What determines the rate of reaction in chemistry? Physical chemistry

16.10.2019

Chemical reaction rate

Chemical reaction rate- change in the amount of one of the reacting substances per unit of time in a unit of reaction space. Is a key concept in chemical kinetics. The rate of a chemical reaction is always a positive quantity, therefore, if it is determined by original substance(the concentration of which decreases during the reaction), then the resulting value is multiplied by −1.

For example for the reaction:

the expression for speed will look like this:

. The rate of a chemical reaction at any given time is proportional to the concentrations of the reactants raised to powers equal to their stoichiometric coefficients.

For elementary reactions, the exponent of the concentration of each substance is often equal to its stoichiometric coefficient; for complex reactions this rule is not observed. In addition to concentration, the following factors influence the rate of a chemical reaction:

the nature of the reactants,
the presence of a catalyst,
temperature (van't Hoff rule),
pressure,
surface area of reacting substances.

If we consider the simplest chemical reaction A + B → C, we will notice that instant The speed of a chemical reaction is not constant.

Literature

Kubasov A. A. Chemical kinetics and catalysis.
Prigogine I., Defey R. Chemical thermodynamics. Novosibirsk: Nauka, 1966. 510 p.
Yablonsky G.S., Bykov V.I., Gorban A.N., Kinetic models of catalytic reactions, Novosibirsk: Nauka (Sib. Department), 1983. - 255 p.

Wikimedia Foundation. 2010.

See what “Rate of a chemical reaction” is in other dictionaries:

Basic concept of chemical kinetics. For simple homogeneous reactions, the rate of a chemical reaction is measured by the change in the number of moles of the reacted substance (at a constant volume of the system) or by the change in the concentration of any of the starting substances... Big Encyclopedic Dictionary

RATE OF CHEMICAL REACTION- the basic concept of chemistry. kinetics, expressing the ratio of the amount of reacted substance (in moles) to the period of time during which the interaction occurred. Since the concentrations of reactants change during interaction, the rate is usually ... Big Polytechnic Encyclopedia

rate of chemical reaction- a quantity characterizing the intensity of a chemical reaction. The rate of formation of a reaction product is the amount of this product as a result of a reaction per unit time per unit volume (if the reaction is homogeneous) or per... ...

A quantity characterizing the intensity of a chemical reaction (See Chemical reactions). The rate of formation of a reaction product is the amount of this product resulting from a reaction per unit time per unit volume (if... ...

Basic concept of chemistry kinetics. For simple homogeneous reactions of S. x. r. measured by the change in the number of moles reacted in va (at a constant volume of the system) or by the change in the concentration of any of the initial va or reaction products (if the volume of the system ...

For complex reactions consisting of several stages (simple or elementary reactions), a mechanism is a set of stages, as a result of which the starting materials are converted into products. Molecules can act as intermediates in these reactions... ... Natural science. Encyclopedic Dictionary

- (eng. nucleophilic substitution reaction) substitution reactions in which the attack is carried out by a nucleophilic reagent carrying a lone pair of electrons. The leaving group in nucleophilic substitution reactions is called a nucleofuge. Everything... Wikipedia

Transformation of some substances into others, different from the original ones chemical composition or building. Total number atoms of each given element, as well as themselves chemical elements, the constituent substances, remain in R. x. unchanged; this R. x... Great Soviet Encyclopedia

drawing speed - linear speed metal movement at the exit from the die, m/s. On modern drawing machines, the drawing speed reaches 50–80 m/s. However, even when drawing wire, the speed, as a rule, does not exceed 30–40 m/s. At… … Encyclopedic Dictionary of Metallurgy

The rate of a chemical reaction is the change in the concentration of reactants per unit time.

In homogeneous reactions, the reaction space refers to the volume of the reaction vessel, and in heterogeneous reactions, the surface on which the reaction occurs. The concentration of reacting substances is usually expressed in mol/l - the number of moles of a substance in 1 liter of solution.

The rate of a chemical reaction depends on the nature of the reactants, concentration, temperature, pressure, contact surface of the substances and its nature, and the presence of catalysts.

An increase in the concentration of substances that enter into a chemical interaction leads to an increase in the rate of the chemical reaction. This happens because all chemical reactions take place between a certain number of reacting particles (atoms, molecules, ions). The more of these particles there are in the volume of the reaction space, the more often they collide and chemical interaction occurs. A chemical reaction can occur through one or more elementary acts (collisions). Based on the reaction equation, we can write down an expression for the dependence of the reaction rate on the concentration of the reactants. If only one molecule participates in an elementary act (during a decomposition reaction), the dependence will have the following form:

v= k*[A]

This is the equation for a monomolecular reaction. When two different molecules interact in an elementary act, the dependence has the form:

v= k*[A]*[B]

The reaction is called bimolecular. In the case of a collision of three molecules, the expression is valid:

v= k*[A]*[B]*[C]

The reaction is called trimolecular. Coefficient designations:

v — reaction speed;

[A], [B], [C] are the concentrations of reacting substances;

k—proportionality coefficient; called the reaction rate constant.

If the concentrations of the reactants are equal to one (1 mol/l) or their product is equal to one, then v = k.. The rate constant depends on the nature of the reactants and on the temperature. The dependence of the rate of simple reactions (i.e. reactions occurring through one elementary act) on concentration is described by the law of mass action: the rate of a chemical reaction is directly proportional to the product of the concentration of reactants raised to the power of their stoichiometric coefficients.

For example, let's look at the reaction 2NO + O 2 = 2NO 2.

In it v= k* 2 *

In the case when the equation of a chemical reaction does not correspond to the elementary act of interaction, but reflects only the relationship between the mass of the substances that reacted and the substances formed, then the powers of the concentrations will not be equal to the coefficients appearing in front of the formulas of the corresponding substances in the reaction equation. For a reaction that occurs in several stages, the rate of the reaction is determined by the rate of the slowest (limiting) stage.

This dependence of the reaction rate on the concentration of reactants is valid for gases and reactions taking place in solution. Reactions involving solids do not obey the law of mass action, since the interaction of molecules occurs only at the interface. Consequently, the rate of a heterogeneous reaction also depends on the size and nature of the contact surface of the reacting phases. How more surface– the faster the reaction will go.

The effect of temperature on the rate of a chemical reaction

The effect of temperature on the rate of a chemical reaction is determined by the Van't Hoff rule: with an increase in temperature for every 10 ° C, the reaction rate increases 2-4 times. Mathematically, this rule is expressed by the following equation:

v t2= v t1*g(t2-t1)/10

Where v t1 And v t2 — reaction rates at temperatures t2 and t1; g - temperature coefficient of reaction - a number showing how many times the reaction rate increases with an increase in temperature for every 10 ° C. Such a significant dependence of the rate of a chemical reaction on temperature is explained by the fact that the formation of new substances does not occur with every collision of reacting molecules. Only those molecules (active molecules) interact that have sufficient energy to break the bonds in the original particles. Therefore, each reaction is characterized by an energy barrier. To overcome it, the molecule needs activation energy - some excess energy that a molecule must have in order for its collision with another molecule to lead to the formation of a new substance. With increasing temperature, the number of active molecules increases rapidly, which leads to a sharp increase in the reaction rate according to Van't Hoff's rule. The activation energy for each specific reaction depends on the nature of the reactants.

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

Catalyst influence

One of the most effective means impact on speed chemical reactions— use of catalysts. TO atalizers - These are substances that change the rate of a reaction, but at the end of the process they themselves remain unchanged in composition and mass. In other words, at the moment of the reaction itself, the catalyst is actively involved in the chemical process, but by the end of the reaction, the reactants change their chemical composition, turning into products, and the catalyst is released in its original form. Typically, the role of a catalyst is to increase the rate of a reaction, although some catalysts slow down the process rather than speed it up. The phenomenon of acceleration of chemical reactions due to the presence of catalysts is called catalysis, and slowdowns - inhibition.

Some substances do not have a catalytic effect, but their additions dramatically increase the catalytic ability of catalysts. Such substances are called promoters. Other substances (catalytic poisons) reduce or even completely block the action of catalysts, this process is called catalyst poisoning.

There are two types of catalysis: homogeneous And heterogeneous. At homogeneous catalysis the reactants, products and catalyst form one phase (gas or liquid). In this case, there is no interface between the catalyst and the reactants.

Peculiarity heterogeneous catalysis is that catalysts (usually solids) are in a different phase state than the reactants and products of the reaction. The reaction usually develops on the surface of a solid.

In homogeneous catalysis, intermediate products are formed between the catalyst and the reactant as a result of a reaction with a lower activation energy. In heterogeneous catalysis, the increase in rate is explained by the adsorption of reactants on the surface of the catalyst. As a result, their concentration increases and the reaction rate increases.

A special case of catalysis is autocatalysis. Its meaning is that a chemical process is accelerated by one of the reaction products.

Chemical reaction rate

Chemical reaction rate- change in the amount of one of the reacting substances per unit of time in a unit of reaction space. Is a key concept in chemical kinetics. The rate of a chemical reaction is always a positive value, therefore, if it is determined by the starting substance (the concentration of which decreases during the reaction), then the resulting value is multiplied by −1.

For example for the reaction:

the expression for speed will look like this:

. The rate of a chemical reaction at any given time is proportional to the concentrations of the reactants raised to powers equal to their stoichiometric coefficients.

the nature of the reactants,
the presence of a catalyst,
temperature (van't Hoff rule),
pressure,
surface area of reacting substances.

If we consider the simplest chemical reaction A + B → C, we will notice that instant The speed of a chemical reaction is not constant.

Literature

Kubasov A. A. Chemical kinetics and catalysis.
Prigogine I., Defey R. Chemical thermodynamics. Novosibirsk: Nauka, 1966. 510 p.
Yablonsky G.S., Bykov V.I., Gorban A.N., Kinetic models of catalytic reactions, Novosibirsk: Nauka (Sib. Department), 1983. - 255 p.

Wikimedia Foundation. 2010.

Welsh dialects of English
Saw (film series)

See what “Rate of a chemical reaction” is in other dictionaries:

RATE OF CHEMICAL REACTION- the basic concept of chemical kinetics. For simple homogeneous reactions, the rate of a chemical reaction is measured by the change in the number of moles of the reacted substance (at a constant volume of the system) or by the change in the concentration of any of the starting substances... Big Encyclopedic Dictionary

rate of chemical reaction- the basic concept of chemical kinetics. For simple homogeneous reactions, the rate of a chemical reaction is measured by the change in the number of moles of the reacted substance (at a constant volume of the system) or by the change in the concentration of any of the starting substances... Encyclopedic Dictionary

Chemical reaction rate- a quantity characterizing the intensity of a chemical reaction (See Chemical reactions). The rate of formation of a reaction product is the amount of this product resulting from a reaction per unit time per unit volume (if... ...

RATE OF CHEMICAL REACTION- basic concept of chemistry kinetics. For simple homogeneous reactions of S. x. r. measured by the change in the number of moles reacted in va (at a constant volume of the system) or by the change in the concentration of any of the initial va or reaction products (if the volume of the system ...

MECHANISM OF CHEMICAL REACTION- For complex reactions consisting of several. stages (simple or elementary reactions), a mechanism is a set of stages, as a result of which the starting materials are converted into products. Molecules can act as intermediates in these reactions... ... Natural science. Encyclopedic Dictionary

Nucleophilic substitution reactions- (eng. nucleophilic substitution reaction) substitution reactions in which the attack is carried out by a nucleophilic reagent carrying a lone pair of electrons. The leaving group in nucleophilic substitution reactions is called a nucleofuge. Everything... Wikipedia

Chemical reactions- transformation of some substances into others, different from the original ones in chemical composition or structure. The total number of atoms of each given element, as well as the chemical elements themselves that make up the substances, remain in R. x. unchanged; this R. x... Great Soviet Encyclopedia

drawing speed- linear speed of metal movement at the exit from the die, m/s. On modern drawing machines, the drawing speed reaches 50–80 m/s. However, even when drawing wire, the speed, as a rule, does not exceed 30–40 m/s. At… … Encyclopedic Dictionary of Metallurgy

7.1. Homogeneous and heterogeneous reactions

Chemicals can be found in different states of aggregation, while their chemical properties in different states are the same, but the activity is different (which was shown in the last lecture using the example of the thermal effect of a chemical reaction).

Let's consider various combinations states of aggregation in which two substances A and B can exist.

A (g.), B (g.)

A (TV), B (TV)

A (w.), B (tv.)

mix

A(tv.), B(g.)

A (f.), B (g.)

mix

(solution)

heterogeneous

homogeneous

heterogeneous

homogeneous

Hg(l) + HNO3

H2O + D2O

Fe + O2

H2S + H2SO4

CO+O2

A phase is a region of a chemical system within which all the properties of the system are constant (identical) or continuously change from point to point. Each of the solids is a separate phase, and there are also solution and gas phases.

Called homogeneous chemical system, in which all substances are in one phase (in solution or gas). If there are several phases, then the system is called

heterogeneous.

Respectively chemical reaction called homogeneous if the reactants are in the same phase. If the reagents are in different phases, then chemical reaction called heterogeneous.

It is not difficult to understand that since contact of reagents is required for a chemical reaction to occur, a homogeneous reaction occurs simultaneously throughout the entire volume of a solution or reaction vessel, while a heterogeneous reaction occurs at a narrow boundary between phases - at the interface. Thus, purely theoretically, a homogeneous reaction occurs faster than a heterogeneous one.

Thus we come to the concept rate of chemical reaction.

The rate of a chemical reaction. Law of mass action. Chemical equilibrium.

7.2. Chemical reaction rate

The branch of chemistry that studies the rates and mechanisms of chemical reactions is a branch of physical chemistry and is called chemical kinetics.

Speed of chemical reaction is the change in the amount of a substance per unit time per unit volume of the reacting system (for a homogeneous reaction) or per unit surface area (for a heterogeneous reaction).

Thus, if the volume	or area	interfaces
do not change, then the expressions for the rates of chemical reactions have the form:


hom o

The ratio of a change in the amount of a substance to the volume of the system can be interpreted as a change in the concentration of a given substance.

Note that for reagents, the expression for the rate of a chemical reaction is written with a minus sign, since the concentration of the reagents decreases, and the rate of the chemical reaction is actually a positive value.

Further conclusions are based on simple physical considerations that consider a chemical reaction as a consequence of the interaction of several particles.

Elementary (or simple) is a chemical reaction that occurs in one stage. If there are several stages, then such reactions are called complex, or composite, or gross reactions.

In 1867, it was proposed to describe the rate of a chemical reaction law of mass action: the rate of an elementary chemical reaction is proportional to the concentrations of the reactants in powers of stoichiometric coefficients.n A +m B P,

A, B – reactants, P – products, n, m – coefficients.

W =k n m

The coefficient k is called the rate constant of a chemical reaction,

characterizes the nature of interacting particles and does not depend on the particle concentration.

The rate of a chemical reaction. Law of mass action. Chemical balance. The quantities n and m are called reaction order by substance A and B respectively, and

their sum (n +m) – reaction order.

For elementary reactions, the reaction order can be 1, 2 and 3.

Elementary reactions with order 1 are called monomolecular, with order 2 - bimolecular, with order 3 - trimolecular, based on the number of molecules involved. Elementary reactions above the third order are unknown - calculations show that the simultaneous meeting of four molecules at one point is too incredible an event.

Since a complex reaction consists of a certain sequence of elementary reactions, its rate can be expressed in terms of the rates of individual stages of the reaction. Therefore, for complex reactions, the order can be any, including fractional or zero (zero order of a reaction indicates that the reaction occurs with constant speed and does not depend on the concentration of reacting particles W = k).

The slowest stage of a complex process is usually called the rate-limiting stage.

Imagine that large number molecules went to a free cinema, but at the entrance there is a controller who checks the age of each molecule. Therefore, a flow of matter enters the cinema doors, and molecules enter the cinema hall one at a time, i.e. very slowly.

Examples of elementary first-order reactions are processes of thermal or radioactive decay; accordingly, the rate constant k characterizes either the probability of breaking a chemical bond or the probability of decay per unit time.

There are a lot of examples of elementary second-order reactions - this is the most familiar way of reactions for us - particle A collided with particle B, some kind of transformation occurred and something happened there (note that products in theory do not affect anything - all attention is given only to reacting particles).

On the contrary, there are quite a few elementary third-order reactions, since it is quite rare for three particles to meet simultaneously.

As an illustration, let's look at the predictive power of chemical kinetics.

The rate of a chemical reaction. Law of mass action. Chemical balance.

First order kinetic equation

(illustrative supplementary material)

Let us consider a homogeneous first-order reaction, the rate constant of which is equal to k, the initial concentration of substance A is equal to [A]0.

By definition, the rate of a homogeneous chemical reaction is equal to

K[A]

change in concentration per unit time. Once substance A –

reagent, put a minus sign.

Such an equation is called differential (there is

derivative)

[A]

To solve it, we transfer the quantities to the left side

concentrations, and on the right - time.

If the derivatives of two functions are equal, then the functions themselves

should differ by no more than a constant.

To solve given equation take the integral of the left side (over

concentration) and the right side (in time). So as not to scare

ln[ A ] = −kt +C

listeners, we will limit ourselves to the answer.

ln icon – natural logarithm, i.e. number b such that

= [A],e = 2.71828…

ln[ A ]- ln0 = - kt

The constant C is found from the initial conditions:

at t = 0 the initial concentration is [A]0

[A]

Times logarithm –

this is a power of a number, we use the properties of powers

[A]0

e a− b=

Now let's get rid of the nasty logarithm (see definition

logarithm 6-7 lines higher),

Why do we raise the number?

to the power of the left side of the equation and the right side of the equation.

[A]

E−kt

Multiply by [A]0

[A]0

First order kinetic equation.

[ A ]= 0 × e − kt

Based on

the obtained kinetic equation of the first

order maybe

calculated

substance concentration

at any time

For the purposes of our course, this conclusion is for informational purposes in order to demonstrate to you the use of mathematical apparatus for calculating the progress of a chemical reaction. Consequently, a competent chemist cannot but know mathematics. Learn math!

The rate of a chemical reaction. Law of mass action. Chemical balance. A graph of the concentration of reagents and products versus time can be qualitatively depicted as follows (using the example irreversible reaction first order)

Factors that affect reaction speed

1. Nature of reactants

For example, the reaction rate of the following substances: H2 SO4, CH3 COOH, H2 S, CH3 OH - with hydroxide ion will vary depending on the strength H-O bonds. To assess the strength of a given bond, you can use the relative positive charge on the hydrogen atom: the greater the charge, the easier the reaction will be.

2. Temperature

Life experience tells us that the rate of reaction depends on temperature and increases with increasing temperature. For example, the process of milk souring occurs faster at room temperature rather than in the refrigerator.

Let us turn to the mathematical expression of the law of mass action.

W =k n m

Since the left side of this expression (reaction rate) depends on temperature, therefore, the right side of the expression also depends on temperature. In this case, the concentration, of course, does not depend on temperature: for example, milk retains its fat content of 2.5% both in the refrigerator and at room temperature. Then, as Sherlock Holmes used to say, the remaining solution is the correct one, no matter how strange it may seem: the rate constant depends on temperature!

The rate of a chemical reaction. Law of mass action. Chemical balance. The dependence of the reaction rate constant on temperature is expressed using the Arrhenius equation:

− E a

k = k0 eRT,

in which

R = 8.314 J mol-1 K-1 – universal gas constant,

E a is the activation energy of the reaction (see below), it is conventionally considered independent of temperature;

k 0 is the pre-exponential factor (i.e. the factor that comes before the exponentiale), the value of which is also almost independent of temperature and is determined, first of all, by the order of the reaction.

Thus, the value of k0 is approximately 1013 s-1 for a first-order reaction, 10 -10 l mol-1 s-1 for a second-order reaction,

for a third order reaction – 10 -33 l2 mol-2 s-1. It is not necessary to remember these values.

The exact values of k0 for each reaction are determined experimentally.

The concept of activation energy becomes clear from the following figure. In fact, activation energy is the energy that a reacting particle must have in order for a reaction to occur.

Moreover, if we heat the system, then the energy of the particles increases (dashed graph), while the transition state (≠) remains at the same level. The energy difference between the transition state and the reactants (activation energy) decreases, and the reaction rate according to the Arrhenius equation increases.

The rate of a chemical reaction. Law of mass action. Chemical balance. In addition to the Arrhenius equation, there is the Van't Hoff equation, which

characterizes the dependence of the reaction rate on temperature through the temperature coefficient γ:

The temperature coefficient γ shows how many times the rate of a chemical reaction will increase when the temperature changes by 10o.

Van't Hoff equation:

T 2− T 1

W (T 2 )= W (T 1 )× γ10

Typically, the coefficient γ is in the range from 2 to 4. For this reason, chemists often use the approximation that an increase in temperature by 20o leads to an increase in the reaction rate by an order of magnitude (i.e., 10 times).

Reaction speed 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. To break bonds in highly polar molecules (HCl, H 2 O) less energy is required, 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 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)
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

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:

		(t 2 - t 1) / 10
Vt 2 / Vt 1	= g

(where Vt 2 and Vt 1 are the reaction rates at temperatures t 2 and t 1, respectively; g is the temperature coefficient of 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.


Exothermic reaction	Endothermic 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). Dramatically slow down the progression of unwanted chemical processes in some cases, it is possible to add inhibitors to the reaction medium (the phenomenon " negative catalysis").