Combustion reaction products. Chemical combustion reactions

During chemical reactions from one substances, others are obtained (not to be confused with nuclear reactions in which one chemical element turns into another).

Any chemical reaction is described by a chemical equation:

Reagents → Reaction Products

The arrow indicates the direction of the reaction.

For example:

In this reaction, methane (CH 4) reacts with oxygen (O 2), as a result of which carbon dioxide is formed (CO 2) and water (H 2 O), or rather water vapor. It is this reaction that happens in your kitchen when you set fire on the gas burner. You should read the equation: one molecule of gaseous methane reaches two molecules of gaseous oxygen, the result is one carbon dioxide molecule and two water molecules (water vapor).

The numbers arranged in front of the components of the chemical reaction are called coefficients reaction.

Chemical reactions There are endothermal (with energy absorption) and exothermic (with energy release). Methane burning is a typical example of an exothermic reaction.

There are several types of chemical reactions. The most common:

• connection reactions;
• decomposition reactions;
• reactions of single substitution;
• double replacement reactions;
• oxidation reactions;
• redox reactions.

Connection reactions

In the connection reactions, at least two elements form one product:

2NA (T) + Cl 2 (g) → 2NACL (T) - Education of cook salt.

Attention should be paid to a significant nuance of reactions of the compound: depending on the conditions of the reaction or the proportions of reactants reacting, there may be different products. For example, as normal conditions Combustion coal Carbon dioxide is obtained:
C (T) + O 2 (g) → CO 2 (g)

If the amount of oxygen is not enough, it is formed deadly dangling gas:
2C (T) + O 2 (g) → 2CO (g)

Reaction decomposition

These reactions are, as if, opposite in essence, connection reactions. As a result of the decomposition reaction, the substance disintegrates two (3, 4 ...) more simple item (Connections):

• 2H 2 O (g) → 2h 2 (g) + o 2 (g) - water decomposition
• 2H 2 O 2 (g) → 2H 2 (g) O + O 2 (g) - Decomposition of hydrogen peroxide

Single replacement reactions

As a result of single substitution reactions, a more active element replaces less active in conjunction:

Zn (T) + CUSO 4 (P-P) → ZNSO 4 (P-P) + Cu (T)

Zinc in the solution of copper sulfate displaces less active copper, as a result of which zinc sulfate solution is formed.

The degree of activity of metals ascending activity:

• Alkaline and alkaline earth metals are most active.

The ion equation of the above reaction will be:

Zn (T) + Cu 2+ + SO 4 2- → Zn 2+ + SO 4 2- + Cu (T)

The ionic connection of Cuso 4 when dissolved in water decomposes on the copper cation (charge 2+) and sulfate anion (charge 2-). As a result of the reaction of substitution, zinc cation is formed (which is the same charge as the copper cation: 2-). Please note that the sulfate anion is present in both parts of the equation, i.e., according to all the rules of mathematics, it can be reduced. As a result, the ion-molecular equation will be obtained:

Zn (T) + Cu 2+ → Zn 2+ + Cu (T)

Double replacement reactions

Two electrons have already reacted in the double substitution reactions. Such reactions are also called exchange reactions. Such reactions are in solution with education:

• insoluble solid (deposition reaction);
• water (neutralization reactions).

Deposition reactions

When mixing the silver nitrate solution (salt) with a solution of sodium chloride, silver chloride is formed:

Molecular equation: KCL (P-P) + AGNO 3 (P-P) → AGCL (T) + KNO 3 (P-P)

Ion equation: K + + Cl - + AG + + NO 3 - → AGCl (T) + K + + NO 3 -

Molecular ion equation: Cl - + AG + → AGCL (T)

If the compound is soluble, it will be in the solution in ion form. If the compound is insoluble, it will be precipitated by forming a solid.

Neutralization reactions

These are the reactions of the interaction of acids and bases, as a result of which water molecules are formed.

For example, the mixing reaction of sulfuric acid solution and sodium hydroxide solution (liquor):

Molecular equation: H 2 SO 4 (P-P) + 2NAOH (P-P) → Na 2 SO 4 (P-P) + 2H 2 O (g)

Ion equation: 2H + + SO 4 2- + 2NA + + 2OH - → 2NA + + SO 4 2- + 2H 2 O (g)

Molecular ion equation: 2H + + 2OH - → 2H 2 O (g) or H + + OH - → H 2 O (g)

Oxidation reactions

This reaction of the reaction of substances with gaseous oxygen in air, in which, as a rule, a large amount of energy is distinguished in the form of heat and light. A typical oxidation reaction is combustion. At the very beginning of this page, the reaction of the interaction of methane with oxygen is shown:

CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (g)

Methane refers to hydrocarbons (compounds of carbon and hydrogen). With the reaction of hydrocarbon with oxygen, a lot of thermal energy is distinguished.

Redox reactions

These are reactions at which the exchange of electrons between the atoms of the reagents occurs. The reactions discussed above are also oxidative and recovery reactions:

• 2NA + CL 2 → 2NACL - connection reaction
• CH 4 + 2O 2 → CO 2 + 2H 2 O - oxidation reaction
• Zn + Cuso 4 → ZNSO 4 + Cu - Single Replacement Reaction

Maximum detailed redox reactions with large quantity Examples of solving equations by electronic balance and the half-formation method described in the section

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 process of burning is studying physical chemistryIn which it is customary to distinguish 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 feature - the presence of a high-temperature region (flame), limited spatially, where and occurs most of Transformation of starting materials (fuel) in combustion products. This process is accompanied by emissions. large number 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 most simple, has permanent speeddepending 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 from physical characteristics Ingredients. In liquid combustion on the speed of combustion big influence The rate of evaporation, 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 gas environment The flame can spread to an extensive distance. It can move by gas at a subsonic speed, and this property Inherent not only with 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 last years This is possible to make some ideas about the nature of the impact of inhibitors on the flames.

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.

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 write down the left part of the reaction equation: the chemical formula of methane plus chemical formulas The substances included in 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 a substance (element) into the sheath of the other atoms.

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 if total use, thereby tightening atoms of molecules ( covalent or atomic Communication):

.

And finally, one atom can give to the overall use of his pair of electrons ( molecular communication):

.

Conclusions from the provisions modern theory Oxidation recovery:

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.

Chemical transformation acts occur with direct contact of the reacting components (molecules, atoms, radicals), but only in cases where their energy exceeds a certain energy limit, called the activation energy E A. I will depict a graphically change in the energy of the reacting components (fuel and oxidizing agent) and the reaction products during burning (Fig. 1)

I will depict a graphically change in the energy of the reacting components (fuel and oxidizing agent) and the reaction products during burning (Fig. 1)

Fig 1. Changes in the energy of reacting substances and reaction products when burning

According to the abscissa axis, the path of burning reaction is depicted along the axis of the ordinate - energy.
- average initial energy of reacting components,
- average energy of combustion products.

Only active particles of combustible and oxidizing agent will enter into the combustion reaction, which will have the energy necessary for the entry into account, i.e. able to overcome the energy barrier
. Excessive energy of active particles compared with medium energy
, called activation energy . Since the reactions occurring during combustion are exothermic
. The difference in the energies of the formed products of combustion and starting materials (fuel and oxidizing agent) determines the thermal effect of the reaction:

D. olya of active molecules increases with increasing temperature of the combustible mixture.

In Fig. 2. depicted the distribution of energies between molecules at temperatures If on the energy axis, mark the value equal to the activation energy , I get the share of active molecules in the mixture at a given temperature . If under the action of the heat source, the temperature of the mixture increased to the value The share of active molecules will also increase, and therefore, the rate of burning reaction.

However, there are chemical reactions that do not need their development in a noticeable preheater. These are chain reactions.

The basis of the theory of chain reactions is the assumption that the initial substances are turned into a final product not immediately, but to form active intermediate products

The product of the primary chemical reaction has a large reserve of energy, which can dissipate in the surrounding space when the molecules of the reaction products or by radiation can be transmitted to the molecules of the reacting components, translating them into an active state. These active molecules (atoms, radicals) of the reactants generate the reaction circuit, where the energy is transmitted from one molecule to another. Therefore, such reactions are called chain.

Chemically active molecules, atoms, radicals formed on the elementary stages of the chain reaction - links are called active centers. Most of the active centers are atoms and radicals that are most reactive. But as a result, they are unstable, because Can enter into recombination reactions with the formation of low-effective products.

The chain length formed by one initial active center can reach several hundred thousand units. The kinetic patterns of chain reactions substantially depend on how many active centers are formed in one chain link. If, with the participation of the original active center, only one active center is formed as a result, this chain reaction is called unbranched, if two or more active centers are formed in one ring of the chain, then such a chain reaction is called branched. The rate of branched chain reactions increases avalanche-like, which is the reason for self-sizing chemical oxidation reactions during combustion, since for most of which the mechanism of branched chain reactions is characterized.

Practically any combustion reaction may have simultaneously signs and a thermal and chain reaction mechanism. The birth of the first active centers may have a thermal character, and the reaction of active particles on the chain mechanism leads to heat release, heating the combustible mixture and the thermal emergence of new active centers.

Any chain reaction consists of elementary stages of birth, continuation and chain break.

Number of chainit is an endothermic reaction. The formation of free radicals (i.e. atoms or groups of atoms having free valence, for example,
) From the molecules of the source substances, it is possible as a result of monomolecular or bimolecular interaction, as well as as a result of any extraneous effects on the combustible mixture - initiation.

Initiation can be carried out by adding a special substance - initiatorseasily forming free radicals (for example, peroxides, chemically active gases
), under the action of ionizing radiation, under the action of light - photochemical initiation. For example, hydrogen interaction with chlorine

under normal conditions proceeds extremely slowly, and with strong lighting ( sunlight, burning magnesium) flows with an explosion.

To reactions continued chains The elementary stages of the chain reaction are preserved with the preservation of free valence and leading to the expenditure of the starting materials and the formation of reaction products.

number of chain:

branch chain:

chain opening:

homogeneous

heterogeneous

When developing a chain, when the concentration of active centers will be quite large, it is possible to form such a link in which the active center will react without generating a new active center. Such a phenomenon is called a circuit break.

Chain break May be homogeneous and heterogeneous.

A homogeneous circuit breakdown is possible either in the interaction of radicals or atoms among themselves with the formation of sustainable products, or with a reaction of an active center with an outsider for the main process of a molecule without generating new active centers.

The heterogeneous circuit break occurs on the walls of the vessel, where the burning reaction or the surface of the solid microparticles present in the gas phase occurs, sometimes specially administered (for example, as with powder extinguishing). The mechanism of heterogeneous circuit breakdown is associated with the adsorption of active centers on the surface of solid particles or materials. The speed of heterogeneous breakage of chains strongly depends on the ratio of the surface area of \u200b\u200bthe walls to the volume of the vessel, where burning occurs. Thus, the decrease in the diameter of the vessel significantly reduces the rate of burning reaction, up to its complete termination. This is based on the creation of fireprocerers.

An example of a branched chain reaction can be the reaction of hydrogen combustion in oxygen.

number of chain:

branch chain:

chain opening:

homogeneous