Speed ​​reaction is determined by a change in the molar concentration of one of the reactants:

V = ± ((C 2 - C 1) / (t 2 - t 1)) = ± (DC / Dt)

Where C 1 and C 2 are the molar concentrations of substances at times t 1 and t 2, respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - by the starting substance).

Reactions occur when molecules of reacting substances collide. Its speed is determined by the number of collisions and the likelihood that they will lead to transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors influencing the rate of chemical reactions.
1. The nature of the reacting substances. Character plays a big role chemical bonds and the structure of reagent molecules. Reactions proceed in the direction of destruction of less strong bonds and the formation of substances with stronger bonds. Thus, breaking bonds in H 2 and N 2 molecules requires high energies; such molecules are slightly reactive. Breaking bonds in highly polar molecules (HCl, H 2 O) requires less energy, and the reaction rate is much higher. Reactions between ions in electrolyte solutions occur almost instantly.
Examples
Fluorine reacts with hydrogen explosively at room temperature, bromine reacts with hydrogen slowly and when heated.
Calcium oxide reacts with water vigorously, releasing heat; copper oxide - does not react.

2. Concentration. With increasing concentration (the number of particles per unit volume), collisions of molecules of reacting substances occur more often - the reaction rate increases.
Law of mass action (K. Guldberg, P. Waage, 1867)
Speed chemical reaction is directly proportional to the product of the concentrations of the reacting substances.

AA + bB + . . . ® . . .

• [A] a [B] b . . .

The reaction rate constant k depends on the nature of the reactants, temperature and catalyst, but does not depend on the concentrations of the reactants.
The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions, the concentration of the solid phase is not included in the expression of the reaction rate.

3. Temperature. For every 10°C increase in temperature, the reaction rate increases by 2-4 times (van't Hoff's rule). As the temperature increases from t 1 to t 2, the change in reaction rate can be calculated using the formula:

(where Vt 2 and Vt 1 are the reaction rates at temperatures t 2 and t 1, respectively; g- temperature coefficient this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:

• e -Ea/RT

Where
A is a constant depending on the nature of the reactants;
R is the universal gas constant;

Ea is the activation energy, i.e. the energy that colliding molecules must have in order for the collision to lead to a chemical transformation.
Energy diagram of a chemical 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. Contact surface of reacting substances. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction occurs. The surface area 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 speed, remaining unchanged at the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. At homogeneous catalysis the reagents and the catalyst constitute one phase (are in the same state of aggregation), with heterogeneous catalysis- different phases (are in different states of aggregation). In some cases, the occurrence of undesirable chemical processes can be sharply slowed down by adding inhibitors to the reaction medium (the phenomenon " negative catalysis").

§ 12. KINETICS OF ENZYMATIVE REACTIONS

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 time.

The rate of an enzymatic reaction depends on the nature of the enzyme, which determines its activity. The higher the enzyme activity, the faster the reaction rate. Enzyme activity is determined by the rate of 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.

During an enzymatic reaction, the enzyme (E) interacts with the substrate (S), resulting in the formation of an enzyme-substrate complex, which then disintegrates to release the enzyme and product (P) of the reaction:

The speed of the enzymatic reaction depends on many factors: the concentration of the substrate and enzyme, temperature, pH of the environment, 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. During myocardial infarction, due to damage and breakdown of the heart muscle, the content of the enzymes aspartate transaminase and alanine aminotransferase in the blood increases sharply. Detection of their activity makes it possible to diagnose this disease.

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

Let's consider the effect of substrate concentration on the rate of the enzymatic reaction (Fig. 30). At low concentrations of the substrate, the speed is directly proportional to its concentration; then, with increasing concentration, the reaction rate increases more slowly, and at very high concentrations of the substrate, the speed is practically independent of its concentration and reaches its maximum value (V max). At such substrate concentrations, 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.

Rice. 30. Dependence of the speed of an enzymatic reaction on the concentration of the substrate

The graph of the dependence of enzyme activity on substrate concentration is described by the Michaelis–Menten equation, which received its name in honor of the outstanding scientists L. Michaelis and M. Menten, who introduced huge contribution in the study of the kinetics of enzymatic reactions,

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

Let's consider physical meaning Michaelis constants. Provided that v = ½ V max , we obtain 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 dependence is straightforward.

Rice. 31. Dependence of the speed of an enzymatic reaction on the concentration of the enzyme

Effect of temperature on the rate of enzymatic reaction

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

Effect of pH on the rate of enzymatic reaction

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

Rice. 33. The influence of pH on the rate of enzymatic reaction

The graph of pH is bell-shaped. The pH value at which enzyme activity 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) conformation of the enzyme and its active center.

Enzyme inhibition

The rate of an enzymatic reaction can be reduced by a number of chemical substances, called inhibitors. Some inhibitors are poisons for humans, for example, cyanide, others are used as medicines.

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 with restoration of enzyme activity is impossible:

An example of an irreversible inhibitor is diisopropyl fluorophosphate (DFP). DPP inhibits the enzyme acetylcholinesterase, which plays important role in the transmission of nerve impulses. This inhibitor interacts with serine in the active center of the enzyme, thereby blocking the activity of the latter. As a result, the ability of the processes is impaired nerve cells neurons conduct nerve impulses. DPP is one of the first nerve agents. Based on it, a number of products that are 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 center 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 to the enzyme reversibly. After dissociation of the EI complex, the enzyme can contact either the substrate and convert it, or an inhibitor (Fig. 34.). Since both the substrate and the inhibitor compete for space at the active site, this inhibition is called competitive.

Rice. 34. Mechanism of action of a competitive inhibitor.

Competitive inhibitors are used in medicine. Sulfonamide drugs were previously widely used to combat infectious diseases. They are close in structure to para-aminobenzoic acid(PABA), an essential growth factor for many pathogenic bacteria. PABA is the predecessor folic acid, which serves as a cofactor for a number of enzymes. Sulfonamide 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.

Noncompetitive inhibitors are not structurally similar to the substrate and, when EI is formed, 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 breakdown, the enzyme is again able to attack the substrate (Fig. 35).

Rice. 35. Mechanism of action of a non-competitive inhibitor

Cyanide CN - can act as a non-competitive 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 Greek words allo – another, stereo – area. 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 - inhibiting, i.e. reducing enzyme activity. Some allosteric enzymes can be affected by two or more effectors.

Rice. 36. Structure of an 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 metabolite pathway. It is capable, by receiving various signals, of both increasing and decreasing the rate of the catalyzed reaction, thereby regulating the speed of the entire process.

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). Nature of 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, the molecules of the reacting substances 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 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 the reactants. Based on experimental material, the Norwegian scientists Guldberg and Waage and, independently of them, the Russian scientist Beketov in 1867 formulated the fundamental law 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 reacting substances to the power of their stoichiometric coefficients. For the general case:

the law of mass action has the form:

The recording of the law of mass action for a given reaction is called basic 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. During such reactions, their products, as they accumulate, react with each other to form the starting substances:

Forward reaction rate:

Feedback speed:

At the moment of equilibrium:

Hence the law of mass action in a state of equilibrium takes 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 when the temperature is exceeded. Let's consider this using the example of the interaction of hydrogen with oxygen.

2H 2 + O 2 = 2H 2 O

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

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

4) Effect of catalysts. The rate of chemical reactions can be controlled using catalysts– substances that change the rate of a reaction and remain after the reaction in unchanged quantities. Changing the rate of a reaction in the presence of a catalyst is called catalysis. Distinguish positive(reaction speed increases) and negative(reaction rate decreases) catalysis. Sometimes a catalyst is formed during a reaction; such processes are called autocatalytic. There are 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 environment, due to the absence of a clearly defined phase interface. Such processes are referred to as microheterogeneous catalysis.

Themes Unified State Exam codifier: Speed ​​reaction. Its dependence on various factors.

The rate of a chemical reaction shows how quickly a particular reaction occurs. Interaction occurs when particles collide in space. In this case, the reaction does not occur at every collision, but only when the particle has the appropriate energy.

Speed ​​reaction – the number of elementary collisions of interacting particles ending in a chemical transformation per unit of time.

Determining the rate of a chemical reaction is related to the conditions under which it is carried out. If the reaction homogeneous– i.e. products and reagents are in the same phase - then the rate of a chemical reaction is defined as the change in substance per unit time:

υ = ΔC / Δt.

If the reactants or products are in different phases, and the collision of particles occurs only at the phase boundary, 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 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 probably know from your physics course, temperature is a measure of the average kinetic energy of motion of particles of a substance. If we increase the temperature, then particles of any substance begin to move faster and, therefore, collide more often.

However, as the temperature increases, 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 increases, 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). The reaction rate is independent of 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 established that most reactions increase their speed approximately equally (about 2-4 times) when the temperature increases by 10 o C. Van't Hoff's rule sounds like this: an increase in temperature by 10 o 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 rate of a chemical reaction 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 – proportionality coefficient, reaction rate constant.

For example, for the reaction of ammonia formation:

N 2 + 3H 2 ↔ 2NH 3

The law of mass action looks like this:

- these are chemical substances participating in a chemical reaction, changing its speed and direction, but non-consumable during the reaction (at the end of the reaction, they do not change either in quantity or composition). An approximate mechanism for the operation of a catalyst for a reaction of type A + B can be chosen 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 specific path.

Based on 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 - 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 effective interaction with reagents. Heterogeneous catalysts include metals, zeolites - crystalline minerals of the aluminosilicate group (compounds of silicon and aluminum), 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 that bind active centers catalyst and blocking its operation ( catalytic poisons). They must be removed regularly by regenerating the catalyst.

In biochemical reactions, catalysts are very effective - enzymes. Enzymatic catalysts act highly efficiently and selectively, with 100% selectivity. Unfortunately, enzymes are very sensitive to increased temperature, acidity of the environment and other factors, so there are a number of limitations for the implementation of processes with enzymatic catalysis on an industrial scale.

Catalysts should not be confused with initiators process and inhibitors. For example, ultraviolet irradiation is necessary to initiate the radical reaction of methane chlorination. This is not a catalyst. Some radical reactions are initiated by peroxide radicals. These are also not catalysts.

Inhibitors- These 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, on the contrary. Reverse catalysis is impossible in principle - the reaction will in any case try to follow the fastest path.

5. Contact area of ​​reacting substances. 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 contact surface area of ​​reacting substances, they use fluidized bed method. For example, in the production of sulfuric acid by the boiling donkey method, pyrites are fired.

6. Nature of reactants . The rate of chemical reactions, other things being equal, is also influenced by Chemical properties, i.e. nature of the reacting substances. 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 enter into chemical reactions much easier and more often.

At low activation energies (less than 40 kJ/mol), the reaction occurs very quickly and easily. A significant part of 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 result in a chemical transformation. The rate of such reactions is negligible. For example, nitrogen practically does not interact with oxygen at normal conditions.

At average activation energies (from 40 to 120 kJ/mol), the reaction rate will be average. Such reactions also occur under normal conditions, but not very quickly, so that they can be observed with the naked eye. Such reactions include the interaction of sodium with water, the interaction of iron with hydrochloric acid and etc.

Substances that are stable under normal conditions usually have high activation energies.

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

The speed of a chemical reaction is influenced by the following factors:

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

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

• Reactions occur when particles of reactants that have a certain energy collide.
• The more reactant particles there are, the closer they are to each other, the more likely they are to collide and react.
• Only effective collisions lead to a reaction, i.e. those in which “old connections” are destroyed or weakened and therefore “new” ones can be formed. To do this, the particles must have sufficient energy.
• The minimum excess energy required for effective collision of reactant particles is called activation energy Ea.
• The activity of chemicals is manifested in the low activation energy of reactions involving them. The lower the activation energy, the higher the reaction rate. For example, in reactions between cations and anions, the activation energy is very low, so such reactions occur almost instantly

The influence of the concentration of reactants on the reaction rate

As the concentration of reactants increases, the reaction rate increases. In order for a reaction to occur, two chemical particles must come together, so the rate of the reaction 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 an increase in the reaction rate.

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

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

Law of mass action (LMA):

The rate of a chemical reaction is proportional to the product of the concentrations of the reacting substances, taken in powers equal to their coefficients in the reaction equation. (“effective mass” is a synonym for the modern concept of “concentration”)

aA +bB =cС +dD, Where k– reaction rate constant

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

Expressions for speeds various types reactions

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

Molecularity of the reaction is the minimum number of molecules involved in an elementary chemical process. Based on 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 ​​contact between substances, i.e. on the degree of grinding of substances and 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 between wood and air, splitting the log into chips, the burning rate will increase.
• Pyrophoric iron is poured onto a sheet of filter paper. During the fall, the iron particles become hot and set fire to the paper.

Effect of temperature on reaction rate

In the 19th century, the Dutch scientist Van't Hoff experimentally discovered that with an increase in temperature by 10 o C, the rates of many reactions increase by 2-4 times.

Van't Hoff's rule

For every 10 ◦ C increase in temperature, 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 ​​from 2 to 4.

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

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

How more E a specific reaction, so 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, this 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 Ea.

Effect of catalyst on reaction rate

You can change the rate of a reaction 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 speed, but at 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 .