Factors on which the rate of a chemical reaction depends. Factors affecting the rate of a chemical reaction
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 higher 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 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 the O-H bond in the CH 3 COOH molecule, in other words, due to the fact that Hcl - 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 NI 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 reactants change only slightly due to thermal expansion or contraction of the liquid.
The most well known fact is that the rate of reactions increases 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, at present, chemical transformations are 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. The dependence of the rate of a 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+ IN- primary reaction,
A + C- secondary reaction,
then A is an activator, IN- 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:
.
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 compounds, 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 steps (links) and proceeding with the preservation of free radicals, lead to the consumption of starting materials and the formation of reaction products. These groups of reactions are called reactions of development (or continuation) of the chain.
The step in the chain reaction in which free radicals are destroyed is called chain break. Chain termination can occur as a result of 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 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.
The modern theory of chain processes was developed by the Nobel Prize winners (1956) 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 IN + + 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.
Name at least 5 factors that affect the rate of a chemical reaction.
- Temperature, pressure, fineness (contact area), presence of catalysts, concentration of reactants
- nightmares)))))))))))) 0
until you read a hundred times pissed off - tin
- 1 catalyst
2 valence
3 chem. activity to other elements
4 impurities
5 temperature
6 external influence - 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. For example, high energies are required to break bonds in H2 and N2 molecules; such molecules are not very reactive. To break bonds in highly polar molecules (HCl, H2O), less energy is required, and the reaction rate is much higher. Reactions between ions in electrolyte solutions proceed almost instantaneously.
Fluorine reacts explosively with hydrogen at room temperature; bromine reacts with hydrogen slowly even 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 molecules of reacting substances 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.
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. With an increase in temperature for every 10C, the reaction rate increases by 2-4 times (Van't Hoff's Rule). As the temperature increases from t1 to t2, the change in the reaction rate can be calculated using the formula:
(t2 - t1) / 10
(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:
A is a constant depending on the nature of the reactants;
R is the universal gas constant 8.314 J / (mol K) \u003d 0.082 l atm / (mol K);
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 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. 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").
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. Electrical conductivity, refractive index, and absorption spectra of substances are also measured.
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, such as food spoilage, metal corrosion, need 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 vwhere c = n / v is the concentration of the substance,
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 SReactions 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.
