Combustion reaction products. Types of chemical reactions

Topic 3. CHEMICAL BASES OF COMBUSTION.

3.1. Chemistry of combustion reactions.

As you have already understood, combustion is a fast chemical reaction accompanied by the release of heat and glow (flame). Usually, this is an exothermic oxidative reaction of the combination of a combustible substance with an oxidizing agent - atmospheric oxygen.

combustible substances can be gases, and liquids, and solids-la. These are H 2, CO, sulfur, phosphorus, metals, C m H n (hydrocarbons in the form of gases, liquids and solids, i.e. organic substances. Natural hydrocarbons, for example, are natural gas, oil, coal). In principle, all substances capable of oxidation can be combustible.

Oxidizers serve: oxygen, ozone, halogens (F, Cl, Br, J), nitrous oxide (NO 2), ammonium nitrate (NH 4 NO 3), etc. In metals, CO 2, H 2 O, N 2 can also be oxidizing agents .

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

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

exothermic Reactions are reactions that release heat.

Endothermic Reactions are reactions that take place with the absorption of heat.

For example:

2H 2 + O 2 \u003d 2H 2 O + Q - exothermic reaction,

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

where: Q – thermal energy.

Thus, endothermic reactions can proceed only with the introduction of external thermal energy, i.e. when heated.

In chemical reactions, according to the law of conservation of mass, the weight of substances before the reaction is equal to the weight of the substances formed after the reaction. When equalizing the chemical equations, we get stoichiometric formulations.

For example, in the reaction

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

we have 1 mol CH 4 + 2 mol O 2 = 1 mol CO 2 + 2 mol H 2 O.

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

Taking into account the concepts of “molar volume”, “molar concentration”, “partial pressure”, we find that for a complete reaction of methane, it is necessary to mix 1 mol of CH 4 with 2 moles of O 2, or 1/3 \u003d 33.3% CH 4 and 2 / 3=66.7% O 2 . Such a composition is called stoichiometric.

If we consider the combustion of CH 4 in air, i.e. in a mixture of 21% O 2 + 79% N 2 or O 2 + 79 / 21N 2 or O 2 + 3.76N 2, then the reaction will be written as follows:

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

1 mol CH 4 +2 mol O 2 +7.52 mol N 2 \u003d 10.52 mol of a mixture of O 2, N 2 and CH 4.

Then the stoichiometric composition of the mixture will be:

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

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

This means that in the most combustible mixture, instead of 100% (CH 4 + O 2) in the reaction with oxygen, there will be 24% (CH 4 + O 2) in the reaction with air, i.e. much less heat will be released.

The same picture will be obtained if we mix arbitrary, non-stoichiometric compositions.

For example, in the reaction 2CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + CH 4 1 mol of CH 4 does not pro-react.

In reaction CH 4 + 4O 2 \u003d CO 2 + 2H 2 O + 2O 2 2 moles of O 2 do not participate in the reaction, but play the role of ballast, requiring some amount of heat for their heating.

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

Less concentrations of reactants in the total mixture;

Part of the heat will be spent on heating the ballast: nitrogen, oxygen or methane.

Let's ask questions:

What energy can be released in the reaction?

What determines the amount of heat, i.e. thermal effect re-

How much heat energy must be added to

endothermic reaction?

For this, the concept of heat content of a substance is introduced.

3.2. Heat content of substances.

Where did the heat come from in the methane combustion reaction? So it was hidden in the CH 4 and O 2 molecules, and now it has been released.

Here is an example of a simpler reaction:

2H 2 + O 2 \u003d 2H 2 O + Q

This means that the energy level of the stoichiometric mixture of hydrogen and oxygen was higher than that of the H 2 O reaction product, and the “extra” energy was released from the substance.

In the reverse reaction of water electrolysis, i.e. decomposition of water with the help of electrical energy, there is a redistribution of atoms in the water molecule with the formation of hydrogen and oxygen. In this case, the heat content of H 2 and O 2 increases.

Thus, each substance during its formation receives or gives away a certain energy, and the measure of thermal energy accumulated by a substance during its formation is called heat content, or enthalpy.

Unlike chemistry, in chemical thermodynamics, the heat of formation of a substance is denoted not by the symbol Q, but by the symbol DH with a sign (+) if the heat is absorbed by a chemical compound, and with a sign (-) if the heat is released during the reaction, that is, it “leaves” from systems.

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

Reference books give the heats of formation of compounds from simple substances.

For example:

At CO 2 \u003d - 393.5 kJ / mol

U H 2 O gas \u003d - 241.8 kJ / mol

But for substances formed during endothermic processes, for example, acetylene C 2 H 2 \u003d + 226.8 kJ / mol, during the formation of a hydrogen atom H + according to the reaction H 2 \u003d H + + H + \u003d + 217.9 kJ/mol.

For pure substances consisting of one chemical element in a stable form (H 2 , O 2 , C, Na, etc.), DH is conventionally taken equal to zero.

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

In chemical reactions, only molecules change. Atoms remain unchanged. Molecule Energy is the binding energy of its atoms, accumulated in the molecule. It is determined by the forces of attraction of atoms to each other. In addition, there is a potential energy of attraction of molecules to each other. It is small in gases, higher in liquids, and even higher in solids.

Each atom has energy, part of which is associated with electrons, and part - with the nucleus. Electrons have the kinetic energy of rotation around the nucleus and the potential electrical energy of attraction to each other and repulsion from each other.

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

If we sum up the heat content of 6.02×10 23 molecules of a substance, we get the molar heat content of this substance.

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

The DH of a chemical element, that is, the energy of its formation, is associated with intranuclear processes. Nuclear energy is associated with the forces of interaction of intranuclear particles and the transformation of one chemical element into another during nuclear reactions. For example, the decay reaction of uranium:

or more simply: U+n®Ba+Kr+3n.

where: n ’ o is a neutron particle with mass 1 and zero charge.

Uranium captures a neutron, as a result of which it splits (decays) into two new elements - barium and krypton - with the formation of 3 neutrons, and nuclear energy is released.

It should be said that millions of times greater energy changes are associated with nuclear reactions than with chemical reactions. Thus, the decay energy of uranium is 4.5×10 9 kcal/mol×uranium. This is 10 million times more than when one mole of coal is burned.

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

The above uranium decay reaction is a classic example of a chain reaction. We will consider the theory of the chain mechanism of the combustion reaction later. But where does the neutron come from and what makes it react with uranium - this is due to the so-called activation energy, which we will consider a little later.

3.3. Thermal effect of the reaction.

The fact that each individual substance contains a certain amount of energy serves as an explanation for the thermal effects of chemical reactions.

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

Consequence 1 of this law: The thermal effect of a chemical reaction is equal to the difference between the sum of the heats of formation of the final products and the sum of the heats of formation of the starting substances, taking into account the coefficients in the formulas of these substances in the reaction equation.

For example, in the reaction 2H 2 +O 2 \u003d 2H 2 O ± DH.

; ; .

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

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

And if DH is signed (-), then the reaction is exothermic.

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

Then the reaction of water decomposition will be:

2H 2 O \u003d 2H 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 + 2H 2 O.

Then the reaction will be written as:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O - 742.3 kJ / mol, which means the reaction is exothermic.

3.4. Kinetic foundations of gas reactions.

According to the law of mass action, the reaction rate at a constant temperature is proportional to the concentration of the reacting substances or, as they say, “acting masses”.

The rate of a chemical reaction ( υ ) it is customary to consider the amount of a substance reacting per unit time ( dt) per unit volume ( dV).

Consider the reaction proceeding according to the equation:

A + B = C + D.

Since the reaction rate characterizes a decrease in the concentration of reactants with time and an increase in the concentration of reaction products, we can write:

, (3.1)

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

Then a direct irreversible reaction at T \u003d const proceeds at a rate of:

, (3.2)

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

According to the law of mass action, the concentrations of the reaction components are included in the kinetic equation to a degree equal to the stoichiometric coefficient of this component.

So for the reaction

aA + bB = cC + dD

The kinetic equation has the form:

The exponents a, b, c, d are usually called the reaction orders for the components A, B, C, D, and the sum of the exponents is called the general order of the reaction.

For example, reactions like

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 interconnected by stoichiometric equations, the simplest kinetic equations of the first order are differential equations of the first order with one independent variable - concentration - and can be integrated.

The simplest kinetic equation is the first order equation of the type

for which . (3.4)

Denote by the concentration of component A before the start of the reaction and, integrating the equation under the boundary condition t=0, [A]=[A 0 ], we obtain:

Or [A]=×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 this way. According to the Arrhenius hypothesis, the reaction between molecules takes place only if they are active, i.e. have excess energy sufficient to break interatomic bonds, the so-called activation energy E A.

Those. the rate of a chemical reaction does not depend on the number of collisions of all molecules, but only of activated ones.

According to Boltzmann's law, the number of active molecules

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

where: E is the activation energy,

T is the temperature of the gas mixture,

n o is the total number of molecules.

Then the number of effective collisions, coinciding with the reaction rate, is equal to:

υ p \u003d Z eff \u003d Z 0 * e - E / RT, (3.7)

where: Z 0 is the total number of molecular collisions.

1) the reaction rate is proportional to the concentration of active molecules, the number of which depends on the temperature and pressure in the mixture, since pressure is the number of molecules colliding with any surface;

2) the reaction is possible only if the interacting molecules receive a certain amount of energy sufficient to break or weaken the interatomic bonds. Activation consists in the transition of molecules to a state in which chemical transformation is possible.

Most often, the activation process proceeds through the formation of intermediate, unstable, but highly active compounds of atoms.

Thus, not only for the occurrence of endothermic processes, an external supply of energy is needed, but also for exothermic ones. For an exothermic reaction to occur, some impulse of thermal energy must be imparted to it. For example, for a combustion reaction to occur in a mixture of hydrogen and oxygen, it must be ignited.

The minimum amount of thermal energy required to “start” a chemical reaction is called the activation energy.

3.5. Reaction activation energy.

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

There is a ball on the platform. The site is located in front of the hill. Therefore, the ball could roll down by itself, if not for the slide. But for a spontaneous descent, it must be raised to the top of the hill. In this case, not only the energy of going up the hill will be released, but also the energy of going down.

Rice. 9. Scheme of activation of the reaction.

Consider two reactions:

1) H 2 + O 2 \u003d H 2 O-

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

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

In general, any reaction

.

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

Thus, the activation energy is the energy necessary for the transformation of reacting substances into the state of an active complex (breaking interatomic bonds, bringing molecules together, accumulating energy in a molecule ...).

With an increase in the temperature of gases, the proportion of active molecules (e -E / RT) sharply increases, and hence the reaction rate according to an exponential dependence. This dependence can be illustrated as follows:

Rice. 10. Dependence of the reaction rate on temperature: 1 - the rate of the 1st reaction, 2 - the rate of the 2nd reaction.

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

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

It can be seen from the foregoing that the magnitude of the activation energy is the most important characteristic of a chemical process. It determines the height of the energy barrier, the overcoming of which is a condition for the reaction to proceed. On the other hand, it characterizes the reaction rate as a function of 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, to speed up a chemical reaction, they use catalysts, i.e. substances that are introduced into the reacting mixture, but are not consumed in the reaction, but accelerate it by lowering the activation energy.

The process of increasing the reaction rate with the help of catalysts is called catalysis.

Catalysts participate in intermediate reactions to create an activated complex by weakening the bonds in the molecules of the starting substances, their decomposition, the adsorption of molecules on the catalyst surface, or the introduction of active catalyst particles.

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

Reaction without a catalyst: A + B = AB.

With catalyst X: A + X = AX ® AX + B = AB + X.

We present a picture similar to that shown in Fig. 9.

Rice. 11. Diagram of the action of the catalyst: E b.cat and E with cat are the activation energies of the reaction without a catalyst and with a catalyst, respectively.

When a catalyst is introduced (Fig. 11), the reaction can proceed along a different path with a lower energy barrier. This pathway corresponds to a new reaction mechanism through the formation of another activated complex. And the new lower energy barrier can overcome more particles, which leads to an increase in the reaction rate.

It should be noted that the activation energy of the reverse reaction decreases by the same amount as the activation energy of the direct reaction, i.e. both reactions speed up equally, and catalysts do not initiate the reaction, they only speed up the reaction, which can occur in their absence, but much more slowly.

Intermediate reaction products can become catalysts, then this reaction is called autocatalytic. So, if the rate of ordinary reactions decreases as the reactants are consumed, then the combustion reaction due to autocatalysis self-accelerates and is autocatalytic.

Most often, solids are used as catalysts, which adsorb the molecules of the reactants. During adsorption, the bonds in the reacting molecules are weakened, and thus the reaction between them is facilitated.

What is adsorption?

3.7. Adsorption.

Adsorption- surface absorption of a substance from a gaseous medium or solution by a surface layer of another substance - a liquid or a solid.

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

Distinguish between physical and chemical adsorption.

At physical adsorption, the trapped particles retain their properties, and when chemical– chemical compounds of the adsorbate with the adsorbent are formed.

The adsorption process is accompanied by the release of heat. For physical adsorption, it is insignificant (1-5 kcal/mol), for chemical adsorption it is much higher (10-100 kcal/mol). Thus, chemical reactions during catalysis can be accelerated.

For combustion and explosion processes, the following examples can be given:

1. The autoignition temperature of a mixture of 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 spontaneous combustion of coal begin with the chemical adsorption of oxygen on the surface of coal particles.

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

4. Oxygen is well adsorbed and absorbed by technical oils with the formation of an explosive mixture. The mixture explodes spontaneously, without an ignition source (chemical absorption).

Combustion

Combustion- a complex physical and chemical process of converting the components of a combustible mixture into combustion products with the release of thermal radiation, light and radiant energy. The nature of combustion can be described as a vigorous oxidation.

Subsonic combustion (deflagration), unlike explosion and detonation, proceeds at low speeds and is not associated with the formation of a shock wave. Subsonic combustion includes normal laminar and turbulent flame propagation, and supersonic combustion includes detonation.

Combustion is divided into thermal and chain. At the core thermal combustion is a chemical reaction capable of proceeding with progressive self-acceleration due to the accumulation of released heat. chain combustion occurs in cases of some gas-phase reactions at low pressures.

Thermal self-acceleration conditions can be provided for all reactions with sufficiently large thermal effects and activation energies.
Combustion can start spontaneously as a result of self-ignition or be initiated by ignition. Under fixed external conditions, continuous combustion can proceed in stationary mode, when the main characteristics of the process - reaction rate, heat release power, temperature and composition of products - do not change in time, or in periodic mode when these characteristics fluctuate around their average values. Due to the strong nonlinear dependence of the reaction rate on temperature, combustion is highly sensitive to external conditions. The same property of combustion determines the existence of several stationary regimes under the same conditions (hysteresis effect).

The combustion process is divided into several types: flash, ignition, ignition, spontaneous combustion, spontaneous ignition, explosion and detonation. In addition, there are special types of combustion: smoldering and cold-flame combustion. Flash - the process of instantaneous combustion of vapors of flammable and combustible liquids, caused by direct exposure to an ignition source. Ignition is the phenomenon of the occurrence of combustion under the influence of an ignition source. Ignition - ignition, accompanied by the appearance of a flame. At the same time, the rest of the mass of the combustible substance remains relatively cold. Spontaneous combustion is a phenomenon of a sharp increase in the rate of exothermic reactions in a substance, leading to combustion in the absence of an ignition source. Self-ignition is spontaneous combustion accompanied by the appearance of a flame. Under production conditions, sawdust, oiled rags can ignite spontaneously. Gasoline, kerosene can ignite spontaneously. An explosion is a rapid chemical transformation of a substance (explosive combustion), accompanied by the release of energy and the formation of compressed gases capable of producing mechanical work.

Flameless burning

Unlike conventional combustion, when zones of oxidizing flame and reducing flame are observed, it is possible to create conditions for flameless combustion. An example is the catalytic oxidation of organics on the surface of a suitable catalyst, such as the oxidation of ethanol on platinum black.

Solid state combustion

These are autowave exothermic processes in mixtures of inorganic and organic powders, which are not accompanied by noticeable gas evolution and lead to the production of exclusively condensed products. Gaseous and liquid phases are formed as intermediate substances that ensure mass transfer, but do not leave the burning system. Examples of reacting powders are known in which the formation of such phases has not been proven (tantalum-carbon).

The trivial terms "gasless combustion" and "solid flame combustion" are used as synonyms.

An example of such processes is SHS (self-propagating high-temperature synthesis) in inorganic and organic mixtures.

Smoldering

A type of combustion in which no flame is formed, and the combustion zone slowly spreads through the material. Smoldering is commonly seen in porous or fibrous materials with a high air content or impregnated with oxidizers.

Autogenous combustion

Self-sustaining combustion. The term is used in waste incineration technologies. The possibility of autogenous (self-sustaining) combustion of waste is determined by the maximum content of ballasting components: moisture and ash. Based on many years of research, the Swedish scientist Tanner proposed to use a triangle scheme with limit values ​​for determining the boundaries of autogenous combustion: more than 25% combustibles, less than 50% moisture, and less than 60% ash.

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See what "Burning" is in other dictionaries:

Physicochemical process in which the transformation of a substance is accompanied by an intense release of energy and heat and mass transfer with the environment. Combustion can start spontaneously as a result of self-ignition or be initiated by ... ... Big Encyclopedic Dictionary

BURNING, burning, pl. no, cf. (book). Action and status according to Ch. burn. Burning gas. Soul burning. Explanatory Dictionary of Ushakov. D.N. Ushakov. 1935 1940 ... Explanatory Dictionary of Ushakov

Glitter, play, enthusiasm, radiance, play, takeoff, elation, uplift, sparkle, brilliance, obsession, fire, passion, spark, inspiration, gleam, inspiration, enthusiasm, liveliness, passion, combustion, rise Vocabulary ... ... Synonym dictionary

Combustion- COMBUSTION, chemical transformation, which is accompanied by intense heat release and heat and mass transfer with the environment. May start spontaneously (spontaneous combustion) or as a result of ignition. A characteristic property of burning ability ... ... Illustrated Encyclopedic Dictionary

Complex chem. a reaction proceeding under conditions of progressive self-acceleration associated with the accumulation of heat or catalyzing reaction products in the system. High (up to several thousand K) temperatures can be achieved during hydrothermal heating, and often there is ... ... Physical Encyclopedia

Physicochemical process in which the transformation of a substance is accompanied by intense energy release and heat and mass transfer with the environment. may start spontaneously as a result of self-ignition, or may be initiated by ... ... Emergencies Dictionary

During chemical reactions, other substances are obtained from one substance (not to be confused with nuclear reactions, in which one chemical element is converted 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), resulting in the formation of carbon dioxide (CO 2) and water (H 2 O), or rather, water vapor. This is exactly the kind of reaction that happens in your kitchen when you light a gas burner. The equation should be read like this: one molecule of methane gas reacts with two molecules of oxygen gas, resulting in one molecule of carbon dioxide and two molecules of water (steam).

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

Chemical reactions are endothermic(with energy absorption) and exothermic(with energy release). The combustion of methane is a typical example of an exothermic reaction.

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

• compound reactions;
• decomposition reactions;
• single substitution reactions;
• double substitution reactions;
• oxidation reactions;
• redox reactions.

Connection reactions

In a compound reaction, at least two elements form one product:

2Na (t) + Cl 2 (g) → 2NaCl (t)- the formation of salt.

Attention should be paid to an essential nuance of compound reactions: depending on the conditions of the reaction or the proportions of the reactants that enter into the reaction, different products can be its result. For example, under normal conditions of combustion of coal, carbon dioxide is obtained:
C (t) + O 2 (g) → CO 2 (g)

If there is not enough oxygen, then deadly carbon monoxide is formed:
2C (t) + O 2 (g) → 2CO (g)

Decomposition reactions

These reactions are, as it were, opposite in essence to the reactions of the compound. As a result of the decomposition reaction, the substance decomposes into two (3, 4...) simpler elements (compounds):

• 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 substitution reactions

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

Zn (t) + CuSO 4 (solution) → ZnSO 4 (solution) + Cu (t)

The zinc in the copper sulfate solution displaces the less active copper, resulting in a zinc sulfate solution.

The degree of activity of metals in ascending order of activity:

• The most active are alkali and alkaline earth metals.

The ionic equation for the above reaction will be:

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

The ionic bond CuSO 4, when dissolved in water, decomposes into a copper cation (charge 2+) and an anion sulfate (charge 2-). As a result of the substitution reaction, a zinc cation is formed (which has the same charge as the copper cation: 2-). Note that the sulfate anion is present on both sides of the equation, i.e., by all the rules of mathematics, it can be reduced. The result is an ion-molecular equation:

Zn (t) + Cu 2+ → Zn 2+ + Cu (t)

Double substitution reactions

In double substitution reactions, two electrons are already replaced. Such reactions are also called exchange reactions. These reactions take place in solution to form:

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

Precipitation reactions

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

Molecular equation: KCl (solution) + AgNO 3 (p-p) → AgCl (t) + KNO 3 (p-p)

Ionic equation: K + + Cl - + Ag + + NO 3 - → AgCl (t) + K + + NO 3 -

Molecular-ionic equation: Cl - + Ag + → AgCl (t)

If the compound is soluble, it will be in solution in ionic form. If the compound is insoluble, it will precipitate, forming a solid.

Neutralization reactions

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

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

Molecular equation: H 2 SO 4 (p-p) + 2NaOH (p-p) → Na 2 SO 4 (p-p) + 2H 2 O (l)

Ionic equation: 2H + + SO 4 2- + 2Na + + 2OH - → 2Na + + SO 4 2- + 2H 2 O (l)

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

Oxidation reactions

These are reactions of interaction of substances with gaseous oxygen in the air, in which, as a rule, a large amount of energy is released 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 given:

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

Methane refers to hydrocarbons (compounds of carbon and hydrogen). When a hydrocarbon reacts with oxygen, a lot of heat energy is released.

Redox reactions

These are reactions in which electrons are exchanged between the atoms of the reactants. The reactions discussed above are also redox reactions:

• 2Na + Cl 2 → 2NaCl - compound reaction
• CH 4 + 2O 2 → CO 2 + 2H 2 O - oxidation reaction
• Zn + CuSO 4 → ZnSO 4 + Cu - single substitution reaction

The most detailed redox reactions with a large number of examples of solving equations by the electron balance method and the half-reaction method are described in the section

Combustion is a chemical reaction of fuel oxidation with oxygen, which proceeds relatively quickly in time with the release of a large amount of heat.

During combustion, the products of combustion are heated to high temperatures.

The general equation for the combustion of any hydrocarbon gas with oxygen is as follows:

where m and n- respectively, the number of carbon and hydrogen atoms in the molecule

Q is the thermal effect of the oxidation reaction.

Table 3.1 shows the combustion reactions of the main combustible gases with oxygen.

Combustion reactions of combustible gases with oxygen

Table 3.1

Table 3.1 shows the oxidation reactions of the most known combustible gases with oxygen. However, in real conditions, the oxidizing agent (oxygen) is supplied to the combustion zone not in its pure form, but as part of the air. It is known that air mainly consists of two parts: oxygen and nitrogen. The composition of the air also includes a small amount of carbon dioxide CO 2, as well as rare gases. Given their small amount in the composition of the air, we neglect them.

Thus, if we take the volume of air as 100%, then the oxygen content will be 21%, and nitrogen 79%. Therefore, on 1 m 3 air oxygen accounted for 79/21 = 3.76 m 3 nitrogen, or 1 m 3 oxygen is contained in 100/21 = 4.76 m 3 air.

Given the above relationships, we can write the general equation for the combustion of hydrocarbons with air:

Table 3.2 shows the equations for the combustion reaction of combustible gases with air.

It should be noted that the equations given in tables 3.1 and 3.2 are stoichiometric, i.e. such a ratio of combustible gas and oxidizing agent (oxygen, air) at which the theoretically required amount of oxidizing agent is supplied to the combustible gas. However, in the practice of gas combustion under real conditions, it is necessary to supply a slightly larger amount of oxidizing agent to the zone than follows from the stoichiometric equations. This is mainly due to the imperfection of the quality of the mixing of the combustible gas and the oxidizer.

Equations for combustion reactions of combustible gases with air

Table 3.2

The ratio of the actual flow rate of the oxidizer (oxygen or air) to the theoretically required one is called the excess air coefficient and is denoted α , i.e.:

where V d– actual air consumption;

V t is the theoretically required amount of air.

Table 3.3 shows the values ​​of the theoretically required amount of oxidant (oxygen and air), as well as the volume of combustion products during combustion 1 m 3 gas and excess air coefficient equal to 1 ( a = 1).

Theoretically required amount of oxidant and the volume of combustion products during combustion 1 m 3 at α = 1

Table 3.3

In practical calculations, sometimes we do not know the chemical composition of gases, but only the heat of combustion is known. It is necessary to determine the theoretically required amount of air required for complete combustion 1 m 3 gas.

For this case, there is an empirical formula D.I. Mendeleev:

where Q n- lower calorific value of gas, kJ/m 3 .

The equations for the reactions of combustion of various gases with oxygen and air reflect only the ratio between the fuel and the oxidizer, and do not explain the mechanism of these reactions. In real conditions, the combustion process is much more complicated.

The Soviet scientist, academician N.N. Semenov. According to his theory, chain reactions of combustion of gases occur in the flame of a gas-air mixture. As a result, intermediate unstable products are formed in the form of free radical atoms. In accordance with the theory of N.N. Semenov's combustion reaction of hydrogen with oxygen is not reduced simply to the combination of two hydrogen molecules and one oxygen to form two water molecules. During the interaction of these two gases, the formation of intermediate substances in the form of hydrogen and oxygen atoms first occurs, and the formation of free OH hydroxyl radicals also occurs.

To start the combustion process, it is necessary to somehow activate the combustible mixture. In other words, it is necessary to create conditions under which the reagents will have a large amount of energy. This energy reserve is necessary for the implementation of the combustion process. The above energy reserve can be created by heating the gas-air mixture to its ignition temperature. This energy, called the activation energy, is needed mainly to break the existing intermolecular bonds in the reagents.

During the combustion process, new bonds are constantly formed along with the destruction of old ones. When new bonds are formed, a significant release of energy occurs, while the breaking of old bonds is always accompanied by energy costs. Due to the fact that during the combustion process the energy released during the formation of new bonds is of great importance compared to the energy spent on breaking old bonds, the total thermal effect remains positive.

The reaction of hydrogen with oxygen is the simplest and most studied. Therefore, consider this branched reaction with an example.

In accordance with the theory of N.N. Semenov at the initial moment of the reaction, as a result of the activation energy and collision of hydrogen and oxygen molecules, two OH hydroxyl radicals are formed:

. (3.5)

The free hydrogen atom H, in turn, reacts with an oxygen molecule. As a result, a OH hydroxyl radical and a free oxygen atom are formed, i.e.:

. (3.7)

The radical can again enter into a chemical reaction with hydrogen and again, as a result of the reaction, form water and free hydrogen, and the oxygen atom, in turn, can react with a hydrogen molecule, which will lead to the formation of another OH radical and a hydrogen atom H , i.e.:

. (3.8)

The above mechanism of the chain reaction of combustion of hydrogen with oxygen shows the possibility of multiple interactions of one OH radical with hydrogen atoms. As a result of this interaction, water molecules are formed.

Therefore, free atoms and radicals are active centers in creating a chain reaction.

The combustion reaction of hydrogen with oxygen, which explains the chain reaction mechanism, can be written as follows:

H 2 O O + (H 2) ...

OH + (H 2) ® H + (O 2) ® OH + (H 2) ...

O + (H 2) ® OH + (H 2) ® H 2 O

H + (O 2) ® OH + H 2 ...

The combustion mechanism of carbon monoxide with oxygen is more complex. According to scientists from the Institute of Chemical Physics of the USSR Academy of Sciences, carbon monoxide does not react with dry oxygen. They also found that the addition of a small amount of hydrogen or moisture to the mixture leads to the onset of the oxidation reaction. As a result, the following sequence of chemical reactions occurs:

H 2 O ® OH + H; (3.10)

OH + CO ® CO 2 + H; (3.11)

H + O 2 ® OH + O; (3.12)

CO + OH ® CO 2 + H; (3.13)

CO + O ® CO 2 ; (3.14)

H + O 2 \u003d OH + O (3.15)

As follows from the above chemical reactions, the presence of a small amount of moisture leads to the formation of hydroxyls and free atoms in the combustion zone. As noted earlier, both hydroxyl radicals and free atoms are the initiators of the creation and carriers of the chain reaction.

An even more complex mechanism for the oxidation of hydrocarbons. Along with some similarities with the mechanism of combustion of hydrogen and carbon monoxide, the mechanism of combustion of hydrocarbons has a number of significant differences. Analyzing the combustion products, it was found that they contain aldehydes and mainly formaldehyde (HCHO).

Consider the mechanism of hydrocarbon oxidation using the simplest of them, methane, as an example. The mechanism of methane oxidation goes through four stages, in each of which the following chemical reactions occur:

At the first stage:

H + O 2 ® OH + O; (3.16)

CH 4 + OH ® CH 3 + H 2 O; (3.17)

CH 4 + O ® CH 2 + H 2 O. (3.18)

At the second stage:

CH 3 + O 2 ® HCHO + OH; (3.19)

CH 2 + O 2 ® HCHO + O; (3.20)

At the third stage:

HCHO + OH ® HCO + H 2 O (3.21)

HCHO + O ®CO + H 2 O; (3.22)

HCO + O 2 ® CO + O + OH (3.23).

At the fourth stage:

CO + O ® CO 2 (3.24)

Combustion (reaction)

(a. burning, burning; n. Brennen, Verbrennung; f. combustion; and. combustion) - a fast-flowing oxidation reaction, accompanied by the release of means. amount of heat; usually accompanied by a bright glow (flame). In most cases, hydrolysis acts as an oxidizing agent, but hydrogenation processes are also possible in reactions of other types (hydrogenation of metals in nitrogen, in halogens). In the physical chemistry to G. include all exothermic. chem. processes, in to-ryh beings. the role is played by the self-acceleration of the reaction, caused by an increase in temperature (thermal mechanism) or the accumulation of active particles (diffusion mechanism).
A characteristic feature of G. is the presence of a spatially limited region of high temperature (flame), in which the main. part of the chem. conversion of starting materials into combustion products and b.h. heat. The appearance of a flame is caused by ignition, which requires the expenditure of a certain energy, but the spread of the flame through a system capable of heating occurs spontaneously, at a rate that depends on the chemical. properties of the system, physical. and gas dynamic processes. Technically important characteristics of gas: the calorific value of the combustible mixture and the theoretical. (adiabatic) temp-pa, which would be achieved with complete combustion of the fuel without heat loss.
From the whole variety of gaseous processes, usually according to the state of aggregation of the fuel and oxidizer, homogeneous gasification of premixed gases and vaporous combustibles in gaseous oxidizers, heterogeneous gasification (solid and liquid combustibles in gaseous oxidizers), and gasification of explosives and gunpowder (going without mass transfer with the environment).
The simplest is homogeneous gassing of mixed gases. The rate of propagation of a laminar flame in such a system is a physical-chemical. constant of the mixture, depending on the composition of the mixture, pressure, temperature and mol. thermal conductivity.
Heterogeneous hydrolysis is the most common process in nature and technology. Its speed is determined by physical. system properties and specific combustion conditions. For gases of liquid combustibles, the rate of their evaporation is of great importance, while for solid gases, the rate of gasification is of great importance. So, at G. coals it is possible to distinguish two stages. At the first stage (under the condition of slow heating), the volatile components of coal are released, and at the second stage, the afterburning of the coke residue.
The spread of the flame through the gas leads to the appearance of gas motion for a mean. distance from the flame front. If the width of the reaction zone is small, then the flame can be represented as gas-dynamic. discontinuity moving through the gas at subsonic speed. This is possible not only in the case of a homogeneous mixture, but also for sufficiently finely dispersed liquid and solid fuels suspended in an oxidizer. Since the component of the flame velocity normal to its front does not depend on the velocity of the gas itself, a completely definite shape of the flame is established in a flow of moving gas during stationary gas flow. G. in such conditions is provided with the corresponding design of furnace devices.
The gas movement caused by the appearance of a flame can be either laminar or turbulent. Turbulence of the flow, as a rule, leads to a sharp acceleration of combustion and the appearance of acoustic. perturbations in the flow, ultimately leading to the appearance of shock, initiating the detonation of the gas mixture. The possibility of gaseous transition to detonation is determined, in addition to the properties of the gas itself, by the size and geometry of the system.
G. fuel processes are used in technology, osn. the task of burning fuel is reduced to achieving max. heat release (combustion efficiency) for a given period of time. In the mountains In fact, the methods of developing p. and. are based on the use of the G. process. ( cm. in-situ combustion). In certain mountains.-geol. conditions spontaneously arising G. ( cm. Spontaneous combustion of coal, Spontaneous combustion of peat) can lead to endogenous fires. L. G. Bolkhovitinov.

Mountain Encyclopedia. - M.: Soviet Encyclopedia. Edited by E. A. Kozlovsky. 1984-1991 .

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