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.
§ 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, and t is the time.
The rate of an enzymatic reaction depends on the nature of the enzyme, which determines its activity. The higher the activity of the enzyme, the higher the rate of the reaction. Enzyme activity is determined by the rate of the reaction catalyzed by the enzyme. A 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.
During the enzymatic reaction, the enzyme (E) interacts with the substrate (S), resulting in the formation of an enzyme-substrate complex, which then decomposes with the release of the enzyme and the product (P) of the reaction:
The rate of an enzymatic reaction depends on many factors: 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. In myocardial infarction due to damage and decay of the heart muscle in the blood, the content of aspartate transaminase and alanine aminotransferase enzymes sharply increases. Identification of their activity allows diagnosing this disease.
Effect of Substrate and Enzyme Concentrations on the Enzymatic Reaction Rate
Consider the effect of 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 (Vmax). At such substrate concentrations, all enzyme molecules are part of the enzyme-substrate complex, and full 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.
Rice. 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 concentration of the substrate 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 rate of the enzymatic reaction; [S] is the substrate concentration; K M is the Michaelis constant.
Let us consider the physical meaning of the Michaelis constant. Provided that v = ½ V max , we get K M = [S]. Thus, the Michaelis constant is equal to the substrate concentration at which the reaction rate is half the maximum.
The rate of the enzymatic reaction also depends on the concentration of the enzyme (Fig. 31). This relationship is linear.
Rice. 31. Dependence of the rate of the enzymatic reaction on the concentration of the enzyme
The effect of temperature on the rate of the enzymatic reaction
The dependence of the rate of the enzymatic reaction on temperature is shown in fig. 32.
Rice. 32. Dependence of the rate of 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 a chemical reaction by 2 - 4 times. At high temperatures above 55 - 60 ° C, the activity of the enzyme decreases sharply due to its thermal denaturation, and, as a result, 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 temperature optimum of the enzymes of thermophilic microorganisms is in the region of higher temperatures.
The effect of pH on the rate of an enzymatic reaction
The graph of the dependence of enzymatic activity on pH is shown in fig. 33.
Rice. 33. Influence of pH on the rate of an enzymatic reaction
The pH-dependence graph is bell-shaped. The pH value at which the activity of the enzyme is maximum is called pH optimum enzyme. The pH optimum values for various 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 site.
Enzyme inhibition
The rate of an enzymatic reaction can be reduced by the action of a number of chemicals called inhibitors. Some inhibitors are poisonous to humans, such as cyanides, while others are used as drugs.
Inhibitors can be divided into two main types: irreversible And reversible. Irreversible inhibitors (I) bind to the enzyme to form a complex, the dissociation of which is impossible with the restoration of enzyme activity:
An example of an irreversible inhibitor is diisopropylfluorophosphate (DFF). DPP inhibits the enzyme acetylcholinesterase, which plays an important role in nerve impulse transmission. 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 impaired. DFF is one of the first nerve agents. Based on it, 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. The activity of the latter is restored:
Reversible inhibitors include competitive And non-competitive inhibitors.
A competitive inhibitor, being a structural analogue of the substrate, interacts with the active site of the enzyme and thus blocks the access of the substrate to the enzyme. In this case, the inhibitor does not undergo chemical transformations and binds to the enzyme reversibly. After the dissociation of the EI complex, the enzyme can bind either to the substrate and transform it, or to the inhibitor (Fig. 34.). Since both the substrate and the inhibitor compete for a place in the active site, this inhibition is called competitive.
Rice. 34. The mechanism of action of a competitive inhibitor.
Competitive inhibitors are used in medicine. Sulfanilamide preparations were previously widely used to combat infectious diseases. They are structurally close to para-aminobenzoic acid(PABA), an essential growth factor for many pathogenic bacteria. PABA is a precursor of folic acid, which serves as a cofactor for a number of enzymes. Sulfanilamide preparations act as a competitive inhibitor of the enzymes for the synthesis of folic acid from PABA and thereby inhibit the growth and reproduction of pathogenic bacteria.
Non-competitive inhibitors are not structurally similar to the substrate, and during the formation of EI they interact not with the active site, but with another site 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; therefore, after its breakdown, the enzyme is again able to attack the substrate (Fig. 35).
Rice. 35. Mechanism of action of a non-competitive inhibitor
CN - cyanide can act as a non-competitive inhibitor. It binds to metal ions that are part of the 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 - another, stereo - area. Thus, allosteric enzymes, along with the active site, have another center called allosteric center(Fig. 36). Substances capable of changing 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 enzyme activity. Some allosteric enzymes may be affected by two or more effectors.
Rice. 36. Structure of an allosteric enzyme.
Regulation of multienzyme systems
Some enzymes act in concert, uniting into multi-enzyme systems, in which each enzyme catalyzes a certain stage of the metabolic pathway:
In a multienzyme system, there is an enzyme that determines the rate of the entire reaction sequence. This enzyme, as a rule, is allosteric and is located at the beginning of the matabolic 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.
The rate of a chemical reaction depends on many factors, including the nature of the reactants, the concentration of 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.) Reactant concentration. In order for a chemical reaction to take place, the molecules of the reactants must collide. 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 the other. Only in this case will the transitions of electrons and the 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 the experimental material, the Norwegian scientists Guldberg and Waage 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 to the power of their stoichiometric coefficients. For the general case:
the law of mass action has the form:
The mass action law for a given reaction is called the main kinetic equation of the reaction. In the basic 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 starting substances:
Forward reaction rate:
Feedback rate:
At the time of equilibrium:
From here, the law of acting masses in a state of equilibrium will take the form:
where K is the equilibrium constant of the reaction.
3) The effect of temperature on the reaction rate. The rate of chemical reactions, as a rule, increases when the temperature is exceeded. 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 C, the reaction rate is almost zero and it would take 54 billion years for the interaction to pass by 15%. At 500 0 C, it takes 50 minutes to form water, and at 700 0 C, the reaction proceeds instantly.
The dependence of the reaction rate on temperature is expressed van't Hoff's rule: with an increase in temperature by 10 about the reaction rate increases by 2 - 4 times. Van't Hoff's rule is written:
4) Influence of catalysts. The rate of chemical reactions can be controlled by catalysts- substances that change the rate of the reaction and remain unchanged after the reaction. The change in the rate of a reaction in the presence of a catalyst is called catalysis. Distinguish positive(reaction rate increases) and negative(reaction rate decreases) catalysis. Sometimes the catalyst is formed during the reaction, such processes are called autocatalytic. Distinguish between homogeneous and heterogeneous catalysis.
At homogeneous In catalysis, the catalyst and reactants are in the same phase. For example:
At heterogeneous In catalysis, the catalyst and reactants 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 functions. In solutions in which enzymatic processes take place, there is no typical heterogeneous medium, due to the absence of a clearly defined phase interface. Such processes are referred to as microheterogeneous catalysis.
Question 1. What substances are called catalysts?
Substances that change the rate of a chemical reaction and remain unchanged at the end of it are called catalysts.
Question 2. What role do enzymes play in the cell?
Enzymes are biological catalysts that speed up chemical reactions in a living cell. Molecules of some enzymes consist only of proteins, others include protein and non-protein compounds (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 participate.
Question 3. On what factors can the rate of enzymatic reactions depend?
The rate of enzymatic reactions largely depends on the concentration of the enzyme, the nature of the substance, temperature, pressure, and the reaction of the medium (acidic or alkaline).
For 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 do most enzymes lose their catalytic properties at high temperatures?
The high temperature of the environment, as a rule, causes protein denaturation, i.e., a violation of its natural structure. Therefore, at high temperatures, most enzymes lose their catalytic properties.
Question 5. Why can a lack of vitamins cause disturbances in the vital processes of the body?
Many vitamins are part of enzymes. Therefore, a lack of vitamins in the body leads to a weakening of the activity of enzymes in cells, and therefore, can cause disturbances in vital processes.
1.8. Biological catalysts
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Topics of the USE codifier:Speed reaction. Its dependence on various factors.
The rate of a chemical reaction indicates how fast a reaction occurs. Interaction occurs when particles collide in space. In this case, the reaction does not occur with every collision, but only when the particles have the appropriate energy.
Speed reaction is the number of elementary collisions of interacting particles, ending in a chemical transformation, per unit of time.
Determination of the rate of a chemical reaction is associated with the conditions for its implementation. If the reaction homogeneous– i.e. products and reactants are in the same phase - then the rate of a chemical reaction is defined as the change in substance per unit time:
υ = ∆C / ∆t.
If the reactants or products are in different phases, and the collision of particles occurs only at the interface, then the reaction is called heterogeneous, and its speed is determined by the change in the amount of substance per unit time per unit of the reaction surface:
υ = Δν / (S Δt).
How to make particles collide more often, i.e. How increase the rate of a chemical reaction?
1. The easiest way is to increase temperature . As you must have known from your physics course, temperature is a measure of the average kinetic energy of the movement of particles of matter. If we raise the temperature, then the particles of any substance begin to move faster, and therefore collide more often.
However, with increasing temperature, the rate of chemical reactions increases mainly due to the fact that the number of effective collisions increases. As the temperature rises, the number of active particles that can overcome the energy barrier of the reaction sharply increases. If we lower the temperature, the particles begin to move more slowly, the number of active particles decreases, and the number of effective collisions per second decreases. Thus, When the temperature rises, the rate of a chemical reaction increases, and when the temperature decreases, it decreases..
Note!
This rule works the same for all chemical reactions (including exothermic and endothermic ones). The reaction rate does not depend on the thermal effect. The rate of exothermic reactions increases with increasing temperature and decreases with decreasing temperature. The rate of endothermic reactions also increases with increasing temperature, and decreases with decreasing temperature.Moreover, back in the 19th century, the Dutch physicist van't Hoff experimentally found that most reactions increase in approximately the same rate (by about 2-4 times) with an increase in temperature by 10 ° C. Van't Hoff's rule sounds like this: an increase in temperature by 10 ° C leads to an increase in the rate of a chemical reaction by 2-4 times (this value is called the temperature coefficient of the chemical reaction rate γ). The exact value of the temperature coefficient is determined for each reaction.
here v is the rate of the chemical reaction,
C A And C B — concentrations of substances A and B, respectively, mol/l
k is the coefficient of proportionality, the rate constant of the reaction.
For example, for the ammonia formation reaction:
N 2 + 3H 2 ↔ 2NH 3
The law of mass action looks like this:
- These are chemicals involved in a chemical reaction, changing its speed and direction, but not expendable during the reaction (at the end of the reaction, they do not change either in quantity or in composition). An approximate mechanism for the operation of a catalyst for a reaction of the type A + B can be depicted as follows:
A+K=AK
AK + B = AB + K
The process of changing the reaction rate when interacting with a catalyst is called catalysis. Catalysts are widely used in industry when it is necessary to increase the rate of a reaction or direct it along a certain path.
According to the phase state of the catalyst, homogeneous and heterogeneous catalysis are distinguished.
homogeneous catalysis - this is when the reactants and the catalyst are in the same phase (gas, solution). Typical homogeneous catalysts are acids and bases. organic amines, etc.
heterogeneous catalysis - this is when the reactants and the catalyst are in different phases. As a rule, heterogeneous catalysts are solids. Because interaction in such catalysts occurs only on the surface of the substance, an important requirement for catalysts is a large surface area. Heterogeneous catalysts are characterized by high porosity, which increases the surface area of the catalyst. Thus, the total surface area of some catalysts sometimes reaches 500 square meters per 1 g of catalyst. Large area and porosity ensure efficient interaction with reagents. Heterogeneous catalysts include metals, zeolites - crystalline minerals of the aluminosilicate group (silicon and aluminum compounds), and others.
Example heterogeneous catalysis - ammonia synthesis:
N 2 + 3H 2 ↔ 2NH 3
Porous iron with Al 2 O 3 and K 2 O impurities is used as a catalyst.
The catalyst itself is not consumed during the chemical reaction, but other substances accumulate on the surface of the catalyst, which bind the active centers of the catalyst and block its operation ( catalytic poisons). They must be removed regularly by regenerating the catalyst.
Catalysts are very effective in biochemical reactions. enzymes. Enzymatic catalysts act highly efficiently and selectively, with a selectivity of 100%. Unfortunately, enzymes are very sensitive to temperature increase, medium acidity, and other factors; therefore, there are a number of limitations for the industrial scale implementation of processes with enzymatic catalysis.
Catalysts should not be confused with initiators process and inhibitors. For example, to initiate a radical reaction of methane chlorination, ultraviolet irradiation is necessary. It's not a catalyst. Some radical reactions are initiated by peroxide radicals. They are also not catalysts.
Inhibitors are substances that slow down a chemical reaction. Inhibitors can be consumed and participate in a chemical reaction. In this case, inhibitors are not catalysts, vice versa. Reverse catalysis is impossible in principle - the reaction will in any case try to follow the fastest path.
5. Area of contact of reactants. For heterogeneous reactions, one way to increase the number of effective collisions is to increase reaction surface area . The larger the contact surface area of the reacting phases, the greater the rate of the heterogeneous chemical reaction. Powdered zinc dissolves much faster in acid than granular zinc of the same mass.
In industry, to increase the area of the contacting surface of the reactants, they use fluidized bed method. For example, in the production of sulfuric acid by the boiling layer method, pyrite is roasted.
6. The nature of the reactants . The rate of chemical reactions, other things being equal, is also influenced by chemical properties, i.e. the nature of the reactants. Less active substances will have a higher activation barrier and react more slowly than more active substances. More active substances have a lower activation energy, and are much easier and more likely to enter into chemical reactions.
At low activation energies (less than 40 kJ/mol), the reaction proceeds very quickly and easily. A significant part of the collisions between particles ends in a chemical transformation. For example, ion exchange reactions occur very quickly under normal conditions.
At high activation energies (more than 120 kJ/mol), only a small number of collisions end in a chemical transformation. The rate of such reactions is negligible. For example, nitrogen practically does not interact with oxygen under normal conditions.
At medium activation energies (from 40 to 120 kJ/mol), the reaction rate will be average. Such reactions also proceed under normal conditions, but not very quickly, so that they can be observed with the naked eye. These reactions include the interaction of sodium with water, the interaction of iron with hydrochloric acid, etc.
Substances that are stable under normal conditions tend to have high activation energies.