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The rate of chemical reactions. Chemical equilibrium

Plan:

1. The concept of speed chemical reaction.

2. Factors affecting the rate of a chemical reaction.

3. Chemical equilibrium. Factors influencing displacement equilibrium. Le Chatelier's principle.

Chemical reactions occur at different rates. Reactions occur very quickly in aqueous solutions. For example, if solutions of barium chloride and sodium sulfate are drained, a white precipitate of barium sulfate immediately precipitates. Ethylene quickly, but not instantly, discolors bromine water. Rust slowly forms on iron objects, plaque appears on copper and bronze products, and foliage rots.

Science studies the rate of a chemical reaction, as well as identifying its dependence on the conditions of the process - chemical kinetics.

If reactions occur in a homogeneous medium, for example, in a solution or gas phase, then the interaction of the reactants occurs throughout the entire volume. Such reactions are called homogeneous.

If a reaction occurs between substances located in different states of aggregation(for example, between a solid and a gas or liquid) or between substances that are not capable of forming a homogeneous medium (for example, between two immiscible liquids), then it passes only on the surface of contact of the substances. Such reactions are called heterogeneous.

υ of a homogeneous reaction is determined by the change in the amount of substance per unit per unit volume:

υ =Δn / Δt ∙V

where Δ n is the change in the number of moles of one of the substances (most often the original, but it can also be a reaction product), (mol);

V – volume of gas or solution (l)

Since Δ n / V = ​​ΔC (change in concentration), then

υ =Δ C / Δt (mol/l∙ s)

υ of a heterogeneous reaction is determined by the change in the amount of substance per unit time on a unit surface of contact of substances.

υ =Δn / Δt ∙ S

where Δ n – change in the amount of substance (reagent or product), (mol);

Δt – time interval (s, min);

S – surface area of ​​​​contact of substances (cm 2, m 2)

Why are the rates of different reactions not the same?

In order for a chemical reaction to begin, the molecules of the reacting substances must collide. But not every collision results in a chemical reaction. In order for a collision to lead to a chemical reaction, the molecules must have sufficiently high energy. Particles that can undergo a chemical reaction upon collision are called active. They have excess energy compared to the average energy of most particles - activation energy E act. There are much fewer active particles in a substance than with average energy, so for many reactions to begin, the system must be given some energy (a flash of light, heating, mechanical shock).

Energy barrier (value E act) is different for different reactions, the lower it is, the easier and faster the reaction proceeds.

2. Factors influencing υ(number of particle collisions and their efficiency).

1) Nature of reactants: their composition, structure => activation energy

▪ the less E act, the greater υ;

If E act < 40 кДж/моль, то это значит, что значительная часть столкновений между частицами реагирующих веществ приводит к их взаимодействию, и скорость такой реакции очень большая. Все реакции ионного обмена протекают практически мгновенно, т.к. в этих реакциях участвуют разноименнозаряженные частицы, и энергия активации в этих случаях ничтожно мала.

If E act> 120 kJ/mol, this means that only a tiny fraction of collisions between interacting particles lead to a reaction. The rate of such reactions is very low. For example, rusting of iron, or

the occurrence of the ammonia synthesis reaction at ordinary temperatures is almost impossible to notice.

If E act have intermediate values(40 – 120 kJ/mol), then the rate of such reactions will be average. Such reactions include the interaction of sodium with water or ethanol, discoloration of bromine water with ethylene, etc.

2) Temperature: at t for every 10 0 C, υ 2-4 times (van't Hoff rule).

υ 2 = υ 1 ∙ γ Δt/10

At t, the number of active particles (s E act) and their active collisions.

Task 1. The rate of a certain reaction at 0 0 C is equal to 1 mol/l ∙ h, the temperature coefficient of the reaction is 3. What will the rate of this reaction be at 30 0 C?

υ 2 = υ 1 ∙ γ Δt/10

υ 2 =1∙3 30-0/10 = 3 3 =27 mol/l∙h

3) Concentration: the more, the more often collisions and υ occur. At constant temperature for the reaction mA + nB = C according to the law of mass action:

υ = k ∙ C A m ∙ C B n

where k is the rate constant;

C – concentration (mol/l)

Law of mass action:

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.

Z.d.m. does not take into account the concentration of reacting substances in the solid state, because they react on surfaces and their concentrations usually remain constant.

Task 2. The reaction proceeds according to the equation A + 2B → C. How many times and how will the reaction rate change when the concentration of substance B increases by 3 times?

Solution:υ = k ∙ C A m ∙ C B n

υ = k ∙ C A ∙ C B 2

υ 1 = k ∙ a ∙ b 2

υ 2 = k ∙ a ∙ 3 in 2

υ 1 / υ 2 = a ∙ in 2 / a ∙ 9 in 2 = 1/9

Answer: will increase 9 times

For gaseous substances, the reaction rate depends on pressure

How more pressure, the higher the speed.

4) Catalysts– substances that change the reaction mechanism, reduce E act => υ .

▪ Catalysts remain unchanged after the reaction is completed

▪ Enzymes are biological catalysts, proteins by nature.

▪ Inhibitors – substances that ↓ υ

5) For heterogeneous reactions, υ also depends:

▪ on the state of the contact surface of the reacting substances.

Compare: equal volumes of sulfuric acid solution were poured into 2 test tubes and at the same time an iron nail was dropped into one and iron filings into the other. Grinding solid leads to an increase in the number of its molecules that can simultaneously react. Therefore, in the second test tube the reaction rate will be greater than in the first.

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 - substance concentration,

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 get 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 speeds. For example, zinc reacts violently with hydrochloric acid, and iron is quite slow.

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.

Chemical methods

Physical methods

Methods for measuring reaction speed

In the above example, the rate of reaction between calcium carbonate and acid was measured by studying the volume of gas released as a function of time. Experimental data on reaction rates can be obtained by measuring other quantities.

If the total amount of gaseous substances changes during a reaction, then its progress can be monitored by measuring the gas pressure at constant volume. In cases where one of the starting materials or one of the reaction products is colored, the progress of the reaction can be monitored by observing the change in color of the solution. Another optical method is to measure the rotation of the plane of polarization of light (if the starting materials and reaction products have different rotating powers).

Some reactions are accompanied by a change in the number of ions in the solution. In such cases, the reaction rate can be studied by measuring the electrical conductivity of the solution. The next chapter will look at some other electrochemical techniques that can be used to measure reaction rates.

The progress of a reaction can be monitored by measuring the concentration of one of the reaction participants over time using a variety of chemical analysis methods. The reaction is carried out in a thermostated vessel. At certain intervals, a sample of the solution (or gas) is taken from the vessel and the concentration of one of the components is determined. To obtain reliable results, it is important that no reaction occurs in the sample taken for analysis. This is achieved by chemically binding one of the reagents, sudden cooling or diluting the solution.

Experimental studies show that the speed of the reaction depends on several factors. Let us first consider the influence of these factors on a qualitative level.

1.The nature of the reacting substances. From laboratory practice we know that neutralization of an acid with a base

H + + OH – ® H 2 O

interaction of salts with the formation of a slightly soluble compound

Ag + + Cl – ® AgCl

and other reactions in electrolyte solutions occur very quickly. The time required to complete such reactions is measured in milliseconds and even microseconds. This is quite understandable, because the essence of such reactions is the approach and combination of charged particles with charges of the opposite sign.

In contrast to ionic reactions, interactions between covalently bonded molecules usually occur much more slowly. Indeed, during the reaction between such particles, the bonds in the molecules of the starting substances must be broken. To do this, the colliding molecules must have a certain amount of energy. In addition, if the molecules are complex enough, in order for a reaction to occur between them, they must be oriented in a certain way in space.

2. Concentration of reactants. The rate of a chemical reaction, other things being equal, depends on the number of collisions of reacting particles per unit time. The probability of collisions depends on the number of particles per unit volume, i.e. on concentration. Therefore, the reaction rate increases with increasing concentration.

3. Physical state of substances. In homogeneous systems, the reaction rate depends on the number of particle collisions in volume of solution(or gas). In heterogeneous systems, chemical interaction occurs at the interface. Increasing the surface area of ​​a solid when it is crushed makes it easier for the reacting particles to reach the particles of the solid, which leads to a significant acceleration of the reaction.

4. Temperature has a significant impact on the rate of various chemical and biological processes. As the temperature increases, the kinetic energy of particles increases, and, consequently, the proportion of particles whose energy is sufficient for chemical interaction increases.

5. Steric factor characterizes the need for mutual orientation of reacting particles. The more complex the molecules, the less likely they are to be properly oriented, and the less efficient the collisions.

6. Availability of catalysts.Catalysts are substances whose presence changes the rate of a chemical reaction. Introduced into the reaction system in small quantities and remaining unchanged after the reaction, they are capable of extremely changing the rate of the process.

The main factors on which the reaction rate depends will be discussed in more detail below.

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. The nature of the chemical bonds and the structure of the reagent molecules play an important role. 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)
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:

(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.

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").

One of the areas of physical chemistry, chemical kinetics, studies the rate of a chemical reaction and the conditions affecting its change. It also examines the mechanisms of these reactions and their thermodynamic validity. These studies are important not only for scientific purposes, but also for monitoring the interaction of components in reactors during the production of all kinds of substances.

The concept of speed in chemistry

The reaction rate is usually called a certain change in the concentrations of the compounds that reacted (ΔC) per unit time (Δt). The mathematical formula for the rate of a chemical reaction is as follows:

ᴠ = ±ΔC/Δt.

The reaction rate is measured in mol/l∙s if it occurs throughout the entire volume (that is, the reaction is homogeneous) and in mol/m 2 ∙s if the interaction occurs on the surface separating the phases (that is, the reaction is heterogeneous). The “-” sign in the formula refers to changes in the concentrations of the initial reactants, and the “+” sign refers to changing concentrations of the products of the same reaction.

Examples of reactions at different rates

Chemical interactions can occur at different rates. Thus, the growth rate of stalactites, that is, the formation of calcium carbonate, is only 0.5 mm per 100 years. Some biochemical reactions occur slowly, such as photosynthesis and protein synthesis. Corrosion of metals occurs at a fairly low rate.

Medium speed can be used to describe reactions that require one to several hours. An example would be cooking, which involves the decomposition and transformation of compounds contained in foods. Synthesis of individual polymers requires heating the reaction mixture for a certain time.

An example of chemical reactions whose speed is quite high is neutralization reactions, the interaction of sodium bicarbonate with a solution of acetic acid, accompanied by the release carbon dioxide. You can also mention the interaction of barium nitrate with sodium sulfate, in which the release of a precipitate of insoluble barium sulfate is observed.

A large number of reactions can occur at lightning speed and are accompanied by an explosion. A classic example is the interaction of potassium with water.

Factors affecting the rate of a chemical reaction

It is worth noting that the same substances can react with each other at different rates. For example, a mixture of gaseous oxygen and hydrogen can be quite long time show no signs of interaction, but when the container is shaken or struck, the reaction becomes explosive. Therefore, chemical kinetics identifies certain factors that have the ability to influence the rate of a chemical reaction. These include:

• the nature of the interacting substances;
• concentration of reagents;
• temperature change;
• presence of a catalyst;
• pressure change (for gaseous substances);
• area of ​​contact of substances (if we are talking about heterogeneous reactions).

Influence of the nature of the substance

So significant difference in the rates of chemical reactions is explained different meanings activation energy (Ea). It is understood as a certain excess amount of energy in comparison with its average value required by a molecule during a collision in order for a reaction to occur. It is measured in kJ/mol and values ​​are usually in the range of 50-250.

It is generally accepted that if E a = 150 kJ/mol for any reaction, then at n. u. it practically does not leak. This energy is spent on overcoming repulsion between the molecules of substances and on weakening the bonds in the original substances. In other words, activation energy characterizes the strength of chemical bonds in substances. Based on the value of activation energy, you can preliminary estimate the rate of a chemical reaction:

• E a< 40, взаимодействие веществ происходят довольно быстро, поскольку почти все столкнове-ния частиц при-водят к их реакции;
• 40-<Е а <120, предполагается средняя реакция, поскольку эффективными будет лишь половина соударений молекул (например, реакция цинка с соляной кислотой);
• E a >120, only a very small part of particle collisions will lead to a reaction, and its speed will be low.

Effect of concentration

The dependence of the reaction rate on concentration is most accurately characterized by the law of mass action (LMA), which states:

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, the values ​​of which are taken in powers corresponding to their stoichiometric coefficients.

This law is suitable for elementary one-stage reactions, or any stage of the interaction of substances characterized by a complex mechanism.

If you need to determine the rate of a chemical reaction, the equation of which can be conditionally written as:

αA+ bB = ϲС, then

in accordance with the above formulation of the law, the speed can be found using the equation:

V=k·[A] a ·[B] b , where

a and b are stoichiometric coefficients,

[A] and [B] are the concentrations of the starting compounds,

k is the rate constant of the reaction under consideration.

The meaning of the rate coefficient of a chemical reaction is that its value will be equal to the rate if the concentrations of the compounds are equal to units. It should be noted that for correct calculation using this formula, it is worth taking into account the state of aggregation of the reagents. The solid concentration is taken to be unity and is not included in the equation because it remains constant during the reaction. Thus, only the concentrations of liquid and gaseous substances are included in the calculations according to the ZDM. Thus, for the reaction of producing silicon dioxide from simple substances, described by the equation

Si (tv) + Ο 2(g) = SiΟ 2(tv) ,

the speed will be determined by the formula:

Typical task

How would the rate of the chemical reaction of nitrogen monoxide with oxygen change if the concentrations of the starting compounds were doubled?

Solution: This process corresponds to the reaction equation:

2ΝΟ + Ο 2 = 2ΝΟ 2.

Let us write down the expressions for the initial (ᴠ 1) and final (ᴠ 2) reaction rates:

ᴠ 1 = k·[ΝΟ] 2 ·[Ο 2 ] and

ᴠ 2 = k·(2·[ΝΟ]) 2 ·2·[Ο 2 ] = k·4[ΝΟ] 2 ·2[Ο 2 ].

ᴠ 1 /ᴠ 2 = (k·4[ΝΟ] 2 ·2[Ο 2 ]) / (k·[ΝΟ] 2 ·[Ο 2 ]).

ᴠ 2 /ᴠ 1 = 4 2/1 = 8.

Answer: increased 8 times.

Effect of temperature

The dependence of the rate of a chemical reaction on temperature was determined experimentally by the Dutch scientist J. H. Van't Hoff. He found that the rate of many reactions increases 2-4 times with every 10 degree increase in temperature. There is a mathematical expression for this rule that looks like:

ᴠ 2 = ᴠ 1 ·γ (Τ2-Τ1)/10, where

ᴠ 1 and ᴠ 2 - corresponding speeds at temperatures Τ 1 and Τ 2;

γ - temperature coefficient, equal to 2-4.

At the same time, this rule does not explain the mechanism of the influence of temperature on the rate of a particular reaction and does not describe the entire set of patterns. It is logical to conclude that with increasing temperature, the chaotic movement of particles intensifies and this provokes a greater number of collisions. However, this does not particularly affect the efficiency of molecular collisions, since it depends mainly on the activation energy. Also, their spatial correspondence to each other plays a significant role in the efficiency of particle collisions.

The dependence of the rate of a chemical reaction on temperature, taking into account the nature of the reagents, obeys the Arrhenius equation:

k = A 0 e -Ea/RΤ, where

A o is a multiplier;

E a - activation energy.

An example of a problem using van't Hoff's law

How should the temperature be changed so that the rate of a chemical reaction, whose temperature coefficient is numerically equal to 3, increases by 27 times?

Solution. Let's use the formula

ᴠ 2 = ᴠ 1 ·γ (Τ2-Τ1)/10.

From the condition ᴠ 2 /ᴠ 1 = 27, and γ = 3. You need to find ΔΤ = Τ 2 -Τ 1.

Transforming the original formula we get:

V 2 /V 1 =γ ΔΤ/10.

We substitute the values: 27 = 3 ΔΤ/10.

From this it is clear that ΔΤ/10 = 3 and ΔΤ = 30.

Answer: the temperature should be increased by 30 degrees.

Effect of catalysts

In physical chemistry, the rate of chemical reactions is also actively studied by a section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Substances that can speed up a reaction, but are not consumed in it themselves, are called catalysts.

It has been proven that catalysts change the mechanism of the chemical interaction itself and contribute to the emergence of new transition states, which are characterized by lower energy barrier heights. That is, they help reduce the activation energy, and therefore increase the number of effective particle impacts. A catalyst cannot cause a reaction that is energetically impossible.

Thus, hydrogen peroxide can decompose to form oxygen and water:

N 2 Ο 2 = N 2 Ο + Ο 2.

But this reaction is very slow and in our first aid kits it exists unchanged for quite a long time. When opening only very old bottles of peroxide, you may notice a slight popping sound caused by the pressure of oxygen on the walls of the vessel. Adding just a few grains of magnesium oxide will provoke active gas release.

The same reaction of peroxide decomposition, but under the influence of catalase, occurs when treating wounds. Living organisms contain many different substances that increase the rate of biochemical reactions. They are usually called enzymes.

Inhibitors have the opposite effect on the course of reactions. However, this is not always a bad thing. Inhibitors are used to protect metal products from corrosion, to extend the shelf life of food, for example, to prevent the oxidation of fats.

Substance contact area

In the event that the interaction occurs between compounds that have different states of aggregation, or between substances that are not capable of forming a homogeneous environment (immiscible liquids), then this factor also significantly affects the rate of the chemical reaction. This is due to the fact that heterogeneous reactions take place directly at the interface between the phases of interacting substances. Obviously, the wider this boundary, the more particles have the opportunity to collide, and the faster the reaction occurs.

For example, it goes much faster in the form of small chips than in the form of a log. For the same purpose, many solids are ground into a fine powder before being added to the solution. Thus, powdered chalk (calcium carbonate) acts faster with hydrochloric acid than a piece of the same mass. However, in addition to increasing the area, this technique also leads to a chaotic rupture of the crystal lattice of the substance, and therefore increases the reactivity of the particles.

Mathematically, the rate of a heterogeneous chemical reaction is found as the change in the amount of substance (Δν) occurring per unit time (Δt) per unit surface

(S): V = Δν/(S·Δt).

Effect of pressure

A change in pressure in the system has an effect only when gases take part in the reaction. An increase in pressure is accompanied by an increase in the molecules of a substance per unit volume, that is, its concentration increases proportionally. Conversely, a decrease in pressure leads to an equivalent decrease in the concentration of the reagent. In this case, the formula corresponding to the ZDM is suitable for calculating the rate of a chemical reaction.

Task. How will the rate of the reaction described by the equation increase?

2ΝΟ + Ο 2 = 2ΝΟ 2,

if the volume of a closed system is reduced by three times (T=const)?

Solution. As volume decreases, pressure increases proportionally. Let's write down the expressions for the initial (V 1) and final (V 2) reaction rates:

V 1 = k 2 [Ο 2 ] and

V 2 = k·(3·) 2 ·3·[Ο 2 ] = k·9[ΝΟ] 2 ·3[Ο 2 ].

To find how many times the new speed is greater than the initial one, you should separate the left and right sides of the expressions:

V 1 /V 2 = (k 9[ΝΟ] 2 3[Ο 2 ]) / (k [ΝΟ] 2 [Ο 2 ]).

The concentration values ​​and rate constants are reduced, and what remains is:

V 2 /V 1 = 9 3/1 = 27.

Answer: the speed has increased 27 times.

To summarize, it should be noted that the speed of interaction of substances, or more precisely, the quantity and quality of collisions of their particles, is influenced by many factors. First of all, these are the activation energy and the geometry of the molecules, which are almost impossible to correct. As for the remaining conditions, to increase the reaction rate one should:

• increase the temperature of the reaction medium;
• increase the concentrations of the starting compounds;
• increase the pressure in the system or reduce its volume if we are talking about gases;
• bring dissimilar substances to one state of aggregation (for example, by dissolving them in water) or increase the area of ​​their contact.