2. starting materials and experimental methods

2.1. Starting materials and their analysis

Phosphorus, fluorine, and lithium were introduced in the form of ammonium dihydrogen phosphate dried at 100°C, and lithium fluoride and lithium carbonate dried at 200°C. Reactive nickel oxide (gray, non-stoichiometric) was calcined at 900°C to convert to green stoichiometric NiO. Reactive cobalt oxide (+2) was used in uncalcined form (X-ray phase analysis verified that it was indeed CoO and not Co 3 O 4). Other reagents were also tested for the introduction of transition metals: cobalt and manganese carbonates, nickel acetate, and also manganese and iron (+2) oxalates precipitated from aqueous solutions. To carry out this part of the experiments, soluble salts were taken: iron sulfate (+2) and manganese chloride (+2), they were dissolved in hot distilled water, and a hot solution of ammonium oxalate was added to them. After cooling, the precipitates were filtered off 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 acquisition of water, hydrolysis, and oxidation are possible. Therefore, it was necessary to analyze them. To do this, three parallel samples of each of the initial substances were calcined to constant weight and weighed in the form of oxides. The calcination temperature was chosen on the basis of literature data on the stability of weight forms: to obtain Fe 2 O 3 , NiO - 900 ° C, to obtain Co 3 O 4 and Mn 2 O 3 - 750 ° C.

2.2. Carrying out syntheses

When lithium fluoride is heated with ammonium dihydrophosphate, volatilization of hydrogen fluoride is possible. Therefore, carrying out the synthesis in one stage is hardly possible. LiMPO 4 must first be obtained, 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 decomposing to an oxide.

(2) LiMPO 4 + LiF ® Li 2 MPO 4 F

Samples of the substances were mixed and ground in a jasper mortar until completely homogeneous, then the tablets were pressed, kept at a temperature of 150-170 ° C for 2 hours to remove most of the volatile components (if heated immediately to higher temperatures, then melting occurs and the uniformity of the tablet is disturbed) . Then the temperature was gradually increased, periodically grinding the mixture, until almost pure LiMPO 4 was obtained. The firings were carried out either in a muffle furnace or in an inert atmosphere in a tube furnace.

Due to the absence of inert gases in the cylinders, nitrogen had to be obtained 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 potassium dichromate sulfate solution to trap possible impurities of ammonia and nitric oxide, followed by an incandescent tube with porous copper granules for purification from oxygen and oxides nitrogen, then with silica gel for rough drying and two washes with concentrated sulfuric acid for a more complete capture of water vapor. These washers were connected to a tube containing compressed mixtures of substances in nickel boats. First, three times the volume of nitrogen was passed through the setup to remove air, and only then heating was started. After firing was completed, the samples were cooled in a stream of nitrogen to prevent air oxidation.

The products were checked by X-ray phase analysis and proceeded to the second stage of experiments; for this, the obtained tablets were ground with a calculated weight of lithium fluoride and, having pressed, continued firing either in a muffle furnace or in an inert atmosphere in a tube furnace according to the technology already considered. To ensure a 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 in which iron and especially cobalt are present, 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 should use an X-ray tube with a different anode, for example, cobalt (but then the same problems would arise with manganese compounds) or install a monochromator on a diffracted beam. But we did not have such an opportunity, therefore, 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 data base.

For the formation of an active complex, it is necessary to overcome a certain energy barrier, spending energy E A. This energy is activation energy - some excess energy, compared with the average energy at a given temperature, that molecules must have in order for their collisions to be effective.

In the general case, for a chemical reaction A + B = C + D, the transition from the starting substances A and B to the reaction products C and D through the state of the active complex A + B = A¼B = C + D can be schematically represented in the form of energy diagrams (Fig. 6.2 ).

Usually reactions between substances with strong covalent bonds are characterized by large values ​​of E A and go slowly, for example:

Low values ​​of E A and very high speeds ionic interactions in electrolyte solutions are characterized. For example:

Ca + 2 + SO \u003d CaSO 4.

This is explained by the fact that oppositely charged ions are attracted to each other and no energy is required to overcome the repulsive forces of the interacting particles.

Influence of the catalyst

A change in the reaction rate under the influence of small additions of special substances, the amount of which does not change during the process, is called catalysis.

Substances that change the rate of a chemical reaction are called catalysts.(substances that change the speed chemical processes in living organisms - enzymes). The catalyst is not consumed in the reactions and is not included in the composition of the final products.

Chemical reactions occurring in the presence of a catalyst are called catalytic. There are positive catalysis - in the presence of a catalyst, the rate of a chemical reaction increases - and negative catalysis (inhibition) - in the presence of a catalyst (inhibitor), the rate of a chemical reaction slows down.

1. Oxidation of sulfur dioxide in the presence of a platinum catalyst:

2SO 2 + O 2 = 2SO 3 - positive catalysis.

2. Slowing down the formation of hydrogen chloride in the presence of oxygen:

H 2 + Cl 2 \u003d 2HCl - negative catalysis.

Distinguish: a) homogeneous catalysis - the reactants and the catalyst form a single-phase system; b) heterogeneous catalysis - the reactants and the catalyst form a system of different phases.

The mechanism of action of the catalyst. The mechanism of action of positive catalysts is reduced to a decrease in the activation energy of the reaction. This forms an active complex with more low level energy and the rate of a chemical reaction is greatly increased. On fig. 6.3 shows the energy diagram of a chemical reaction occurring in the absence (1) and in the presence (2) of a catalyst.

If the slow reaction A + B = AB is carried out in the presence of catalyst K, then the catalyst enters into a chemical interaction with one of the starting materials, forming an unstable intermediate compound: A + K = AK.

The activation energy of this process is low. The intermediate compound AA is reactive; it reacts with another starting material, while the catalyst is released and leaves the reaction zone:

Summing up both processes, we obtain the equation for a fast reaction: A + B + (K) = AB + (K).

Example. Oxidation of sulfur dioxide with the participation of the NO catalyst: 2SO 2 + O 2 \u003d 2SO 3 - a slow reaction;

With the introduction of a catalyst - NO - an intermediate compound is formed: 2NO + O 2 \u003d 2NO 2.

In heterogeneous catalysis, the accelerating action is associated with adsorption. Adsorption - the phenomenon of absorption of gases, vapors, solutes by the surface solid body. The surface of the catalyst is not uniform. It has the so-called active centers on which the adsorption of reacting substances occurs, which increases their concentration.

There are also substances that enhance the effect of a catalyst, although they are not catalysts themselves. These substances are called promoters.

CHEMICAL EQUILIBRIUM

">24. "> "> Signs of reversible and irreversible reactions. Equilibrium criteria. Equilibrium constant. Le Chatelier principle.

;color:#000000;background:#ffffff">1. The reaction is called;color:#000000;background:#ffffff">reversible;color:#000000;background:#ffffff">, if its direction depends on the concentrations of substances participating in the reaction. For example, N;vertical-align:sub;color:#000000;background:#ffffff">2;color:#000000;background:#ffffff"> + 3H;vertical-align:sub;color:#000000;background:#ffffff">2;color:#000000;background:#ffffff"> = 2NH;vertical-align:sub;color:#000000;background:#ffffff">3;color:#000000;background:#ffffff"> at a low concentration of ammonia in the gas mixture and high concentrations of nitrogen and hydrogen, ammonia is formed; on the contrary, at a high concentration of ammonia, it decomposes, the reaction proceeds in the opposite direction. Upon completion reversible reaction, i.e., when the chemical equilibrium is reached, the system contains both the initial substances and the reaction products.

;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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"> The state of chemical equilibrium is quantitatively characterized by the equilibrium constant, which is the ratio of the constants of the straight line (" xml:lang="en-US" lang="en-US">K;vertical-align:sub">1 ">) and reverse ( " xml:lang="en-US" lang="en-US">K;vertical-align:sub">2 ">) reactions.

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"> The equilibrium constant depends on the temperature and nature of the reacting substances. The larger the equilibrium constant, the more the equilibrium is shifted towards the formation of direct reaction products.

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1. "> For exothermic p-th increase. T shifts to the left
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" xml:lang="en-US" lang="en-US">→ t◦→

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AT modern science distinguish between chemical and nuclear reactions that occur as a result of the interaction of the starting substances, which are commonly called reagents. As a result, other chemicals are formed, which are called products. All interactions occur under certain conditions (temperature, radiation, the presence of catalysts, etc.). The atomic nuclei of the reactants of chemical reactions do not change. In nuclear transformations, new nuclei and particles are formed. There are several different signs by which the types of chemical reactions are determined.

The classification can be based on the number of initial and formed substances. In this case, all types of chemical reactions are divided into five groups:

1. Decompositions (several new ones are obtained from one substance), for example, decomposition when heated to potassium chloride and oxygen: KCLO3 → 2KCL + 3O2.
2. Compounds (two or more compounds form one new one), interacting with water, calcium oxide turns into calcium hydroxide: H2O + CaO → Ca(OH)2;
3. Substitutions (the number of products is equal to the number of starting substances in which one component is replaced by another), iron in copper sulfate, replacing copper, forms ferrous sulfate: Fe + CuSO4 → FeSO4 + Cu.
4. Double exchange (molecules of two substances exchange the parts that leave them), metals in and exchange anions, forming precipitated silver iodide and cadium nitrate: KI + AgNO3 → AgI↓ + KNO3.
5. Polymorphic transformation (there is a transition of a substance from one crystalline form to another), color iodide, when heated, turns into yellow mercury iodide: HgI2 (red) ↔ HgI2 (yellow).

If chemical transformations are considered on the basis of changes in the oxidation state of elements in the reacting substances, then the types of chemical reactions can be divided into groups:

1. With a change in the degree of oxidation - redox reactions (ORD). As an example, consider the interaction of iron with hydrochloric acid: Fe + HCL → FeCl2 + H2, as a result, the oxidation state of iron (the reducing agent that donates electrons) changed from 0 to -2, and hydrogen (the oxidizing agent that accepts electrons) from +1 to 0 .
2. No change in oxidation state (i.e., no OVR). For example, the reactions of acid-base interaction of hydrogen bromide with sodium hydroxide: HBr + NaOH → NaBr + H2O, as a result of such reactions, salt and water are formed, and the oxidation states chemical elements, included in the original substances, do not change.

If we consider the flow rate in the forward and reverse directions, then all types of chemical reactions can also be divided into two groups:

1. Reversible - those that flow in two directions at the same time. Most reactions are reversible. An example is the dissolution of carbon dioxide in water with the formation of unstable carbonic acid, which decomposes into the starting substances: H2O + CO2 ↔ H2CO3.
2. Irreversible - flow only in the forward direction, after the complete consumption of one of the starting substances, they are completed, after which only products and the starting substance, taken in excess, are present. Usually one of the products is either precipitated insoluble matter or evolved gas. For example, when sulfuric acid and barium chloride react: H2SO4 + BaCl2 + → BaSO4↓ + 2HCl, an insoluble

The types of chemical reactions in organic chemistry can be divided into four groups:

1. Substitution (one atoms or groups of atoms are replaced by others), for example, when chloroethane interacts with sodium hydroxide, ethanol and sodium chloride are formed: C2H5Cl + NaOH → C2H5OH + NaCl, that is, the chlorine atom is replaced by a hydrogen atom.
2. Attachment (two molecules react and form one), for example, bromine joins at the site of the double bond break in the ethylene molecule: Br2 + CH2=CH2 → BrCH2—CH2Br.
3. Cleavage (a molecule decomposes into two or more molecules), for example, under certain conditions, ethanol decomposes into ethylene and water: C2H5OH → CH2=CH2 + H2O.
4. Rearrangement (isomerization, when one molecule turns into another, but the qualitative and quantitative composition of the atoms in it does not change), for example, 3-chlororuten-1 (C4H7CL) turns into 1 chlorobutene-2 ​​(C4H7CL). Here the chlorine atom moved from the third carbon atom in the hydrocarbon chain to the first, and the double bond connected the first and second carbon atoms, and then began to connect the second and third atoms.

Other types of chemical reactions are also known:

1. By flowing with absorption (endothermic) or release of heat (exothermic).
2. According to the type of reactants or products formed. Interaction with water - hydrolysis, with hydrogen - hydrogenation, with oxygen - oxidation or combustion. The splitting off of water is dehydration, hydrogen is dehydrogenation, and so on.
3. According to the conditions of interaction: in the presence of catalysts (catalytic), under the action of low or high temperature, when pressure changes, in the light and so on.
4. According to the mechanism of the reaction: ionic, radical chain or chain reactions.

For the formation of an active complex, it is necessary to overcome a certain energy barrier, spending energy E A. This energy is activation energy - some excess energy, compared with the average energy at a given temperature, that molecules must have in order for their collisions to be effective.

In the general case, for a chemical reaction A + B = C + D, the transition from the starting substances A and B to the reaction products C and D through the state of the active complex A + B = A¼B = C + D can be schematically represented in the form of energy diagrams (Fig. 6.2 ).

Ionic interactions in electrolyte solutions are characterized by low values ​​of E A and very high rates. For example:

Ca + 2 + SO \u003d CaSO 4.

This is explained by the fact that oppositely charged ions are attracted to each other and no energy is required to overcome the repulsive forces of the interacting particles.

Influence of the catalyst

A change in the reaction rate under the influence of small additions of special substances, the amount of which does not change during the process, is called catalysis.

Substances that change the rate of a chemical reaction are called catalysts.(substances that change the rate of chemical processes in living organisms - enzymes). The catalyst is not consumed in the reactions and is not included in the composition of the final products.

Chemical reactions occurring in the presence of a catalyst are called catalytic. There are positive catalysis - in the presence of a catalyst, the rate of a chemical reaction increases - and negative catalysis (inhibition) - in the presence of a catalyst (inhibitor), the rate of a chemical reaction slows down.

1. Oxidation of sulfur dioxide in the presence of a platinum catalyst:

2SO 2 + O 2 = 2SO 3 - positive catalysis.

2. Slowing down the formation of hydrogen chloride in the presence of oxygen:

H 2 + Cl 2 \u003d 2HCl - negative catalysis.

Distinguish: a) homogeneous catalysis - the reactants and the catalyst form a single-phase system; b) heterogeneous catalysis - the reactants and the catalyst form a system of different phases.

The mechanism of action of the catalyst. The mechanism of action of positive catalysts is reduced to a decrease in the activation energy of the reaction. In this case, an active complex with a lower energy level is formed and the rate of the chemical reaction increases greatly. On fig. 6.3 shows the energy diagram of a chemical reaction occurring in the absence (1) and in the presence (2) of a catalyst.

If the slow reaction A + B = AB is carried out in the presence of catalyst K, then the catalyst enters into a chemical interaction with one of the starting materials, forming an unstable intermediate compound: A + K = AK.

The activation energy of this process is low. The intermediate compound AA is reactive; it reacts with another starting material, while the catalyst is released and leaves the reaction zone:

Summing up both processes, we obtain the equation for a fast reaction: A + B + (K) = AB + (K).

Example. Oxidation of sulfur dioxide with the participation of the NO catalyst: 2SO 2 + O 2 \u003d 2SO 3 - a slow reaction;

With the introduction of a catalyst - NO - an intermediate compound is formed: 2NO + O 2 \u003d 2NO 2.

In heterogeneous catalysis, the accelerating action is associated with adsorption. Adsorption is the phenomenon of absorption of gases, vapors, dissolved substances by the surface of a solid body. The surface of the catalyst is not uniform. It has the so-called active centers on which the adsorption of reacting substances occurs, which increases their concentration.

Some substances reduce or completely destroy the activity of a solid catalyst - catalytic poisons (these include compounds of lead, arsenic, mercury, cyanide compounds). Platinum catalysts are especially sensitive to catalytic poisons.

There are also substances that enhance the effect of a catalyst, although they are not catalysts themselves. These substances are called promoters.

CHEMICAL EQUILIBRIUM

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