The rate of chemical reaction depends on many factors, including the nature of the reactant substances, the concentration of reacting substances, the temperature, the presence of catalysts. Consider these factors.

1). Nature of reacting substances. If there is an interaction between substances with ion bond, the reaction proceeds faster than between substances with a covalent bond.

2.) Concentration of reactant substances. To have a chemical reaction, the collision of the molecules of reacting substances is necessary. That is, the molecules should be so close to each other so that the atoms of one particle felt on themselves the electric fields of the other. Only in this case, electrons will be possible and the corresponding rearrangements of atoms will be possible, as a result of which the molecules of new substances are formed. Thus, the rate of chemical reactions is proportional to the number of collisions, which occurs between molecules, and the number of collisions, in turn, is proportional to the concentration of reacting substances. Based on the experimental material, Norwegian scientists Guldberg and Vaug and independently of them, Russian scientists of Beketov in 1867 formulated the main law of chemical kinetics - the law of acting masses (ZDM): At a constant temperature, the rate of the chemical reaction is directly proportional to the product of the concentrations of the reactant substances into the degree of their stoichiometric coefficients. For general case:

the law of the existing masses has the form:

The record of the law of the active masses for this reaction is called the main kinetic reaction equation. Basically, the kinetic equation K is a reaction rate constant, which depends on the nature of the reacting substances and temperature.

Most chemical reactions are reversible. During such reactions, their products are reacting with each other to the formation of starting materials:

Direct reaction speed:

Reverse reaction rate:

At the time of equilibrium:

Hence the law of the active masses in the equilibrium state takes the form:

where k is the reaction equilibrium constant.

3) The effect of temperature on the reaction rate. The speed of chemical reactions, as a rule, is increasing when the temperature is exceeded. Consider this on the example of the interaction of hydrogen with oxygen.

2N 2 + O 2 \u003d 2N 2

At 20 0, the reaction rate is almost equal to zero and it would take 54 billion years so that the interaction passed by 15%. At 500 0 C for water formation, 50 minutes will need, and at 700 0, the reaction processes instantly.

The dependence of the reaction rate on temperature is expressed vant-Gooff Rule: With an increase in temperature, 10 o reaction rate increases in 2-4 times. The rule of Vant-Hoffa is recorded:

4) Influence of catalysts. Chemical reaction speed can be adjusted using catalysts - substances that change the rate of reaction and remaining after the reaction in a constant quantity. Changing the reaction rate in the presence of a catalyst is called catalysis. Distinguish positive (the reaction rate increases) and negative (The reaction rate decreases) catalysis. Sometimes the catalyst is formed during the reaction, such processes are called autocatalytic. Distinguish homogeneous and heterogeneous catalysis.

For homogenic Catalysis catalyst and reactant substances are in the same phase. For example:

For heterogeneous Catalysis catalyst and reactant substances are in different phases. For example:

Heterogeneous catalysis is associated with enzymatic processes. All chemical processes occurring in living organisms are catalyzed by enzymes, which are proteins with certain specialized features. In solutions in which enzymatic processes are coming, there is no typical heterogeneous medium due to the lack of a clearly pronounced surface of the phase partition. Such processes refer to microheterogenic catalysis.

The mechanisms of leakage of chemical transformations and their speed studies the chemical kinetics. Chemical processes occur in time with different speeds. Some happening quickly, almost instantly, it takes a very long time for the flow of others.

In contact with

Speed \u200b\u200breaction - The rate of which reagents is consumed (their concentration is reduced) or the reaction products are formed in a unit of volume.

Factors that can affect the rate of chemical reaction

To how fast chemical interaction will occur, the following factors may affect:

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

Thus, changing certain conditions for the flow of the chemical process, it is possible to affect how quickly the process will flow.

In the process of chemical interaction, the particles of reacting substances are faced 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 proportionately molar concentrations of reagents.

The law of acting masses Describes the dependence of the reaction rate from the molar concentrations of substances entering into interaction.

For an elementary reaction (A + V → ...), this law is expressed by the formula:

υ \u003d k ∙ with a ∙ s b,

where k is a speed constant; C A and C B - molar concentrations of reagents, A and V.

If one of the reactors is in a solid state, the interaction occurs on the surface of the phase partition, in connection with this, the concentration of the solid substance is not included in the equation of the kinetic law of the existing mass. To understand the physical meaning of the velocity constant, it is necessary to accept C, A and C equal to 1. Then it becomes clear that the rate constant is equal to the reaction rate at concentrations of reagents equal to one.

Nature reagents

Since the chemical bonds of reacting substances are destroyed in the process of interaction and new bonds of the reaction products are formed, the nature of the connections participating in the reaction of compounds and the structure of the reacting substances molecules will be played.

Surface surface of the contact of reagents

This characteristic as the surface area of \u200b\u200bthe surface of solid reagents is affected by the reaction, sometimes quite significantly. Grinding a solid substance allows you to increase the surface area of \u200b\u200bthe contact of the reagents, and therefore accelerate the course of the process. The area of \u200b\u200bcontact of soluble substances is easily increased by dissolving the substance.

Reaction temperature

With increasing temperature, the energy of the encountered particles will increase, it is obvious that with increasing temperature and the chemical process will be accelerated. A visual example of how the increase in temperature affects the process of interaction of substances, you can read the data given in the table.

Table 1. Effect of temperature change on the water formation rate (O 2 + 2N 2 → 2N 2 O)

For a quantitative description of how the temperature can affect the rate of interaction of the substances use the Vant-Gooff rule. The Vant-Hoffa rule is that with an increase in temperature by 10 degrees, acceleration occurs 2-4 times.

The mathematical formula describing the Vant-Gooff rule is as follows:

Where γ is the temperature coefficient of the chemical reaction rate (γ \u003d 2-4).

But much more accurately describes the temperature dependence of the velocity constant Equation of Arrhenius:

Where R is a universal gas constant, A is a multiplier determined by the reaction, E, A - activation energy.

The activation energy is called such an energy that molecule must acquire, so that the chemical transformation occur. That is, it is a kind of energy barrier, which will be necessary to overcome molecules encountered in the reaction volume to redistribute connections.

The activation energy does not depend on external factors, and depends on the nature of the substance. The value of activation energy up to 40 - 50 kJ / mol allows substances to respond with each other quite actively. If activation energy exceeds 120 kJ / molThe substances (at normal temperatures) will react very slowly. The change in temperature leads to a change in the number of active molecules, that is, molecules that have reached the energy greater than the activation energy, which means that are capable of chemical transformations.

Catalyst action

The catalyst is called a substance capable of accelerating the process, but not part of its products. Catalysis (acceleration of chemical transformation) is separated by · homogeneous, · heterogeneous. If reagents and catalyst are in the same aggregate states, the catalysis is called homogeneous, if in various, then heterogeneous. The mechanisms of the action of catalysts are diverse and quite complex. In addition, it is worth noting that the selection of action is characterized by catalysts. That is, the same catalyst, accelerating one reaction, may not change the speed of the other.

Pressure

If gaseous substances are involved in the transformation, then the process of pressure changes in the system will influence the process rate . This is becauseThat for gaseous reagents, the change in pressure leads to a change in concentration.

Experimental determination of the speed of the chemical reaction

It is possible to determine the speed of the chemical transformation experimentally, having obtained data on how the concentration of substances entering into the reaction, or products changes per unit of time. Methods for obtaining such data are divided into

• chemical
• physico-chemical.

Chemical methods are quite simple, available and accurate. With their help, the speed is determined, directly measuring the concentration or the amount of substance of reagents or products. In the case of a slow reaction, to control how the reagent is consumed by samples. After that determine the content in the reagent sample. After selecting samples at equal periods of time, it is possible to obtain data on the change in the amount of substance in the process of interaction. Most often use such types of analysis as titrimimetry and gravimetry.

If the reaction proceeds quickly, then to select the sample, it has to be stopped. This can be done by cooling, sharp removal of catalystYou can also dilute or translate one of the reagents to non-reactive state.

Methods of physicochemical analysis in modern experimental kinetics are used more often than chemical. With their help, it is possible to observe the change in the concentrations of substances in real time. At the same time, the reaction is not necessary to stop and select samples.

Physico-chemical methods are based on the measurement of the physical property depending on the quantitative content in the system of a particular connection and varying with time. For example, if gases are involved in the reaction, then this property may be pressure. Also measured electrical conductivity, refractive index, absorption spectra of substances.

Some chemical reactions occur almost instant (the explosion of the oxygen-hydrogen mixture, the reaction of the ion exchange in aqueous solution), the second - quickly (combustion of substances, the interaction of zinc with acid), the third is slow (iron rusting, rotting organic residues). There are so slow reactions that the person simply cannot see them. For example, the transformation of granite in sand and clay occurs over thousands of years.

In other words, chemical reactions can flow with different speed.

But what is speed reaction? What is the exact definition of this value and, most importantly, its mathematical expression?

The reaction rate is called a change in the amount of substance per unit of time in one unit of volume. Mathematically, this expression is written as:

Where n 1 and N 2 - the amount of substance (mole) at time t 1 and t 2, respectively, in the system volume V..

What sign plus or minus (±) will stand before expressing speed depends on the change in the number of which substances we look - the product or reagent.

It is obvious that during the reaction there is a consumption of reagents, that is, their number decreases, therefore, for reagents, the expression (N 2 - N 1) always matters less than zero. Since the speed cannot be a negative value, in this case before the expression you need to put a "minus" sign.

If we look at changing the amount of product, not a reagent, then before the expression for calculating the speed sign "minus" is not required, since the expression (N 2 - N 1) is always positive in this case, because The amount of product as a result of the reaction can only increase.

The ratio of the amount of substance n. To the volume in which this amount of substance is called the molar concentration FROM:

Thus, using the concept of molar concentration and its mathematical expression, you can write another option for determining the reaction speed:

The reaction rate is called the change in the molar concentration of the substance as a result of the flow of a chemical reaction for one time unit:

Factors affecting the reaction rate

It is often extremely important to know from which the speed of one or another reaction depends and how to influence it. For example, the oil refining industry literally beats for each additional half-circuit of the product per unit of time. After all, given the enormous amount of oil processed, even half apler flows into a major financial annual profit. In some cases, it is extremely important to slow down any reaction, in particular corrosion of metals.

So what does the reaction rate depend on? It depends, oddly enough, from many different parameters.

In order to understand this issue, first of all, let us imagine what happens as a result of a chemical reaction, for example:

The equation written above reflects the process in which molecules of substances A and B, facing each other, form molecules of substances C and D.

That is, undoubtedly, in order for the reaction to pass, at least, it is necessary to collide the molecules of the starting materials. Obviously, if we increase the amount of molecules in a unit of volume, the number of collisions will increase in the same way as the frequency of your collisions with passengers in a crowded bus will increase compared to the semi-empty.

In other words, the reaction rate increases with increasing concentration of reacting substances.

In the case when one of the reagents or several are immediately gases, the reaction rate increases with the increase in pressure, since the gas pressure is always directly proportional to the concentration of its molecules.

However, the collision of particles is necessary, but not at all in sufficient condition of the reaction. The fact is that according to the calculations, the number of collisions of molecules of reacting substances at their reasonable concentration is so large that all reactions should flow in an instant. However, in practice it does not happen. What is the matter?

The fact is that not all collision of the reagents molecules will necessarily be effective. Many collisions are elastic - molecules bounce apart from each other like balls. In order for the reaction to pass, molecules must have sufficient kinetic energy. The minimum energy to which molecules of the reacting substances should have to ensure that the reaction passes is called the activation energy and is indicated as e a. In a system consisting of a large number of molecules, there is a distribution of energy molecules, some of them have low energy, part high and middle. Of all these molecules, only a small part of the energy molecules exceeds the activation energy.

As is known from the course of physics, the temperature in fact is the measure of the kinetic energy of the particles, of which the substance consists. That is, the faster the particles are moving, the component of the substance, the higher its temperature. Thus, it is obviously increasing the temperature in fact, we increase the kinetic energy of molecules, as a result of which the proportion of molecules with an energy exceeding E A and their collision will lead to a chemical reaction.

The fact of the positive effect of the temperature on the rate of flow of reaction back in the 19th century empirically set the Dutch chemist Vant Hoff. Based on the studies held by him, he formulated the rule that still carries his name, and it sounds as follows:

The rate of any chemical reaction increases 2-4 times with an increase in temperature by 10 degrees.

Mathematical display of this rule is written as:

where V 2. and V 1. - Speed \u200b\u200bat a temperature T 2 and T 1, respectively, and γ is the temperature coefficient of the reaction, the value of which most often lies in the range from 2 to 4.

Often the speed of many reactions can be enhanced using catalysts.

Catalysts - substances accelerating any reaction and are not consistent.

But how does the catalyst manage to increase the reaction rate?

Recall the activation energy E a. Molecules with energy are less than activation energy in the absence of a catalyst can not interact with each other. Catalysts, change the path through which the reaction flows is just as an experienced conductor will pave the expedition route not directly through the mountain, but with the help of a bypass trail, with the result that even those satellites that did not have enough energy to climb the mountain will be able to move to another Her side.

Despite the fact that the catalyst is not spent during the reaction, nevertheless it takes an active part in it, forming intermediate compounds with reagents, but by the end of the reaction returns to its original state.

In addition to the above factors affecting the reaction rate, if there is a separation boundary (heterogeneous reaction) between the reactors, the reaction rate will depend on the area of \u200b\u200bcontact of the reagents. For example, imagine a granule of metal aluminum, which was thrown into the tube with an aqueous solution of hydrochloric acid. Aluminum is an active metal that is able to react with acids with non-oxidants. With hydrochloric acid, the reaction equation is as follows:

2Al + 6HCl → 2AlCl 3 + 3H 2

Aluminum is a solid, and this means that the reaction with hydrochloric acid goes only on its surface. Obviously, if we increase the surface area, after having rolled the aluminum granules in Foil, we thereby provide a greater number of aluminum atoms available for the reaction with acid. As a result, the reaction rate will increase. Similarly, the increase in the surface of the solid can be achieved by grinding it into powder.

Also, the speed of the heterogeneous reaction, in which the solid reacts with gaseous or liquid reacts, often has a positive effect, which is due to the fact that, as a result of the mixing, removal from the reaction zone accumulating molecules of reaction products is achieved and the new portion of the reagent molecules is reached.

The latter should also be noted a huge effect on the rate of reaction and the nature of reagents. For example, the lower in the Mendeleev table there is an alkaline metal, the faster it reacts with water, the fluorine among all halogen reacts the most quickly with gaseous hydrogen, etc.

Summarizing all of the above, the reaction rate depends on the following factors:

1) Concentration of reagents: the higher, the greater the reaction rate.

2) Temperature: with increasing temperature, the speed of any reaction increases.

3) The area of \u200b\u200bcontact of the reactant substances: the larger the area of \u200b\u200bcontact of the reagents, the higher the reaction rate.

4) Stirring if the reaction occurs honey with solid substance and liquid or gas mixing can speed it up.

§ 12. Kinetics of enzymatic reactions

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

where V is the speed of the enzymatic reaction, the change in the concentration of the substrate or the reaction product, T is time.

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

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

The rate of the enzymatic reaction depends on many factors: from the concentration of substrate and enzyme, temperature, pH of the medium, the presence of various regulators capable of increased or reduce 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 aspartattransaminase enzymes and alaninotransferase is sharply increasing. The identification of their activity allows you to diagnose this disease.

The effect of the concentration of the substrate and the enzyme for the rate of enzymatic reaction

Consider the influence of the substrate concentration on the rate of enzymatic reaction (Fig. 30.). At low concentrations of the substrate, the speed is directly proportional to its concentration, further with increasing concentration, the reaction rate increases slower, and at very high concentrations of the substrate, the speed almost does not depend on its concentration and reaches its maximum value (V MAX). With such concentrations of the substrate, all enzyme molecules are located as part of an enzyme-substrate complex, and a complete saturation of the active enzyme centers is achieved, which is why the reaction rate in this case is practically independent of the concentration of the substrate.

Fig. 30. The dependence of the rate of enzymatic reaction from the substrate concentration

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

where V is the rate of enzymatic reaction; [S] - the concentration of the substrate; K M - Mikhailis Constant.

Consider the physical meaning of the constant of Michaelis. Provided that V \u003d ½ V MAX, we obtain k m \u003d [s]. Thus, the Mihaelis constant is equal to the substrate concentration, in which the reaction rate is equal to half the maximum.

The rate of the enzymatic reaction depends on the concentration of the enzyme (Fig. 31). This dependence is rectilinear.

Fig. 31. The dependence of the rate of enzymatic reaction from the enzyme concentration

The effect of temperature for the rate of enzymatic reaction

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

Fig. 32. The dependence of the speed of the enzymatic reaction on temperature.

At low temperatures (approximately 40 - 50 ° C), the temperature increases for every 10 ° C in accordance with the Vant-Hoff rule is accompanied by an increase in the speed of the chemical reaction in 2-4 times. At high temperatures, more than 55 - 60 o with the activity of the enzyme is sharply reduced due to its thermal denaturation, and, as a result, there is a sharp decrease in the rate of enzymatic reaction. The maximum activity of enzymes is usually observed within 40 - 60 o C. The temperature in which the activity of the enzyme is maximal, is called temperature optimum. The temperature optimality of the enzymes of thermophilic microorganisms is in the field of higher temperatures.

Influence of pH on the rate of enzymatic reaction

The graph of the dependence of enzymatic activity from the pH is presented in Fig. 33.

Fig. 33. Influence of pH for an enzymatic reaction rate

The pH dependence schedule has a bell-shaped form. The pH value in which the enzyme activity is maximum called pH-optimum enzyme. The pH-optimum values \u200b\u200bfor various enzymes fluctuate widely.

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

a) ionization of amino acid residues involved in catalysis,

b) the ionization of the substrate,

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

Inhibition of enzymes

The rate of enzymatic reaction can be reduced by a number of chemicals called inhibitors. Some inhibitors are poisons for a person, for example, cyanides, others - are used as medicinal preparations.

Inhibitors can be divided into two main types: irreversible and reversible. The irreversible inhibitors (i) are associated with the enzyme with the formation of the complex, the dissociation of which with the restoration of the enzyme activity is impossible:

An example of an irreversible inhibitor is diisopropylfluorphosphate (DFF). The DFF inhibits an enzyme acetylcholinesterase, which plays an important role in the transmission of the nerve impulse. This inhibitor interacts with the serine active center of the enzyme, thereby blocking the activity of the latter. As a result, the ability of neurons of neurons nerve cells is disturbed to carry out a nervous impulse. DFF is one of the first substances of neuro-paralytic action. Based on it created a number of relatively non-toxic for humans and animals insecticides -substances poisonous for insects.

Reversible inhibitors, in contrast to irreversible, under certain conditions can be easily separated from the enzyme. The activity of the latter is restored:

Among reversible inhibitors allocate competitive and non-competitive Inhibitors.

A competitive inhibitor, being a structural analogue of the substrate, interacts with the active center of the enzyme and thus overlaps the substrate access to the enzyme. In this case, the inhibitor is not subjected to chemical transformations and binds to the enzyme reversibly. After the dissociation of the EI complex, the enzyme can be contacted either with the substrate and convert it or with an inhibitor (Fig. 34.). Since the substrate and the inhibitor compete for a place in the active center, such inhibition is called competitive.

Fig. 34. The mechanism of action of the competitive inhibitor.

Competitive inhibitors are used in medicine. Sulfonamide preparations were previously widely used to combat infectious diseases. They are close in their structure to para-aminobenzoic acid (PABK), the necessary factor of the growth of many pathogenic bacteria. PABK is a precursor of folic acid, which serves as a cofactor of a number of enzymes. Sulfanimide drugs act as a competitive inhibitor of folic acid synthesis enzymes from PABK and thereby suppress the growth and reproduction of pathogenic bacteria.

Non-competitive inhibitors on the structure are not similar to the substrate and in the formation of EI interacts not with the active center, but with another part of the enzyme. The interaction of an inhibitor with an enzyme leads to a change in the structure of the latter. The formation of the EI complex is reversible, so after its decay the enzyme is again able to attack the substrate (Fig. 35).

Fig. 35. The mechanism of action of a non-competitive inhibitor

CN cyanide can act as a non-competitive inhibitor. It binds to metal ions belonging to the promotional groups and suppresses the activity of these enzymes. Cyanide poisoning is extremely dangerous. They can lead to a fatal outcome.

Alosteric enzymes

The term "Alosteric" comes from the Greek words Allo - the other, Stereo - plot. Thus, alto-solid enzymes along with an active center have another center called alosteric center (Fig. 36). A substances capable of changing the activity of enzymes are associated with the Alosteric Center, these substances call alosteric effectors. Effectors are positive - enzyme activating, and negative - inhibitory, i.e. Lowering the activity of the enzyme. Some alto-solid enzymes can be exposed to two or more effectors.

Fig. 36. Structure of Alosteric Enzyme.

Regulation of multimenme systems

Some enzymes act agreed, uniting in multimenimen systems in which each enzyme catalyzes a certain stage of the metabolic path:

The multimenza system has an enzyme that determines the speed of the entire reaction sequence. This enzyme is usually altogether and is at the beginning of the matabolic path. It is capable of obtaining various signals, both to increase and reduce the speed of the catalyzed reaction, thereby adjusting the speed of the entire process.

Question 1. What substances are called catalysts?

Substances that change the speed of the chemical reaction, remaining by the end of it unchanged, are called catalysts.

Question 2. What role do the enzymes play in the cell?

Enzymes - biological catalysts accelerating chemical reactions in a living cell. The molecules of alone enzymes consist only of proteins, others include protein and a compound of non-virological nature (organic - coenzyme or inorganic - ions of various metals). Enzymes are strictly specific: each enzyme catalyzes a certain type of reactions in which certain types of substrate molecules are involved.

Question 3. From which factors may depend on the rate of enzymatic reactions?

The rate of enzymatic reactions largely depends on the concentration of the enzyme, the nature of the substance, temperature, pressure, medium reaction (acid or alkaline).

In many enzymes under certain conditions, for example, in the presence of molecules of certain substances, the configuration of the active center changes, which allows them to provide the greatest enzymatic activity.

Question 4. Why most enzymes at high temperature loses catalytic properties?

High temperature medium, as a rule, causes protein denaturation, i.e. violation of its natural structure. Therefore, at high temperatures, most enzymes lose their catalytic properties.

Question 5. Why can the lack of vitamins cause violations in the processes of the body's life?

Many vitamins are part of enzymes. Therefore, the disadvantage in the body of vitamins leads to the weakening of the activity of enzymes in cells, and therefore, can cause violations in the processes of vital activity.

1.8. Biological catalysts

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