reaction rate ( υ ) It is customary to consider the number of things that responds to a unit of time ( d.t.) per unit volume ( dV).

Consider the reaction flowing through the equation:

A + B \u003d C + D.

Since the reaction rate characterizes a decrease in the time of the concentration of reacting substances and an increase in the concentration of reaction products, you can write:

, (3.1)

where minuses in derivatives are indicated about the direction of change in the concentration of components, and the concentrations of components are indicated in square brackets.

Then the direct irreversible reaction at T \u003d const flows with Sprot:

, (3.2)

where: k. - Chemical reaction rate constant. It does not depend on the concentration of components, but changes only with temperature.

According to the law of the current mass concentration of the reaction components, it is necessary to the kinetic equation to the degree equal to the stoichiometric coefficient of this component.

So, for reaction

aA + BB \u003d CC + DD

The kinetic equation is:

The indicators of the degrees A, B, C, D are taken to be called the reaction orders of components A, B, C, D, and the amount of indicators - the general order of reaction.

For example, type reactions

A ® BB + CC - I order

2a \u003d BB + CC - II order,

A + b \u003d cc + DD - III order.

Since the concentrations of all reacting components are associated with the stoichiometric equations, the simplest kinetic equations I of the order are differential equations I of order with a single independent variable - concentration - and may be integrid-rovany.

The simplest kinetic equation is the type I equation i

for which . (3.4)

Denote by the concentration of component A before the reaction and, injecting the equation under the boundary condition T \u003d 0, [A] \u003d [A 0], we obtain:

Or [a] \u003d × e - kt. (3.5)

Thus, the dependence of the reaction rate on the concentration of substances is exponential.

The kinetic energy of gases explains it so. According to the hypothesis of Arrhenius, the reaction between molecules passes only if they are active, i.e. have excessive energy sufficient to break the interatomic bonds, the so-called activation energy E A.

Those. The speed of the chemical reaction depends not on the number of collisions of all molecules, but only activated.

According to the Boltzmann law, the number of active molecules

n a \u003d n o * e - E / RT, (3.6)

where: e is the activation energy,

T - the temperature of the gas mixture,

n O is the total number of molecules.

Then the number of effective collisions coincides with the reaction rate is:

υ p \u003d z Eff \u003d z 0 * E - E / RT, (3.7)

where: z 0 is the total number of collisions of molecules.

1) the reaction rate is proportional to the concentration of active mole-cooler, the number of which depends on the temperature and pressure in the mixture, since the pressure and the number of molecules encountered from any surface;

2) The reaction is possible only if the interacting Moles receive a certain stock of energy, sufficient to break or weaken interatomic bonds. Activation lies in the transition of molecules into such a state in which the chemical transformation is possible.

Most often, the activation process is based on the formation of intermediate unstable, but highly active compounds of atoms.

Thus, not only for the flow of endothermic processes, an outer supply of energy is needed, but also for exothermic. To make an exothermic reaction, it is necessary to tell her some kind of thermal energy pulse. For example, to flow the combustion reaction in a mixture of hydrogen with a circuit, it is necessary to set fire to it.

The minimum amount of thermal energy required for the "launcher" of the chemical reaction is called activation energy.

3.5. Reaction activation energy.

To explain this phenomenon, often use the following example (Fig. 9):

The playground lies the ball. The site is located in front of the slide. Therefore, the ball could ride himself down if it were not a slide. But for spontaneous descent, it should be raised to the top of the slide. At the same time, not only the energy of lifting is free, but also the energy of the descent is down.

Fig. 9. Reaction activation scheme.

Consider two reactions:

1) H 2 + O 2 \u003d H 2

2) H 2 O \u003d H 2 + O 2 +

As can be seen from the figure, E 2 \u003d + E 1;

In general, with any reaction

.

And on the difference E 1 and E 2, which are always positive, the sign of the tech effect depends.

Thus, activation energy is the energy required for the pre-rotation of the reactant substances into the state of the active complex (the gap of the interatomic bonds, the convergence of molecules, the accumulation of energy in the molecule ...).

The proportion of active molecules (E -E / RT) increases with an increase in gases, which means the reaction rate of exponential dependence. This dependence can be illustrated as follows:

Fig. 10. The dependence of the reaction rate of the reaction from temperature: 1 is the rate of 1st reaction, 2 is the 2nd reaction rate.

As can be seen from Figure 10, the rate of the first reaction is less than the speed of the second reaction, and the activation energy of the 1st reaction is greater than the second. And at the same temperature T 2 υ 2 > υ 1 . The greater the activation energy, the higher the temperature necessary to achieve this reaction rate.

The reason for this is that when E is larger, then existing intergatase bonds in the molecules of reacting components are stronger, and more energy to overcome these forces. In this case, the proportion of active molecules is less than.

It shows that the amount of activation energy is the most important characteristic of the chemical process. It determines the height of the energy barrier, the overcoming of which is a condition that flows the reaction. On the other hand, it characterizes the reaction rate on temperature, i.e. The higher the activation energy, the higher the temperature to achieve a given reaction.

3.6. Catalysis.

In addition to increasing the temperature and concentration of substances, the chemical reaction is used for acceleration catalysts. Substances that are introduced into the reacting mixture, but are not spent in reaction, but accelerates it by reducing activation energy.

The process of increasing the reaction rate with catalysts is naked - catalysis.

Catalysts are involved in intermediate reactions to create an ac-studied complex due to the weakening of bonds in the molecules of the source substances, their decomposition, adsorption of molecules on the surface of the catalyst, or input of the active particles of the catalyst.

The nature of the participation of the catalyst can be illustrated by the following scheme:

Reaction without a catalyst: A + B \u003d AB.

With catalyst X: a + x \u003d ah ® ah + B \u003d Av + \u200b\u200bH.

We give a picture like presented in Fig. nine.

Fig. 11. Scheme of the action of kata-licker: E B.Kat and E with cat- Energy activation of the reaction without ka-altar and with a catalyst, respectively.

When entering the catalyst (Fig. 11), the reaction may flow over another way with a smaller energy barrier. This path corresponds to a new reaction mechanism through the formation of another activated complex. And a new lower energy barrier can overcome a larger number of particles, which leads to an increase in the reaction rate.

It should be noted that the activation energy of the reverse reaction lowers the same value as the activation energy of the direct reaction, i.e. Both reactions are accelerated the same, and the catalysts do not initiate the reaction, they will only speed up the reaction, which can occur in their absence, but much slower.

Intermediate reaction products can be catalysts, then this reaction is called autocatalytic. So, if the speed of ordinary re-stocks decreases as the reacting substances exemplies are spent, the combustion reaction due to autocatalysis is self-esteem and is autocatalytic.

Most often, solid affairs are used as catalysts that adsorb the molecules of the reactants. During adsorption, communications in reacting molecules weaken, and thus facilitates the reaction between them.

What is adsorption?

3.7. Adsorption.

Adsorption - Surface absorption of any substance from a ga-zoo-shaped medium or a solution with a surface layer of another substance - liquid or solid.

For example, adsorption of toxic gases on the surface of the activated carbon used in the gas masks.

There are physical and chemical adsorption.

For physical adsorption captured particles retain their properties, and when chemical - Adsorbate chemical compounds are formed.

The adsorption process is accompanied by the release of heat. In physical adsorption, it is insignificant (1-5 kcal / mol), in chemical - significantly more (10-100 kcal / mol). Thus, chemical reactions during catalysis can accelerate.

For burning and explosion processes, you can bring the following examples:

1. The temperature of the self-ignition temperature of the mixture H 2 + O 2 is 500 0 C. In the presence of a palladium catalyst, it decreases to 100 0 C.

2. The processes of self-burning coal begin with the chemical adsorption of oxygen on the surface of coal particles.

3. When working with pure oxygen on clothing, oxygen (physical adsorption) is well adsorbed. And in the presence of a spark or flame clothing easily flashes.

4. Oxygen is well adsorbed and absorbed by technical masses to form an explosive mixture. The mixture explodes spontaneously, without a source of ignition (chemical absorption).