The dependence of the reaction rate on temperature. The temperature coefficient of the reaction rate and its features for biochemical processes
The rate of a chemical reaction depends on the temperature, and as the temperature rises, the rate of the reaction increases. The Dutch scientist Van't Hoff showed that when the temperature rises by 10 degrees, the rate of most reactions increases by 2-4 times;
VT 2 = VT 1 *y (T2-T1)/10
Where VT 2 and VT 1 are the reaction rates at temperatures T 2 and T 1; y is the temperature coefficient of the reaction rate, which shows how many times the reaction rate increased with an increase in temperature by 10K.
At a reactant concentration of 1 mol/l, the reaction rate is numerically equal to the rate constant k. Then the equation shows that the rate constant depends on temperature in the same way as the rate of the process.
3. Write a variant of the reaction of elimination (elimination) with the release of hydrogen halide.
C 2 H 5 Cl \u003d C 2 H 4 + HCl
Ticket number 4
1. What is "atomic mass", "molecular mass", "mole of substance" and what is taken as an atomic mass unit (a.m.u.)?
ATOMIC MASS - the mass of an atom in atomic mass units (a.m.u.). per unit a. e. m., 1/12 of the mass of the carbon-12 isotope is accepted.
a.u.m. \u003d 1/12 m 12 6 C \u003d 1.66 * 10 -24
MOLECULAR WEIGHT - The molar mass of a compound, referred to 1/12 of the molar mass of a carbon-12 atom.
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 denoted 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 With. 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.
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 of 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 thermal effect: H \u003d E a (ex.) - 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.
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 Ea 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 Ea. The more Ea, the smaller the number of active molecules and the slower the reaction proceeds. When decreasing Ea speed increases and Ea= 0 the reaction proceeds instantaneously.
Value Ea 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 Ea:
γ is the temperature coefficient of the chemical reaction rate. The van't Hoff rule has limited application, since the value of γ depends on temperature, and outside the region Ea= 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 Ea, 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
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 regulations 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 processes, γ 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).
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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.
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- a constant value. Let us compare this expression with the Arrhenius equation (4.2), taking the value ~ e = 2.718:
.
Let's take natural logarithm both parts of this approximate equality:
.
Differentiating the obtained relation with respect to temperature, we find the desired relationship between the activation energy and temperature:
If the activation energy and temperature approximately satisfy this relation, then the van't Hoff rule can be used to estimate the effect of temperature on the reaction rate.
Example 4-2. The first order reaction at 70°C is 40% complete in 60 minutes. At what temperature will the reaction be 80% complete in 120 min if the activation energy is 60 kJ/mol?
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. Temperature coefficient speed is 3. (answer)
4-2. The half-life of a substance at 323 K is 100 minutes, and at 353 K it is 15 minutes. Determine the temperature coefficient of speed. (Answer)
4-3. What should be the activation energy in order for the reaction rate to increase by 3 times with an increase in temperature by 10 0 С a) at 300 K; b) at 1000 K? (answer)
4-4. The first order reaction has an activation energy of 25 kcal/mol and a pre-exponential factor of 5 . 10 13 sec -1 . At what temperature will the half-life for this reaction be: a) 1 min; b) 30 days? (answer)
4-5. In which of the two cases does the 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. In the 1st order irreversible reaction in 20 min at 125°C, the degree of conversion of the starting material was 60%, and at 145°C the same degree of conversion was achieved in 5.5 min. Find the rate constants and activation energy of this reaction. (Answer)
4-9. The reaction of the 1st order at a temperature of 25 ° C is completed by 30% in 30 minutes. At what temperature will the reaction be 60% complete in 40 minutes if the activation energy is 30 kJ/mol? (Answer)
4-10. The reaction of the 1st order at a temperature of 25 ° C is completed by 70% in 15 minutes. At what temperature will the reaction be 50% complete in 15 minutes if the activation energy is 50 kJ/mol? (Answer)
4-11. The rate constant of the first order reaction is 4.02. 10 -4 s -1 at 393 K and 1.98 . 10 -3 s -1 at 413 K. Calculate the pre-exponential factor for this reaction. (Answer)
4-12. For the reaction H 2 + I 2 2HI, the rate constant at a temperature of 683 K is 0.0659 l / (mol. min), and at a temperature of 716 K - 0.375 l / (mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 700 K. (Answer)
4-13. For the reaction 2N 2 O 2N 2 + O 2, the rate constant at a temperature of 986 K is 6.72 l / (mol. min), and at a temperature of 1165 K - 977.0 l / (mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 1053.0 K. (Answer)
4-14. Trichloroacetate ion in ionizing solvents containing H + decomposes according to the equation
H + + CCl 3 COO - CO 2 + CHCl 3
The rate-determining step is the monomolecular cleavage of the C-C bond in the trichloroacetate ion. The reaction proceeds in the first order, and the rate constants have the following values: k= 3.11 . 10 -4 s -1 at 90 o C, k= 7.62. 10 -5 s -1 at 80 o C. Calculate a) activation energy, b) rate constant at 60 o C. (answer)
4-15. For the reaction CH 3 COOC 2 H 5 + NaOH * CH 3 COONa + C 2 H 5 OH, the rate constant at a temperature of 282.6 K is 2.307 l / (mol. min), and at a temperature of 318.1 K - 21.65 l /(mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 343 K. (Answer)
4-16. For the reaction C 12 H 22 O 11 + H 2 O C 6 H 12 O 6 + C 6 H 12 O 6, the rate constant at a temperature of 298.2 K is 0.765 l / (mol. min), and at a temperature of 328.2 K - 35.5 l/(mol min). Find the activation energy of this reaction and the rate constant at a temperature of 313.2 K. (Answer)
4-17. The substance decomposes in two parallel paths with rate constants k 1 and k 2. What is the difference between the activation energies of these two reactions, if at 10 o C k 1 /k 2 = 10, and at 40 o C k 1 /k 2 = 0.1? (answer)
4-18. In two reactions of the same order, the difference in activation energies is E 2 - E 1 = 40 kJ/mol. At a temperature of 293 K, the ratio of the rate constants is k 1 /k 2 \u003d 2. At what temperature will the rate constants become equal? (Answer)
4-19. Decomposition of acetone dicarboxylic acid in aqueous solution is a first order reaction. The rate constants of this reaction were measured at different temperatures:
Calculate the activation energy and the pre-exponential factor. What is the half-life at 25°C?
