Effect of volume on reaction rate. The rate of a chemical reaction and factors influencing it. Measuring the speed of a process

We are constantly faced with various chemical interactions. The combustion of natural gas, the rusting of iron, the souring of milk - these are not all the processes that are studied in detail in a school chemistry course.

Some reactions take fractions of seconds to occur, while some interactions take days or weeks.

Let's try to identify the dependence of the reaction rate on temperature, concentration, and other factors. The new educational standard allocates a minimum amount of teaching time to this issue. The tests of the Unified State Exam include tasks on the dependence of the reaction rate on temperature, concentration, and even offer calculation problems. Many high school students experience certain difficulties in finding answers to these questions, so we will analyze this topic in detail.

Relevance of the issue under consideration

Information about the reaction rate has important practical and scientific significance. For example, in the specific production of substances and products, the productivity of equipment and the cost of goods directly depend on this value.

Classification of ongoing reactions

There is a direct relationship between the state of aggregation of the initial components and the products formed during heterogeneous interactions.

In chemistry, a system usually means a substance or a combination of them.

A system that consists of one phase (the same state of aggregation) is considered homogeneous. As an example, we can mention a mixture of gases and several different liquids.

A heterogeneous system is a system in which reacting substances are in the form of gases and liquids, solids and gases.

There is not only a dependence of the reaction rate on temperature, but also on the phase in which the components entering into the analyzed interaction are used.

A homogeneous composition is characterized by the process occurring throughout the entire volume, which significantly improves its quality.

If the starting substances are in different phase states, then the maximum interaction is observed at the phase interface. For example, when an active metal is dissolved in an acid, the formation of a product (salt) is observed only on the surface of their contact.

Mathematical relationship between process speed and various factors

What does the equation for the dependence of the rate of a chemical reaction on temperature look like? For a homogeneous process, the rate is determined by the amount of substance that interacts or is formed during the reaction in the volume of the system per unit time.

For a heterogeneous process, the rate is determined in terms of the amount of substance reacting or produced in the process per unit area in a minimum period of time.

Factors affecting the rate of a chemical reaction

The nature of the reacting substances is one of the reasons for the different rates of processes. For example, alkali metals form alkalis with water at room temperature, and the process is accompanied by intense release of hydrogen gas. Noble metals (gold, platinum, silver) are not capable of such processes either at room temperature or when heated.

The nature of the reactants is a factor that is taken into account in the chemical industry to increase the profitability of production.

A relationship was revealed between the concentration of reagents and the speed of the chemical reaction. The higher it is, the more particles will collide, therefore, the process will proceed faster.

The law of mass action in mathematical form describes a directly proportional relationship between the concentration of starting substances and the speed of the process.

It was formulated in the mid-nineteenth century by the Russian chemist N. N. Beketov. For each process, a reaction constant is determined, which is not related to temperature, concentration, or the nature of the reactants.

In order to speed up the reaction in which a solid substance is involved, you need to grind it to a powder state.

In this case, the surface area increases, which has a positive effect on the speed of the process. For diesel fuel, a special injection system is used, due to which, when it comes into contact with air, the rate of combustion of the hydrocarbon mixture increases significantly.

Heating

The dependence of the rate of a chemical reaction on temperature is explained by molecular kinetic theory. It allows you to calculate the number of collisions between reagent molecules under certain conditions. If you are armed with such information, then under normal conditions all processes should proceed instantly.

But if we consider a specific example of the dependence of the reaction rate on temperature, it turns out that for interaction it is necessary to first break the chemical bonds between atoms so that new substances are formed from them. This requires significant energy expenditure. What is the dependence of the reaction rate on temperature? The activation energy determines the possibility of rupture of molecules; it is precisely this energy that characterizes the reality of the processes. Its units are kJ/mol.

If the energy is insufficient, the collision will be ineffective, so it is not accompanied by the formation of a new molecule.

Graphical representation

The dependence of the rate of a chemical reaction on temperature can be represented graphically. When heated, the number of collisions between particles increases, which accelerates the interaction.

What does a graph of reaction rate versus temperature look like? The energy of molecules is displayed horizontally, and the number of particles with a high energy reserve is indicated vertically. A graph is a curve by which one can judge the speed of a particular interaction.

The greater the difference in energy from the average, the further the point of the curve is located from the maximum, and the smaller percentage of molecules have such an energy reserve.

Important aspects

Is it possible to write down the equation for the dependence of the reaction rate constant on temperature? Its increase is reflected in an increase in the speed of the process. This dependence is characterized by a certain value called the temperature coefficient of the process rate.

For any interaction, the dependence of the reaction rate constant on temperature was revealed. If it increases by 10 degrees, the speed of the process increases by 2-4 times.

The dependence of the rate of homogeneous reactions on temperature can be represented in mathematical form.

For most interactions at room temperature, the coefficient is in the range from 2 to 4. For example, with a temperature coefficient of 2.9, an increase in temperature of 100 degrees speeds up the process by almost 50,000 times.

The dependence of the reaction rate on temperature can easily be explained by different activation energies. It has a minimum value during ionic processes, which are determined only by the interaction of cations and anions. Numerous experiments indicate the instantaneous occurrence of such reactions.

At a high activation energy, only a small number of collisions between particles will lead to interaction. At an average activation energy, the reactants will interact at an average rate.

Tasks on the dependence of the reaction rate on concentration and temperature are considered only at the senior level of education, and often cause serious difficulties for children.

Measuring the speed of a process

Those processes that require significant activation energy involve an initial rupture or weakening of bonds between atoms in the starting substances. In this case, they transition to a certain intermediate state called the activated complex. It is an unstable state, quite quickly decomposes into reaction products, the process is accompanied by the release of additional energy.

In its simplest form, an activated complex is a configuration of atoms with weakened old bonds.

Inhibitors and catalysts

Let us analyze the dependence of the rate of the enzymatic reaction on the temperature of the medium. Such substances function as process accelerators.

They themselves are not participants in the interaction; their number remains unchanged after the process is completed. While catalysts help increase the reaction rate, inhibitors, on the contrary, slow down this process.

The essence of this lies in the formation of intermediate compounds, as a result of which a change in the speed of the process is observed.

Conclusion

Various chemical interactions occur every minute in the world. How to establish the dependence of the reaction rate on temperature? The Arrhenius equation is a mathematical explanation of the relationship between the rate constant and temperature. It gives an idea of ​​those values ​​of activation energy at which the destruction or weakening of bonds between atoms in molecules and the distribution of particles into new chemical substances is possible.

Thanks to the molecular kinetic theory, it is possible to predict the probability of interactions between the initial components and calculate the rate of the process. Among those factors that affect the reaction rate, of particular importance are changes in temperature, percentage concentration of interacting substances, contact surface area, the presence of a catalyst (inhibitor), as well as the nature of the interacting components.

In life we ​​encounter different chemical reactions. Some of them, like the rusting of iron, can last for several years. Others, such as fermenting sugar into alcohol, take several weeks. Firewood in a stove burns in a couple of hours, and gasoline in an engine burns in a split second.

To reduce equipment costs, chemical plants increase the speed of reactions. And some processes, for example, food spoilage and metal corrosion, need to be slowed down.

Chemical reaction rate can be expressed as change in the amount of matter (n, modulo) per unit of time (t) - compare the speed of a moving body in physics as a change in coordinates per unit of time: υ = Δx/Δt. So that the speed does not depend on the volume of the vessel in which the reaction takes place, we divide the expression by the volume of the reacting substances (v), i.e. we get change in the amount of a substance per unit time per unit volume, or change in the concentration of one of the substances per unit time:

where c = n / v is the concentration of the substance,

Δ (read “delta”) is a generally accepted designation for a change in value.

If substances have different coefficients in the equation, the reaction rate for each of them calculated using this formula will be different. For example, 2 moles of sulfur dioxide reacted completely with 1 mole of oxygen in 10 seconds in 1 liter:

2SO2 + O2 = 2SO3

The oxygen rate will be: υ = 1: (10 1) = 0.1 mol/l s

Speed ​​for sulfur dioxide: υ = 2: (10 1) = 0.2 mol/l s- this does not need to be memorized and said during the exam, the example is given so as not to be confused if this question arises.

The rate of heterogeneous reactions (involving solids) is often expressed per unit area of ​​contacting surfaces:

Reactions are called heterogeneous when the reactants are in different phases:

• a solid with another solid, liquid or gas,
• two immiscible liquids
• liquid with gas.

Homogeneous reactions occur between substances in one phase:

• between well-mixed liquids,
• gases,
• substances in solutions.

Conditions affecting the rate of chemical reactions

1) The reaction speed depends on nature of reactants. Simply put, different substances react at different rates. For example, zinc reacts violently with hydrochloric acid, while iron reacts rather slowly.

2) The higher the reaction speed, the faster concentration substances. Zinc will react much longer with a highly dilute acid.

3) The reaction speed increases significantly with increasing temperature. For example, for fuel to burn, it is necessary to ignite it, i.e., increase the temperature. For many reactions, a 10°C increase in temperature is accompanied by a 2–4-fold increase in rate.

4) Speed heterogeneous reactions increases with increasing surfaces of reacting substances. Solids are usually ground for this purpose. For example, in order for iron and sulfur powders to react when heated, the iron must be in the form of fine sawdust.

Please note that in this case formula (1) is implied! Formula (2) expresses the speed per unit area, therefore it cannot depend on the area.

5) The rate of reaction depends on the presence of catalysts or inhibitors.

Catalysts- substances that accelerate chemical reactions, but are not consumed. An example is the rapid decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:

2H 2 O 2 = 2H 2 O + O 2

Manganese(IV) oxide remains at the bottom and can be reused.

Inhibitors- substances that slow down the reaction. For example, corrosion inhibitors are added to a water heating system to extend the life of pipes and batteries. In cars, corrosion inhibitors are added to brake and coolant fluid.

A few more examples.

Main concepts studied:

Rate of chemical reactions

Molar concentration

Kinetics

Homogeneous and heterogeneous reactions

Factors affecting the rate of chemical reactions

Catalyst, inhibitor

Catalysis

Reversible and irreversible reactions

Chemical equilibrium

Chemical reactions are reactions as a result of which other substances are obtained from one substance (new substances are formed from the original substances). Some chemical reactions occur in a fraction of a second (explosion), while others take minutes, days, years, decades, etc.

For example: the combustion reaction of gunpowder occurs instantly with ignition and explosion, and the reaction of darkening of silver or rusting of iron (corrosion) occurs so slowly that its result can be monitored only after a long time.

To characterize the speed of a chemical reaction, the concept of chemical reaction speed - υ is used.

Chemical reaction rate is the change in the concentration of one of the reactants of a reaction per unit time.

Formula for calculating the rate of a chemical reaction:

c 1 – molar concentration of the substance at the initial time t 1

c 2 – molar concentration of the substance at the initial time t 2

since the rate of a chemical reaction is characterized by a change in the molar concentration of the reactants (starting substances), then t 2 > t 1, and c 2 > c 1 (the concentration of the starting substances decreases as the reaction proceeds).

Molar concentration (s)– is the amount of substance per unit volume. The unit of measurement for molar concentration is [mol/l].

The branch of chemistry that studies the rate of chemical reactions is called chemical kinetics. Knowing its laws, a person can control chemical processes and set them at a certain speed.

When calculating the rate of a chemical reaction, it is necessary to remember that reactions are divided into homogeneous and heterogeneous.

Homogeneous reactions– reactions that occur in the same environment (i.e., the reactants are in the same state of aggregation; for example: gas + gas, liquid + liquid).

Heterogeneous reactions– these are reactions occurring between substances in a heterogeneous medium (there is a phase interface, i.e. the reacting substances are in different states of aggregation; for example: gas + liquid, liquid + solid).

The above formula for calculating the rate of a chemical reaction is valid only for homogeneous reactions. If the reaction is heterogeneous, then it can only occur at the surface of the reactants.

For a heterogeneous reaction, the rate is calculated using the formula:

∆ν – change in the amount of substance

S – interface area

∆ t – time period during which the reaction took place

The rate of chemical reactions depends on various factors: the nature of the reactants, the concentration of the substances, temperature, catalysts or inhibitors.

Dependence of reaction rates on the nature of the reacting substances.

Let's analyze this dependence of the reaction rate using an example: let’s drop metal granules of equal area into two test tubes containing the same amount of hydrochloric acid (HCl) solution: an iron (Fe) granule into the first test tube, and a magnesium (Mg) granule into the second. As a result of observations, based on the rate of hydrogen release (H2), it can be noted that magnesium reacts with hydrochloric acid at the highest speed than iron. The rate of this chemical reaction is influenced by the nature of the metal (i.e. magnesium is a more reactive metal than iron, and therefore reacts more vigorously with the acid).

Dependence of the rate of chemical reactions on the concentration of reactants.

The higher the concentration of the reacting (starting) substance, the faster the reaction proceeds. Conversely, the lower the concentration of the reactant, the slower the reaction.

For example: pour a concentrated solution of hydrochloric acid (HCl) into one test tube, and a dilute solution of hydrochloric acid into the other. Let's put a zinc granule (Zn) in both test tubes. We will observe, by the rate of hydrogen evolution, that the reaction will proceed faster in the first test tube, because the concentration of hydrochloric acid in it is greater than in the second test tube.

To determine the dependence of the rate of a chemical reaction, use law of action of (acting) masses : the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken in powers that are equal to their coefficients.

For example, for a reaction proceeding according to the scheme: nA + mB → D, the rate of a chemical reaction is determined by the formula:

υ h.r. = k · C (A) n · C (B) m , Where

υ x.r - rate of chemical reaction

C (A) – A

C (B) – molar concentration of a substance IN

n and m – their coefficients

k – rate constant of a chemical reaction (reference value).

The law of mass action does not apply to substances in a solid state, because their concentration is constant (due to the fact that they react only on the surface, which remains unchanged).

For example: for reaction 2 Cu + O 2 = 2 CuO the reaction rate is determined by the formula:

υ h.r. = k C(O 2)

PROBLEM: The rate constant for the reaction 2A + B = D is 0.005. calculate the reaction rate at the molar concentration of substance A = 0.6 mol/l, substance B = 0.8 mol/l.

Dependence of the rate of a chemical reaction on temperature.

This dependence is determined van't Hoff rule (1884): with every 10°C increase in temperature, the rate of a chemical reaction increases on average by 2–4 times.

Thus, the interaction of hydrogen (H 2) and oxygen (O 2) at room temperature almost does not occur, the rate of this chemical reaction is so low. But at a temperature of 500 C o this reaction takes place in 50 minutes, and at a temperature of 700 C o it occurs almost instantly.

Formula for calculating the rate of a chemical reaction according to the Van't Hoff rule:

where: υ t 1 and υ t 2 - rates of chemical reactions at t 2 and t 1

γ is the temperature coefficient, which shows how many times the reaction rate increases with an increase in temperature by 10 C o.

Changing reaction speed:

2. Substitute the data from the problem statement into the formula:

Dependence of reaction rates on special substances - catalysts and inhibitors.

Catalyst- a substance that increases the rate of a chemical reaction, but does not itself participate in it.

Inhibitor- a substance that slows down a chemical reaction, but does not itself participate in it.

Example: into a test tube with a solution of 3% hydrogen peroxide (H 2 O 2), which has been heated, add a smoldering splinter - it will not light up, because the reaction rate of the decomposition of hydrogen peroxide into water (H 2 O) and oxygen (O 2) is very low, and the resulting oxygen is not enough to carry out a high-quality reaction to oxygen (sustaining combustion). Now let’s add a little black powder of manganese (IV) oxide (MnO 2) into the test tube and see that the rapid release of gas bubbles (oxygen) has begun, and the smoldering splinter brought into the test tube flares up brightly. MnO 2 is the catalyst for this reaction; it accelerated the rate of the reaction, but did not participate in it itself (this can be proven by weighing the catalyst before and after the reaction - its mass will not change).

Goal of the work: study of the rate of a chemical reaction and its dependence on various factors: the nature of the reactants, concentration, temperature.

Chemical reactions occur at different rates. Speed ​​of chemical reaction is called the change in the concentration of a reactant per unit time. It is equal to the number of interaction events per unit time per unit volume for a reaction occurring in a homogeneous system (for homogeneous reactions), or per unit interface surface for reactions occurring in a heterogeneous system (for heterogeneous reactions).

Average reaction speed v avg. in the time interval from t 1 before t 2 is determined by the relation:

Where C 1 And C 2– molar concentration of any reaction participant at time points t 1 And t 2 respectively.

The “–” sign before the fraction refers to the concentration of the starting substances, Δ WITH < 0, знак “+” – к концентрации продуктов реакции, ΔWITH > 0.

The main factors influencing the rate of a chemical reaction: the nature of the reactants, their concentration, pressure (if gases are involved in the reaction), temperature, catalyst, interface area for heterogeneous reactions.

Most chemical reactions are complex processes occurring in several stages, i.e. consisting of several elementary processes. Elementary or simple reactions are reactions that occur in one step.

For elementary reactions, the dependence of the reaction rate on concentration is expressed by the law of mass action.

At constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken in powers equal to the stoichiometric coefficients.

For the reaction in general form

a A + b B… → c C,

according to the law of mass action v expressed by the ratio

v = К∙с(А) а ∙ с(В) b,

Where c(A) And s(B)– molar concentrations of reactants A and B;

TO– rate constant of this reaction, equal to v, If c(A)a=1 and c(B)b=1, and depending on the nature of the reactants, temperature, catalyst, and interface area for heterogeneous reactions.

The expression of the reaction rate as a function of concentration is called the kinetic equation.

In the case of complex reactions, the law of mass action applies to each individual stage.

For heterogeneous reactions, the kinetic equation includes only the concentrations of gaseous and dissolved substances; yes, for burning coal

C (k) + O 2 (g) → CO 2 (g)

the velocity equation has the form

v = K∙s(O 2)

A few words about the molecularity and kinetic order of the reaction.

Concept "molecularity of reaction" apply only to simple reactions. The molecularity of a reaction characterizes the number of particles participating in an elementary interaction.

There are mono-, bi- and trimolecular reactions, in which one, two and three particles participate, respectively. The probability of three particles colliding simultaneously is small. The elementary process of interaction of more than three particles is unknown. Examples of elementary reactions:

N 2 O 5 → NO + NO + O 2 (monomolecular)

H 2 + I 2 → 2HI (bimolecular)

2NO + Cl 2 → 2NOCl (trimolecular)

The molecularity of simple reactions coincides with the general kinetic order of the reaction. The order of the reaction determines the nature of the dependence of rate on concentration.

The general (total) kinetic order of a reaction is the sum of the exponents at the concentrations of the reactants in the reaction rate equation, determined experimentally.

As temperature increases, the rate of most chemical reactions increases. The dependence of the reaction rate on temperature is approximately determined by the Van't Hoff rule.

For every 10-degree increase in temperature, the rate of most reactions increases by 2–4 times.

where and are the reaction rate, respectively, at temperatures t 2 And t 1 (t 2 >t 1);

γ is the temperature coefficient of the reaction rate, this is a number showing how many times the rate of a chemical reaction increases when the temperature increases by 10 0.

Using Van't Hoff's rule, it is only possible to approximately estimate the effect of temperature on the reaction rate. A more accurate description of the dependence of the temperature reaction rate is feasible within the framework of the Arrhenius activation theory.

One of the methods of accelerating a chemical reaction is catalysis, which is carried out using substances (catalysts).

Catalysts- these are substances that change the rate of a chemical reaction due to repeated participation in intermediate chemical interactions with reaction reagents, but after each cycle of intermediate interaction they restore their chemical composition.

The mechanism of action of the catalyst is reduced to a decrease in the activation energy of the reaction, i.e. reducing the difference between the average energy of active molecules (active complex) and the average energy of molecules of the starting substances. The rate of the chemical reaction increases.

Knowledge of the rates of chemical reactions is of great theoretical and practical importance. For example, in the chemical industry, during the production of a substance, the size and productivity of the equipment and the amount of the resulting product depend on the reaction rate.

Different chemical reactions have different rates. Some reactions occur within a fraction of a second, while others take months or even years to complete. The speed of chemical reactions studies chemical kinetics.

The basic concepts with which chemical kinetics operates are chemical system And phase:

• Chemical system- substance (a set of substances);
• Chemical phase- part of a system separated from other parts interface.

Systems consisting of one phase are called homogeneous or homogeneous, for example, gas mixtures or solutions. Reactions occurring in homogeneous systems are called homogeneous reactions, such reactions occur throughout the entire volume of the mixture.

Systems consisting of several phases are called heterogeneous or heterogeneous, for example, liquid + solid. Reactions occurring in heterogeneous systems are called heterogeneous reactions, such reactions occur only at the interface.

Homogeneous reaction rate

The rate of a homogeneous reaction is the amount of substance (ν) formed as a result of a reaction per unit time (t) per unit volume of the system (V):

• ν 1 - number of moles of substance at time t 1;
• ν 2 - number of moles of substance at time t 2 ;

Mole-volume concentration substance (C, mol/l) - the ratio of the number of moles of a substance (ν) to the entire volume of the reaction mixture (V): С=ν/V.

The rate of a homogeneous reaction is equal to the change in the concentration of the reactant per unit time.

In the event that we are talking about the concentration of one of the reaction products, a “plus” sign is put in the expression, if we are talking about the concentration of one of the original substances, a “minus” sign is put in the expression.

Heterogeneous reaction rate

As mentioned above, the main difference between heterogeneous reactions and homogeneous ones is that the reaction occurs at the phase boundary.

The rate of a heterogeneous reaction (v het) is the amount of substance (ν) formed per unit time (t) per unit interface surface (S).

The main factors influencing the speed of reactions:

• the nature of the reacting substances;
• concentration;
• temperature;
• catalysts;
• reagent particle sizes;
• pressure.

The last two points relate to heterogeneous reactions.

Nature of reactants

A necessary condition for chemical interaction between molecules of substances is their collision with each other in the “right” part of the molecule, called area with high reactivity. It’s like in boxing: if a boxer’s blow hits the opponent’s gloves, there will be no reaction; but if the blow lands on the opponent’s head, then the probability of a knockout (reaction) increases significantly; and if the impact force (the force of collisions of molecules) is high, then knockout (reaction) becomes inevitable.

Based on the above, we can conclude that the more complex the molecule, the smaller its highly reactive region. Hence, the larger and more complex the molecules of the reacting substances, the slower the reaction rate.

Reagent concentration

The rate of reaction is directly proportional to the number of collisions of molecules. The higher the concentration of reagents, the more collisions, the higher the rate of chemical reaction. For example, combustion in pure oxygen occurs much faster than in ordinary air.

However, it should be said that in complex reactions occurring in several stages; such dependence is not respected. This allows you to determine which of the reagents is not involved in the slowest stage of the reaction, which determines the reaction rate itself.

The dependence of the reaction rate on the concentration of reactants is expressed law of mass action, which was discovered in 1867 by Norwegian scientists Guldberg and Waage.

The speed (v) of the conditioned reaction described by the equation aA+bB=cC+dD, in accordance with the law of mass action, will be calculated using a formula called kinetic reaction equation:

V=k·[A] a ·[B] b

• [A], [B] - concentrations of starting substances;
• k is the reaction rate constant, equal to the rate of this reaction at concentrations of reactants equal to 1 mol each.

k does not depend on the concentration of the reacting substances, but depends on their nature and temperature.

Using the kinetic equation of a reaction, you can determine the rate of change of the reaction depending on the change in the concentration of the reactants.

Examples of kinetic equations:

2SO 2 (g)+O 2 (g)=2SO 3 (g) v=k 2 CuO(s)+H 2 (g)=Cu(s)+H 2 O(g) v=k

Note that the kinetic equations do not include concentrations of solids, only gaseous and dissolved ones.

Reagent temperature

As the temperature rises, the molecules move faster, hence the number of their collisions with each other increases. In addition, the kinetic energy of molecules increases, which increases the efficiency of collisions, which ultimately determine the rate of reaction.

According to activation theory, only molecules with energy that exceeds a certain average value can take part in a chemical reaction. The amount of excess of the average energy of molecules is called activation energies. This energy is necessary to weaken the chemical bonds in the molecules of the starting substances. Molecules that have the necessary excess energy to allow them to react are called active molecules. The higher the temperature, the more active molecules, the higher the reaction rate.

The dependence of the reaction rate on temperature is characterized van't Hoff's rule:

Mathematically, van't Hoff's rule is expressed by the following formula:

• γ is a temperature coefficient showing an increase in the reaction rate with an increase in temperature by 10°C;
• v 1 - reaction rate at temperature t 1;
• v 2 - reaction rate at temperature t 2 ;

Catalysts

Catalysts- these are substances that affect the rate of reaction, but are not consumed themselves.

Reactions that occur with the participation of catalysts are called catalytic reactions.

The main effect of a catalyst is to reduce the activation energy of the reaction, as a result of which the number of effective collisions of molecules increases.

Catalysts can speed up reactions millions of times!

There are two types of catalysis:

• homogeneous (uniform) catalysis- the catalyst and reagents form one phase: gas or solution;
• heterogeneous (heterogeneous) catalysis- the catalyst is in the form of an independent phase.

The mechanism of catalytic reactions is very complex and completely unknown. According to one scientific hypothesis, in catalytic reactions, a catalyst and a reagent react to form an intermediate compound, which reacts much more actively with another starting substance to form the final reaction product, while the catalyst itself is released in a free state.

Typically, catalysts are understood as substances that accelerate a reaction, but there are substances that slow down the course of a reaction - they are called inhibitors.

Biological catalysts are called enzymes. Enzymes are proteins.

Reagent particle size

Let's take a match and bring it to a piece of coal. It is unlikely that the coal will have time to ignite before the match goes out. Let's grind the coal and repeat the experiment - the coal dust will not just ignite, but will ignite very quickly - an explosion will occur (the main danger in coal mines). What's going on?

By grinding the coal, we will dramatically increase its surface area. The larger the surface area on which molecules collide, the faster the reaction rate.

Reagent pressure

The pressure of gaseous reagents is similar to their concentration - the higher the pressure, the higher the concentration - the higher the reaction rate, because the number of molecular collisions increases. Like concentration, the pressure of reactants does not “work” in complex reactions.