· Reaction rate constant k depends on the nature of the reactants, temperature and catalyst, but does not depend on the value
reagent concentrations.
The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions, the concentration of the solid phase is not included in the reaction rate expression.
· Law of acting masses establishes the ratio between the masses of reactants in chemical reactions at
equilibrium, as well as the dependence of the rate of a chemical reaction on the concentration of the starting substances.
The activation energy of a chemical reaction. active molecules. activated complex.
· Activation energy in chemistry- the minimum amount of energy that needs to be reported to the system (in chemistry
expressed in joules per mole) for the reaction to occur. The term was introduced by Svante August Arrhenius in 1889. A typical designation for the reaction energy is Ea.
In the chemical model known as Theory of active collisions(TAC), there are three conditions necessary for a reaction to occur:
- The molecules must collide. This is an important condition, but it is not sufficient, since a reaction will not necessarily occur during a collision.
- Molecules must have the necessary energy (activation energy). In the course of a chemical reaction, the interacting molecules must pass through an intermediate state, which may have a higher energy. That is, the molecules must overcome the energy barrier; if this does not happen, the reaction will not start.
The molecules must be correctly oriented relative to each other.
At a low (for a certain reaction) temperature, most molecules have an energy less than the activation energy and are unable to overcome the energy barrier. However, in a substance there will always be individual molecules whose energy is much higher than the average. Even at low temperatures, most reactions continue to go on. Increasing the temperature makes it possible to increase the proportion of molecules with sufficient energy to overcome the energy barrier. Thus, the rate of the reaction is increased.
· active radicals, according to one of the theories of aging of the body, are the cause of aging. They form as a by-product
products of various chemical reactions in the body and oxidize it. Therefore, it is necessary to get rid of them as soon as possible. One way is to take antioxidants, which pharmaceuticals deal with. Also, these active particles can be used as disinfectant solutions.
· Activated complex, the grouping of atoms at the decisive moment of the elementary act of a chemical reaction. The concept of
The activated complex is widely used in the theory of chemical reaction rates.
Question number 3
What factors affect the rate constant of a chemical reaction?
Reaction rate constant (specific reaction rate) is the coefficient of proportionality in the kinetic equation.
The physical meaning of the reaction rate constant k follows from the equation of the law of mass action: k numerically equal to the reaction rate at a concentration of each of the reactants equal to 1 mol / l.
The reaction rate constant depends on the temperature, on the nature of the reactants, on the presence of a catalyst in the system, but does not depend on their concentration.
1. Temperature. With an increase in temperature for every 10 ° C, the reaction rate increases by 2-4 times (Van't Hoff's Rule). With an increase in temperature from t1 to t2, the change in the reaction rate can be calculated by the formula: (t2 - t1) / 10 Vt2 / Vt1 = g (where Vt2 and Vt1 are the reaction rates at temperatures t2 and t1, respectively; g is the temperature coefficient of this reaction). Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation: k = A e –Ea/RT where A is a constant depending on the nature of the reactants; R is the universal gas constant; Ea is the activation energy, i.e., the energy that colliding molecules must have in order for the collision to lead to a chemical transformation. Energy diagram of a chemical reaction. Exothermic reaction Endothermic reaction A - reagents, B - activated complex (transition state), C - products. The greater the activation energy Ea, the more the reaction rate increases with increasing temperature. 2. The contact surface of the reactants. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction proceeds. The surface of solids can be increased by grinding them, and for soluble substances by dissolving them. 3. Catalysis. Substances that participate in reactions and increase its rate, remaining unchanged by the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. In homogeneous catalysis, the reactants and the catalyst make up one phase (they are in the same state of aggregation), while in heterogeneous catalysis they are different phases (they are in different states of aggregation). In some cases, the course of undesirable chemical processes can be drastically slowed down by adding inhibitors to the reaction medium (the phenomenon of "negative catalysis").
Question number 4
Formulate and write down the law of mass action for the reaction:
2 NO+O2=2NO2
LAW OF MASS ACTION: The rate of a chemical reaction is proportional to the product of the concentrations of the reactants. for the reaction 2NO + O2 2NO2, the law of mass action will be written as follows: v=kС2(NO)·С(O2), where k is the rate constant, depending on the nature of the reactants and temperature. The rate in reactions involving solids is determined only by the concentration of gases or dissolved substances: C + O2 \u003d CO2, v \u003d kCO2
The mechanisms of chemical transformations and their rates are studied by chemical kinetics. Chemical processes proceed in time at different rates. Some happen quickly, almost instantly, while others take a very long time to occur.
In contact with
Speed reaction- the rate at which reagents are consumed (their concentration decreases) or reaction products are formed per unit volume.
Factors that can affect the rate of a chemical reaction
The following factors can affect how quickly a chemical interaction occurs:
- concentration of substances;
- the nature of the reagents;
- temperature;
- the presence of a catalyst;
- pressure (for reactions in a gaseous medium).
Thus, by changing certain conditions for the course of a chemical process, it is possible to influence how quickly the process will proceed.
In the process of chemical interaction, the particles of the reacting substances collide with each other. The number of such coincidences is proportional to the number of particles of substances in the volume of the reacting mixture, and hence proportional to the molar concentrations of the reagents.
Law of acting masses describes the dependence of the reaction rate on the molar concentrations of the reacting substances.
For an elementary reaction (A + B → ...), this law is expressed by the formula:
υ \u003d k ∙С A ∙С B,
where k is the rate constant; C A and C B are the molar concentrations of the reactants, A and B.
If one of the reacting substances is in a solid state, then the interaction occurs at the phase interface, and therefore the concentration of the solid substance is not included in the equation of the kinetic law of acting masses. To understand the physical meaning of the rate constant, it is necessary to take C, A and C B equal to 1. Then it becomes clear that the rate constant is equal to the reaction rate at reagent concentrations equal to unity.
The nature of the reagents
Since the chemical bonds of the reacting substances are destroyed in the process of interaction and new bonds of the reaction products are formed, the nature of the bonds participating in the reaction of the compounds and the structure of the molecules of the reacting substances will play an important role.
Surface area of contact of reagents
Such a characteristic as the surface area of contact of solid reagents, sometimes quite significantly, affects the course of the reaction. Grinding a solid allows you to increase the surface area of contact of the reagents, and hence speed up the process. The area of contact of solutes is easily increased by the dissolution of the substance.
Reaction temperature
As the temperature increases, the energy of the colliding particles will increase, it is obvious that with an increase in temperature, the chemical process itself will accelerate. A clear example of how an increase in temperature affects the process of interaction of substances can be considered the data given in the table.
Table 1. Effect of temperature change on the rate of water formation (О 2 +2Н 2 →2Н 2 О)
For a quantitative description of how temperature can affect the rate of interaction of substances, the van't Hoff rule is used. Van't Hoff's rule is that when the temperature rises by 10 degrees, there is an acceleration of 2-4 times.
The mathematical formula describing the van't Hoff rule is as follows:
Where γ is the temperature coefficient of the chemical reaction rate (γ = 2−4).
But the Arrhenius equation describes the temperature dependence of the rate constant much more accurately:
Where R is the universal gas constant, A is a factor determined by the type of reaction, E, A is the activation energy.
The activation energy is the energy that a molecule must acquire in order for a chemical transformation to occur. That is, it is a kind of energy barrier that will need to be overcome by molecules colliding in the reaction volume in order to redistribute bonds.
The activation energy does not depend on external factors, but depends on the nature of the substance. The value of the activation energy up to 40 - 50 kJ / mol allows substances to react with each other quite actively. If the activation energy exceeds 120 kJ/mol, then the substances (at ordinary temperatures) will react very slowly. A change in temperature leads to a change in the number of active molecules, that is, molecules that have reached an energy greater than the activation energy, and therefore capable of chemical transformations.
Catalyst action
A catalyst is a substance that can speed up a process, but is not part of its products. Catalysis (acceleration of the course of a chemical transformation) is divided into · homogeneous, · heterogeneous. If the reactants and the catalyst are in the same state of aggregation, then catalysis is called homogeneous, if in different states, then heterogeneous. The mechanisms of action of catalysts are diverse and quite complex. In addition, it should be noted that catalysts are characterized by selectivity of action. That is, the same catalyst, accelerating one reaction, may not change the rate of another in any way.
Pressure
If gaseous substances are involved in the transformation, then the rate of the process will be affected by a change in pressure in the system . This happens because that for gaseous reactants, a change in pressure leads to a change in concentration.
Experimental determination of the rate of a chemical reaction
It is possible to determine the rate of a chemical transformation experimentally by obtaining data on how the concentration of reacting substances or products changes per unit time. Methods for obtaining such data are divided into
- chemical,
- physical and chemical.
Chemical methods are quite simple, affordable and accurate. With their help, the speed is determined by directly measuring the concentration or amount of a substance of reactants or products. In the case of a slow reaction, samples are taken to monitor how the reagent is consumed. After that, the content of the reagent in the sample is determined. By sampling at regular intervals, it is possible to obtain data on the change in the amount of a substance during the interaction. The most commonly used types of analysis are titrimetry and gravimetry.
If the reaction proceeds quickly, then in order to take a sample, it has to be stopped. This can be done by cooling abrupt removal of the catalyst, it is also possible to dilute or transfer one of the reagents to a non-reactive state.
Methods of physicochemical analysis in modern experimental kinetics are used more often than chemical ones. With their help, you can observe the change in the concentrations of substances in real time. There is no need to stop the reaction and take samples.
Physico-chemical methods are based on the measurement of a physical property that depends on the quantitative content of a certain compound in the system and changes with time. For example, if gases are involved in the reaction, then pressure can be such a property. Also measure electrical conductivity, refractive index, absorption spectra of substances.
Rice. 40. Dependence of the value of the reciprocal concentration of the reagent on time for a second-order reaction
Rice. Fig. 39. Dependence of the logarithm of the concentration of the reagent on the flow time for the reaction of the first order
Rice. 38. Change in the concentration of the starting substance over time in a first-order reaction
Rice. 37. Change in the concentration of the starting substance over time in a zero-order reaction
Mathematically, this linear dependence will be written as follows
where k is the rate constant, C 0 is the initial molar concentration of the reactant, C is the concentration at time t.
From it, you can derive a formula for calculating the rate constant of a zero-order chemical reaction.
Is the zero order rate constant measured in mol/L? s (mol l -1 s -1).
The half-life for a zero order reaction is proportional to the concentration of the starting material
For first-order reactions, the kinetic curve in the C,t coordinates is exponential and looks like this (Fig. 38) Mathematically, this curve is described by the following equation
C \u003d C 0 e - kt
In practice, for first-order reactions, the kinetic curve is most often plotted in the coordinates lnC, t. In this case, a linear dependence of lnС on time is observed (Fig. 39)
ln C \u003d lnC 0 - kt
|
Accordingly, the value of the rate constant and the half-life can be calculated using the following formulas
k = ln or k = 2.303lg
(when moving from a decimal logarithm to a natural one).
The first-order reaction rate constant has the dimension t -1 , i.e. 1/s and does not depend on concentration units. It shows the proportion of molecules that have reacted per unit of time from the total number of reagent molecules in the system. Thus, in the first-order reactions, the same fractions of the taken amount of the starting material are spent over the same time intervals.
The second distinctive feature of first-order reactions is that t ½ for them does not depend on the initial concentration of the reagent, but is determined only by the rate constant.
We will consider the form of the equation for the dependence of concentration on time for second-order reactions only for the simplest case, when 2 identical molecules, or molecules of different substances, participate in an elementary act, but their initial concentrations (C 0) are equal. In this case, a linear dependence is observed in the coordinates 1/C, t (Fig. 40). The mathematical equation of this dependence will be written as follows
and is measured in l?s -1? mol -1, i.e. its numerical value depends on the units in which the concentration of the substance is measured.
The half-life of second-order reactions is inversely proportional to the initial concentration of the reagent
This is due to the fact that the rate of second-order reactions strongly depends on the number of collisions between the molecules of the reacting substances per unit time, which, in turn, is proportional to the number of molecules per unit volume, i.e. substance concentration. Thus, the greater the concentration of a substance in the system, the more often the molecules collide with each other and the shorter the time period, half of them will have time to react.
Third-order reactions, as mentioned earlier, are extremely rare and are of no practical interest. Therefore, in this regard, we will not consider them.
Reaction rate constant (specific reaction rate) is the coefficient of proportionality in the kinetic equation.
The physical meaning of the reaction rate constant k follows from the equation of the law of mass action: k numerically equal to the reaction rate at a concentration of each of the reactants equal to 1 mol / l.
The reaction rate constant depends on the temperature, on the nature of the reactants, on the catalyst, but does not depend on their concentration. For a reaction of the form 2A+2B->3C+D, the rate of formation of reaction products and the rate of consumption of reagents can be represented as: d[A]/(2*dt)=d[B]/(2*dt)=d[C] /(3*dt)=d[D]/dt Thus, in order to avoid using several forms of recording the rate for the same reaction, a chemical variable is used that determines the degree of the reaction and does not depend on stoichiometric coefficients: ξ=(Δn) /ν where ν is the stoichiometric coefficient. Then the reaction rate: v=(1/V)*dξ/dt where V is the volume of the system.
Dimension
The dimension of the reaction rate constant depends on the order of the reaction. If the concentration of reactants is measured in mol l −1 (M):
- For a first order reaction, k has dimension c −1
- For a second order reaction, k has the dimension l mol −1 s −1 (or M −1 s −1)
- For a third order reaction, k has the dimension l 2 mol −2 s −1 (or M −2 s −1)
see also
Write a review on the article "Reaction rate constant"
Notes
An excerpt characterizing the reaction rate constant
Likhachev got up and rummaged through his packs, and Petya soon heard the warlike sound of steel on a bar. He climbed onto the wagon and sat on its edge. The Cossack sharpened his saber under the wagon.- And what, the good fellows sleep? Petya said.
- Who is sleeping, and who is like this.
- Well, what about the boy?
- Is it spring? He was there, in the hallways, collapsed. Sleeping with fear. It was glad.
For a long time after that Petya was silent, listening to the sounds. Footsteps were heard in the darkness and a black figure appeared.
- What are you sharpening? the man asked, approaching the wagon.
- But the master sharpen his saber.
“It’s a good thing,” said the man, who seemed to be a hussar to Petya. - Do you have a cup left?
“At the wheel.
The hussar took the cup.
“It’s probably light soon,” he said, yawning, and went somewhere.
Petya should have known that he was in the forest, in the party of Denisov, a verst from the road, that he was sitting on a wagon recaptured from the French, near which horses were tied, that the Cossack Likhachev was sitting under him and sharpening his saber, that a large black spot to the right - a guardhouse, and a bright red spot below to the left - a dying fire, that the man who came for a cup was a hussar who wanted to drink; but he knew nothing and did not want to know it. He was in a magical realm, in which there was nothing like reality. A big black spot, maybe it was definitely a guardhouse, or maybe there was a cave that led into the very depths of the earth. The red spot may have been fire, or perhaps the eye of a huge monster. Maybe he’s definitely sitting on a wagon now, but it’s very possible that he’s not sitting on a wagon, but on a terribly high tower, from which if you fall, you would fly to the ground all day, a whole month - all fly and you will never reach . It may be that just the Cossack Likhachev is sitting under the wagon, but it may very well be that this is the kindest, bravest, most wonderful, most excellent person in the world, whom no one knows. Perhaps it was the hussar who was exactly passing for water and went into the hollow, or perhaps he had just disappeared from sight and completely disappeared, and he was not there.
