Temperature coefficient of the rate of a chemical reaction (van't Hoff rule). Reaction Rate Calculations Using the Temperature Coefficient of the Reaction Rate
Factors affecting the course of the reaction
In the human body, thousands of enzymatic reactions take place in a living cell. However, in a multistage chain of processes, the difference between the rates of individual reactions is quite large. Thus, the synthesis of protein molecules in a cell is preceded by at least two more stages: the synthesis of transfer RNA and the synthesis of ribosomes. But the time during which the concentration of tRNA molecules doubles is 1.7 minutes, protein molecules - 17 minutes, and ribosomes - 170 minutes. The rate of the overall process of the slow (limiting) stage, in our example, the rate of ribosome synthesis. The presence of a limiting reaction provides high reliability and flexibility in controlling thousands of reactions occurring in the cell. It is enough to keep under observation and regulate only the slowest of them. This method of controlling the rate of multi-stage synthesis is called the minimum principle. It allows to significantly simplify and make more reliable the system of autoregulation in the cell.
Classifications of reactions used in kinetics: reactions, homogeneous, heterogeneous and microheterogeneous; simple and complex reactions (parallel, sequential, conjugated, chain). Molecularity of the elementary act of the reaction. Kinetic equations. Reaction order. Half life
Microheterogeneous reactions -
The molecularity of the reaction is determined by the number of molecules that enter into chemical interaction in the elementary act of the reaction. On this basis, the reactions are divided into monomolecular, bimolecular and trimolecular.
Then reactions of type A -> B will be monomolecular, for example:
a) C 16 H 34 (t ° C) -> C g H 18 + C 8 H 16 - hydrocarbon cracking reaction;
b) CaC0 3 (t ° C) -> CaO + C0 2 - thermal decomposition of calcium carbonate.
Reactions like A + B -> C or 2A -> C - are bimolecular, for example:
a) C + 0 2 -> C0 2; b) 2Н 2 0 2 -> 2Н 2 0 + 0 2 etc.
Trimolecular reactions are described general equations type:
a) A + B + C D; b) 2A + B D; c) 3A D.
For example: a) 2Н 2 + 0 2 2Н 2 0; b) 2NO + H 2 N 2 0 + H 2 0.
The reaction rate depending on the molecularity will be expressed by the equations: a) V = k C A - for a monomolecular reaction; b) V \u003d to C A C in or c) V \u003d to C 2 A - for a bimolecular reaction; d) V \u003d k C C in C e) V \u003d k C 2 A C in or e) V \u003d k C 3 A - for a trimolecular reaction.
Molecularity is the number of molecules that react in one elementary chemical act.
Often the molecularity of the reaction is difficult to establish, so more formal sign- order chemical reaction.
Reaction order is equal to the sum indicators of degrees of concentration in the equation expressing the dependence of the reaction rate on the concentration of reactants (kinetic equation).
The order of the reaction most often does not coincide with the molecularity due to the fact that the reaction mechanism, i.e., the "elementary act" of the reaction (see the definition of the sign of molecularity), is difficult to establish.
Consider a number of examples illustrating this position.
1. The rate of dissolution of crystals is described by the equations of zero-order kinetics, despite the monomolecular nature of the reaction: AgCl (TB) -> Ag + + CI", V = k C (AgCl (TB p = k" C (AgCl (ra)) - p - density and is a constant value, i.e., the dissolution rate does not depend on the amount (concentration) of the dissolved substance.
2. The reaction of sucrose hydrolysis: CO + H 2 0 -> C 6 H 12 0 6 (glucose) + C 6 H 12 0 6 (fructose) is a bimolecular reaction, but its kinetics is described by a first-order kinetic equation: V \u003d k * C cax , since under experimental conditions, including in the body, the concentration of water is a constant value С(Н 2 0) - const.
3. 
The decomposition reaction of hydrogen peroxide, which proceeds with the participation of catalysts, both inorganic ions Fe 3+, Cu 2+ of metal platinum, and biological enzymes, such as catalase, has general form:
2H 2 0 2 -\u003e 2H 2 0 + O e, i.e., is bimolecular.
Dependence of reaction rate on concentration. Kinetic equations of reactions of the first, second and zero orders. Experimental methods for determining the rate and rate constant of reactions.
The dependence of the reaction rate on temperature. Van't Hoff rule. Temperature coefficient reaction rate and its features for biochemical processes.
γ is the temperature coefficient of the reaction rate.
physical meaning The value of γ lies in the fact that it shows how many times the reaction rate changes with a change in temperature for every 10 degrees.
15. The concept of the theory of active collisions. Energy profile of the reaction; activation energy; Arrhenius equation. The role of the steric factor. The concept of the theory of the transition state.
The relationship of the rate constant, activation energy and temperature is described by the Arrhenius equation: k T \u003d k 0 *Ae ~ E / RT, where k t and k 0 are the rate constants at temperature T and T e e is the base of the natural logarithm, A is the steric factor.
The steric factor A determines the probability of a collision of two reacting particles in active center molecules. This factor is especially important for biochemical reactions with biopolymers. At acid-base reactions H + -ion must react with the terminal carboxyl group - COO. "However, not every collision of H + -ion with a protein molecule will lead to this reaction. Only those collisions that are directly carried out at some points of macromolecules, called active centers, will be effective.
It follows from the Arrhenius equation that the higher the rate constant, the lower the value of the activation energy E and the higher the temperature T of the process.
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, then, accordingly, the value of the reaction rate constant will also increase. 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 normal conditions does not leak (it is possible only with enough low temperatures). In the presence of a catalyst, the activation energy decreases and the reaction rate increases. But, both before the use of the catalyst and in the presence of it, the thermal effect of the reaction does not change, the reaction has the form:
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 E a are related by a relation called the Arrhenius equation:
Here A is the pre-exponential factor, R is the universal gas constant, T is the absolute temperature.
Thus, at a constant temperature, the reaction rate determines E a. The more E a, the smaller the number of active molecules and the slower the reaction proceeds. When decreasing E a speed increases and E a= 0 the reaction proceeds instantaneously.
Value E a characterizes the nature of the reacting substances and is determined experimentally from the dependence k = f(T). Writing equation (5.3) in logarithmic form and solving it for constants at two temperatures, we find E a:
γ 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 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 fraction of collisions favorable in spatial relation(takes values from 0 to ), is the fraction 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 carbon-containing inorganic substances in oxygen, indicating the oxidizing agent and reducing agent, as well as the oxidation states of carbon before and after the reaction.
C - reducing agent, oxidation process
O - oxidizing agent, reduction process
Ticket number 5
1. What is the "electronegativity", "valency", "oxidation state" of an element and what are the basic rules for determining them?
OXIDATION STATE - the conditional charge of an atom of an element, obtained on the assumption that the compound consists of ions. It can be positive, negative, zero, fractional and is indicated by an Arabic numeral with a “+” or “-” sign in the form of the upper right index of the element symbol: C 1-, O 2-, H +, Mg 2+, N 3-, N 5+ , Cr 6+ .
To determine the oxidation state (s. o.) of an element in a compound (ion), use the following rules:
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.
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, then, accordingly, the value of the reaction rate constant will also increase. 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 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.