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 - activation energy 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. Activation energy also independent of temperature.

This dependency is set to late XIX in. 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 the greater the activation energy 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.

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. Temperature coefficient 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 number 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 detach an atom from a molecule to form final product- , as a result of which a 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 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 proceeding according to a complex mechanism, but is not universal, since there are reactions in which one simple reactions affect the course of others (for example, coupled reactions.)

Important in the study of complex chemical reactions is the principle microreversibility or detailed balance:

if in a complex process it is established chemical equilibrium, 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 it is necessary full analysis the entire kinetic scheme.

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.

The increase in the reaction rate with increasing temperature is usually characterized by the temperature coefficient of the reaction rate, a number showing how many times the rate of a given reaction increases with an increase in the temperature of the system by 10 ° C. The temperature coefficient of different reactions is different. At ordinary temperatures, its value for most reactions is in the range of 2 ... 4.

The temperature coefficient is determined in accordance with the so-called "van't Hoff rule", which is mathematically expressed by the equation

v 2 /v 1 = g ( T 2 – T 1)/10 ,

where v 1 and v 2 – reaction rates at temperatures T 1 and T 2; g is the temperature coefficient of the reaction.

So, for example, if g = 2, then for T 2 - T 1 = 50°C v 2 /v 1 = 2 5 = 32, i.e. the reaction accelerated by 32 times, and this acceleration does not depend on absolute values ​​in any way T 1 and T 2 but only on their difference.

activation energy, the difference between the values ​​of the average energy of particles (molecules, radicals, ions, etc.) entering into an elementary act of a chemical reaction and the average energy of all particles in the reacting system. For various chemical reactions E. and. varies widely - from a few to ~ 10 j./mol. For the same chemical reaction, the value of E. a. depends on the type of distribution functions of molecules in terms of the energies of their translational motion and internal degrees of freedom (electronic, vibrational, rotational). As a statistical value E. a. should be distinguished from the threshold energy, or energy barrier - the minimum energy that one pair of colliding particles must have for a given elementary reaction to occur.

Arrhenius equation, temperature dependence rate constants to elemental chem. reactions:

where A is a pre-exponential factor (the dimension is the same as the dimension of k), E a-activation energy, usually accepting positive. values, T-abs. temperature, k-Boltzmann constant. It is customary to cite E a per molecule. and on the number of particles N A\u003d 6.02 * 10 23 (Avogadro's constant) and expressed in kJ / mol; in these cases, in the Arrhenius equation, the value k replace the gas constant R. Graph of 1nk versus 1 /kT(Arrhenius plot) - a straight line, the negative slope of which is determined by the activation energy E a and characterizes positive. temperature dependence to.

Catalyst - Chemical substance, which accelerates the reaction, but is not part of the reaction products. The amount of catalyst, unlike other reagents, does not change after the reaction. It is important to understand that the catalyst is involved in the reaction. Providing more fast track for the reaction, the catalyst reacts with the starting material, the resulting intermediate compound undergoes transformations and is finally split into a product and a catalyst. Then the catalyst again reacts with the starting material, and this catalytic cycle is repeated many times (up to a million times) [ source?] is repeated.

Catalysts are classified into homogeneous and heterogeneous. A homogeneous catalyst is in the same phase with the reactants, a heterogeneous one forms an independent phase separated by an interface from the phase in which the reactants are located. Typical homogeneous catalysts are acids and bases. Metals, their oxides and sulfides are used as heterogeneous catalysts.

Reactions of the same type can proceed with both homogeneous and heterogeneous catalysts. So, along with acid solutions, those having acid properties solid Al 2 O 3 , TiO 2 , ThO 2 , aluminosilicates, zeolites. Heterogeneous catalysts with basic properties: CaO, BaO, MgO.

Heterogeneous catalysts, as a rule, have a highly developed surface, for which they are distributed on an inert carrier (silica gel, alumina, activated carbon, etc.).

For each type of reaction, only certain catalysts are effective. In addition to those already mentioned acid-base, there are catalysts redox; they are characterized by the presence of a transition metal or its compound (Co +3, V 2 O 5 + MoO 3). In this case, catalysis is carried out by changing the oxidation state of the transition metal.

Dispersed system- these are formations of two or more phases (bodies) that do not mix at all or practically and do not chemically react with each other. The first of the substances dispersed phase) is finely distributed in the second ( dispersion medium). If there are several phases, they can be physically separated from each other (by centrifugation, separation, etc.).

Usually dispersed systems are colloidal solutions, sols. Dispersed systems also include the case of a solid dispersed medium in which the dispersed phase is located.

Most general classification dispersed systems is based on the difference in state of aggregation dispersion medium and dispersed phase. Combinations of three types of aggregate state make it possible to distinguish nine types of dispersed systems. For brevity, they are usually denoted by a fraction, the numerator of which indicates the dispersed phase, and the denominator indicates the dispersion medium, for example, for the “gas in liquid” system, the designation G/L is adopted.

colloidal solutions. The colloidal state is characteristic of many substances if their particles have a size of 1 to 500 nm. It is easy to show that total surface these particles is huge. If we assume that the particles have the shape of a ball with a diameter of 10 nm, then with the total volume of these particles 1 cm 3 they will have

surface area of ​​about 10 m2. As mentioned earlier, the surface layer is characterized by surface energy and the ability to adsorb certain particles, including ions

from a solution. A characteristic feature of colloidal particles is the presence of a charge on their surface due to the selective adsorption of ions. A colloidal particle has a complex structure. It includes the nucleus, adsorbed ions, counterins and solvent. There are lyophilic (guid.

rophilic) colloids, in which the solvent interacts with the particle nuclei, ilnophobic (hydrophobic) colloids, in which the solvent does not interact with the nuclei

particles. The solvent is included in the composition of hydrophobic particles only as a solvate shell of adsorbed ions or in the presence of stabilizers (surfactants) having lyophobic and lyophilic parts.

Here are some examples of colloidal particles:

How. it can be seen that the core consists of an electrically neutral aggregate of particles with adsorbed ions of the elements that make up the core (in these examples, Ag +, HS-, Fe 3+ ions). A colloidal particle, in addition to the nucleus, has counterions and solvent molecules. Adsorbed ions and counterions form an adsorbed layer with the solvent. The total charge of the particle is equal to the difference between the charges of adsorbed ions and counterions. Around the particles there is a diffuse layer of ions, the charge of which is equal to the number of the colloidal particle. Colloidal particle and diffuse layers form an electrically neutral micelle

Micelles(diminutive of lat. mica- particle, grain) - particles in colloidal systems, consist of a very small nucleus insoluble in a given medium, surrounded by a stabilizing shell of adsorbed ions and solvent molecules. For example, an arsenic sulfide micelle has the structure:

((As 2 S 3) m nHS − (n-x)H + ) x- xH +

The average size micelles from 10 −5 to 10 −7 cm.

Coagulation- separation of a colloidal solution into two phases - a solvent and a gelatinous mass, or thickening of the solution as a result of the coarsening of the particles of the solute

Peptization is the process of transition of a colloidal precipitate or gel into a colloidal solution under the action of a liquid or substances added to it that are well adsorbed by the precipitate or gel, in this case called peptizers (for example, peptization of fats under the action of bile).
Peptization - separation of aggregates of particles of gels (jelly) or loose sediments under the influence of certain substances - peptizers after coagulation of colloidal solutions. As a result of peptization, the precipitate (or gel) passes into a suspended state.

SOLUTIONS, single-phase systems consisting of two or more components. According to their state of aggregation, solutions can be solid, liquid or gaseous.

Solubility, the ability of a substance to form with another substance (or substances) homogeneous mixtures with a dispersed distribution of components (see Solutions). Usually, a solvent is considered a substance that exists in its pure form in the same state of aggregation as the resulting solution. If, before dissolution, both substances were in the same state of aggregation, the solvent is considered to be a substance present in the mixture in a significantly larger amount.

Solubility is determined by the physical and chemical affinity of the molecules of the solvent and the solute, the ratio of energies by the interaction of homogeneous and dissimilar components of the solution. As a rule, they are well soluble in each other, similar in physical. and chem. the properties of matter (the empirical rule "like dissolves in like"). In particular, substances containing polar molecules, and substances with ion type connections well sol. in polar solvents (water, ethanol, liquid ammonia), and non-polar substances are well sol. in non-polar solvents (benzene, carbon disulfide).

The solubility of a given substance depends on temperature and pressure corresponds to the general principle of shifting equilibria (see the Le Chatelier-Brown principle). The concentration of a saturated solution under given conditions numerically determines the R. of a substance in a given solvent and is also called. solubility. Supersaturated solutions contain a larger amount of solute than corresponds to its solubility, the existence of supersaturated solutions is due to kinetic. difficulties of crystallization (see the Origin of a new phase). To characterize the solubility of poorly soluble substances, the product of PA activities is used (for solutions close in their properties to the ideal, the product of the solubility of PR).

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 at 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 thermal motion 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 starting 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 significant 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).

Problem 336.
At 150°C, some reaction is complete in 16 minutes. Taking the temperature coefficient of the reaction rate equal to 2.5, calculate how long this reaction will end if it is carried out: a) at 20 0 °С; b) at 80°C.
Solution:
According to the van't Hoff rule, the dependence of velocity on temperature is expressed by the equation:

v t and k t - the rate and rate constant of the reaction at a temperature of t°C; v (t + 10) and k (t + 10) the same values ​​at temperature (t + 10 0 C); - the temperature coefficient of the reaction rate, the value of which for most reactions lies in the range of 2 - 4.

a) Given that the rate of a chemical reaction at a given temperature is inversely proportional to the duration of its course, we substitute the data given in the condition of the problem into a formula that quantitatively expresses the van't Hoff rule, we get:

b) Since this reaction proceeds with a decrease in temperature, then at a given temperature the rate of this reaction is directly proportional to the duration of its course, we substitute the data given in the condition of the problem into a formula that quantitatively expresses the van't Hoff rule, we get:

Answer: a) at 200 0 С t2 = 9.8 s; b) at 80 0 С t3 = 162 h 1 min 16 s.

Problem 337.
Will the value of the reaction rate constant change: a) when replacing one catalyst with another; b) when the concentrations of reactants change?
Solution:
The reaction rate constant is a value that depends on the nature of the reactants, on the temperature and on the presence of catalysts, and does not depend on the concentration of the reactants. It can be equal to the reaction rate in the case when the concentrations of the reactants are equal to unity (1 mol/l).

a) When one catalyst is replaced by another, the rate of a given chemical reaction will change, or it will increase. If a catalyst is used, the rate of a chemical reaction will increase, and the value of the reaction rate constant will increase accordingly. A change in the value of the reaction rate constant will also occur when one catalyst is replaced by another, which will increase or decrease the rate of this reaction relative to the original catalyst.

b) When the concentration of the reactants changes, the values ​​of the reaction rate will change, and the value of the reaction rate constant will not change.

Problem 338.
Does the thermal effect of a reaction depend on its activation energy? Justify the answer.
Solution:
The thermal effect of the reaction depends only on the initial and final state of the system and does not depend on the intermediate stages of the process. Activation energy is the excess energy that molecules of substances must have in order for their collision to lead to the formation of a new substance. The activation energy can be changed by raising or lowering the temperature, respectively lowering or increasing it. Catalysts lower the activation energy, while inhibitors lower it.

Thus, a change in the activation energy leads to a change in the reaction rate, but not to a change in the heat of the reaction. The thermal effect of a reaction is a constant value and does not depend on a change in the activation energy for a given reaction. For example, the reaction for the formation of ammonia from nitrogen and hydrogen is:

This reaction is exothermic, > 0). The reaction proceeds with a decrease in the number of moles of reacting particles and the number of moles gaseous substances, which brings the system from a less stable state to a more stable one, the entropy decreases,< 0. Данная реакция в обычных условиях не протекает (она возможна только при достаточно низких температурах). В присутствии катализатора энергия активации уменьшается, и скорость реакции возрастает. Но, как до применения катализатора, так и в присутствии его тепловой эффект реакции не изменяется, реакция имеет вид:

Problem 339.
For which reaction, direct or reverse, is the activation energy greater if the direct reaction proceeds with the release of heat?
Solution:
The difference between the activation energies of the direct and reverse reactions is equal to the thermal effect: H \u003d E a (pr.) - E a (arr.) . This reaction proceeds with the release of heat, i.e. is exothermic,< 0 Исходя из этого, энергия активации прямой реакции имеет меньшее значение, чем энергия активации обратной реакции:
E a(ex.)< Е а(обр.) .

Answer: E a(ex.)< Е а(обр.) .

Problem 340.
How many times will the rate of a reaction proceeding at 298 K increase if its activation energy is reduced by 4 kJ/mol?
Solution:
Let us denote the decrease in the activation energy by Ea, and the rate constants of the reaction before and after the decrease in the activation energy, respectively, by k and k. Using the Arrhenius equation, we obtain:

E a is the activation energy, k and k" are the reaction rate constants, T is the temperature in K (298).
Substituting the data of the problem into the last equation and, expressing the activation energy in joules, we calculate the increase in the reaction rate:

Answer: 5 times.

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 a 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 molar mass 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 carbonaceous inorganic substances in oxygen, indicating the oxidizing agent and reducing agent, as well as the oxidation state 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 V simple substances(H2, S8, P4) s. 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.