Calculate the temperature coefficient of the reaction rate. Calculation of the rate, constant and temperature of the reaction rate coefficient

Temperature and reaction rate

At a fixed temperature, a reaction is possible if the interacting molecules have a certain amount of energy. Arrhenius called this excess energy activation energy , and the molecules themselves activated.

According to Arrhenius, the rate constant k and activation energy E a are related by a relation called the Arrhenius equation:

Here A is the pre-exponential factor, R is the universal gas constant, T is the absolute temperature.

Thus, at a constant temperature, the reaction rate determines E a. The more E a, the smaller the number of active molecules and the slower the reaction proceeds. When decreasing E a speed increases and E a= 0 the reaction proceeds instantaneously.

Value E a characterizes the nature of the reacting substances and is determined experimentally from the dependence k = f(T). Writing equation (5.3) in logarithmic form and solving it for constants at two temperatures, we find E a:

γ – temperature coefficient speed chemical reaction. The van't Hoff rule has limited application, since the value of γ depends on temperature, and outside the region E a= 50–100 kJ ∙ mol–1 this rule is not fulfilled at all.

On fig. 5.4 it can be seen that the energy spent on the transfer of the initial products to the active state (A * - activated complex) is then fully or partially re-emitted during the transition to the final products. The difference between the energies of the initial and final products determines Δ H reaction that does not depend on the activation energy.

Thus, on the way from the initial state to the final state, the system must overcome the energy barrier. Only active molecules possessing at the moment of collision the necessary energy excess equal to E a, can overcome this barrier and enter into a chemical interaction. As the temperature rises, the proportion of active molecules in the reaction medium increases.

Preexponential multiplierA characterizes the total number of collisions. For reactions with simple molecules A close to theoretical collision magnitude Z, i.e. A = Z calculated from the kinetic theory of gases. For complex molecules A ≠ Z, so it is necessary to introduce the steric factor P:

Here Z is the number of all collisions, P is the proportion of spatially favorable collisions (takes values ​​from 0 to ), is the proportion of active, i.e., energetically favorable collisions.

The dimension of the rate constant is obtained from the relation

Analyzing expression (5.3), we come to the conclusion that there are two fundamental possibilities for accelerating the reaction:
a) an increase in temperature,
b) decrease in activation energy.

Tasks and tests on the topic "Chemical kinetics. Temperature and reaction rate"

• The rate of a chemical reaction. Catalysts - Classification of chemical reactions and patterns of their course Grade 8–9

  Lessons: 5 Assignments: 8 Quizzes: 1

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 C; k 2 = 137.4 10 -4 min. -1 Determine the temperature coefficient of the rate constant of a chemical reaction.

Solution. 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.

Solution. 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?

Solution.

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;

Acceleration or slowdown of the process when small amounts of substances are added to the reaction mixture;

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 used: “If several simple reactions occur simultaneously in the system, then the main postulate chemical kinetics applied to each of them as if the given 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 required.

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.

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.

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- a change in the rate of chemical reactions in the presence of substances that, after the 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 reaction mixture), and in the presence of manganese(IV) oxide - an example of heterogeneous catalysis (an aqueous solution of hydrogen peroxide is a liquid phase, manganese oxide is a 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 the pure solvent; -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.

The rate of chemical reactions increases with increasing temperature. The increase in the reaction rate with temperature can be estimated using the van't Hoff rule. According to the rule, an increase in temperature by 10 degrees increases the rate constant of the reaction by 2-4 times:

This rule is not fulfilled at high temperatures, when the rate constant hardly changes with temperature.

Van't Hoff's rule allows you to quickly determine the expiration date of a drug. An increase in temperature increases the rate of decomposition of the drug. This shortens the time to determine the expiration date of the drug.

The method consists in the fact that the drug is kept at elevated temperature T for a certain time tT, the amount of decomposed drug m is found and recalculated to a standard storage temperature of 298K. Considering the process of decomposition of the drug as a first-order reaction, the rate is expressed at the selected temperature T and T = 298K:

Considering the mass of the decomposed drug to be the same for standard and real storage conditions, the decomposition rates can be expressed by the equations:

Assuming T=298+10n, where n = 1,2,3…,

Get the final expression for the shelf life of the drug under standard conditions 298K:

Theory of active collisions. Activation energy. Arrhenius equation. Relationship between reaction rate and activation energy.

The theory of active collisions was formulated by S. Arrhenius in 1889. This theory is based on the idea that for a chemical reaction to occur, a collision between the molecules of the initial substances is necessary, and the number of collisions is determined by the intensity of the thermal motion of the molecules, i.e. temperature dependent. But not every collision of molecules leads to a chemical transformation: only active collision leads to it.

Active collisions are collisions that occur, for example, between molecules A and B with a large amount of energy. The minimum amount of energy that the molecules of the initial substances must have in order for their collision to be active is called the energy barrier of the reaction.

Activation energy is the excess energy that can be communicated or transferred to one mole of a substance.

The activation energy significantly affects the value of the reaction rate constant and its dependence on temperature: the larger Ea, the lower the rate constant and the more significantly the change in temperature affects it.

The reaction rate constant is related to the activation energy by a complex relationship described by the Arrhenius equation:

k=Ae–Ea/RT, where A is the pre-exponential factor; Ea is the activation energy, R is the universal gas constant equal to 8.31 j/mol; T is the absolute temperature;

e is the base of natural logarithms.

However, the observed reaction rate constants are generally much smaller than those calculated using the Arrhenius equation. Therefore, the equation for the reaction rate constant is modified as follows:

(minus before whole fraction)

The multiplier causes the temperature dependence of the rate constant to differ from the Arrhenius equation. Since the Arrhenius activation energy is calculated as the slope of the logarithmic dependence of the reaction rate on the reciprocal temperature, then doing the same with the equation , we get:

Features of heterogeneous reactions. The rate of heterogeneous reactions and factors determining it. Kinetic and diffusion regions of heterogeneous processes. Examples of heterogeneous reactions of interest to pharmacy.

HETEROGENEOUS REACTIONS, chem. reactions involving substances in decomp. phases and constituting together a heterogeneous system. Typical heterogeneous reactions: thermal. decomposition of salts to form gaseous and solid products (e.g. CaCO3 -> CaO + CO2), reduction of metal oxides with hydrogen or carbon (e.g. PbO + C -> Pb + CO), dissolution of metals in acids (e.g. Zn + + H2SO4 -> ZnSO4 + H2), interaction. solid reagents (A12O3 + NiO -> NiAl2O4). In a special class, heterogeneous catalytic reactions occurring on the catalyst surface are distinguished; in this case, the reactants and products may not be in different phases. Direction, in the reaction N2 + + 3H2 -> 2NH3 occurring on the surface of an iron catalyst, the reactants and the reaction product are in the gas phase and form a homogeneous system.

The features of heterogeneous reactions are due to the participation of condensed phases in them. This makes it difficult to mix and transport reactants and products; activation of reagent molecules on the interface is possible. The kinetics of any heterogeneous reaction is defined as the rate of the chemical itself. transformations and transfer processes (diffusion) necessary to replenish the consumption of reactants and remove reaction products from the reaction zone. In the absence of diffusion hindrances, the rate of a heterogeneous reaction is proportional to the size of the reaction zone; this is the name of the specific reaction rate calculated per unit surface (or volume) of the reaction. zones, does not change in time; for simple (single-step) reactions, it can be determined on the basis of the acting masses of the law. This law is not satisfied if the diffusion of substances proceeds more slowly than chemical. district; in this case, the observed rate of the heterogeneous reaction is described by the equations of diffusion kinetics.

The rate of a heterogeneous reaction is the amount of a substance that enters into a reaction or is formed during a reaction per unit time per unit area of ​​the phase surface.

Factors affecting the rate of a chemical reaction:

The nature of the reactants

The concentration of reagents,

Temperature,

The presence of a catalyst.

Vheterog = Δp(S Δt), where Vheterog is the reaction rate in a heterogeneous system; n is the number of moles of any of the substances resulting from the reaction; V is the volume of the system; t - time; S is the surface area of ​​the phase on which the reaction proceeds; Δ - increment sign (Δp = p2 - p1; Δt = t2 - t1).