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, topics less number 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 is the temperature coefficient of 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 total number 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 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.
MOL - 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.
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 of the rate constant 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 catalyst 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 support (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.
The most general classification dispersed systems 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).
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.
Solution. Let us represent the van't Hoff rule (4.1) as a power-law dependence of the rate constant:
,
where B - constant. 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 relationship, 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?
Solution. 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.
Solution. 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. The temperature coefficient of the rate 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 rate constant of a reaction increase in 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 the complex reaction in terms of the corresponding quantities related to elementary stages. (Answer)
4-8. AT irreversible reaction 1st order for 20 min at 125 o C, the degree of conversion of the starting material was 60%, and at 145 o 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?
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.
