Chemical reaction rate - change in the amount of one of the reactants per unit time in a unit of reaction space.

The following factors influence the rate of a chemical reaction:

• the nature of the reactants;
• concentration of reactants;
• contact surface of reactants (in heterogeneous reactions);
• temperature;
• the action of catalysts.

Active collision theory allows to explain the influence of some factors on the rate of a chemical reaction. The main provisions of this theory:

• Reactions occur when particles of reagents collide, which have a certain energy.
• The more reagent particles, the closer they are to each other, the more chances they have to collide and react.
• Only effective collisions lead to a reaction, i.e. such that “old ties” are destroyed or weakened and therefore “new” ones can form. For this, the particles must have sufficient energy.
• The minimum excess energy required for effective collision of reagent particles is called activation energy Еа.
• The activity of chemicals is manifested in the low activation energy of reactions with their participation. The lower the activation energy, the higher the reaction rate. For example, in the reactions between cations and anions, the activation energy is very small, so such reactions proceed almost instantly.

Influence of the concentration of reactants on the reaction rate

With an increase in the concentration of reactants, the reaction rate increases. In order to react, two chemical particles must move closer together, so the reaction rate depends on the number of collisions between them. An increase in the number of particles in a given volume leads to more frequent collisions and to an increase in the reaction rate.

An increase in the rate of the reaction proceeding in the gas phase will result in an increase in pressure or a decrease in the volume occupied by the mixture.

On the basis of experimental data in 1867, the Norwegian scientists K. Guldberg and P Vaage, and independently of them in 1865, the Russian scientist N.I. Beketov formulated the basic law of chemical kinetics, establishing dependence of the reaction rate on the concentration of reactants

Mass Action Law (ZDM):

The rate of a chemical reaction is proportional to the product of the concentrations of the reactants taken in powers equal to their coefficients in the reaction equation. ("Active mass" is a synonym modern concept "concentration")

aA +bВ \u003dcC +dD,where k- reaction rate constant

ZDM is performed only for elementary chemical reactionsproceeding in one stage. If the reaction proceeds sequentially through several stages, then the total rate of the entire process is determined by its slowest part.

Expressions for the rates of various types of reactions

ZDM refers to homogeneous reactions. If the reaction is heterogeneous (reagents are in different states of aggregation), then only liquid or only gaseous reagents enter into the ZDM equation, and solid reagents are excluded, affecting only the rate constant k.

Molecularity of the reaction Is the minimum number of molecules participating in an elementary chemical process. In terms of molecularity, elementary chemical reactions are divided into molecular (A →) and bimolecular (A + B →); trimolecular reactions are extremely rare.

Rate of heterogeneous reactions

• Depends on surface area of \u200b\u200bcontact of substances, i.e. on the degree of grinding of substances, the completeness of mixing of reagents.
• An example is wood burning. A whole log burns relatively slowly in air. If you increase the surface of contact of wood with air, splitting the log into chips, the burning rate will increase.
• The pyrophoric iron is poured onto a sheet of filter paper. During the fall, the iron particles heat up and set the paper on fire.

Effect of temperature on reaction rate

In the 19th century, the Dutch scientist Van't Hoff empirically found that when the temperature rises by 10 ° C, the rates of many reactions increase 2-4 times.

Van't Hoff's rule

With an increase in temperature for every 10 ° C, the reaction rate increases by 2-4 times.

Here γ (Greek letter "gamma") - the so-called temperature coefficient or Van't Hoff coefficient, takes values \u200b\u200bfrom 2 to 4.

For each specific reaction, the temperature coefficient is determined empirically. It shows how many times the rate of a given chemical reaction (and its rate constant) increases with every 10 degrees increase in temperature.

The Van't Hoff rule is used to approximate the change in the reaction rate constant with increasing or decreasing temperature. A more accurate relationship between the rate constant and temperature was established by the Swedish chemist Svante Arrhenius:

Than more E a specific reaction, the less (at a given temperature) will be the rate constant k (and rate) of this reaction. An increase in T leads to an increase in the rate constant, which is explained by the fact that an increase in temperature leads to a rapid increase in the number of "energetic" molecules capable of overcoming the activation barrier E a.

Effect of the catalyst on the reaction rate

It is possible to change the reaction rate by using special substances that change the reaction mechanism and direct it along an energetically more favorable path with a lower activation energy.

Catalysts- these are substances that participate in a chemical reaction and increase its rate, but after the end of the reaction, they remain unchanged qualitatively and quantitatively.

Inhibitors- substances that slow down chemical reactions.

Changing the rate of a chemical reaction or its direction using a catalyst is called catalysis .

The rate of a chemical reaction depends on many factors, including the nature of the reactants, the concentration of the reactants, temperature, and the presence of catalysts. Let's consider these factors.

1). The nature of the reactants... If there is an interaction between substances with an ionic bond, then the reaction proceeds faster than between substances with a covalent bond.

2.) Concentration of reactants... For a chemical reaction to take place, a collision of molecules of reacting substances is necessary. That is, the molecules must come so close to each other that the atoms of one particle experience the action of the electric fields of another. Only in this case will electron transitions and corresponding rearrangements of atoms be possible, as a result of which molecules of new substances are formed. Thus, the rate of chemical reactions is proportional to the number of collisions that occur between molecules, and the number of collisions, in turn, is proportional to the concentration of reactants. On the basis of experimental material, the Norwegian scientists Guldberg and Vaage and independently of them the Russian scientist Beketov in 1867 formulated the basic law of chemical kinetics - law of mass action (ZDM): at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants in the power of their stoichiometric coefficients. For the general case:

the law of mass action has the form:

The record of the law of mass action for this reaction is called the basic kinetic equation of the reaction... Basically kinetic equation k is the reaction rate constant, which depends on the nature of the reactants and temperature.

Most chemical reactions are reversible. In the course of such reactions, their products, as they accumulate, react with each other to form the initial substances:

Forward reaction speed:

Feedback rate:

At the moment of balance:

Hence the law of mass action in a state of equilibrium will take the form:

where K is the reaction equilibrium constant.

3) Effect of temperature on reaction rate... The rate of chemical reactions, as a rule, increases with temperature rise. Let us consider this using the example of the interaction of hydrogen with oxygen.

2H 2 + O 2 \u003d 2H 2 O

At 20 0 С, the reaction rate is practically zero and it would take 54 billion years for the interaction to pass by 15%. At 500 0 С it will take 50 minutes for water to form, and at 700 0 С the reaction proceeds instantly.

The dependence of the reaction rate on temperature is expressed van't Hoff rule: with an increase in temperature by 10 °, the reaction rate increases 2 - 4 times. Van't Hoff's rule is written:

4) Effect of catalysts... The rate of chemical reactions can be adjusted using catalysts - substances that change the reaction rate and remain unchanged after the reaction. Changing the reaction rate in the presence of a catalyst is called catalysis. Distinguish positive (reaction rate increases) and negative (the reaction rate decreases) catalysis. Sometimes the catalyst is formed during the reaction, such processes are called autocatalytic. Distinguish between homogeneous and heterogeneous catalysis.

When homogeneous By catalysis, the catalyst and reactants are in the same phase. For instance:

When heterogeneous catalysis, the catalyst and reactants are in different phases. For instance:

Heterogeneous catalysis is associated with enzymatic processes. All chemical processes in living organisms are catalyzed by enzymes, which are proteins with specific specialized functions. In solutions in which enzymatic processes take place, there is no typical heterogeneous environment, due to the absence of a clearly defined interface. Such processes are referred to as microheterogeneous catalysis.

The mechanisms of the occurrence of chemical transformations and their rates are studied by chemical kinetics. Chemical processes take place in time at different rates. Some happen quickly, almost instantly, while others take a very long time to proceed.

In contact with

Speed \u200b\u200breaction - the rate at which the reagents are consumed (their concentration decreases) or the reaction products are formed per unit volume.

Factors that can affect the rate of a chemical reaction

The following factors can affect how quickly the chemical interaction takes place:

• concentration of substances;
• the nature of the reagents;
• temperature;
• the presence of a catalyst;
• pressure (for reactions in a gas environment).

Thus, by changing certain conditions of the 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 therefore is proportional to the molar concentrations of the reagents.

Mass action law describes the dependence of the reaction rate on the molar concentrations of the interacting 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 - molar concentrations of reagents, A and B.

If one of the reacting substances is in a solid state, then the interaction occurs at the interface, and therefore the concentration solid matter is not included in the equation of the kinetic law of mass action. 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 in the process of interaction they are destroyed chemical bonds reacting substances and new bonds of reaction products are formed, the nature of the bonds involved in the reaction of the compounds and the structure of the molecules of the reacting substances will play an important role.

Surface area of \u200b\u200bcontact of reagents

Such a characteristic as the surface area of \u200b\u200bcontact of solid reagents affects the course of the reaction, at times, quite significantly. Grinding a solid substance allows you to increase the surface area of \u200b\u200bcontact between the reagents, and therefore speed up the process. The contact area of \u200b\u200bsoluble substances is easily increased by dissolving the substance.

Reaction temperature

As the temperature rises, the energy of the colliding particles will increase; it is obvious that as the temperature rises, the chemical process itself will accelerate. The data given in the table can be considered an illustrative example of how an increase in temperature affects the process of interaction of substances.

Table 1. Influence 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, the acceleration occurs 2-4 times.

The mathematical formula describing the Van't Hoff rule is as follows:

Where γ is the temperature coefficient of the rate of a chemical reaction (γ \u003d 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.

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 activation energy value up to 40-50 kJ / mol allows substances to react with each other rather actively. If the activation energy exceeds 120 kJ / mol, then the substances (at normal 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 are capable of chemical transformations.

Catalyst action

A catalyst is a substance that can speed up the process, but is not part of its products. Catalysis (acceleration of chemical transformation) is divided into · homogeneous, · heterogeneous. If the reactants and the catalyst are in the same state of aggregation, then the catalysis is called homogeneous, if in different, then heterogeneous. The mechanisms of action of catalysts are varied and rather complex. In addition, it should be noted that catalysts are characterized by selectivity of action. That is, one and the same catalyst, while accelerating one reaction, may not change the speed of another in any way.

Pressure

If gaseous substances participate in the transformation, then the change in pressure in the system will affect the rate of the process ... This is becausethat 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 experimentally the rate of the chemical transformation by obtaining data on how the concentration of the reacting substances or products changes per unit time. Methods for obtaining such data are divided into

• chemical,
• physical and chemical.

Chemical methods are simple, affordable, and accurate. With their help, the rate is determined by directly measuring the concentration or amount of a substance of reagents or products. In the case of a slow reaction, samples are taken to control how the reagent is consumed. Then the content of the reagent in the sample is determined. By taking samples 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 must be stopped. This can be done with cooling, abrupt catalyst removal, you can also make dilution or transfer one of the reagents to an inactive state.

The 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 concentration of substances in real time. In this case, the reaction does not need to be stopped and samples taken.

Physicochemical methods are based on measurement physical properties, depending on the quantitative content in the system of a certain compound and changing over time. For example, if gases are involved in the reaction, then this property may be pressure. They also measure electrical conductivity, refractive index, absorption spectra of substances.

§ 12. KINETICS OF ENZYMATIVE REACTIONS

The kinetics of enzymatic reactions is the science of the rates of enzymatic reactions, their dependence on various factors. The rate of an enzymatic reaction is determined by the chemical amount of the reacted substrate or the resulting reaction product per unit time per unit volume under certain conditions:

where v is the rate of the enzymatic reaction, is the change in the concentration of the substrate or reaction product, t is the time.

The speed of the enzymatic reaction depends on the nature of the enzyme, which determines its activity. The higher the enzyme activity, the higher the reaction rate. Enzyme activity is determined by the rate of the reaction catalyzed by the enzyme. The measure of enzyme activity is one standard unit of enzyme activity. One standard unit of enzyme activity is the amount of enzyme that catalyzes the conversion of 1 μmol of substrate in 1 minute.

In the course of the enzymatic reaction, the enzyme (E) interacts with the substrate (S), as a result, an enzyme-substrate complex is formed, which then decomposes with the release of the enzyme and the product (P) of the reaction:

The speed of the enzymatic reaction depends on many factors: on the concentration of the substrate and enzyme, temperature, pH of the medium, the presence of various regulatory substances that can increase or decrease the activity of enzymes.

Interesting to know! Enzymes are used in medicine to diagnose various diseases. With myocardial infarction due to damage and decay of the heart muscle in the blood, the content of the enzymes aspartate transaminase and alanine aminotransferase increases sharply. Revealing their activity allows you to diagnose this disease.

Influence of substrate and enzyme concentration on the rate of enzymatic reaction

Let us consider the effect of the substrate concentration on the rate of the enzymatic reaction (Fig. 30.). At low substrate concentrations, the rate is directly proportional to its concentration; then, with increasing concentration, the reaction rate increases more slowly, and at very high substrate concentrations, the rate is practically independent of its concentration and reaches its maximum value (V max). At such concentrations of the substrate, all enzyme molecules are part of the enzyme-substrate complex, and complete saturation of the active centers of the enzyme is achieved, which is why the reaction rate in this case is practically independent of the substrate concentration.

Figure: 30. Dependence of the rate of the enzymatic reaction on the concentration of the substrate

The graph of the dependence of the enzyme activity on the substrate concentration is described by the Michaelis-Menten equation, which got its name in honor of the outstanding scientists L. Michaelis and M. Menten, who made a great contribution to the study of the kinetics of enzymatic reactions,

where v is the speed of the enzymatic reaction; [S] is the concentration of the substrate; K M - Michaelis constant.

Consider the physical meaning of the Michaelis constant. Provided that v \u003d ½ V max, we obtain K M \u003d [S]. Thus, the Michaelis constant is equal to the substrate concentration at which the reaction rate is half the maximum.

The speed of the enzymatic reaction also depends on the concentration of the enzyme (Fig. 31). This relationship is straightforward.

Figure: 31. Dependence of the rate of the enzymatic reaction on the concentration of the enzyme

Effect of temperature on the rate of enzymatic reaction

The temperature dependence of the enzymatic reaction rate is shown in Fig. 32.

Figure: 32. Dependence of the rate of the enzymatic reaction on temperature.

At low temperatures (up to approximately 40-50 ° C), an increase in temperature for every 10 ° C in accordance with the Van't Hoff rule is accompanied by an increase in the rate of the chemical reaction by 2 to 4 times. At high temperatures above 55-60 ° C, the enzyme activity decreases sharply due to its thermal denaturation, and, as a consequence, a sharp decrease in the rate of the enzymatic reaction is observed. The maximum activity of enzymes is usually observed in the range of 40 - 60 o C. The temperature at which the activity of the enzyme is maximum is called the temperature optimum. The optimum temperature for enzymes of thermophilic microorganisms is in the region of higher temperatures.

Effect of pH on the rate of enzymatic reaction

The graph of the dependence of enzymatic activity on pH is shown in Fig. 33.

Figure: 33. Effect of pH on the rate of enzymatic reaction

The graph of dependence on pH is bell-shaped. The pH value at which the enzyme activity is maximal is called pH optimum enzyme. The optimum pH values \u200b\u200bfor different enzymes vary widely.

The nature of the dependence of the enzymatic reaction on pH is determined by the fact that this indicator affects:

a) ionization of amino acid residues involved in catalysis,

b) ionization of the substrate,

c) the conformation of the enzyme and its active center.

Enzyme inhibition

The speed of the enzymatic reaction can be reduced by the action of a number of chemicals called inhibitors... Some inhibitors are poisons to humans, such as cyanides, while others are used as medicines.

Inhibitors can be classified into two main types: irreversible and reversible... Irreversible inhibitors (I) bind to the enzyme with the formation of a complex, the dissociation of which with the restoration of the enzyme activity is impossible:

An example of an irreversible inhibitor is diisopropyl fluorophosphate (DFP). DPP inhibits the enzyme acetylcholinesterase, which plays an important role in the transmission of nerve impulses. This inhibitor interacts with the serine of the active site of the enzyme, thereby blocking the activity of the latter. As a result, the ability of the processes of nerve cells of neurons to conduct a nerve impulse is disrupted. DFF is one of the first nerve agents. On its basis, a number of relatively non-toxic for humans and animals have been created. insecticides -substances poisonous to insects.

Reversible inhibitors, unlike irreversible ones, can be easily separated from the enzyme under certain conditions. At the same time, the activity of the latter is restored:

Among the reversible inhibitors, there are competitive and non-competitive inhibitors.

The competitive inhibitor, being a structural analog of the substrate, interacts with the active site of the enzyme and thus blocks the substrate's access to the enzyme. In this case, the inhibitor does not undergo chemical transformations and binds reversibly to the enzyme. After the dissociation of the EI complex, the enzyme can bind either to the substrate and transform it, or to an inhibitor (Fig. 34). Since both the substrate and the inhibitor compete for a site in the active site, this inhibition is called competitive.

Figure: 34. Mechanism of action of a competitive inhibitor.

Competitive inhibitors are used in medicine. For the fight against infectious diseases, sulfa drugs were previously widely used. They are close in structure to para-aminobenzoic acid (PABA), an essential growth factor for many pathogenic bacteria. PABA is a precursor of folate, which is a cofactor for a number of enzymes. Sulfanilamide drugs act as a competitive inhibitor of enzymes for the synthesis of folic acid from PABA and thereby inhibit the growth and reproduction of pathogenic bacteria.

Structurally, noncompetitive inhibitors are not similar to the substrate and, during the formation of EI, they interact not with the active center, but with another site of the enzyme. The interaction of the inhibitor with the enzyme leads to a change in the structure of the latter. The formation of the EI complex is reversible; therefore, after its degradation, the enzyme is again capable of attacking the substrate (Fig. 35).

Figure: 35. Mechanism of action of a non-competitive inhibitor

Cyanide CN - can act as a noncompetitive inhibitor. It binds to metal ions that are part of prosthetic groups and inhibits the activity of these enzymes. Cyanide poisoning is extremely dangerous. They can be fatal.

Allosteric enzymes

The term "allosteric" comes from the Greek words allo - other, stereo - plot. Thus, allosteric enzymes, along with the active center, have another center, called allosteric center (fig. 36). Substances that can change the activity of enzymes bind to the allosteric center, these substances are called allosteric effectors... Effectors are positive - activating the enzyme, and negative - inhibitory, i.e. reducing the activity of the enzyme. Some allosteric enzymes can be exposed to two or more effectors.

Figure: 36. The structure of the allosteric enzyme.

Regulation of multienzyme systems

Some enzymes act in concert, combining into multienzyme systems, in which each enzyme catalyzes a specific stage of the metabolic pathway:

In a multienzyme system, there is an enzyme that determines the rate of the entire sequence of reactions. This enzyme is usually allosteric and is located at the beginning of the metabolic pathway. He is able, receiving various signals, both to increase and decrease the rate of the catalyzed reaction, thereby regulating the rate of the entire process.