What is the temperature coefficient of the reaction. Temperature dependence of the reaction rate

Van't Hoff's rule:

when the temperature rises by 10 degrees, the rate of a homogeneous chemical reaction increases by 2-4 times.

where V2 is the reaction rate at temperature T2, V1 is the reaction rate at temperature T1, is the temperature coefficient of the reaction (if it is equal to 2, for example, then the reaction rate will increase by 2 times when the temperature rises by 10 degrees).

From the van't Hoff equation temperature coefficient calculated by the formula:

The theory of active collisions generalizes the regularities the dependence of the speed of chem.r-and on temperature:

1. Not all molecules can react, but only those in a special active state

2.Activation of a molecule occurs as a result of a biomolecular collision.

3. When particles with approximately the same amount of energy collide, it is redistributed, as a result of which the energy of one of the molecules reaches a value corresponding to the activation energy.

4. Influence of temperature on the reaction rate: a shift in the equilibrium between ordinary and active molecules towards an increase in the concentration of the former.

Energy profile of the reaction (plot of potential energy versus reaction coordinate)

Activation energy Ea- the minimum additional energy that must be imparted to the molecule in excess of its average value in order to make chem. interaction.

Arrhenius equation establishes the dependence of the rate constant of a chemical reaction k on temperature T.

Here A characterizes the frequency of collisions of reacting molecules, R is the universal gas constant.

7. Catalysis. Homogeneous and heterogeneous catalysis. Features of the catalytic activity of enzymes. Catalysis- speed change chemical reactions in the presence of substances that, after completion of the reaction, remain unchanged in form and quantity. An increase in the rate of a reaction is called positive catalysis, decrease - negative catalysis (or inhibition). Catalysts name substances that cause positive catalysis; substances that slow down reactions inhibitors. Distinguish between homogeneous and heterogeneous catalysis. The acceleration of the disproportionation reaction of hydrogen peroxide in an aqueous solution in the presence of dichromate ions is an example of homogeneous catalysis (the catalyst forms one phase with the reaction mixture), and in the presence of manganese(IV) oxide it is an example of heterogeneous catalysis (an aqueous solution of hydrogen peroxide-liquid phase, manganese oxide - solid). Catalysts of biochemical reactions are of a protein nature and are called enzymes. Enzymes differ from conventional catalysts in a number of ways: 1) they have a much higher catalytic efficiency; 2) high specificity, i.e. selectivity of action; 3) many enzymes exhibit catalytic activity with respect to only one substrate; 4) enzymes show maximum efficiency only in mild conditions, characterized by a small range of temperatures and pH values. Enzyme activity \u003d Zero-order reaction rate. 8. Chemical balance. Reversible and irreversible in the direction of the reaction. Chemical equilibrium: dynamic state in which the rates of the forward and reverse reactions are equal. Equilibrium constant: under constant external conditions in equilibrium, the ratio of the product of product concentrations to the product of reactant concentrations, taking into account stoichiometry, is a constant value, independent of the chemical composition of the system. K c is related to the Gibbs standard E by: Le Chatelier's principle: the impact of some factor (t, c, p) on the equilibrium system stimulates the shift of equilibrium in such a direction, which contributes to the restoration of the initial characteristics of the system. Thermodynamic equilibrium conditions: G 2 -G 1 \u003d 0S 2 -S 1 \u003d 0 Reversible p-tion: under these conditions, spontaneously flowing both in the forward and in the opposite direction .Run through conditions: - Slightly soluble precipitate - gas - low dissociating substance (water) - stable complex compound Irreversible district: under given conditions flows in one direction. The position of chemical equilibrium depends on the following reaction parameters: temperature, pressure and concentration. The influence that these factors have on a chemical reaction is subject to rules that were expressed in general view in 1884 by the French scientist Le Chatelier. The modern formulation of Le Chatelier's principle is as follows:

9. The role of water and solutions in life. Thermodynamics of dissolution.Solution is a homogeneous system of variable composition of two or more substances in a state of equilibrium. Classification: 1) weigh(coarse-dispersed system): suspensions (solids in liquid) and emulsions (liquid in liquid) 2) colloids, sols(fine-dispersed systems). The value of solutions in life: many chemical processes proceed only if the substances involved in them are in a dissolved state. The most important biological fluids (blood, lymph, urine, saliva, sweat) are solutions of salts, proteins, carbohydrates, lipids in water. Assimilation of food is associated with the transition of nutrients into a dissolved state. Biochemical reactions in living organisms proceed in solutions. Biofluids are involved in the transport of nutrients (fats, amino acids, oxygen), drugs to organs and tissues, as well as in the excretion of metabolites from the body. In the liquid media of the body, the constancy of acidity, salt concentration and organic matter(concentration homeostasis). The most common solvent on our planet is water. Water features: surpasses all substances in its heat capacity; anomalous cooling behavior - water condenses, begins to sink, then rises (all other substances sink when compacted); can sublimate (sublimation of water) - sublimation (under certain conditions, ice can turn into steam without first turning into liquid water, i.e. without melting); water dissolves all substances (the only question is how much?); high dielectric constant of water (a value showing how many times the interaction force between two charges in a given substance is less than in vacuum); high critical temperature; water is ampholyte (not acid, not basic); participates in the creation of polymeric structures of the body (protein, lipids ...); basis of membrane transport. Dissolution thermodynamics: according to the 2nd law of thermodynamics at p, T=const substances can spontaneously dissolve in any solvent if, as a result of this process, the Gibbs energy of the system decreases, i.e. . G=( H - T S)<0 . (H- enthalpy factor, T S is the entropy factor of dissolution). When dissolving liquid and solid substances S>0. Dissolving gases in liquid S<0. The enthalpy change is the algebraic sum of the enthalpy change H cr as a result of the destruction of the crystal lattice and the change in enthalpy H sol due to solvation by solvent particles H sol = H kr + H Sol . When dissolving gases, the enthalpy H cr = 0, because no need to expend energy to destroy the crystal lattice. During dissolution, both entropy and enthalpy can change. 10 . Ideal Solution- the enthalpy of mixing is 0 (homogeneous mixtures of hydrocarbons; hypothetical solution, where the equality of all forces of intermolecular interaction.) Solubility constant or PR- this is the product of the concentrations of ions of a sparingly soluble electrolyte in a saturated solution at a given temperature - a constant value BaCO 3 \u003d Ba + CO 3, Ks \u003dDissolution and Precipitation Conditions Precipitation and dissolution - exchange reactions occurring in an electrolyte solution --- 1) The electrolyte will precipitate if the product of the concentration of its ions in the solution is greater than the solubility constant c (Ba) * c (CO 3)> Kpr 2) Its precipitate will dissolve if all vice versa 11. Coligative properties of solutions. Colligative properties of solutions- these are their properties that, under given conditions, turn out to be equal and independent of the chemical nature of the dissolved substance; properties of solutions that depend only on the number of kinetic units and on their thermal motion. Raoult's law and its consequences A vapor in equilibrium with a liquid is called saturated. The pressure of such a vapor over a pure solvent (p0) is called the pressure or saturated vapor pressure of a pure solvent. The vapor pressure of a solution containing a non-volatile solute is directly proportional to the mole fraction of the solvent in that solution: p = p0 χr-l, where p is the vapor pressure over the solution, PA; p0 is the vapor pressure over a pure solvent; χr-l is the molar fraction of the solvent. For electrolyte solutions, a slightly different form of the equation is used, which allows adding an isotonic coefficient to it: Δp = i p0 χv -va, where Δp is the actual change in pressure compared to a pure solvent; χv-va is the mole fraction of a substance in solution. From Raoult's law there are two consequences. According to one of them, the boiling point of the solution is higher than the boiling point of the solvent. This is due to the fact that the saturated vapor pressure of the solvent over the solution becomes equal to atmospheric pressure (liquid boiling condition) at a higher temperature than in the case of a pure solvent. The increase in boiling point Tboil is proportional to the molality of the solution:. Tkip = Ke cm where Ke is the ebullioscopic solvent constant, cm is the molal concentration. According to second investigation from Raoult's law, the freezing (crystallization) temperature of a solution is lower than the freezing (crystallization) temperature of a pure solvent. This is due to the lower vapor pressure of the solvent over the solution than over the solvent. The decrease in freezing point (crystallization) Тzam is proportional to the molality of the solution : Tzam = Kk cm where Kk is the cryoscopic constant of the solution Lowering the crystallization temperature of solutions. Condition crystallization is the equality of the saturated vapor pressure of the solvent over the solution to the vapor pressure over the solid solvent. Since the vapor pressure of a solvent over a solution is always lower than over a pure solvent, this equality will always be achieved at a temperature lower than the freezing point of the solvent. So, ocean water begins to freeze at a temperature of about minus 2 ° C. The difference between the crystallization temperature of the solvent and the temperature of the beginning of the crystallization of the solution is a decrease in the crystallization temperature. Increasing the boiling point of solutions Liquid boils at the temperature at which the total saturation vapor pressure becomes equal to the external pressure. the pressure of saturated vapors over a solution at any temperature will be less than over a pure solvent, and equality to its external pressure will be achieved at a higher temperature. Thus, the boiling point of a solution of a non-volatile substance T is always higher than the boiling point of a pure solvent at the same pressure T °. The increase in the boiling point of infinitely dilute solutions of non-volatile substances does not depend on the nature of the solute and is directly proportional to the molar concentration of the solution. The spontaneous passage of a solvent through a semi-permeable membrane separating a solution and a solvent or two solutions with different concentrations of a solute is called osmosis. Osmosis is due to the diffusion of solvent molecules through a semi-permeable barrier that allows only solvent molecules to pass through. Solvent molecules diffuse from a solvent into a solution or from a less concentrated solution to a more concentrated one. Osmosis is quantitatively characterized osmotic pressure, equal to the force per unit surface area, and forcing the solvent molecules to penetrate through a semipermeable partition. It is equal to the pressure of the solution column in the osmometer with height h. At equilibrium, the external pressure balances the osmotic pressure. In this case, the rates of direct and reverse transitions of molecules through a semipermeable partition become the same. Osmotic pressure increases with increasing solute concentration and temperature. Van't Hoff suggested that for the osmotic pressure one can apply the equation of state of an ideal gas: pV = nRT or p = (n/V) RT whence p = with RT, where p is the osmotic pressure (kPa), c is the molar concentration of the solution. Osmotic pressure is directly proportional to the molar concentration of the solute and temperature. Osmosis plays very important role in biological processes, ensuring the flow of water into cells and other structures. Solutions with the same osmotic pressure are called isotonic. If the osmotic pressure is higher than intracellular, then it is called hypertonic, if it is lower than intracellular, it is called hypotonic. The isotonic coefficient (also the van't Hoff factor; denoted i) is a dimensionless parameter that characterizes the behavior of a substance in solution. It is numerically equal to the ratio of the value of some colligative property of a solution of a given substance and the value of the same colligative property of a non-electrolyte of the same concentration, with other system parameters unchanged. Isoosmia-relative constancy of osmotic pressure in liquid media and tissues of the body, due to the maintenance of the concentrations of the substances contained in them at a given level: electrolytes, proteins. This is one of the most important physiological constants of the body, provided by the mechanisms of self-regulation (Homeostasis). HEMOLYSIS- destruction of red blood cells, accompanied by the release of hemoglobin from them. Physical reasons refers to the action of high and low temperatures, ultrasound, to chemical - hemolytic poisons, certain drugs, etc. Hemolysis can occur during transfusion of incompatible blood, the introduction of hypotonic solutions. Plasmolysis- when cells are placed in a hypertonic solution, water from the cells goes into a more concentrated solution and wrinkling of the cells is observed.

Elements of the theory of electrolyte solutions. Strong and weak electrolytes. Ionization constant of a weak electrolyte. Ostwald's breeding law. Ionic strength of the solution. Activity and activity coefficient of ions. Electrolytes in the body, saliva as an electrolyte.

electrolytes- These are substances with ionic or highly polar covalent bonds in aqueous solutions that undergo electrolytic dissociation, resulting in the formation of cations and anions.

Strong electrolytes- substances capable of dissociating completely. These include most salts, as well as some substances of a molecular structure (HCl).

Weak electrolytes dissociate to an insignificant degree, and their predominant form is molecular (H2S, organic acids).

Quantitatively, the ability of a molecular electrolyte to dissociate is determined by degree of ionization ( it depends on the electrolyte concentration ):

where Ntot is the total number of molecules in the solution; N ionization is the number of molecules decomposed into ions.

Ionization constant:

Where [A], [B] are decayed ions

- a substance that has not broken down into ions.

Ostwald's dilution law:

K= α 2 c/1- α ,

Where α is the degree of ionization

C - molar concentration

Ionic strength of solution:

I=0.5∑s i z i 2 ,

Where c i is the molar concentration of the ion in the solution, mol/l

z i is the ion charge.

Ion activity is its effective concentration.

Activity is related to molar concentration as follows:

where f is activity factor

electrolytes in the body: Na and Cl participate in maintaining the acid-base balance, osmotic balance in the body. Sa plays an important role in the construction of bone tissue and teeth, in the regulation of blood acidity and its coagulation, in the excitability of muscle and nervous tissue. To It is located mainly in body fluids and soft tissues, where it is a necessary element for maintaining osmotic pressure and regulating blood pH. mg is a cofactor in many enzymatic reactions, is necessary at all stages of protein synthesis. in living organisms Fe is an important trace element that catalyzes the processes of oxygen exchange. Co is part of vitamin B 12, is involved in hematopoiesis, functions nervous system and liver, enzymatic reactions. Zn essential for the metabolism of vitamin E, is involved in the synthesis of various anabolic hormones in the body, including insulin, testosterone and growth hormone. Mn affects growth, blood formation and gonadal function.

Saliva as an electrolyte is a complex biochemical environment. The number of H + and OH ions "determines the pH of saliva, which is normally 6.9. The value of the pH value varies depending on the nature of the pathological process in the oral cavity. So. with infectious diseases saliva is acidic. Of the inorganic substances in saliva, anions of chlorine, bromine, iodine, and fluorine are contained. Anions of phosphates, fluorine contribute to an increase in electrochemical potentials, anion of chlorine - the transfer of ionic charges and is a depolarizer (a factor that accelerates anodic and cathodic processes). Microelements are determined in saliva: iron, copper, silver, manganese, aluminum, etc. - and macroelements: calcium, potassium, sodium, magnesium, phosphorus.

From qualitative considerations, it is clear that the rate of reactions should increase with increasing temperature, since in this case, the energy of the colliding particles increases and the probability that a chemical transformation occurs during the collision increases. For a quantitative description of temperature effects in chemical kinetics, two basic relationships are used - the van't Hoff rule and the Arrhenius equation.

Van't Hoff's rule lies in the fact that when heated by 10 ° C, the rate of most chemical reactions increases by 2-4 times. Mathematically, this means that the reaction rate depends on temperature in a power-law manner:

, (4.1)

where is the temperature coefficient of speed ( = 24). Van't Hoff's rule is very rough and is applicable only in a very limited temperature range.

Much more accurate is Arrhenius equation describing the temperature dependence of the rate constant:

, (4.2)

where R- universal gas constant; A- pre-exponential factor, which does not depend on temperature, but is determined only by the type of reaction; E A - activation energy, which can be characterized as some threshold energy: roughly speaking, if the energy of colliding particles is less than E A, then the reaction will not occur during the collision if the energy exceeds E A, the reaction will occur. The activation energy does not depend on temperature.

Graphically dependency k(T) as follows:

At low temperatures, chemical reactions almost do not occur: k(T) 0. At very high temperatures, the rate constant tends to the limit value: k(T)A. This corresponds to the fact that all molecules are chemically active and each collision leads to a reaction.

The activation energy can be determined by measuring the rate constant at two temperatures. Equation (4.2) implies:

. (4.3)

More precisely, the activation energy is determined from the values ​​of the rate constant at several temperatures. To do this, the Arrhenius equation (4.2) is written in the logarithmic form

and write the experimental data in coordinates ln k - 1/T. The tangent of the slope of the resulting straight line is - E A / R.

For some reactions, the pre-exponential factor depends only slightly on temperature. In this case, the so-called experimental activation energy:

. (4.4)

If the pre-exponential factor is constant, then the experimental activation energy is equal to the Arrhenius activation energy: E op = E A.

Example 4-1. Using the Arrhenius equation, estimate at what temperatures and activation energies the van't Hoff rule is valid.

Decision. Let us represent the van't Hoff rule (4.1) as a power-law dependence of the rate constant:

,

where B- a constant value. Let us compare this expression with the Arrhenius equation (4.2), taking the value ~ e = 2.718:

.

Let's take the natural logarithm of both parts of this approximate equality:

.

Differentiating the obtained relation with respect to temperature, we find the desired relationship between the activation energy and temperature:

If the activation energy and temperature approximately satisfy this relation, then the van't Hoff rule can be used to estimate the effect of temperature on the reaction rate.

Example 4-2. The first order reaction at 70°C is 40% complete in 60 minutes. At what temperature will the reaction be 80% complete in 120 min if the activation energy is 60 kJ/mol?

Decision. For a first order reaction, the rate constant is expressed in terms of the degree of conversion as follows:

,

where a = x/a- the degree of transformation. We write this equation at two temperatures, taking into account the Arrhenius equation:

where E A= 60 kJ/mol, T 1 = 343K, t 1 = 60 min, a 1 = 0.4, t 2 = 120 min, a 2 = 0.8. Divide one equation by the other and take the logarithm:

Substituting the above quantities into this expression, we find T 2 \u003d 333 K \u003d 60 o C.

Example 4-3. The rate of bacterial hydrolysis of fish muscles doubles when moving from a temperature of -1.1 o C to a temperature of +2.2 o C. Estimate the activation energy of this reaction.

Decision. The increase in the rate of hydrolysis by 2 times is due to the increase in the rate constant: k 2 = 2k one . The activation energy in relation to the rate constants at two temperatures can be determined from equation (4.3) with T 1 = t 1 + 273.15 = 272.05K T 2 = t 2 + 273.15 = 275.35K:

130800 J/mol = 130.8 kJ/mol.

4-1. Using the van't Hoff rule, calculate at what temperature the reaction will end after 15 minutes, if at 20 ° C it takes 2 hours. Temperature coefficient speed is 3. (answer)

4-2. The half-life of a substance at 323 K is 100 minutes, and at 353 K it is 15 minutes. Determine the temperature coefficient of speed. (Answer)

4-3. What should be the activation energy in order for the reaction rate to increase by 3 times with an increase in temperature by 10 0 С a) at 300 K; b) at 1000 K? (answer)

4-4. The first order reaction has an activation energy of 25 kcal/mol and a pre-exponential factor of 5 . 10 13 sec -1 . At what temperature will the half-life for this reaction be: a) 1 min; b) 30 days? (answer)

4-5. In which of the two cases does the reaction rate constant increase more times: when heated from 0 o C to 10 o C or when heated from 10 o C to 20 o C? Justify your answer using the Arrhenius equation. (Answer)

4-6. The activation energy of some reaction is 1.5 times greater than the activation energy of another reaction. When heated from T 1 to T 2 the rate constant of the second reaction increased in a once. How many times did the rate constant of the first reaction increase when heated from T 1 to T 2 ? (answer)

4-7. The rate constant of a complex reaction is expressed in terms of the rate constants of the elementary steps as follows:

Express the activation energy and the pre-exponential factor of a complex reaction in terms of the corresponding quantities related to elementary stages. (Answer)

4-8. In the 1st order irreversible reaction in 20 min at 125°C, the degree of conversion of the starting material was 60%, and at 145°C the same degree of conversion was achieved in 5.5 min. Find the rate constants and activation energy of this reaction. (Answer)

4-9. The reaction of the 1st order at a temperature of 25 ° C is completed by 30% in 30 minutes. At what temperature will the reaction be 60% complete in 40 minutes if the activation energy is 30 kJ/mol? (Answer)

4-10. The reaction of the 1st order at a temperature of 25 ° C is completed by 70% in 15 minutes. At what temperature will the reaction be 50% complete in 15 minutes if the activation energy is 50 kJ/mol? (Answer)

4-11. The rate constant of the first order reaction is 4.02. 10 -4 s -1 at 393 K and 1.98 . 10 -3 s -1 at 413 K. Calculate the pre-exponential factor for this reaction. (Answer)

4-12. For the reaction H 2 + I 2 2HI, the rate constant at a temperature of 683 K is 0.0659 l / (mol. min), and at a temperature of 716 K - 0.375 l / (mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 700 K. (Answer)

4-13. For the reaction 2N 2 O 2N 2 + O 2, the rate constant at a temperature of 986 K is 6.72 l / (mol. min), and at a temperature of 1165 K - 977.0 l / (mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 1053.0 K. (Answer)

4-14. Trichloroacetate ion in ionizing solvents containing H + decomposes according to the equation

H + + CCl 3 COO - CO 2 + CHCl 3

The rate-determining step is the monomolecular cleavage of the C-C bond in the trichloroacetate ion. The reaction proceeds in the first order, and the rate constants have the following values: k= 3.11 . 10 -4 s -1 at 90 o C, k= 7.62. 10 -5 s -1 at 80 o C. Calculate a) activation energy, b) rate constant at 60 o C. (answer)

4-15. For the reaction CH 3 COOC 2 H 5 + NaOH * CH 3 COONa + C 2 H 5 OH, the rate constant at a temperature of 282.6 K is 2.307 l / (mol. min), and at a temperature of 318.1 K - 21.65 l /(mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 343 K. (Answer)

4-16. For the reaction C 12 H 22 O 11 + H 2 O C 6 H 12 O 6 + C 6 H 12 O 6, the rate constant at a temperature of 298.2 K is 0.765 l / (mol. min), and at a temperature of 328.2 K - 35.5 l/(mol min). Find the activation energy of this reaction and the rate constant at a temperature of 313.2 K. (Answer)

4-17. The substance decomposes in two parallel paths with rate constants k 1 and k 2. What is the difference between the activation energies of these two reactions, if at 10 o C k 1 /k 2 = 10, and at 40 o C k 1 /k 2 = 0.1? (answer)

4-18. In two reactions of the same order, the difference in activation energies is E 2 - E 1 = 40 kJ/mol. At a temperature of 293 K, the ratio of the rate constants is k 1 /k 2 \u003d 2. At what temperature will the rate constants become equal? ​​(Answer)

4-19. Decomposition of acetone dicarboxylic acid in aqueous solution is a first order reaction. The rate constants of this reaction were measured at different temperatures:

Calculate the activation energy and the pre-exponential factor. What is the half-life at 25°C?

As the temperature rises, the rate of a chemical process usually increases. In 1879, the Dutch scientist J. van't Hoff formulated an empirical rule: with an increase in temperature by 10 K, the rate of most chemical reactions increases by 2-4 times.

Mathematical notation of the rule I. van't Hoff:

γ 10 \u003d (k t + 10) / k t, where k t is the rate constant of the reaction at temperature T; k t+10 - reaction rate constant at temperature T+10; γ 10 - Van't Hoff temperature coefficient. Its value ranges from 2 to 4. For biochemical processesγ 10 varies from 7 to 10.

All biological processes proceed in a certain temperature range: 45-50°C. The optimum temperature is 36-40°C. In the body of warm-blooded animals, this temperature is maintained constant due to the thermoregulation of the corresponding biosystem. When studying biosystems, temperature coefficients γ 2 , γ 3 , γ 5 are used. For comparison, they are brought to γ ​​10 .

The dependence of the reaction rate on temperature, in accordance with the van't Hoff rule, can be represented by the equation:

V 2 /V 1 \u003d γ ((T 2 -T 1) / 10)

Activation energy. A significant increase in the reaction rate with increasing temperature cannot be explained only by an increase in the number of collisions between particles of reacting substances, since, in accordance with the kinetic theory of gases, the number of collisions increases slightly with increasing temperature. The increase in the reaction rate with increasing temperature is explained by the fact that a chemical reaction does not occur with any collision of particles of reacting substances, but only with a meeting of active particles that have the necessary excess energy at the moment of collision.

The energy required to turn inactive particles into active particles is called activation energy (Ea). Activation energy - excess, compared with the average value, the energy required for the entry of reacting substances into a reaction when they collide. The activation energy is measured in kilojoules per mole (kJ/mol). Usually E is from 40 to 200 kJ/mol.

The energy diagram of the exothermic and endothermic reactions is shown in fig. 2.3. For any chemical process, it is possible to distinguish the initial, intermediate and final states. At the top of the energy barrier, the reactants are in an intermediate state called the activated complex, or transition state. The difference between the energy of the activated complex and the initial energy of the reagents is Ea, and the difference between the energy of the reaction products and starting materials (reagents) is ΔН, the heat of the reaction. The activation energy, in contrast to ΔH, is always a positive value. For an exothermic reaction (Fig. 2.3, a), the products are located at a lower energy level than the reactants (Ea< ΔН).

Rice. 2.3. Energy diagrams of reactions: A - exothermic B - endothermic

A B

Ea is the main factor determining the reaction rate: if Ea > 120 kJ/mol (higher energy barrier, fewer active particles in the system), the reaction is slow; and vice versa, if Ea< 40 кДж/моль, реакция осуществляется с большой скоростью.

For reactions involving complex biomolecules, one should take into account the fact that in an activated complex formed during the collision of particles, the molecules must be oriented in space in a certain way, since only the reacting region of the molecule undergoes transformation, which is small in relation to its size.

If the rate constants k 1 and k 2 are known at temperatures T 1 and T 2 , the value of Ea can be calculated.

In biochemical processes, the activation energy is 2-3 times less than in inorganic ones. At the same time, the Ea of reactions involving foreign substances, xenobiotics, significantly exceeds the Ea of conventional biochemical processes. This fact is the natural bioprotection of the system from the influence of foreign substances, i.e. reactions natural for the body occur under favorable conditions with low Ea, and for foreign reactions, Ea is high. This is a gene barrier that characterizes one of the main features of the course of biochemical processes.

The rate of a chemical reaction depends on the temperature, and as the temperature rises, the rate of the reaction increases. The Dutch scientist van't Hoff showed that when the temperature rises by 10 degrees, the rate of most reactions increases by 2-4 times;

VT 2 = VT 1 *y (T2-T1)/10

Where VT 2 and VT 1 are the reaction rates at temperatures T 2 and T 1; y is the temperature coefficient of the reaction rate, which shows how many times the reaction rate increased with an increase in temperature by 10K.

At a reactant concentration of 1 mol/l, the reaction rate is numerically equal to the rate constant k. Then the equation shows that the rate constant depends on temperature in the same way as the rate of the process.

3. Write a variant of the reaction of elimination (elimination) with the release of hydrogen halide.

C 2 H 5 Cl \u003d C 2 H 4 + HCl

Ticket number 4

1. What is "atomic mass", "molecular mass", "mole of substance" and what is taken as an atomic mass unit (a.m.u.)?

ATOMIC MASS - the mass of an atom in atomic mass units (a.m.u.). per unit a. e. m., 1/12 of the mass of the carbon-12 isotope is accepted.

a.u.m. \u003d 1/12 m 12 6 C \u003d 1.66 * 10 -24

MOLECULAR WEIGHT - The molar mass of a compound, referred to 1/12 of the molar mass of a carbon-12 atom.

MOLE - the amount of a substance containing the same number of particles or structural units (atoms, ions, molecules, radicals, electrons, equivalents, etc.) as in 12 a. e.m. isotope carbon-12.

The formula for increasing the rate of a reaction in the presence of a catalyst.

You can change the value of Ea (activation energy) using catalysts. Substances that take part, but are not consumed in the reaction process, are called catalysts. This phenomenon itself is called catalysis. The increase in the reaction rate in the presence of a catalyst is determined by the formula

Depending on whether the catalyst is in the same phase as the reactants or forms an independent phase, one speaks of homogeneous or heterogeneous catalysis. The mechanism of catalytic action for them is not the same, however, in both cases, the reaction is accelerated due to a decrease in Ea. There are a number of specific catalysts - inhibitors that reduce the reaction rate.

where are the parameters of the catalytic process, V, k, Ea- non-catalytic process.

Write the reactions of combustion of carbon-containing inorganic substances in oxygen, indicating the oxidizing agent and reducing agent, as well as the oxidation states of carbon before and after the reaction.

C - reducing agent, oxidation process

O - oxidizing agent, reduction process

Ticket number 5

1. What is the "electronegativity", "valency", "oxidation state" of an element and what are the basic rules for determining them?

OXIDATION STATE - the conditional charge of an atom of an element, obtained on the assumption that the compound consists of ions. It can be positive, negative, zero, fractional and is indicated by an Arabic numeral with a “+” or “-” sign in the form of the upper right index of the element symbol: C 1-, O 2-, H +, Mg 2+, N 3-, N 5+ , Cr 6+ .

To determine the oxidation state (s. o.) of an element in a compound (ion), the following rules are used:

1 In simple substances (H2, S8, P4) p. about. equals zero.

2 Constant p. about. have alkaline (E+) and alkaline earth (E2+) elements, as well as fluorine P-.

3 Hydrogen in most compounds has s. about. H + (H2O, CH4, HC1), in hydrides - H- (-NaH, CaH2); with. about. oxygen, as a rule, is equal to -2 (O2-), in peroxides (-O-O-) - 1 (O-).

4 In binary compounds of non-metals, negative p. about. assigned to the element on the right).

5 Algebraic sum p. about. molecule is zero, ion - its charge.

The ability of an atom to attach or replace a certain number of other atoms is called VALENCE. The measure of valency is the number of hydrogen or oxygen atoms attached to an element, provided that hydrogen is one- and oxygen is divalent.

The rate of most chemical reactions increases with increasing temperature. Since the concentration of reactants is practically independent of temperature, in accordance with the kinetic equation of the reaction, the main effect of temperature on the reaction rate is through a change in the reaction rate constant. As the temperature increases, the energy of the colliding particles increases and the probability that a chemical transformation occurs during the collision increases.

The dependence of the reaction rate on temperature can be characterized by the value of the temperature coefficient.

Experimental data on the effect of temperature on the rate of many chemical reactions at ordinary temperatures (273–373 K), in a small temperature range, showed that an increase in temperature by 10 degrees increases the reaction rate by 2–4 times (van't Hoff rule).

According to van't Hoff temperature coefficient of rate constant(Van't Hoff coefficient)is the increase in the rate of a reaction with an increase in temperature by 10degrees.

(4.63)

where and are the rate constants at temperatures and ; is the temperature coefficient of the reaction rate.

When the temperature rises to n tens of degrees, the ratio of the rate constants will be equal to

where n can be either an integer or a fractional number.

Van't Hoff's rule is an approximate rule. It is applicable in a narrow temperature range, since the temperature coefficient changes with temperature.

A more accurate dependence of the reaction rate constant on temperature is expressed by the semi-empirical Arrhenius equation

where A is a pre-exponential factor which does not depend on temperature, but is determined only by the type of reaction; E - the activation energy of a chemical reaction. The activation energy can be represented as a certain threshold energy that characterizes the height of the energy barrier on the reaction path. The activation energy also does not depend on temperature.

This dependence was established at the end of the 19th century. Dutch scientist Arrhenius for elementary chemical reactions.

Direct activation energy ( E 1) and reverse ( E 2) the reaction is related to the thermal effect of the reaction D H ratio (see Fig. 1):

E 1 – E 2=D N.

If the reaction is endothermic and D H> 0, then E 1 > E 2 and the activation energy of the forward reaction is greater than the reverse. If the reaction is exothermic, then E 1 < Е 2 .

Arrhenius equation (101) in differential form can be written:

It follows from the equation that more energy activation of E, the faster the reaction rate increases with temperature.

Separating variables k and T and considering E constant value, after integrating equation (4.66) we get:

Rice. 5. Graph ln k–1/T.

, (4.67)

where A is a pre-exponential factor having the dimension of the rate constant. If this equation is valid, then on the graph in coordinates, the experimental points are located on a straight line at an angle a to the abscissa axis and the slope () is equal to , which makes it possible to calculate the activation energy of a chemical reaction from the dependence of the rate constant on temperature according to the equation .

The activation energy of a chemical reaction can be calculated from the values ​​of the rate constants at two different temperatures using the equation

. (4.68)

The theoretical derivation of the Arrhenius equation is made for elementary reactions. But experience shows that the vast majority of complex reactions also obey this equation. However, for complex reactions, the activation energy and the pre-exponential factor in the Arrhenius equation do not have a definite physical meaning.

The Arrhenius equation (4.67) makes it possible to give a satisfactory description of a wide range of reactions in a narrow temperature range.

To describe the dependence of the reaction rate on temperature, the modified Arrhenius equation is also used

, (4.69)

which already includes three parameters : BUT, E and n.

Equation (4.69) is widely used for reactions occurring in solutions. For some reactions, the dependence of the reaction rate constant on temperature differs from the dependences given above. For example, in third-order reactions, the rate constant decreases with increasing temperature. In chain exothermic reactions, the reaction rate constant increases sharply at a temperature above a certain limit (thermal explosion).

4.5.1. Examples of problem solving

Example 1 The rate constant of some reaction with increasing temperature changed as follows: t 1 = 20°C;

k 1 \u003d 2.76 10 -4 min. -one ; t 2 \u003d 50 0 С; k 2 = 137.4 10 -4 min. -1 Determine the temperature coefficient of the rate constant of a chemical reaction.

Decision. The van't Hoff rule makes it possible to calculate the temperature coefficient of the rate constant from the relation

g n= =2 ¸ 4, where n = = =3;

g 3 \u003d \u003d 49.78 g \u003d 3.68

Example 2 Using the van't Hoff rule, calculate at what temperature the reaction will end in 15 minutes, if it took 120 minutes at a temperature of 20 0 C. The temperature coefficient of the reaction rate is 3.

Decision. Obviously, the shorter the reaction time ( t), the greater the rate constant of the reaction:

3n = 8, n ln3 = ln8, n== .

The temperature at which the reaction will end in 15 minutes is:

20 + 1.9 × 10 \u003d 39 0 C.

Example 3 The rate constant of the reaction of saponification of acetic-ethyl ester with an alkali solution at a temperature of 282.4 K is equal to 2.37 l 2 / mol 2 min. , and at a temperature of 287.40 K it is equal to 3.2 l 2 / mol 2 min. Find the temperature at which the rate constant of this reaction is 4?

Decision.

1. Knowing the values ​​of the rate constants at two temperatures, we can find the activation energy of the reaction:

= = 40.8 kJ/mol.

2. Knowing the value of the activation energy, from the Arrhenius equation

,

Questions and tasks for self-control.

1. What quantities are called "Arrhenius" parameters?

2. What is the minimum amount of experimental data needed to calculate the activation energy of a chemical reaction?

3. Show that the temperature coefficient of the rate constant depends on temperature.

4. Are there deviations from the Arrhenius equation? How can the dependence of the rate constant on temperature be described in this case?

Kinetics of complex reactions

Reactions, as a rule, do not proceed through the direct interaction of all initial particles with their direct transition into reaction products, but consist of several elementary stages. This primarily applies to reactions in which, according to their stoichiometric equation, more than three particles take part. However, even reactions of two or one particle often do not proceed by a simple bi- or monomolecular mechanism, but by a more complex path, that is, through a number of elementary stages.

Reactions are called complex if the consumption of starting materials and the formation of reaction products occur through a series of elementary stages that can occur simultaneously or sequentially. At the same time, some stages take place with the participation of substances that are neither starting substances nor reaction products (intermediate substances).

As an example of a complex reaction, we can consider the reaction of chlorination of ethylene with the formation of dichloroethane. Direct interaction must go through a four-membered activated complex, which is associated with overcoming a high energy barrier. The speed of such a process is low. If atoms are formed in the system in one way or another (for example, under the action of light), then the process can proceed according to a chain mechanism. The atom easily joins at the double bond to form a free radical - . This free radical can easily tear off an atom from a molecule to form the final product - , as a result of which the free atom is regenerated.

As a result of these two stages, one molecule and one molecule are converted into a product molecule - , and the regenerated atom interacts with the next ethylene molecule. Both stages have low activation energies, and this way provides a fast reaction. Taking into account the possibility of recombination of free atoms and free radicals, the complete scheme of the process can be written as:

With all the variety, complex reactions can be reduced to a combination of several types of complex reactions, namely parallel, sequential and series-parallel reactions.

The two stages are called successive if the particle formed in one stage is the initial particle in another stage. For example, in the above scheme, the first and second stages are sequential:

.

The two stages are called parallel, if the same particles take part as initial in both. For example, in the reaction scheme, the fourth and fifth stages are parallel:

The two stages are called series-parallel, if they are parallel with respect to one and sequential with respect to the other of the particles participating in these stages.

An example of series-parallel steps are the second and fourth steps of this reaction scheme.

To characteristics The fact that the reaction proceeds according to a complex mechanism includes the following signs:

Mismatch of reaction order and stoichiometric coefficients;

Changing the composition of products depending on temperature, initial concentrations and other conditions;

Speeding up or slowing down the process when added to reaction mixture small amounts of substances;

Influence of the material and dimensions of the vessel on the reaction rate, etc.

In the kinetic analysis of complex reactions, the principle of independence is applied: “If several simple reactions occur simultaneously in the system, then the basic postulate of chemical kinetics applies to each of them, as if this reaction were the only one.” This principle can also be formulated as follows: "The value of the rate constant of an elementary reaction does not depend on whether other elementary reactions proceed simultaneously in a given system."

The principle of independence is valid for most reactions that proceed according to a complex mechanism, but is not universal, since there are reactions in which some simple reactions affect the course of others (for example, conjugated reactions.)

Importance in the study of complex chemical reactions has the principle microreversibility or detailed balance:

if a chemical equilibrium is established in a complex process, then the rates of the forward and reverse reactions must be equal for each of the elementary stages.

The most common case for a complex reaction to occur is when the reaction proceeds through several simple steps proceeding at different rates. The difference in rates leads to the fact that the kinetics of obtaining the reaction product can be determined by the laws of only one reaction. For example, for parallel reactions, the rate of the entire process is determined by the rate of the fastest stage, and for sequential reactions, the slowest one. Therefore, when analyzing the kinetics of parallel reactions with a significant difference in the constants, the rate of the slow stage can be neglected, and when analyzing sequential reactions, it is not necessary to determine the rate of the fast reaction.

In sequential reactions, the slowest reaction is called limiting. The limiting stage has the smallest rate constant.

If the values ​​of the rate constants of the individual stages of a complex reaction are close, then a complete analysis of the entire kinetic scheme is necessary.

The introduction of the concept of a rate-determining stage in many cases simplifies the mathematical side of considering such systems and explains the fact that sometimes the kinetics of complex, multi-stage reactions is well described by simple equations, for example, of the first order.