The rate of a chemical reaction is determined by the equation. The rate of a chemical reaction and the factors that affect it
Objective: speed study chemical reaction and its dependence on various factors: the nature of the reactants, concentration, temperature.
Chemical reactions proceed at different rates. The rate of a chemical reaction is called the change in the concentration of the reactant per unit time. It is equal to the number of interaction acts per unit time per unit volume for a reaction occurring in a homogeneous system (for homogeneous reactions), or per unit interface for reactions occurring in a heterogeneous system (for heterogeneous reactions).
average speed reactions v cf. in the time interval from t1 before t2 is determined by the relation:
where From 1 and From 2 is the molar concentration of any participant in the reaction at time points t1 and t2 respectively.
The “–“ sign in front of the fraction refers to the concentration of the starting substances, Δ WITH < 0, знак “+” – к концентрации продуктов реакции, ΔWITH > 0.
The main factors affecting the rate of a chemical reaction are: the nature of the reactants, their concentration, pressure (if gases are involved in the reaction), temperature, catalyst, interface area for heterogeneous reactions.
Most chemical reactions are complex processes that occur in several stages, i.e. consisting of several elementary processes. Elementary or simple reactions are reactions that occur in one stage.
For elementary reactions, the dependence of the reaction rate on concentration is expressed by the law of mass action.
At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of reactants, taken in powers equal to stoichiometric coefficients.
For a reaction in general view
a A + b B ... → c C,
according to the law of mass action v is expressed by the relation
v = K∙s(A) a ∙ c(B) b,
where c(A) and c(B) are the molar concentrations of reactants A and B;
TO is the rate constant of this reaction, equal to v, if c(A) a=1 and c(B) b=1, and depending on the nature of the reactants, temperature, catalyst, surface area of the interface for heterogeneous reactions.
Expressing the dependence of the reaction rate on concentration is called the kinetic equation.
In the case of complex reactions, the law of mass action applies to each individual stage.
For heterogeneous reactions, the kinetic equation includes only the concentrations of gaseous and dissolved substances; yes, for burning coal
C (c) + O 2 (g) → CO 2 (g)
the velocity equation has the form
v \u003d K s (O 2)
A few words about the molecularity and kinetic order of the reaction.
concept "molecularity of the reaction" apply only to simple reactions. The molecularity of a reaction characterizes the number of particles participating in an elementary interaction.
There are mono-, bi- and trimolecular reactions, in which one, two and three particles, respectively, participate. The probability of simultaneous collision of three particles is small. The elementary process of interaction of more than three particles is unknown. Examples of elementary reactions:
N 2 O 5 → NO + NO + O 2 (monomolecular)
H 2 + I 2 → 2HI (bimolecular)
2NO + Cl 2 → 2NOCl (trimolecular)
The molecularity of simple reactions coincides with the overall kinetic order of the reaction. The order of the reaction determines the nature of the dependence of the rate on the concentration.
The overall (total) kinetic order of a reaction is the sum of the exponents at the concentrations of the reactants in the reaction rate equation, determined experimentally.
As the temperature rises, the rate of most chemical reactions increases. The dependence of the reaction rate on temperature is approximately determined by the van't Hoff rule.
For every 10 degrees increase in temperature, the rate of most reactions increases by a factor of 2–4.
where and are the reaction rates, respectively, at temperatures t2 and t1 (t2>t1);
γ is the temperature coefficient of the reaction rate, this is a number showing how many times the rate of a chemical reaction increases with an increase in temperature by 10 0.
Using the van't Hoff rule, it is only possible to approximately estimate the effect of temperature on the reaction rate. A more accurate description of the dependence of the temperature reaction rate is feasible within the framework of the Arrhenius activation theory.
One of the methods of accelerating a chemical reaction is catalysis, which is carried out with the help of substances (catalysts).
Catalysts- these are substances that change the rate of a chemical reaction due to repeated participation in the intermediate chemical interaction with the reaction reagents, but after each cycle of the intermediate interaction they restore their chemical composition.
The mechanism of action of the catalyst is reduced to a decrease in the activation energy of the reaction, i.e. a decrease in the difference between the average energy of the active molecules (active complex) and the average energy of the molecules of the starting substances. This increases the rate of the chemical reaction.
Speed reaction is determined by the change in the molar concentration of one of the reactants:
V \u003d ± ((C 2 - C 1) / (t 2 - t 1)) \u003d ± (DC / Dt)
Where C 1 and C 2 are the molar concentrations of substances at times t 1 and t 2, respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - by the original substance).
Reactions occur when molecules of reactants collide. Its speed is determined by the number of collisions and the likelihood that they will lead to a transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors affecting the rate of chemical reactions.
1. The nature of the reactants. An important role is played by the nature of chemical bonds and the structure of the molecules of the reagents. Reactions proceed in the direction of the destruction of less strong bonds and the formation of substances with stronger bonds. Thus, high energies are required to break bonds in H 2 and N 2 molecules; such molecules are not very reactive. To break bonds in highly polar molecules (HCl, H 2 O), less energy is required, and the reaction rate is much higher. Reactions between ions in electrolyte solutions proceed almost instantaneously.
Examples
Fluorine reacts explosively with hydrogen room temperature, bromine interacts with hydrogen slowly and when heated.
Calcium oxide reacts vigorously with water, releasing heat; copper oxide - does not react.
2. Concentration. With an increase in concentration (the number of particles per unit volume), collisions of reactant molecules occur more often - the reaction rate increases.
The law of active masses (K. Guldberg, P. Waage, 1867)
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.
AA + bB + . . . ® . . .
- [A] a [B] b . . .
The reaction rate constant k depends on the nature of the reactants, temperature, and catalyst, but does not depend on the concentrations of the reactants.
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.
3. Temperature. For every 10°C increase in temperature, the reaction rate increases by a factor of 2-4 (Van't Hoff's Rule). With an increase in temperature from t 1 to t 2, the change in the reaction rate can be calculated by the formula:
| |
|
(t 2 - t 1) / 10 |
| Vt 2 / Vt 1 | = g | |
(where Vt 2 and Vt 1 are the reaction rates at temperatures t 2 and t 1, 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:
- 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 result in a chemical transformation.
Energy diagram of a chemical reaction.
| exothermic reaction | Endothermic reaction |
A - reagents, B - activated complex (transition state), C - products.
The higher the activation energy Ea, the more the reaction rate increases with increasing temperature.
4. 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.
5. 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. At homogeneous catalysis the reagents and the catalyst constitute one phase (they are in the same state of aggregation), with heterogeneous catalysis- different phases (they are in different states of aggregation). Dramatically slow down the course of unwanted chemical processes in some cases, it is possible to add inhibitors to the reaction medium (the " negative catalysis").
Topics of the USE codifier:Speed reaction. Its dependence on various factors.
The rate of a chemical reaction indicates how fast a reaction occurs. Interaction occurs when particles collide in space. In this case, the reaction does not occur with every collision, but only when the particles have the appropriate energy.
Speed reaction is the number of elementary collisions of interacting particles, ending in a chemical transformation, per unit of time.
Determination of the rate of a chemical reaction is associated with the conditions for its implementation. If the reaction homogeneous– i.e. products and reactants are in the same phase - then the rate of a chemical reaction is defined as the change in substance per unit time:
υ = ∆C / ∆t.
If the reactants or products are in different phases, and the collision of particles occurs only at the interface, then the reaction is called heterogeneous, and its speed is determined by the change in the amount of substance per unit time per unit of the reaction surface:
υ = Δν / (S Δt).
How to make particles collide more often, i.e. how increase the rate of a chemical reaction?
1. The easiest way is to increase temperature . As you must have known from your physics course, temperature is a measure of the average kinetic energy of the movement of particles of matter. If we raise the temperature, then the particles of any substance begin to move faster, and therefore collide more often.
However, with increasing temperature, the rate of chemical reactions increases mainly due to the fact that the number of effective collisions increases. As the temperature rises, the number of active particles that can overcome the energy barrier of the reaction sharply increases. If we lower the temperature, the particles begin to move more slowly, the number of active particles decreases, and the number of effective collisions per second decreases. In this way, When the temperature rises, the rate of a chemical reaction increases, and when the temperature falls, it decreases..
Note! This rule works the same for all chemical reactions (including exothermic and endothermic ones). The reaction rate does not depend on the thermal effect. The rate of exothermic reactions increases with increasing temperature and decreases with decreasing temperature. The rate of endothermic reactions also increases with increasing temperature, and decreases with decreasing temperature.
Moreover, back in the 19th century, the Dutch physicist van't Hoff experimentally found that most reactions increase in approximately the same rate (by about 2-4 times) with an increase in temperature by 10 ° C. Van't Hoff's rule sounds like this: an increase in temperature by 10 ° C leads to an increase in the rate of a chemical reaction by 2-4 times (this value is called the temperature coefficient of the chemical reaction rate γ). Exact value temperature coefficient is determined for each reaction.
Here v 2 - reaction rate at temperature T 2, v 1 - reaction rate at temperature T 1, γ is the temperature coefficient of the reaction rate, the van't Hoff coefficient.
In some situations, it is not always possible to increase the reaction rate with the help of temperature, because. some substances decompose when the temperature rises, some substances or solvents evaporate at elevated temperatures, etc., i.e. process conditions are violated.
2. Concentration. You can also increase the number of effective collisions by changing concentration reactants . usually used for gases and liquids, as In gases and liquids, particles move rapidly and are actively mixed. The higher the concentration of reactants (liquids, gases), the more number effective collisions, and the higher the rate of the chemical reaction.
Based a large number experiments in 1867 in the works of the Norwegian scientists P. Guldenberg and P. Waage and, independently of them, in 1865 by the Russian scientist N.I. Beketov derived the basic law of chemical kinetics, which establishes the dependence of the rate of a chemical reaction on the concentration of reactants:
The rate of a chemical reaction is directly proportional to the product of the concentrations of reactants in powers equal to their coefficients in the chemical reaction equation.
For a chemical reaction of the form: aA + bB = cC + dD the law of mass action is written as follows:
here v is the rate of the chemical reaction,
C A and C B — concentrations of substances A and B, respectively, mol/l
k is the coefficient of proportionality, the rate constant of the reaction.
for instance, for the ammonia formation reaction:
N 2 + 3H 2 ↔ 2NH 3
The law of mass action looks like this:
The reaction rate constant shows how fast substances will react if their concentrations are 1 mol / l, or their product is 1. The rate constant of a chemical reaction depends on temperature and does not depend on the concentration of the reactants.
The law of mass action does not take into account the concentration of solids, because they react, as a rule, on the surface, and the number of reacting particles per unit surface does not change.
In most cases, a chemical reaction consists of several simple steps, in this case, the chemical reaction equation shows only the total or final equation of the ongoing processes. In this case, the rate of a chemical reaction in a complex way depends (or does not depend) on the concentration of reactants, intermediates, or catalyst, so the exact form of the kinetic equation is determined experimentally, or based on an analysis of the proposed reaction mechanism. Generally, the rate of a complex chemical reaction is determined by the rate of its slowest step ( limiting stage).
3. Pressure. For gases, the concentration directly depends on pressure. As pressure increases, the concentration of gases increases. The mathematical expression of this dependence (for an ideal gas) is the Mendeleev-Clapeyron equation:
pV=νRT
Thus, if among the reactants there is a gaseous substance, then at When pressure is increased, the rate of a chemical reaction increases; when pressure is reduced, it decreases. .
For instance. How will the rate of the reaction of fusion of lime with silicon oxide change:
CaCO 3 + SiO 2 ↔ CaSiO 3 + CO 2
with increasing pressure?
The correct answer would be - no way, because. there are no gases among the reagents, and calcium carbonate is a solid salt, insoluble in water, silicon oxide is a solid. The gas will be the product - carbon dioxide. But products do not affect the rate of the forward reaction.
Another way to increase the rate of a chemical reaction is to direct it along a different path, replacing the direct interaction, for example, of substances A and B with a series of sequential reactions with a third substance K, which require much less energy (have a lower activation energy barrier) and proceed at given conditions faster than the direct reaction. This third substance is called catalyst .
- it chemical substances participating in a chemical reaction, changing its speed and direction, but not expendable during the reaction (at the end of the reaction, they do not change either in quantity or in composition). An approximate mechanism for the operation of a catalyst for a reaction of the type A + B can be depicted as follows:
A+K=AK
AK + B = AB + K
The process of changing the reaction rate when interacting with a catalyst is called catalysis. Catalysts are widely used in industry when it is necessary to increase the rate of a reaction or direct it along a certain path.
According to the phase state of the catalyst, homogeneous and heterogeneous catalysis are distinguished.
homogeneous catalysis - this is when the reactants and the catalyst are in the same phase (gas, solution). Typical homogeneous catalysts are acids and bases. organic amines, etc.
heterogeneous catalysis - this is when the reactants and the catalyst are in different phases. As a rule, heterogeneous catalysts are solids. Because interaction in such catalysts occurs only on the surface of the substance, an important requirement for catalysts is a large surface area. Heterogeneous catalysts are characterized by high porosity, which increases the surface area of the catalyst. Thus, the total surface area of some catalysts sometimes reaches 500 square meters per 1 g of catalyst. Large area and porosity ensure efficient interaction with reagents. Heterogeneous catalysts include metals, zeolites - crystalline minerals of the aluminosilicate group (silicon and aluminum compounds), and others.
Example heterogeneous catalysis - ammonia synthesis:
N 2 + 3H 2 ↔ 2NH 3
Porous iron with Al 2 O 3 and K 2 O impurities is used as a catalyst.
The catalyst itself is not consumed during the chemical reaction, but other substances accumulate on the surface of the catalyst, which bind the active centers of the catalyst and block its operation ( catalytic poisons). They must be removed regularly by regenerating the catalyst.
Catalysts are very effective in biochemical reactions. enzymes. Enzymatic catalysts act highly efficiently and selectively, with a selectivity of 100%. Unfortunately, enzymes are very sensitive to temperature increase, medium acidity, and other factors; therefore, there are a number of limitations for the industrial scale implementation of processes with enzymatic catalysis.
Catalysts should not be confused with initiators process and inhibitors. for instance, to initiate a radical reaction of methane chlorination, ultraviolet irradiation is necessary. It's not a catalyst. Some radical reactions are initiated by peroxide radicals. They are also not catalysts.
Inhibitors are substances that slow down a chemical reaction. Inhibitors can be consumed and participate in a chemical reaction. In this case, inhibitors are not catalysts, vice versa. Reverse catalysis is impossible in principle - the reaction will in any case try to follow the fastest path.
5. Area of contact of reactants. For heterogeneous reactions, one way to increase the number of effective collisions is to increase reaction surface area . How more area the contact surface of the reacting phases, the greater the rate of a heterogeneous chemical reaction. Powdered zinc dissolves much faster in acid than granular zinc of the same mass.
In industry, to increase the area of the contacting surface of the reactants, they use fluidized bed method. for instance, in the production of sulfuric acid by the boiling layer method, pyrite is roasted.
6. The nature of the reactants . Other things being equal, the rate of chemical reactions is also influenced by Chemical properties, i.e. the nature of the reactants. Less active substances will have a higher activation barrier, and react more slowly than more active substances. More active substances have a lower activation energy, and are much easier and more likely to enter into chemical reactions.
At low activation energies (less than 40 kJ/mol), the reaction proceeds very quickly and easily. A significant part of the collisions between particles ends in a chemical transformation. For example, ion exchange reactions occur when normal conditions very fast.
At high activation energies (more than 120 kJ/mol), only a small number of collisions end in a chemical transformation. The rate of such reactions is negligible. For example, nitrogen practically does not interact with oxygen under normal conditions.
At medium activation energies (from 40 to 120 kJ/mol), the reaction rate will be average. Such reactions also proceed under normal conditions, but not very quickly, so that they can be observed with the naked eye. These reactions include the interaction of sodium with water, the interaction of iron with hydrochloric acid, etc.
Substances that are stable under normal conditions tend to have high activation energies.
In life, we are faced with different chemical reactions. Some of them, like the rusting of iron, can go on for several years. Others, such as the fermentation of sugar into alcohol, take several weeks. Firewood in the stove burns out in a couple of hours, and gasoline in the engine burns out in a split second.
To reduce equipment costs, chemical plants increase the rate of reactions. And some processes, for example, damage food products, corrosion of metals - needs to be slowed down.
The rate of a chemical reaction can be expressed as change in the amount of matter (n, modulo) per unit time (t) - compare the speed of a moving body in physics as a change in coordinates per unit time: υ = Δx/Δt . So that the rate does not depend on the volume of the vessel in which the reaction takes place, we divide the expression by the volume of reacting substances (v), i.e., we obtain change in the amount of a substance per unit time per unit volume, or change in the concentration of one of the substances per unit time:
n 2 − n 1
υ = –––––––––– = –––––––– = Δс/Δt (1)
(t 2 − t 1) v Δt v
where c = n / v - substance concentration,
Δ (pronounced "delta") is the generally accepted designation for a change in magnitude.
If substances have different coefficients in the equation, the reaction rate for each of them, calculated by this formula, will be different. For example, 2 moles of sulfur dioxide reacted completely with 1 mole of oxygen in 10 seconds in 1 liter:
2SO 2 + O 2 \u003d 2SO 3
The oxygen velocity will be: υ \u003d 1: (10 1) \u003d 0.1 mol / l s
Sour gas speed: υ \u003d 2: (10 1) \u003d 0.2 mol / l s- this does not need to be memorized and spoken in the exam, an example is given in order not to get confused if this question arises.
The rate of heterogeneous reactions (involving solids) is often expressed per unit area of contacting surfaces:
Δn
υ = –––––– (2)
Δt S
Reactions are called heterogeneous when the reactants are in different phases:
- a solid with another solid, liquid or gas,
- two immiscible liquids
- gas liquid.
Homogeneous reactions occur between substances in the same phase:
- between well-miscible liquids,
- gases,
- substances in solutions.
Conditions affecting the rate of chemical reactions
1) The reaction rate depends on the nature of the reactants. Simply put, different substances react at different rates. For example, zinc reacts violently with hydrochloric acid, while iron reacts rather slowly.
2) The reaction rate is greater, the higher concentration substances. With a highly dilute acid, the zinc will take significantly longer to react.
3) The reaction rate increases significantly with increasing temperature. For example, in order to burn fuel, it is necessary to set it on fire, that is, to increase the temperature. For many reactions, an increase in temperature by 10°C is accompanied by an increase in the rate by a factor of 2–4.
4) Speed heterogeneous reactions increases with increasing surfaces of reactants. Solids for this are usually crushed. For example, in order for iron and sulfur powders to react when heated, iron must be in the form of small sawdust.
Note that formula (1) is implied in this case! Formula (2) expresses the speed per unit area, therefore it cannot depend on the area.
5) The reaction rate depends on the presence of catalysts or inhibitors.
Catalysts Substances that speed up chemical reactions but are not themselves consumed. An example is the rapid decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:
2H 2 O 2 \u003d 2H 2 O + O 2
Manganese (IV) oxide remains on the bottom and can be reused.
Inhibitors- substances that slow down the reaction. For example, to extend the life of pipes and batteries, corrosion inhibitors are added to the water heating system. In automobiles, corrosion inhibitors are added to the brake fluid.
A few more examples.
The rate of a chemical reaction depends on the following factors:
1) The nature of the reactants.
2) The contact surface of the reagents.
3) The concentration of reactants.
4) Temperature.
5) The presence of catalysts.
The rate of heterogeneous reactions also depends on:
a) the magnitude of the phase separation surface (with an increase in the phase separation surface, the rate of heterogeneous reactions increases);
b) the rate of supply of reactants to the interface and the rate of removal of reaction products from it.
Factors affecting the rate of a chemical reaction:
1. The nature of the reagents. An important role is played by the nature of chemical bonds in compounds, the structure of their molecules. For example, the release of hydrogen by zinc from a solution of hydrochloric acid occurs much faster than from a solution of acetic acid, since the polarity of the H-C1 bond is greater than O-N connections in the CH 3 COOH molecule, in other words, due to the fact that Hcl is a strong electrolyte, and CH 3 COOH is a weak electrolyte in an aqueous solution.
2. Reagent contact surface. The larger the contact surface of the reactants, the faster the reaction proceeds. The surface of solids can be increased by grinding them, and for soluble substances by dissolving them. Reactions in solutions proceed almost instantaneously.
3. The concentration of reagents. For an interaction to occur, the particles of reactants in a homogeneous system must collide. With an increase reactant concentrations the rate of reactions increases. This is explained by the fact that with an increase in the amount of a substance per unit volume, the number of collisions between the particles of the reacting substances increases. The number of collisions is proportional to the number of particles of reactants in the volume of the reactor, i.e., their molar concentrations.
Quantitatively, the dependence of the reaction rate on the concentration of the reactants is expressed law of acting masses (Guldberg and Waage, Norway, 1867): the rate of a chemical reaction is proportional to the product of the concentrations of the reactants.
For reaction:
aA + bB ↔ cC + dD
the reaction rate in accordance with the law of mass action is equal to:
υ = k[A]υ a[B]υ b ,(9)
where [A] and [B] are the concentrations of the initial substances;
k-reaction rate constant, which is equal to the reaction rate at concentrations of reactants [A] = [B] = 1 mol/l.
The reaction rate constant depends on the nature of the reactants, temperature, but does not depend on the concentration of substances.
Expression (9) is called the kinetic equation of the reaction. The kinetic equations include the concentrations of gaseous and dissolved substances, but do not include the concentrations of solids:
2SO 2 (g) + O 2 (g) \u003d 2SO 3 (g); υ = k 2 · [O 2 ];
CuO (tv.) + H 2 (g) \u003d Cu (tv) + H 2 O (g); υ = k.
According to the kinetic equations, it is possible to calculate how the reaction rate changes with a change in the concentration of the reactants.
Influence of the catalyst.
5. Reaction temperature. Theory of active collisions
In order for an elementary act of chemical interaction to take place, the reacting particles must collide with each other. However, not every collision results in a chemical interaction. Chemical interaction occurs when particles approach at distances at which the redistribution of electron density and the emergence of new chemical bonds are possible. Interacting particles must have enough energy to overcome the repulsive forces that arise between their electron shells.
transition state- the state of the system, in which the destruction and creation of a connection are balanced. The system is in the transition state for a short (10 -15 s) time. The energy required to bring the system into a transition state is called activation energy. In multistep reactions that include several transition states, the activation energy corresponds to the highest energy value. After overcoming the transition state, the molecules fly apart again with the destruction of old bonds and the formation of new ones or with the transformation of the original bonds. Both options are possible, as they occur with the release of energy. There are substances that can reduce the activation energy for a given reaction.
active molecules A 2 and B 2 upon collision combine into an intermediate active complex A 2 ... B 2 with weakening and then breaking of the A-A and B-B bonds and strengthening of the A-B bonds.
The "activation energy" of the HI formation reaction (168 kJ/mol) is much less than the energy required to completely break the bond in the initial H 2 and I 2 molecules (571 kJ/mol). Therefore, the reaction path through the formation active (activated) complex energetically more favorable than the path through the complete breaking of bonds in the original molecules. The vast majority of reactions occur through the formation of intermediate active complexes. The provisions of the active complex theory were developed by G. Eyring and M. Polyani in the 30s of the XX century.
Activation energy represents the excess of the kinetic energy of the particles relative to the average energy required for the chemical transformation of the colliding particles. Reactions are characterized by different values of activation energy (E a). In most cases, the activation energy of chemical reactions between neutral molecules ranges from 80 to 240 kJ/mol. For biochemical processes values E a often lower - up to 20 kJ / mol. This can be explained by the fact that the vast majority of biochemical processes proceed through the stage of enzyme-substrate complexes. Energy barriers limit the reaction. Due to this, in principle possible reactions(at Q< 0) практически всегда не протекают или замедляются. Реакции с энергией активации выше 120 кДж/моль настолько медленны, что их протекание трудно заметить.
In order for a reaction to occur, the molecules must be oriented in a certain way and have sufficient energy upon collision. The probability of proper orientation in a collision is characterized by activation entropy ∆S a. The redistribution of the electron density in the active complex is favored by the condition that, upon collision, the molecules A 2 and B 2 are oriented, as shown in Fig. 3a, while with the orientation shown in Fig. 3b, the reaction probability is still much less - in Fig. 3c.
Rice. Fig. 3. Favorable (a) and unfavorable (b, c) orientations of A 2 and B 2 molecules upon collision
The equation characterizing the dependence of the rate and reaction on temperature, activation energy and activation entropy has the form:
(10)
where k- reaction rate constant;
A- in the first approximation, the total number of collisions between molecules per unit time (second) per unit volume;
e- base of natural logarithms;
R- universal gas constant;
T- absolute temperature;
E a- activation energy;
∆S a- change in entropy of activation.
Equation (11) was derived by Arrhenius in 1889. Preexponential multiplier A proportional to the total number of collisions between molecules per unit time. Its dimension coincides with the dimension of the rate constant and depends on the total order of the reaction.
Exhibitor is equal to the fraction of active collisions from their total number, i.e. the colliding molecules must have sufficient interaction energy. The probability of their desired orientation at the moment of impact is proportional to .
When discussing the law of mass action for velocity (9), it was specially stipulated that the rate constant is a constant value that does not depend on the concentrations of reagents. It was assumed that all chemical transformations proceed at a constant temperature. At the same time, the rate of chemical transformation can change significantly with a decrease or increase in temperature. From the point of view of the law of mass action, this change in velocity is due to the temperature dependence of the rate constant, since the concentrations of the reactants change only slightly due to thermal expansion or contraction of the liquid.
Most well known fact is the increase in the rate of reactions with increasing temperature. This type of temperature dependence of velocity is called normal (Fig. 3a). This type of dependence is characteristic of all simple reactions.
Rice. 3. Types of temperature dependence of the rate of chemical reactions: a - normal;
b - abnormal; c - enzymatic
However, chemical transformations are now well known, the rate of which decreases with increasing temperature, this type of temperature dependence of the rate is called anomalous . An example is the gas-phase reaction of nitrogen (II) oxide with bromine (Fig. 3b).
Of particular interest to physicians is the temperature dependence of the rate of enzymatic reactions, i.e. reactions involving enzymes. Almost all reactions occurring in the body belong to this class. For example, in the decomposition of hydrogen peroxide in the presence of the enzyme catalase, the rate of decomposition depends on temperature. In the range 273-320 TO temperature dependence is normal. As the temperature increases, the speed increases, and as the temperature decreases, it decreases. When the temperature rises above 320 TO there is a sharp anomalous drop in the peroxide decomposition rate. A similar picture takes place for other enzymatic reactions (Fig. 3c).
From the Arrhenius equation for k it is clear that, since T included in the exponent, the rate of a chemical reaction is very sensitive to changes in temperature. Speed dependency homogeneous reaction on temperature can be expressed by the van't Hoff rule, according to which with an increase in temperature for every 10 °, the reaction rate increases by 2-4 times; the number showing how many times the rate of a given reaction increases with an increase in temperature by 10 ° is called temperature coefficient of the reaction rate -γ.
This rule is mathematically expressed by the following formula:
(12)
where γ is the temperature coefficient, which shows how many times the reaction rate increases with an increase in temperature by 10 0; υ 1 -t 1 ; υ 2 - reaction rate at temperature t2.
As the temperature rises in an arithmetic progression, the speed increases exponentially.
For example, if γ = 2.9, then with an increase in temperature by 100 ° the reaction rate increases by a factor of 2.9 10, i.e. 40 thousand times. Deviations from this rule are biochemical reactions, the rate of which increases tenfold with a slight increase in temperature. This rule is valid only in a rough approximation. Reactions involving large molecules (proteins) are characterized by a large temperature coefficient. The rate of protein denaturation (ovalbumin) increases 50 times with a temperature increase of 10 °C. After reaching a certain maximum (50-60 °C), the reaction rate decreases sharply as a result of thermal denaturation of the protein.
For many chemical reactions, the law of mass action for velocity is unknown. In such cases, the following expression can be used to describe the temperature dependence of the conversion rate:
pre-exponent A with does not depend on temperature, but depends on concentration. The unit of measure is mol/l∙s.
The theoretical dependence makes it possible to pre-calculate the velocity at any temperature if the activation energy and the pre-exponential are known. Thus, the effect of temperature on the rate of chemical transformation is predicted.
Complex reactions
The principle of independence. Everything discussed above referred to relatively simple reactions, but so-called complex reactions are often encountered in chemistry. These reactions include those discussed below. When deriving the kinetic equations for these reactions, the principle of independence is used: if several reactions take place in the system, then each of them is independent of the others and its rate is proportional to the product of the concentrations of its reactants.
Parallel Reactions are reactions that take place simultaneously in several directions.
The thermal decomposition of potassium chlorate occurs simultaneously in two reactions: 
Successive reactions are reactions that proceed in several stages. There are many such reactions in chemistry.
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Associated reactions. If several reactions take place in the system and one of them cannot occur without the other, then these reactions are called conjugated , and the phenomenon itself by induction .
2HI + H 2 CrO 4 → I 2 + Cr 2 O 3 + H 2 O.
This reaction is practically not observed under normal conditions, but if FeO is added to the system, then the following reaction occurs:
FeO + H 2 CrO 4 → Fe 2 O 3 + Cr 2 O 3 + H 2 O
and the first reaction goes along with it. The reason for this is the formation in the second reaction of intermediate products involved in the first reaction:
FeO 2 + H 2 CrO 4 → Cr 2 O 3 + Fe 5+;
HI + Fe 5+ → Fe 2 O 3 + I 2 + H 2 O.
Chemical induction- a phenomenon in which one chemical reaction (secondary) depends on another (primary).
A+ V- primary reaction,
A + C- secondary reaction,
then A is an activator, V- inductor, C - acceptor.
During chemical induction, in contrast to catalysis, the concentrations of all participants in the reaction decrease.
Induction factor is determined from the following equation:
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Depending on the value of the induction factor, the following cases are possible.
I> 0 - fading process. The reaction rate decreases with time.
I < 0 - ускоряющийся процесс. Скорость реакции увеличивается со временем.
The phenomenon of induction is important because in some cases the energy of the primary reaction can compensate for the energy expended in the secondary reaction. For this reason, for example, it is thermodynamically possible to synthesize proteins by polycondensation of amino acids.
Chain reactions. If a chemical reaction proceeds with the formation of active particles (ions, radicals), which, entering into subsequent reactions, cause the appearance of new active particles, then such a sequence of reactions is called chain reaction.
The formation of free radicals is associated with the expenditure of energy to break bonds in a molecule. This energy can be imparted to molecules by illumination, electric discharge, heating, irradiation with neutrons, α- and β-particles. To carry out chain reactions at low temperatures, initiators are introduced into the reacting mixture - substances that easily form radicals: sodium vapor, organic peroxides, iodine, etc.
The reaction of the formation of hydrogen chloride from simple connections activated by light.
Total reaction:
H 2 + C1 2 2HC1.
Separate stages:
Сl 2 2Сl∙ photoactivation of chlorine (initiation)
Cl ∙ + H 2 \u003d Hcl + H ∙ chain development
H ∙ + Cl 2 \u003d Hcl + Cl ∙, etc.
H ∙ + Cl ∙ \u003d Hcl open circuit
Here H∙ and Сl∙ are active particles (radicals).
Three groups of elementary steps can be distinguished in this reaction mechanism. The first is a photochemical reaction chain origin. Chlorine molecules, having absorbed a quantum of light, dissociate into free atoms with a high reactivity. Thus, when a chain is nucleated, free atoms or radicals are formed from valence-saturated molecules. The chain generation process is also called initiation. Chlorine atoms, having unpaired electrons, are able to react with molecular hydrogen, forming molecules of hydrogen chloride and atomic hydrogen. Atomic hydrogen, in turn, interacts with a chlorine molecule, as a result of which a hydrogen chloride molecule and atomic chlorine are again formed, etc.
These processes, characterized by the repetition of the same elementary stages (links) and proceeding with the preservation of free radicals, lead to the consumption of starting substances and the formation of reaction products. These groups of reactions are called reactions of development (or continuation) of the chain.
Stage chain reaction at which free radicals are destroyed is called chain break. Chain termination can occur as a result of the recombination of free radicals, if the energy released in this case can be given to some third body: the vessel wall or molecules of inert impurities (stages 4, 5). That is why the rate of chain reactions is very sensitive to the presence of impurities, to the shape and dimensions of the vessel, especially at low pressures.
The number of elementary links from the moment the chain is born to its break is called the chain length. In the example under consideration, up to 10 5 HCl molecules are formed for each light quantum.
Chain reactions, during which there is no "multiplication" of the number of free radicals, are called unbranched or simple chain reactions . In each elementary stage of the unbranched chain process, one radical "gives birth" to one molecule of the reaction product and only one new radical (Fig. 41).
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Other examples of simple chain reactions: a) chlorination of paraffinic hydrocarbons Cl ∙ + CH 4 → CH 3 ∙ + HC1; CH 3 ∙ + Cl - → CH 3 Cl + Cl ∙ etc.; b) radical polymerization reactions, for example, polymerization of vinyl acetate in the presence of benzoyl peroxide, which easily decomposes into radicals; c) the interaction of hydrogen with bromine, proceeding according to a mechanism similar to the reaction of chlorine with hydrogen, only with a shorter chain length due to its endothermicity.
If two or more active particles appear as a result of the act of growth, then this chain reaction is branched.
In 1925, N. N. Semenov and his collaborators discovered reactions containing elementary stages, as a result of which not one, but several chemically active particles, atoms, or radicals, arise. The appearance of several new free radicals leads to the appearance of several new chains, i.e. one chain forks. Such processes are called branched chain reactions (Fig. 42).
An example of a highly branched chain process is the oxidation of hydrogen at low pressures and a temperature of about 900°C. The reaction mechanism can be written as follows.
1. H 2 + O 2 OH∙ + OH∙ chain initiation
2. OH ∙ + H 2 → H 2 O + H ∙ chain development
3. H ∙ + O 2 → OH ∙ + O: chain branching
4. O: + H 2 → OH ∙ + H ∙
5. OH ∙ + H 2 → H 2 O + H ∙ chain continuation
6. H∙ + H∙ + wall → H 2 open circuit on the vessel wall
7. H ∙ + O 2 + M → HO 2 ∙ + M chain termination in bulk.
M is an inert molecule. The HO 2 ∙ radical, which is formed during a triple collision, is inactive and cannot continue the chain.
At the first stage of the process, hydroxyl radicals are formed, which provide the development of a simple chain. In the third stage, as a result of interaction with the initial molecule of one radical, two radicals are formed, and the oxygen atom has two free valences. This provides branching of the chain.
As a result of chain branching, the reaction rate rapidly increases in the initial period of time, and the process ends with a chain ignition-explosion. However, branched chain reactions end in an explosion only when the branching rate is greater than the chain termination rate. Otherwise, the process is slow.
When the reaction conditions change (changes in pressure, temperature, mixture composition, size and condition of the walls of the reaction vessel, etc.), a transition from a slow reaction to an explosion can occur and vice versa. Thus, in chain reactions there are limiting (critical) states in which chain ignition occurs, from which one should distinguish thermal ignition that occurs in exothermic reactions as a result of ever-increasing heating of the reacting mixture with poor heat removal.
According to the branched chain mechanism, oxidized vapors of sulfur, phosphorus, carbon monoxide (II), carbon disulfide, etc. occur.
Modern theory chain processes developed by laureates Nobel Prize(1956) by the Soviet academician N. N. Semenov and the English scientist Hinshelwood.
Chain reactions should be distinguished from catalytic reactions, although the latter are also cyclic in nature. The most significant difference between chain reactions and catalytic ones is that with a chain mechanism, the reaction can proceed in the direction of increasing the energy of the system due to spontaneous reactions. A catalyst does not cause a thermodynamically impossible reaction. In addition, in catalytic reactions there are no such process steps as chain nucleation and chain termination.
polymerization reactions. A special case of a chain reaction is the polymerization reaction.
Polymerization is a process in which the reaction of active particles (radicals, ions) with low molecular weight compounds (monomers) is accompanied by the sequential addition of the latter with an increase in the length of the material chain (the length of the molecule), i.e., with the formation of a polymer.
Monomers are organic compounds, as a rule, containing unsaturated (double, triple) bonds in the composition of the molecule.
The main stages of the polymerization process:
1. Initiation(under the action of light, heat, etc.):
A: A→A" + A"- homolytic decomposition with the formation of radicals (active valence-unsaturated particles).
A: B→ A - + B +- heterolytic decomposition with the formation of ions.
2. Chain growth: A "+ M→ AM"
(or A - + M→ AM", or V + + M→ VM +).
3. Open circuit: AM" + AM"→ polymer
(or AM" + B +→ polymer, VM + + A"→ polymer).
The speed of a chain process is always greater than that of a non-chain process.
