Classification of chemical reactions. Initial substances activated complex reaction products
DEFINITION
Chemical reaction are called transformations of substances in which a change in their composition and (or) structure occurs.
Most often, chemical reactions are understood as the process of converting starting substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations containing the formulas of the starting substances and reaction products. According to the law conservation of mass, the number of atoms of each element on the left and right sides chemical equation the same. Typically, the formulas of the starting substances are written on the left side of the equation, and the formulas of the products on the right. The equality of the number of atoms of each element on the left and right sides of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.
Chemical equations may contain additional information about the characteristics of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or “below”) the equal sign.
All chemical reactions can be grouped into several classes, which have certain characteristics.
Classification of chemical reactions according to the number and composition of starting and resulting substances
According to this classification, chemical reactions are divided into reactions of combination, decomposition, substitution, and exchange.
As a result compound reactions from two or more (complex or simple) substances one new substance is formed. IN general view The equation for such a chemical reaction will look like this:
For example:
CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2
SO 3 + H 2 O = H 2 SO 4
2Mg + O 2 = 2MgO.
2FeCl 2 + Cl 2 = 2FeCl 3
The reactions of the compound are in most cases exothermic, i.e. proceed with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox reactions (ORR), i.e. occur with changes in the oxidation states of elements. It is impossible to say unambiguously whether the reaction of a compound between complex substances will be classified as ORR.
Reactions that result in the formation of several other new substances (complex or simple) from one complex substance are classified as decomposition reactions. In general, the equation for the chemical reaction of decomposition will look like this:
For example:
CaCO 3 CaO + CO 2 (1)
2H 2 O = 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O = CuSO 4 + 5H 2 O (3)
Cu(OH) 2 = CuO + H 2 O (4)
H 2 SiO 3 = SiO 2 + H 2 O (5)
2SO 3 =2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 +4H 2 O (7)
Most decomposition reactions occur when heated (1,4,5). Possible decomposition under the influence of electric current (2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) occurs without changing the oxidation states of the elements, i.e. these reactions are not related to ODD. ORR decomposition reactions include the decomposition of oxides, acids and salts formed by elements in higher oxidation states (6).
Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):
C 18 H 38 = C 9 H 18 + C 9 H 20 (8)
C 4 H 10 = C 4 H 6 + 2H 2 (9)
At substitution reactions a simple substance interacts with a complex substance, forming a new simple and a new complex substance. In general, the equation for a chemical substitution reaction will look like this:
For example:
2Al + Fe 2 O 3 = 2Fe + Al 2 O 3 (1)
Zn + 2HCl = ZnСl 2 + H 2 (2)
2KBr + Cl 2 = 2KCl + Br 2 (3)
2КlO 3 + l 2 = 2KlO 3 + Сl 2 (4)
CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (5)
Ca 3 (PO 4) 2 + 3SiO 2 = 3СаSiO 3 + P 2 O 5 (6)
CH 4 + Cl 2 = CH 3 Cl + HCl (7)
Most substitution reactions are redox (1 – 4, 7). Examples of decomposition reactions in which no change in oxidation states occurs are few (5, 6).
Exchange reactions are reactions that occur between complex substances in which they exchange their constituent parts. Typically this term is used for reactions involving ions in aqueous solution. In general, the equation for a chemical exchange reaction will look like this:
AB + CD = AD + CB
For example:
CuO + 2HCl = CuCl 2 + H 2 O (1)
NaOH + HCl = NaCl + H 2 O (2)
NaHCO 3 + HCl = NaCl + H 2 O + CO 2 (3)
AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)
CrCl 3 + ZNaON = Cr(OH) 3 ↓+ ZNaCl (5)
Exchange reactions are not redox. Special case these exchange reactions are neutralization reactions (reactions between acids and alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5) or a poorly dissociating compound, most often water (1, 2).
Classification of chemical reactions according to changes in oxidation states
Depending on the change in the oxidation states of the elements that make up the reagents and reaction products, all chemical reactions are divided into redox reactions (1, 2) and those occurring without changing the oxidation state (3, 4).
2Mg + CO 2 = 2MgO + C (1)
Mg 0 – 2e = Mg 2+ (reducing agent)
C 4+ + 4e = C 0 (oxidizing agent)
FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)
Fe 2+ -e = Fe 3+ (reducing agent)
N 5+ +3e = N 2+ (oxidizing agent)
AgNO 3 +HCl = AgCl ↓ + HNO 3 (3)
Ca(OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)
Classification of chemical reactions by thermal effect
Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conventionally divided into exothermic (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during a reaction is called the thermal effect of the reaction. If the equation indicates the amount of heat released or absorbed, then such equations are called thermochemical.
N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)
2Mg + O 2 = 2MgO + 602.5 kJ (2)
N 2 + O 2 = 2NO – 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
Based on the direction of the reaction, a distinction is made between reversible (chemical processes whose products are capable of reacting with each other under the same conditions in which they were obtained to form the starting substances) and irreversible (chemical processes whose products are not able to react with each other to form the starting substances). ).
For reversible reactions, the equation in general form is usually written as follows:
A + B ↔ AB
For example:
CH 3 COOH + C 2 H 5 OH ↔ H 3 COOC 2 H 5 + H 2 O
Examples of irreversible reactions include the following reactions:
2КlО 3 → 2Кl + ЗО 2
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
Evidence of the irreversibility of a reaction can be the release of a gaseous substance, a precipitate, or a poorly dissociating compound, most often water, as reaction products.
Classification of chemical reactions according to the presence of a catalyst
From this point of view, catalytic and non-catalytic reactions are distinguished.
A catalyst is a substance that speeds up the progress of a chemical reaction. Reactions that occur with the participation of catalysts are called catalytic. Some reactions cannot take place at all without the presence of a catalyst:
2H 2 O 2 = 2H 2 O + O 2 (MnO 2 catalyst)
Often one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):
MeO+ 2HF = MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1
Solid starting materials can react with each other and when separated spatially. In this regard, unlike conventional solid-phase reactions, it is not necessary to use starting materials in stoichiometric quantities. The final product, regardless of the ratio of the starting substances, will have a stoichiometric composition.
Solid starting materials and reaction products do not affect the shift of heterogeneous chemical equilibrium.
Solid starting materials can react with each other and when separated spatially. In this regard, in ex. The final product, regardless of the ratio of the starting substances, will have a stoichiometric composition.
Reactions between solid starting materials can be accelerated due to the fact that the solids bind to each other through a transport reaction. It can be foreseen that this principle will be carried over to numerous reactions between solids. At the same time, it is especially favorable that it is possible to select the appropriate transport reactions based on simple theoretical concepts.
The granulometric composition of the loaded particles of the solid starting material and the hydrodynamic regime of the process do not change.
Only those molecules of the solid starting substance AI that enter the adsorption centers filled with the substance AZ participate in the chemical reaction.
Thus, the composition of the melt with a continuous supply of solid starting materials is determined by the ratio PiSy / p2sH, and with different sizes of pieces of lime and carbon we will obtain different melt compositions.
To obtain an aqueous extract, 50 - 80 mg of the solid starting material is boiled for several minutes with 3 ml of water, which is replenished dropwise as the solution evaporates. An aqueous extract that has a neutral reaction (neutral aqueous extract) may contain interfering cations that must be removed with soda in the same way as is done if the object under study is a liquid (see page. As a result of neutralization of an alkaline (after action with soda) liquid and separating the precipitate, the prepared solution is obtained.
Rate-time curves for silver oxalate degradation. G110 S. dots indicate the results of experiments without breaks, circles indicate experiments with breaks of 60 minutes. (/ and 30 minutes (/ /. Such experiments show at the same time that simple mixing of a solid starting material with a solid product may not be enough to detect the autocatalytic effect of the latter.
Chemical technological process, in which gaseous starting substances are blown through holes at the bottom of the apparatus, and the solid starting substances in it seem to boil, being in a suspended state all the time. In this case, the reactions take place in the fluidized bed itself.
Chemist is a technological process in which gaseous starting substances are blown through holes at the bottom of the apparatus, and the solid starting substances in it seem to boil, being constantly in suspension. In this case, reactions take place in the fluidized bed itself.
Typical curves a f (t of the process of thermal dissociation of solids. Explanations are given in the text. When describing the course of thermal dissociation, the reaction rate is most often made dependent on the composition of the solid phase, expressed by the degree of transformation (decomposition) a of the solid starting substance. In Fig. VIII- Figure 12 shows the most typical dependences of a on reaction time.
In table 22 summarizes data concerning the possibility of finding anions in the analytical fractions described above, resulting from the preparation of a solution from the solid starting material to be analyzed.
In the dehydration of manganese oxalate dihydrate, studied from the point of view of Volmer's theory, for which the formation of an amorphous product and its subsequent crystallization was x-ray proven, the growth of nuclei of a solid, amorphous product was observed before the formation of a crystalline product, which proves the special catalytic properties of the interface: solid initial substance/solid and for the radiographically amorphous state. Crystallization of an amorphous product may, however, be important for explaining the dependence of rate on vapor pressure during the decomposition of crystalline hydrates. In these cases, the formation of a layer of an amorphous product that is difficult to penetrate for water molecules can lead to a decrease in the reaction rate.
Ft - flow of solid matter entering the apparatus, kg/hour; Fg (0) - flow of gaseous substance entering the apparatus, kg/hour; Fg - flow of gaseous substance entering into chemical interaction, kg/hour; Fr is the volume occupied by the gas phase in the reaction volume of the apparatus, m3; GT is the weight of the solid starting material in the reaction volume of the apparatus, kg; GT is the weight of the gaseous starting substance in the reaction volume of the apparatus, kg; скв - equivalent concentration of the gaseous starting substance in the reaction volume of the apparatus, kg/m8; a is the stoichiometric coefficient of transition from the substance flow Ft to the flow Fg; &g, / sg - solid and gaseous phase unloading coefficients, l / hour; K is the reaction rate constant; F (n) - function reflecting the order of the reaction; X - output coordinate (temperature); Ta is the time constant of the thermal model of the reaction volume of the apparatus; K7 is the gain coefficient of the thermal model of the reaction volume of the apparatus.
A mixture of 5 1 g of cyclopentadienyl manganese tricarbonyl, 13 7 g of phosphorus trichloride, 4 25 g of aluminum chloride and 15 ml of isopentane was heated with vigorous stirring and kept at a temperature of 45 - 50 C for 3 hours. Before heating, the mixture is a suspension of solid starting materials in a yellow solution.
It is important to determine which ions are missing in the sample. Preliminary tests) are mainly carried out with solid starting materials, the solutions are evaporated.
Very often, the rate of dissolution of the starting material is so insignificant or the reaction product is so slightly soluble that the new phase is densely deposited on the original one and, due to this, its external shape repeats the shape of the original substance. Such transformations, which occur at the interface of a solid starting material and lead to the production of solid final products, are called topochemical reactions in the narrow sense of the word. In contrast to reactions occurring in the bulk of a solution, the degree of dispersion of the reaction products in this case is similar to the dispersion of the starting substances. The topochemical method of consideration is therefore special, but applicable in the description of catalysts, electrolytic separation of metals and in matters of corrosion.
If vapor pressure promotes reactions between solids, then we should expect the same for chemical transport reactions. What opportunities do transport reactions provide as a means of interaction between solid starting substances?
In solid-phase reactions, the transformation can begin only in the bulk of the phase, and then develop at the interface between the new and old phases. Such reactions, where the transformation zone or front passes along the interface between the solid starting material and the solid product, are called topochemical. An example of such reactions is the weathering of crystalline hydrates. Faraday also noticed that well-cut transparent crystals of Cu2SO4 - 5H2O do not lose water in dry air for a long time. If a scratch is applied to their surface or a break is made, then rapid dehydration of the crystal immediately begins, which always spreads from the damaged area.
The fact that many anions can be detected fractionally does not mean that the discovery of anions is an easier task than the discovery of cations. Even with the limited number of anions that are studied in this textbook, the analysis presents great difficulties if the starting material for research is a solid starting material that is insoluble in water. Such a substance must be treated with soda (soda extract), which is associated with a number of complications in the work.
When writing reactions between solutions of electrolytes, each time you need to imagine whether there is any reason interfering with the actual occurrence of this or that reaction. For example, if an electrolyte solution interacts with solid substances and one of the products is slightly soluble, then the reaction can quickly stop due to the fact that a layer of also a solid reaction product is formed on the surface of the solid starting substance, preventing its further progress. That's why to get carbon dioxide When using acid on marble, use hydrochloric acid rather than sulfuric acid, since in the case of sulfuric acid the marble is quickly covered with a layer of gypsum (CaSO4 - 2H2O) and the reaction practically does not occur.
To react bismuth with fluorine, a fluidized bed reactor is used. The fluidized bed synthesis technique, borrowed from technology, has the following advantages: rapid establishment of thermal equilibrium in the reaction mixture, absence of sintering of solid reaction products, good heat exchange with the walls of the tube, large surface area of the solid starting materials and therefore rapid conversion.
For the g - t system, an increase in the contact surface of the phases is achieved by grinding the solid phase. The gaseous substance is brought into contact with the crushed starting material in a variety of ways, for example, solid particles of the substance are placed on the shelves of the reactor, and the gas flow moves over the shelves. In other cases, a finely divided solid starting material is sprayed into a stream of gaseous starting material in a hollow volume; This is how pulverized fuel is burned in the furnaces of steam boilers.
In fast industrial processes, reactions in mixtures of solids usually proceed at rates thousands of times greater than would be possible with direct interaction of solid phases. The thickness of the layer of the resulting product is almost the same over the entire surface of the grain it covers. This is explained by the fact that reactions occurring between solid starting substances actually occur with the participation of gaseous or liquid phases.
In the development of the chemistry of solid-phase reactions, discussions often arose on the question of whether solid substances could react with each other without the participation of a liquid or gas. This issue has now been resolved in favor of the existence of purely solid-phase reactions. It is interesting, however, that it can be shown in a number of transformations with solid starting materials that some liquid or gaseous phase nevertheless participates as a reaction mediator. However, generalizations in this area should be avoided; on the contrary, it is necessary to experimentally study the state of the system in each individual case. Budnikov and Ginstling carried out such research in particular detail.
If the problem of the initial substance for oil and gas formation can be considered solved in general, then the problem of the mechanism of oil and gas formation, which is key, still requires a solution in detail. Common composition organic matter, sedimentary rocks and hydrocarbons (HC) is an important argument in favor of a biosphere source of oil and gas. The role of thermal energy (heating) for the production of liquid and gas hydrocarbons from a solid starting material is also obvious. These circumstances made it possible to create a concept about hydrocarbon generation centers and formulate ideas about the main phases of gas and oil formation, which have become widespread throughout the world.
The rate of reactions occurring without the participation of gaseous and liquid phases is so low that they cannot be of great practical importance in fast industrial processes. But in practice, reactions in mixtures of solids usually proceed at rates thousands of times greater than would be possible with direct interaction of solids. The thickness of the layer of the resulting product is almost the same over the entire surface of the grain it covers. This is explained by the fact that reactions occurring between solid starting substances actually occur with the participation of gaseous or liquid phases.
The rate of such reactions, occurring without the participation of gaseous and liquid phases, is so low that they cannot be of great practical importance in fast industrial processes, carried out, in particular, in the production of salts. In practice, reactions in mixtures of solids usually occur at rates thousands of times greater than would be possible with direct interaction of solids. The thickness of the layer of the resulting product is almost the same over the entire surface of the grain it covers. This is explained by the fact that reactions occurring between solid starting substances actually occur with the participation of gaseous or liquid phases.
The rate of reactions occurring without the participation of gaseous and liquid phases is so low that they cannot be of great practical importance in fast industrial processes. But in practice, reactions in mixtures of solids usually proceed at rates thousands of times higher, or than would be possible with direct interaction of solids. The thickness of the layer of the resulting product is almost the same over the entire surface of the grain it covers. This is explained by the fact that reactions occurring between solid starting substances actually occur with the participation of gaseous or liquid phases.
It is unlikely that these compressive stresses, in relation to which solids are stronger than in relation to tension, would reach the magnitude necessary to destroy microscopic crystals. Direct experiments to study the dependence of the rate of decomposition of potassium permanganate on the size of the surface, which is inversely propo. This shows that fragmentation itself is not always the cause of the observed acceleration of the reaction. Explaining the acceleration of the reaction of solids by the existence of branched chain reactions also encounters some difficulties. Conditions in the solid phase differ significantly from those in the gas or liquid phase due to their heterogeneity. If a chain mechanism exists, then such a reaction is still limited to the interface between the solid starting material and the reaction product. Consequently, even in the presence of a chain mechanism, the question arises about the reasons special properties interface: initial solid/solid product.
2. Starting materials and experimental methods
2.1. Starting substances and their analysis
Phosphorus, fluorine and lithium were introduced in the form of ammonium dihydrogen phosphate, dried at 100 °C, and lithium fluoride and carbonate, dried at 200 °C. Reactive nickel oxide (grey, non-stoichiometric) was calcined at 900 °C to convert to green stoichiometric NiO. Reactive cobalt oxide (+2) was used in uncalcined form (it was verified by X-ray phase analysis that it was indeed CoO and not Co 3 O 4). Other reagents have also been tested for the introduction of transition metals: cobalt and manganese carbonates, nickel acetate, as well as manganese and iron (+2) oxalates precipitated from aqueous solutions. To carry out this part of the experiments, we took soluble salts: iron sulfate (+2) and manganese chloride (+2), dissolved them in hot distilled water and added a hot solution of ammonium oxalate to them. After cooling, the precipitates were filtered on a Buchner funnel, washed with distilled water until sulfate or chloride ions were removed, and dried at room temperature for several days.
There is no certainty that these carbonates, oxalates and acetate exactly correspond to the ideal formulas: during storage, loss or gain of water, hydrolysis, and oxidation are possible. Therefore, it was necessary to analyze them. To do this, three parallel samples of each of the starting substances were calcined to constant weight and weighed in the form of oxides. The calcination temperature was chosen based on literature data on the stability of weight forms: for the production of Fe 2 O 3, NiO - 900 ° C, for the production of Co 3 O 4 and Mn 2 O 3 - 750 ° C.
2.2. Carrying out syntheses
When lithium fluoride is heated with ammonium dihydrogen phosphate, hydrogen fluoride may volatilize. Therefore, carrying out the synthesis in one stage is hardly possible. First you need to get LiMPO 4, and only after complete removal of water can lithium fluoride be added.
Thus, two stages can be distinguished.
(1) 2NH 4 H 2 PO 4 + Li 2 CO 3 + 2MO ® 2 LiMPO 4 + 2NH 3 + CO 2 + 2H 2 O.
Here MO is either an oxide (NiO, CoO) or a compound that decomposes to an oxide.
(2) LiMPO 4 + LiF ® Li 2 MPO 4 F
Weighed amounts of substances were mixed and ground in a jasper mortar until completely homogeneous, then the tablets were pressed and kept at a temperature of 150-170 °C for 2 hours to remove most of the volatile components (if immediately heated to higher temperatures, melting occurs and the uniformity of the tablet is disrupted) . Then the temperature was gradually increased, periodically grinding the mixture, until almost pure LiMPO 4 was obtained. Firings were carried out either in a muffle furnace or in an inert atmosphere in a tube furnace.
Due to the lack of inert gases in the cylinders, it was necessary to obtain nitrogen by heating an aqueous solution of ammonium chloride and barium nitrite. The flask in which the main reaction to produce nitrogen took place (an exothermic reaction, slight heating) was connected to two washers with a sulfate solution of potassium dichromate to trap possible impurities of ammonia and nitrogen oxide, followed by a heated tube with porous copper granules to remove oxygen and oxides nitrogen, then with silica gel for rough drying and two washes with concentrated sulfuric acid for more complete capture of water vapor. These washers were connected to a tube that contained mixtures of substances compressed in nickel boats. First, three times the volume of nitrogen was passed through the installation to remove air, and only then heating began. After completion of firing, the samples were cooled in a stream of nitrogen to prevent oxidation by air.
The products were checked by X-ray phase analysis and proceeded to the second stage of the experiments; for this, the resulting tablets were ground with a calculated portion of lithium fluoride and, after being pressed, firing continued either in a muffle furnace or in an inert atmosphere in a tubular furnace using the technology already discussed. To ensure more complete binding of phosphate, lithium fluoride was introduced in a five percent excess. This excess is only 0.7 wt. % of the mixture and is less significant than the admixture of unreacted phosphate.
2.3. Radiography
X-ray phase analysis was carried out on a DRON-2.0 diffractometer in copper Ka radiation. This radiation is not very suitable for compounds containing iron and especially cobalt, since it is strongly absorbed by the atoms of these elements and excites their own x-ray radiation. As a result, the diffraction maxima are weakened and the background sharply increases. Therefore, the sensitivity of phase analysis decreases, the number of observed reflections decreases, and the accuracy of their measurement deteriorates due to strong intensity fluctuations. To overcome these difficulties, one would have to use an X-ray tube with a different anode, for example, cobalt (but then the same problems with manganese compounds would arise) or install a monochromator on a diffracted beam. But we did not have such an opportunity, so to reduce statistical errors, the shooting of the cobalt compound had to be repeated several times.
The phase analysis used the PDF-2 powder diffraction database.
Glava 6
Chemical kinetics. Chemical balance.
6.1.Chemicalkinetics.
Chemical kinetics- a branch of chemistry that studies the rates and mechanisms of chemical processes, as well as their dependence on various factors.
Study of kinetics chemical reactions allows you to both determine the mechanisms of chemical processes and control chemical processes during their practical implementation.
Any chemical process is the transformation of reagents into reaction products:
reactants→ transition state→ reaction products.
Reagents (starting materials) – substances that enter into the process of chemical interaction.
Reaction products– substances formed at the end of a chemical transformation process. In reversible processes, the products of the direct reaction are reagents of the reverse reaction.
Irreversible reactions– reactions occurring under given conditions in almost the same direction (denoted by the sign →).
For example:
CaCO 3 → CaO + CO 2
Reversible reactions– reactions occurring simultaneously in two opposite directions (indicated by a sign).
Transition state (activated complex) is a state of a chemical system that is intermediate between the starting substances (reagents) and the reaction products. In this state, old chemical bonds are broken and new chemical bonds are formed. Next, the activated complex is converted into reaction products.
Most chemical reactions are complex and consist of several stages called elementary reactions .
Elementary reaction- a single act of formation or rupture chemical bond. The set of elementary reactions that make up a chemical reaction determines mechanism of chemical reaction.
The equation of a chemical reaction usually indicates the initial state of the system (starting substances) and its final state (reaction products). At the same time, the actual mechanism of a chemical reaction can be quite complex and include a number of elementary reactions. Complex chemical reactions include reversible, parallel, sequential And other multi-step reactions (chain reactions , coupled reactions etc.).
If the rates of different stages of a chemical reaction differ significantly, then the rate of a complex reaction as a whole is determined by the rate of its slowest stage. This stage (elementary reaction) is called limiting stage.
Depending on the phase state of the reacting substances, two types of chemical reactions are distinguished: homogeneous And heterogeneous.
Phase called a part of a system that differs in its physical and chemical properties from other parts of the system and separated from them by the interface. Systems consisting of one phase are called homogeneous systems, from several phases – heterogeneous. An example of a homogeneous system would be air, which is a mixture of substances (nitrogen, oxygen, etc.) in the same gas phase. A suspension of chalk (solid) in water (liquid) is an example of a heterogeneous system consisting of two phases.
Accordingly, reactions in which the interacting substances are in the same phase are called homogeneous reactions. The interaction of substances in such reactions occurs throughout the entire volume of the reaction space.
Heterogeneous reactions include reactions occurring at the interface. An example of a heterogeneous reaction is the reaction of zinc (solid phase) with a solution of hydrochloric acid (liquid phase). In a heterogeneous system, a reaction always occurs at the interface between two phases, since only here can reactants located in different phases collide with each other.
Chemical reactions are usually distinguished by their molecularity, those. by the number of molecules participating in each elementary act of interaction . On this basis, reactions are distinguished between monomolecular, bimolecular and trimolecular.
Monomolecular are called reactions in which the elementary act is a chemical transformation of one molecule , For example:
Bimolecular are considered reactions in which the elementary act occurs when two molecules collide, for example:
IN trimolecular In reactions, an elementary act occurs during the simultaneous collision of three molecules, for example:
The collision of more than three molecules at the same time is almost impossible, so reactions of greater molecularity do not occur in practice.
The rates of chemical reactions can vary significantly. Chemical reactions can proceed extremely slowly, over a period of whole geological periods, such as weathering rocks, which represents the transformation of aluminosilicates:
K 2 O Al 2 O 3 6SiO 2 + CO 2 + 2H 2 O → K 2 CO 3 + 4SiO 2 + Al 2 O 3 2SiO 2 2H 2 O.
orthoclase – feldspar, potash quartz. sand kaolinite (clay)
Some reactions occur almost instantly, for example, the explosion of black powder, which is a mixture of coal, sulfur and saltpeter:
3C + S + 2KNO3 = N2 + 3CO2 + K2S.
The rate of a chemical reaction serves as a quantitative measure of the intensity of its occurrence.
In general under the speed of a chemical reaction understand the number of elementary reaction acts occurring per unit of time in a unit of reaction space.
Since for homogeneous processes the reaction space is the volume of the reaction vessel, then
for homogeneous reactions With The speed of a chemical reaction is determined by the amount of substance reacted per unit time in a unit volume.
Considering that the amount of a substance contained in a certain volume characterizes the concentration of the substance, then
the reaction rate is a value indicating the change in the molar concentration of one of the substances per unit time.
If, at constant volume and temperature, the concentration of one of the reactants decreases from With 1 to With 2 for the period from t 1 to t 2, then, in accordance with the definition, the reaction rate for a given period of time (average reaction rate) is equal to:
Typically for homogeneous reactions the rate dimension V[mol/l·s].
Since for heterogeneous reactions the reaction space is surface , on which the reaction occurs, then for heterogeneous chemical reactions, the reaction rate refers to the unit surface area on which the reaction occurs. Accordingly, the average rate of a heterogeneous reaction has the form:
Where S– surface area on which the reaction occurs.
The speed dimension for heterogeneous reactions is [mol/l·s·m2].
The speed of a chemical reaction depends on a number of factors:
the nature of the reacting substances;
concentrations of reactants;
pressure (for gas systems);
system temperature;
surface area (for heterogeneous systems);
the presence of a catalyst and other factors in the system.
Since every chemical interaction is the result of a collision of particles, an increase in concentration (the number of particles in a given volume) leads to more frequent collisions and, as a consequence, an increase in the reaction rate. The dependence of the rate of chemical reactions on the molar concentrations of the reactants is described by the basic law of chemical kinetics - law of mass action , which was formulated in 1865 by N.N. Beketov and in 1867 by K.M. Guldberg and P. Waage.
Law of mass action reads: the rate of an elementary chemical reaction at a constant temperature is directly proportional to the product of the molar concentrations of the reactants in powers equal to their stoichiometric coefficients.
The equation expressing the dependence of the reaction rate on the concentration of each substance is called kinetic equation of the reaction .
It should be noted that the law of mass action is fully applicable only to the simplest homogeneous reactions. If a reaction occurs in several stages, then the law is valid for each stage, and the speed of a complex chemical process is determined by the speed of the slowest reaction, which is limiting stage the whole process.
In general, if an elementary reaction occurs simultaneously T molecules of matter A And n molecules of matter IN:
mA + nIN = WITH,
then the equation for the reaction rate is (kinetic equation) has the form:
Where k– proportionality coefficient, which is called rate constant chemical reaction; [ A A; [B] – molar concentration of the substance B; m And n– stoichiometric coefficients in the reaction equation.
To understand physical meaning of the reaction rate constant , it is necessary to take in the equations written above the concentrations of the reacting substances [ A] = 1 mol/l and [ IN] = 1 mol/l (or equate their product to unity), and then:
From here it is clear that reaction rate constant k is numerically equal to the reaction rate in which the concentrations of reactants (or their product in kinetic equations) are equal to unity.
Reaction rate constant k depends on the nature of the reactants and temperature, but does not depend on the concentration of the reagents.
For heterogeneous reactions, the concentration of the solid phase is not included in the expression for the rate of a chemical reaction.
For example, in the methane synthesis reaction:
If a reaction occurs in the gas phase, then its rate is significantly affected by a change in pressure in the system, since a change in pressure in the gas phase leads to a proportional change in concentration. Thus, an increase in pressure leads to a proportional increase in concentration, and a decrease in pressure, accordingly, reduces the concentration of the gaseous reactant.
Changes in pressure have virtually no effect on the concentration of liquid and solid substances (condensed state of matter) and have no effect on the rate of reactions occurring in the liquid or solid phases.
Chemical reactions are carried out due to the collision of particles of reacting substances. However, not every collision of reactant particles is effective , i.e. leads to the formation of reaction products. Only particles with increased energy - active particles , are capable of carrying out a chemical reaction. With increasing temperature, the kinetic energy of particles increases and the number of active ones increases, therefore, the rate of chemical processes increases.
The dependence of the reaction rate on temperature is determined van't Hoff's rule : for every 10 0 C increase in temperature, the rate of a chemical reaction increases two to four times.
V 1 – reaction rate at the initial temperature of the system t 1 , V 2 – reaction rate at the final temperature of the system t 2 ,
γ – temperature coefficient of reaction (van’t Hoff coefficient), equal to 2÷4.
Knowing the value of the temperature coefficient γ makes it possible to calculate the change in reaction rate with increasing temperature from T 1 to T 2. In this case, you can use the formula:
It is obvious that with increasing temperature in arithmetic progression the reaction rate increases exponentially. The effect of temperature on the reaction rate is greater, the more more value temperature coefficient of reaction g.
It should be noted that Van't Hoff's rule is approximate and is applicable only for an approximate assessment of the effect of small changes in temperature on the reaction rate.
The energy required for reactions to occur can be provided by various influences (heat, light, electric current, laser radiation, plasma, radioactive radiation, high pressure, etc.).
Reactions can be divided into thermal, photochemical, electrochemical, radiation-chemical etc. With all these influences, the proportion of active molecules that have energy equal to or greater the minimum energy required for a given interaction E min.
When active molecules collide, a so-called activated complex , within which the redistribution of atoms occurs.
The energy required to increase the average energy of the molecules of the reacting substances to the energy of the activated complex is called the activation energy Ea.
Activation energy can be considered as a certain additional energy that reagent molecules must acquire in order to overcome a certain energy barrier . Thus, E a ra in the difference between the average energy of the reacting particles E ref and energy of the activated complex E min. The activation energy is determined by the nature of the reagents. Meaning E a ranges from 0 to 400 kJ. If the value E a exceeds 150 kJ, then such reactions practically do not occur at temperatures close to the standard one.
The change in energy of a system during a reaction can be graphically represented using the following energy diagram (Figure 6.1).
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Rice. 6.1. Energy diagram of an exothermic reaction:
E out is the average energy of the starting substances; Econd – average energy of reaction products; E min – energy of the activated complex; E act – activation energy; ΔH р – thermal effect of a chemical reaction
From the energy diagram it is clear that the difference between the energy values of the reaction products and the energy of the starting substances will represent the thermal effect of the reaction.
E cont. – E ref. = ΔН р.
According to Arrhenius equation, the higher the activation energy value E act, the greater the rate constant of the chemical reaction k depends on temperature:
E- activation energy (J/mol),
R - universal gas constant,
T– temperature in K,
A- Arrhenius constant,
e= 2.718 – the base of natural logarithms.
Catalysts- These are substances that increase the rate of a chemical reaction. They interact with reagents to form an intermediate chemical compound and are released at the end of the reaction. The effect that catalysts have on chemical reactions is called catalysis.
For example, a mixture of aluminum powder and crystalline iodine at room temperature shows no noticeable signs of interaction, but a drop of water is enough to cause a violent reaction:
Distinguish homogeneous catalysis (the catalyst forms a homogeneous system with the reacting substances, for example, a gas mixture) and heterogeneous catalysis (the catalyst and reactants are in different phases and the catalytic process occurs at the phase interface).
To explain the mechanism of homogeneous catalysis greatest distribution received intermediate theory (proposed by the French researcher Sabatier and developed in the works of the Russian scientist N.D. Zelinsky). According to this theory, a slow process, for example, the reaction:
in the presence of a catalyst occurs quickly, but in two stages. In the first stage of the process, an intermediate compound of one of the reagents with the catalyst is formed A...kat.
First stage:
A + kat = A.∙. kat.
At the second stage, the resulting compound forms an activated complex with another reagent [ A.∙.kat.∙.B], which turns into the final product AB with catalyst regeneration kat.
Second stage:
A.∙.kat + B = = AB + kat.
Intermediate interaction of the catalyst with the reagents directs the process to new way, characterized by a lower energy barrier. Thus, The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds.
An example would be a slow reaction:
2SO2 + O2 = 2SO3 slowly.
In the industrial nitrous method for producing sulfuric acid, nitrogen oxide (II) is used as a catalyst, which significantly speeds up the reaction:
Heterogeneous catalysis is widely used in oil refining processes. Catalysts include platinum, nickel, aluminum oxide, etc. Hydrogenation of vegetable oil occurs on a nickel catalyst (nickel on kieselguhr), etc.
An example of heterogeneous catalysis is the oxidation of SO 2 to SO 3 on a V 2 O 5 catalyst in the production of sulfuric acid by the contact method.
Substances that increase the activity of the catalyst are called promoters (or activators). At the same time, promoters themselves may not have catalytic properties.
Catalytic poisons - foreign impurities in the reaction mixture leading to partial or complete loss of catalyst activity. Thus, traces of phosphorus and arsenic cause quick loss catalyst for V 2 O 5 activity in the oxidation reaction of SO 2 to SO 3 .
Many important chemical productions, such as the production of sulfuric acid, ammonia, nitric acid, synthetic rubber, a number of polymers, etc., are carried out in the presence of catalysts.
Biochemical reactions in plant and animal organisms accelerate biochemical catalysts – enzymes.
Sharp It is possible to slow down the occurrence of undesirable chemical processes by adding special substances to the reaction medium - inhibitors. For example, to inhibit undesirable processes of corrosion destruction of metals, various metal corrosion inhibitors .
6.1.1. Questions for self-control of theory knowledge
on the topic "Chemical kinetics"
1. What does chemical kinetics study?
2. What is commonly understood by the term “reagents”?
3. What is commonly understood by the term “reaction products”?
4. How are reversible processes designated in chemical reactions?
5. What is commonly understood by the term “activated complex”?
6. What is an elementary reaction?
7. What reactions are considered complex?
8. Which stage of reactions is called the limiting stage?
9. Define the concept of “phase”?
10. What systems are considered homogeneous?
11. What systems are considered heterogeneous?
12. Give examples of homogeneous systems.
13. Give examples of heterogeneous systems.
14. What is considered the “molecularity” of a reaction?
15. What is meant by the term “rate of a chemical reaction”?
16. Give examples of fast and slow reactions.
17. What is meant by the term “rate of a homogeneous chemical reaction”?
18. What is meant by the term “rate of a heterogeneous chemical reaction”?
19. On what factors does the rate of a chemical reaction depend?
20. Formulate the basic law of chemical kinetics.
21. What is the rate constant of chemical reactions?
22.What factors does the rate constant of chemical reactions depend on?
23. The concentrations of which substances are not included in the kinetic equation of chemical reactions?
24. How does the rate of a chemical reaction depend on pressure?
25. How does the rate of a chemical reaction depend on temperature?
26. How is the “Van’t Hoff Rule” formulated?
27. What is the “temperature coefficient of a chemical reaction”?
28. Define the concept of “activation energy”.
29. Define the concept of “catalyst for a chemical reaction”?
30. What is homogeneous catalysis?
31. What is heterogeneous catalysis?
32. How is the mechanism of action of a catalyst in homogeneous catalysis explained?
33. Give examples of catalytic reactions.
34. What are enzymes?
35. What are promoters?
6.1.2. Examples of solving typical problems
on the topic "Chemical kinetics"
Example 1. The reaction rate depends on the contact surface area of the reactants:
1) sulfuric acid with barium chloride solution,
2) combustion of hydrogen in chlorine,
3) sulfuric acid with potassium hydroxide solution,
4) combustion of iron in oxygen.
The rate of heterogeneous reactions depends on the contact surface area of the reacting substances. Among the above reactions, a heterogeneous reaction, i.e. characterized by the presence of different phases is the combustion reaction of iron (solid phase) in oxygen (gas phase).
Answer. 3.
Example 2. How will the reaction rate change?
2H 2(g) + O 2(G) = 2H 2 O (g)
when the concentration of the starting substances doubles?
Let us write the kinetic equation of the reaction, which establishes the dependence of the reaction rate on the concentration of the reactants:
V 1 = k [N 2 ] 2 · [O 2 ].
If the concentrations of the starting substances are doubled, the kinetic equation will take the form:
V 2 = k (2 [N 2 ]) 2 2 [O 2 ] = 8 k [N 2 ] 2 · [O 2 ], i.e.
When the concentration of the starting substances was doubled, the rate of this reaction increased 8 times.
Answer. 8.
Example 3. How will the reaction rate change if the total pressure in the system CH 4 (G) + 2O 2 (G) = CO 2 (G) + 2H 2 O (G) is reduced by 5 times?
In accordance with the kinetic equation of the reaction, the rate of this reaction will be determined:
V 1 = k[CH 4 ] · [O 2 ] 2 .
When the pressure decreases by a factor of five, the concentration of each gaseous substance will also decrease by a factor of five. The kinetic equation of the reaction under these conditions will be as follows:
it can be determined that the reaction rate has decreased by 125 times.
Answer. 125.
Example 4. How will the rate of a reaction characterized by a reaction temperature coefficient of 3 change if the temperature in the system increases from 20 to 60°C?
Solution. According to van't Hoff's rule
When the temperature increased by 40 0 C, the rate of this reaction increased 81 times
Answer. 81.
6.1.3. Questions and exercises for self-study
Rate of chemical reactions
1. Depending on the physical state of the reacting substances, chemical reactions are divided into:
1) exothermic and endothermic,
2) reversible and irreversible,
3) catalytic and non-catalytic,
4) homogeneous and heterogeneous.
2. Indicate the number or sum of conventional numbers under which homogeneous reactions are given:
3. Indicate the number or sum of conventional numbers under which expressions are given that can be used to calculate the rate of a homogeneous reaction:
4. The unit of measurement for the rate of a homogeneous reaction can be:
1) mol/l s,
3) mol/l·,
4) l/mol·s.
5. Indicate the number or sum of conditional numbers under which fair expressions are given. During a homogeneous reaction
A + 2B® 2 C + D:
1) concentration A And IN are decreasing
2) concentration WITH increases faster than concentration D,
4) concentration IN decreases faster than concentration A,
8) the reaction rate remains constant.
6. What number is shown on the line that correctly reflects the change over time in the concentration of the substance formed in the reaction:
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7. Change over time in the concentration of the starting substance in a reaction that proceeds to completion, right describes the curve:
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9. Indicate the number or sum of conventional numbers under which reactions are given, the speed of which does not depend on what substance it is calculated from?
10. Indicate the number or sum of conditional numbers under which the factors influencing the reaction rate are given:
1) the nature of the reacting substances,
2) concentration of reacting substances,
4) temperature of the reaction system,
8) the presence of a catalyst in the reaction system.
11. The basic law of chemical kinetics establishes the dependence of the reaction rate on:
1) temperatures of reacting substances,
2) concentrations of reacting substances,
3) the nature of the reacting substances,
4) reaction time.
12. Indicate the number or sum of conditional numbers under which the correct statements are given. Chemical kinetics:
1) section of physics,
2) studies the rate of a chemical reaction,
4) uses the law of mass action,
8) studies the dependence of the rate of reactions on the conditions of their occurrence.
13. Ya.Kh. Van't Hoff:
1) first laureate Nobel Prize in chemistry,
2) studied the dependence of the reaction rate on temperature,
4) studied the dependence of the reaction rate on the concentration of substances,
8) formulated the law of mass action.
14. Under the same conditions, the reaction proceeds faster:
1) Ca + H 2 O ®
3) Mg + H 2 O ®
4) Zn + H 2 O ®
15. The rate of hydrogen evolution is highest in the reaction:
1) Zn + HCl (5% solution) ®
2) Zn + HCl (10% solution) ®
3) Zn + HCl (15% solution) ®
4) Zn + HCl (30% solution) ®
16. Concentration of reactant does not affect on the reaction rate if this substance is taken into the reaction in:
1) solid state,
2) gaseous state,
3) dissolved state.
17. Calculate the average rate of reaction A + B = C (mol/l×s), if it is known that the initial concentration of A was 0.8 mol/l, and after 10 seconds it became 0.6 mol/l.
1) 0,2, 2) 0,01, 3) 0,1, 4) 0,02.
18. By how much mol/l did the concentrations of substances A and B decrease in the reaction? A + 2B® 3 C, if it is known that during the same time the concentration WITH increased by 4.5 mol/l?
D WITH A D WITH B
19. Calculate the average rate of the reaction 2CO + O 2 ® 2CO 2 (mol/l×s), if it is known that the initial concentration of CO was 0.60 mol/l, and after 10 seconds it became 0.15 mol/l. By how many mol/l did the CO 2 concentration change over this period of time?
3) 0,045; 0,045,
20. How many degrees does the system need to be heated so that the rate of the reaction occurring in it increases by 2–4 times?
1) 150, 2) 10, 3) 200, 4) 50.
21. The reaction rate at 20°C is 0.2 mol/l×s. Determine the reaction rate at 60°C (mol/l×s) if the temperature coefficient of the reaction rate is 3.
1) 16,2, 2) 32,4, 3) 8,1, 4) 4,05.
22. Empirical dependence of reaction rate on temperature right reflects the equation:
23. The reaction rate at 20°C is 0.08 mol/l×s. Calculate the reaction rate at 0°C (mol/l×s), if the temperature coefficient of the reaction rate is 2.
1) 0,16, 2) 0,04, 3) 0,02, 4) 0,002.
24. How many times will the reaction rate increase when the temperature increases by 40°C, if the temperature coefficient of the reaction rate is 3?
1) 64, 2) 243, 3) 81, 4) 27.
25. By how many degrees should the temperature be increased to increase the reaction rate by 64 times, if the temperature coefficient of the reaction rate is 4?
1) 60, 2) 81, 3) 27, 4) 30.
26. Calculate the temperature coefficient of the reaction rate if it is known that when the temperature increases by 50 o C, the reaction rate increases by 32 times.
1) 3, 2) 2, 3) 4, 4) 2,5.
27. The reason for the increase in reaction rate with increasing temperature is an increase in:
1) the speed of movement of molecules,
2) the number of collisions between molecules,
3) the proportion of active molecules,
4) stability of the molecules of the reaction products.
28. Indicate the number or sum of conventional numbers under which the reactions for which MnO 2 is a catalyst are given:
1) 2KClO 3 ® 2KCl + 3O 2,
2) 2Al + 3I 2 ® 2AlI 3,
4) 2H 2 O 2 ® 2H 2 O + O 2,
8) 2SO 2 + O 2 ® 2SO 3 .
29. Indicate the number or sum of conventional numbers under which the correct answers are given. Using catalytic reactions in industry, the following is obtained:
1) hydrochloric acid,
2) sulfuric acid,
4) ammonia,
8) nitric acid.
30. Indicate the number or sum of conventional numbers under which the correct answers are given. Catalyst:
1) participates in the reaction,
2) used only in solid state,
4) is not consumed during the reaction,
8) necessarily contains a metal atom in its composition.
31. Indicate the number or sum of conventional numbers under which the correct answers are given. The following are used as catalysts:
32. Substances that reduce the activity of a catalyst are called:
1) promoters,
2) regenerators,
3) inhibitors,
4) catalytic poisons.
33. Catalytic is not reaction:
1) (C 6 H 10 O 5) n + n H2O® n C6H12O6,
cellulose
2) 2SO 2 + O 2 ® 2SO 3,
3) 3H 2 + N 2 ® 2NH 3 ,
4) NH 3 + HCl ® NH 4 Cl.
34. Under what number is the equation of homogeneous catalysis given:
35. The mechanism of action of the catalyst correctly reflects the statement. Catalyst:
1) increasing kinetic energy initial particles, increases the number of their collisions,
2) forms intermediate compounds with starting substances that are easily converted into final substances,
3) without interacting with the starting substances, it directs the reaction along a new path,
4) decreasing the kinetic energy of the initial particles, increases the number of their collisions.
36. The role of a promoter in a catalytic reaction is that it:
1) reduces the activity of the catalyst,
2) increases the activity of the catalyst,
3) leads the reaction in the desired direction,
4) protects the catalyst from catalytic poisons.
37. Enzymes:
1) biological catalysts,
2) have a protein nature,
4) do not differ in the specificity of action,
8) accelerate biochemical processes in living organisms.
38. The reaction is heterogeneous:
39. Indicate the number or sum of conventional numbers under which the correct answers are given. To increase the burning rate of coal: C + O 2 ® CO 2, you need to:
1) increase the concentration of O 2,
2) increase the concentration of coal,
4) grind the coal,
8) increase the concentration of carbon dioxide.
40. If reactant A is taken into the reaction: A t + X gas ® in the solid state, then the reaction rate is affected by:
1) concentration A,
2) surface area of contact between A and X,
4) molar mass A,
8) concentration of substance X.
41. The dimension of the rate of a heterogeneous reaction is:
1) mol/l, 2) mol/cm 3 ×s,
3) mol/l×s 4) mol/cm 2 ×s.
42. Indicate the number or sum of conventional numbers under which the correct answers are given. The fluidized bed principle is used:
1) to increase the contact surface of the reagents,
2) when firing pyrites,
4) during catalytic cracking of petroleum products,
8) to regenerate catalyst activity.
43. The least
1) Na + H 2 O ® 2) Ca + H 2 O ®
3) K + H 2 O ® 4) Mg + H 2 O ®
44. The graph shows energy diagrams of the non-catalytic and catalytic reaction of hydrogen iodide decomposition. The change in activation energy reflects the energy segment:
1) b, 2) c, 3) d, 4) b–c.
45. The greatest The reaction described by the scheme has activation energy:
1) AgNO 3 + KCl ® AgCl + KNO 3,
2) BaCl 2 + K 2 SO 4 ® BaSO 4 + 2KCl,
3) 2Na + 2H 2 O ® 2NaOH + 2H 2,
6.2. Chemical balance.
Along with practically irreversible chemical reactions:
СaCl 2 + 2AgNO 3 = Ca(NO 3) 2 + 2AgCl↓, etc.
Numerous processes are known when the chemical transformation does not reach completion, but an equilibrium mixture of all participants and products of the reaction occurs, located both on the left and on the right sides of the stoichiometric reaction equation. Thus, under standard conditions the system is reversible:
Let us consider the features of the occurrence of reversible processes using the example of a system, which, in general, has the form:
Provided that the forward → and reverse ← reactions occur in one stage, according to the law of mass action, the speed values for the forward reaction ( V direct) and reverse ( V the reactions are described by the following kinetic equations:
Where k straight And k arr - rate constants, respectively, of forward and reverse reactions.
At the initial moment of time (see Fig. 6.2), the concentrations of the starting substances [A] and [B], and therefore the rate of the direct reaction, have a maximum value. The concentrations of the reaction products [C] and [D] and the rate of the reverse reaction at the initial moment are zero. During the reaction, the concentrations of the starting substances decrease, which leads to a decrease in the rate of the forward reaction. The concentrations of reaction products, and, consequently, the rate of the reverse reaction increase. Finally, there comes a point at which the rates of the forward and reverse reactions become equal.
The state of the system in which V straight = V arr. called chemical equilibrium. This balance is dynamic , since a two-way reaction takes place in the system - in direct ( A And B– reagents, C And D– products) and in reverse ( A And B– products, C and D– reagents) directions.
V arr.
Reaction time
Rice. 6.2. Dependence of the rates of forward and reverse reactions
from the time of their occurrence.
In a reversible system in equilibrium, the concentrations of all participants in the process are called equilibrium concentrations, since in this case both forward and reverse reactions occur constantly and at the same speed.
A quantitative characteristic of chemical equilibrium can be derived using the appropriate kinetic equations :
Since the reaction rate constants at a fixed temperature are constant, the ratio will also be constant
called chemical equilibrium constant. Equating the right-hand sides of the kinetic equations for the forward and reverse reactions, we can obtain:
Where K r– chemical equilibrium constant, expressed in terms of equilibrium concentrations of reaction participants.
The chemical equilibrium constant is the ratio of the product of the equilibrium concentrations of reaction products to the product of the equilibrium concentrations of the starting substances in powers of their stoichiometric coefficients.
For example, for a reversible reaction
the expression for the equilibrium constant has the form:
If two or more phases are involved in the process of a chemical transformation, then the expression for the equilibrium constant should take into account only those of them in which changes in the concentrations of the reactants occur. For example, in the expression for the equilibrium constant for the system
the total number of moles of gaseous substances before and after the reaction remains constant and the pressure in the system does not change. The equilibrium in this system does not shift when pressure changes.
The influence of temperature changes on the shift in chemical equilibrium.
In each reversible reaction, one of the directions corresponds to an exothermic process, and the other to an endothermic process. So in the reaction of ammonia synthesis, the forward reaction is exothermic, and the reverse reaction is endothermic.
1) the concentrations of H 2, N 2 and NH 3 do not change over time,
3) the number of NH 3 molecules decaying per unit time is equal to half the total number of H 2 and N 2 molecules formed during this time,
4) the total number of H 2 and N 2 molecules converted into NH 3 per unit time is equal to the number of NH 3 molecules formed during the same time.
49. Indicate the number or sum of conventional numbers under which the correct answers are given. Chemical equilibrium in the system: 2SO 2 + O 2 2SO 3 ∆Н ˂0 will be disrupted by:
1) reducing the pressure in the system,
2) heating,
4) increase in oxygen concentration.
50. Indicate the number or sum of conventional numbers under which the correct answers are given. To shift the equilibrium in the system N 2 + 3H 2 2NH 3 ∆H ˂0 to the left, you need to:
1) introduce H 2 into the system,
2) remove NH 3 from the system,
4) increase blood pressure,
8) increase the temperature.
51. To shift the equilibrium of the reaction 2SO 2 + O 2 2SO 3 ∆Н ˂0 to the right, it is necessary:
1) heat the system,
2) introduce O 2 into the system,
4) introduce SO 3 into the system,
8) reduce the pressure in the system.
52. Le Chatelier's rule (principle) does not match statement:
1) an increase in temperature shifts the equilibrium towards an endothermic reaction;
2) a decrease in temperature shifts the equilibrium towards an exothermic reaction;
3) an increase in pressure shifts the equilibrium towards a reaction leading to an increase in volume;
N 2 + O 2 ∆H ˂0.2H 2 O (steam), 2NH 3 cat. 3H 2 + N 2 . B,
2) k 1 H = k 2 2 ,
67. For the equilibrium constant ( K p) affects:
1) pressure,
2) temperature,
3) concentration,
4) catalyst.
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;color:#000000;background:#ffffff">Irreversible reactions;color:#000000;background:#ffffff"> reactions in which the taken substances are completely converted into reaction products that do not react with each other under given conditions, for example;background:#ffffff">, ;color:#000000;background:#ffffff">burning;background:#ffffff"> ;color:#000000;background:#ffffff">hydrocarbons;background:#ffffff">, ;color:#000000;background:#ffffff">education;color:#000000;background:#ffffff">low dissociating;background:#ffffff"> ;color:#000000;background:#ffffff">compounds, precipitation, formation of gaseous substances.
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