Tasks for the section chemical kinetics and equilibrium of a chemical reaction. Chemical kinetics and equilibrium Chemical kinetics. Chemical equilibrium

The purpose of the work: to study the effect of temperature on the reaction rate of concentration on a shift in chemical equilibrium. Theoretical justification: The rate of a chemical reaction is the amount of a substance entering into a reaction or formed as a result of a reaction per unit of time per unit volume for homogeneous reactions or per unit interface for heterogeneous reactions. If in a period of time...

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"Ufa State Oil Technical University"

Department of General and Analytical Chemistry

REPORT

According to laboratory work No. 1

"Chemical Kinetics and Equilibrium"

Student of the group _______________ E.V. Beletskova

BTS-14-01

Associate Professor _______________ S.B.Denisova

2014

Goal of the work : study of the effect of temperature on the reaction rate, concentration on the shift of chemical equilibrium.

Theoretical justification:

The rate of a chemical reactioncalled the amount of a substance that enters into a reaction or is formed as a result of a reaction per unit time per unit volume (for homogeneous reactions) or per unit interface of phases (for heterogeneous reactions).

If during the time interval ∆τ = τ 2 - τ 1 the concentration of one of the substances participating in the reaction decreases by ∆С = С 2 - C 1 , then the average rate of a chemical reaction over a specified period of time is equal to

V value expresses the rate of a chemical process over a certain period of time. Therefore, the smaller ∆τ, the closer the average speed will be to the true one.

The rate of a chemical reaction depends on the following factors:

nature and concentration of reacting substances;
reaction system temperature;
the presence of a catalyst;
pressure,
the magnitude of the phase separation surface and the mixing rate of the system (for heterogeneous reactions);
solvent type.

Influence of the concentration of reagents. The reaction rate is proportional to the number of collisions of the molecules of the reactants. The number of collisions, in turn, the greater, the higher the concentration of each of the starting substances.

The general formulation of the effect of concentration on the rate of a chemical reaction is given bylaw of mass action(1867, Guldberg, Waage, Beketov).

At a constant temperature, the rate of a chemical reaction is proportional to the product of the concentrations of the reactants, taken in powers of their equalizing (stoichiometric) coefficients.

For the reaction aA + bB = cC V = K [ A ] a [ B ] in ,

where K coefficient of proportionality or rate constant;

 reagent concentration in mol/l.

If [A] = 1 mol/l, [B] = 1 mol/l, then V=K , hence the physical meaning

rate constants K: the rate constant is equal to the reaction rate at concentrations of reactants equal to unity.

Effect of temperature on the reaction rate. As the temperature rises, the frequency of collisions of the reacting molecules increases, and consequently, the rate of the reaction increases.

The effect of temperature on the rate of homogeneous reactions can be quantified by the van't Hoff rule.

In accordance with the van't Hoff rule, with an increase (decrease) in temperature by 10 degrees, the rate of a chemical reaction increases (decreases) by 2-4 times:

or ,

where V (t 2 ) and V (t 1 ) chemical reaction rates at appropriate temperatures;τ (t 2 ) and τ (t 1 ) the duration of a chemical reaction at appropriate temperatures;γ van't Hoff temperature coefficient, which can take a numerical value in the range 2-4.

Activation energy. The excess energy that molecules must have in order for their collision to lead to the formation of a new substance is called the activation energy of this reaction (expressed in kJ / mol). One way to activate is to increase the temperature: as the temperature rises, the number of active particles increases greatly, due to which the reaction rate increases sharply.

The dependence of the reaction rate on temperature is expressed by the Arrhenius equation:

where K is the rate constant of a chemical reaction; E a activation energy;

R universal gas constant; A constant; exp base of natural logarithms.

The value of the activation energy can be determined if two values of the rate constant K are known 1 and K 2 at a temperature respectively T 1 and T 2 , according to the following formula:

chemical balance.

All chemical reactions can be divided into two groups: irreversible and reversible. Irreversible reactions proceed to the end until one of the reactants is completely consumed, i.e. flow in only one direction. Reversible reactions do not proceed to completion. In a reversible reaction, none of the reactants is completely consumed. A reversible reaction can proceed in both forward and reverse directions.

Chemical equilibrium is the state of a system in which the rates of the forward and reverse reactions are equal.

For a reversible reaction

m A + n B ⇄ p C + q D

the chemical equilibrium constant is

In reversible chemical reactions, equilibrium is established at the moment when the ratio of the product of the concentrations of the products, raised to powers equal to stoichiometric coefficients, to the product of the concentrations of the starting substances, also raised to the appropriate powers, is equal to a certain constant value called the chemical equilibrium constant.

The chemical equilibrium constant depends on the nature of the reactants and on the temperature. The concentrations at which equilibrium is established are called equilibrium. A change in external conditions (concentration, temperature, pressure) causes a shift in the chemical equilibrium in the system and its transition to a new equilibrium state.

Such a transition of the reaction system from one state to another is called a shift (or shift) of chemical equilibrium.

The direction of the chemical equilibrium shift is determined by Le Chatelier's principle:If a system in a state of chemical equilibrium is subjected to some external influence (to change the concentration, temperature, pressure), then processes spontaneously arise in this system that seek to weaken the effect produced.

Increasing the concentration of one of the initial reagents shifts the equilibrium to the right (the forward reaction intensifies); an increase in the concentration of reaction products shifts the equilibrium to the left (reverse reaction intensifies).

If the reaction proceeds with an increase in the number of gas molecules (i.e., on the right side of the reaction equation, the total number of gas molecules is greater than the number of molecules of gaseous substances on the left side), then an increase in pressure prevents the reaction, and a decrease in pressure favors the reaction.

When the temperature rises, the equilibrium shifts towards an endothermic reaction, and when it decreases, it shifts towards an exothermic reaction.

A catalyst changes the rate of both the forward and reverse reactions by the same factor. Therefore, the catalyst does not cause a shift in equilibrium, but only reduces or increases the time required to reach equilibrium.

Experience No. 1 Dependence of the rate of a homogeneous reaction on the concentration of the initial reagents.

Devices, equipment: test tubes, stopwatch, solutions of sodium thiosulfate ( III ) , razb. sulfuric acid (1M), water.
Procedure: This dependence can be studied on the classical example of a homogeneous reaction of the interaction of sodium thiosulfate with sulfuric acid, proceeding according to the equation

Na 2 S 2 O 3 + H 2 SO 4 = Na 2 SO 4 + S↓ + SO 2 + H 2 O.

Sulfur at the first moment forms a colloidal solution with water (hardly perceptible turbidity). It is necessary to measure with a stopwatch the time from the moment of draining to the appearance of a barely noticeable turbidity. Knowing the reaction time (in seconds), one can determine the relative rate of the reaction, i.e. reciprocal of time: .

For the experiment, three dry, clean test tubes should be prepared and numbered. In the first, add 4 drops of sodium thiosulfate solution and 8 drops of water; in the second 8 drops of sodium thiosulfate and 4 drops of water; in the third 12 drops of sodium thiosulfate. Shake the tubes.

If we conditionally designate the molar concentration of sodium thiosulfate in test tube 1 through "s", then, accordingly, in test tube 2 it will be 2 s mol, in test tube 3 3 s mol.

Add one drop of sulfuric acid to test tube 1, at the same time turn on the stopwatch: while shaking the test tube, monitor the appearance of turbidity in the test tube, keeping it at eye level. When the slightest turbidity appears, stop the stopwatch, note the reaction time and write it down in the table.

Carry out similar experiments with the second and third test tubes. Enter the data of the experiment in the laboratory journal in the form of a table ...

tube number	Number of drops Na 2 S 2 O 3	Number of water drops	Number of drops H2SO4	Na 2 S 2 O 3 concentration in moles	Reaction time τ , s	Relative speed V =1/ τ , c-1
					26,09	3,83
					12,19
					8,27	12,09

Graph of reaction rate versus sodium thiosulfate concentration.

Conclusion: with an increase in the concentration of sodium thiosulfate, the rate of this reaction increases. The dependency graph is a straight line passing through the origin.

Experiment No. 2. Studying the dependence of the rate of a homogeneous reaction on temperature.

Instruments and equipment: test tubes, stopwatch, thermometer, sodium thiosulfate solutions ( III ), sulfuric acid (1M)
Methodology:

Prepare three clean dry test tubes, number them. Add 10 drops of sodium thiosulfate solution to each of them. Place test tube No. 1 in a glass of water at room temperature and note the temperature after 1–2 minutes. Then add one drop of sulfuric acid to the test tube, turn on the stopwatch at the same time and stop it when a weak, barely noticeable turbidity appears. Note the time in seconds from the moment the acid is added to the test tube until the appearance of turbidity. Record the result in a table.

Then raise the temperature of the water in the glass by exactly 10 0 either by heating on a hot plate or by mixing with hot water. Place test tube No. 2 in this water, hold for several minutes and add one drop of sulfuric acid, turning on the stopwatch at the same time, shake the test tube with the contents in a glass of water until turbidity appears. When a barely noticeable turbidity appears, turn off the stopwatch and enter the stopwatch readings into the table. Carry out a similar experiment with the third test tube. Raise the temperature in the beaker by another 10 0 , place test tube No. 3 in it, hold for several minutes and add one drop of sulfuric acid, simultaneously turning on the stopwatch and shaking the test tube.

Express the results of the experiments in a graph, plotting the velocity along the ordinate axis, and the temperature along the abscissa axis.

Determine the temperature coefficient of the reaction γ

test tubes

Temperature

t , 0 C

Reaction time

τ, s

Relative speed

reactions

1/τ,s -1

Temperature coefficient

26,09

17,22

10,74

3,83

5,81

9,31

1,51

1,55

Graph of reaction rate versus temperature.

Conclusion: during the experiment, the average temperature coefficient was calculated, which turned out to be 1.55. Ideally, it is

2-4. The deviation from the ideal can be explained by the error in measuring the cloudiness of the solution. The graph of the dependence of the reaction rate on temperature has the form of a parabola branch that does not pass through 0. With increasing temperature, the reaction rate increases

Experience No. 3 The influence of the concentration of reacting substances on chemical equilibrium.

Devices and equipment: test tubes, potassium chloride (crystal), ferric chloride solutions ( III ), potassium thiocyanate (sat.), distilled water, cylinder
Methodology:

A classic example of a reversible reaction is the interaction between ferric chloride and potassium thiocyanate:

FeCl 3 + 3 KCNS ⇄ Fe(CNS) 3 + 3 KCl.

Red

The resulting iron thiocyanate has a red color, the intensity of which depends on the concentration. By changing the color of the solution, one can judge the shift in chemical equilibrium depending on the increase or decrease in the content of iron thiocyanate in the reaction mixture. Write an equation for the equilibrium constant of this process.

Pour 20 ml of distilled water into a measuring cup or cylinder and add one drop of a saturated solution of ferric chloride ( III ) and one drop of a saturated solution of potassium thiocyanate. Pour the resulting colored solution into four test tubes equally. Number tubes.

In the first test tube add one drop of saturated ferric chloride solution ( III ).Add one drop of a saturated solution of potassium thiocyanate to the second test tube. Add crystalline potassium chloride to the third test tubeand shake vigorously. Fourth test tube- for comparison.

Based on Le Chatelier's principle, explain what caused the color change in each individual case.

Record the results of the experiment in a table in the form

test tubes

What

added

Change

intensity

coloring

Direction of equilibrium shift

(right left)

In the first case, in the second case, we increased the concentration of the initial substances, so a more intense color is obtained. Moreover, in the second case, the color is darker, because the concentration KSCN changes with cubic speed. In the third experiment, we increased the concentration of the final substance, so the color of the solution is lighter.

Conclusion: with an increase in the concentration of the starting substances, the equilibrium shifts towards the formation of reaction products. With an increase in the concentration of products, the equilibrium shifts towards the formation of the starting substances.

General conclusions: in the course of the experiments, we experimentally established the dependence of the reaction rate on the concentration of the starting substances (the higher the concentration, the higher the reaction rate); the dependence of the reaction rate on temperature (the higher the temperature, the greater the reaction rate); how the concentration of reacting substances affects the chemical equilibrium (with an increase in the concentration of the starting substances, the chemical equilibrium shifts towards the formation of products; with an increase in the concentration of products, the chemical equilibrium shifts towards the formation of the starting substances)

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Chemical kinetics

Chemical equilibrium

Chemical kinetics is a branch of chemistry that studies the rate of a chemical reaction and the factors that affect it.

The fundamental feasibility of the process is judged by the value of the change in the Gibbs energy of the system. However, it does not say anything about the real possibility of the reaction under the given conditions, does not give an idea of the speed and mechanism of the process.

The study of reaction rates makes it possible to elucidate the mechanism of complex chemical transformations. This creates a perspective for the control of the chemical process, allows for mathematical modeling of processes.

Reactions can be:

1. homogeneous– flow in one medium (in the gas phase); pass in its entirety;

2. heterogeneous- do not occur in the same medium (between substances in different phases); pass through the interface.

Under chemical reaction rate understand the number of elementary reactions taking place per unit time per unit volume (for homogeneous reactions) and per unit surface (for heterogeneous reactions).

Since the concentration of the reactants changes during the reaction, the rate is usually defined as the change in the concentration of the reactants per unit time and is expressed in. In this case, there is no need to monitor the change in the concentration of all substances involved in the reaction, since the stoichiometric coefficient in the reaction equation establishes the ratio between the concentrations, i.e. at the rate of accumulation of ammonia is twice the rate of consumption of hydrogen.

, , because cannot be negative, so put "-".

Speed in time interval – true instantaneous speed– 1st derivative of concentration with respect to time.

The rate of chemical reactions depends :

1. from the nature of the reacting substances;

2. on the concentration of reagents;

3. from the catalyst;

4. on temperature;

5. on the degree of grinding of a solid substance (heterogeneous reactions);

6. from the environment (solutions);

7. from the form of the reactor (chain reactions);

8. from lighting (photochemical reactions).

The basic law of chemical kinetics is law of mass action: the rate of a chemical reaction is proportional to the product of the concentrations of the reactants in the reaction

where is the rate constant of the chemical reaction

Physical meaning at .

If more than 2 particles participate in the reaction, then: ~ in powers equal to stoichiometric coefficients, i.e.: , Where

- an indicator of the order of the reaction as a whole (reactions of the first, second, third ... orders).

The number of particles involved in this reaction act determines reaction molecularity :

Monomolecular ()

Bimolecular ( )

Trimolecular.

More than 3 does not happen, because collision of more than 3 particles at once is unlikely.

When the reaction goes in several stages, then the total reaction = the slowest stage (limiting stage).

The dependence of the reaction rate on temperature is determined by the empirical van't Hoff's rule: with an increase in temperature by , the rate of a chemical reaction increases by 2–4 times: .

where is the temperature coefficient of the chemical reaction rate .

Not every collision of molecules is accompanied by their interaction. Most molecules bounce off like elastic balls. And only those that are active in a collision interact with each other. Active molecules have some excess compared to inactive molecules, so in active molecules the bonds between them are weakened.

The energy to transfer a molecule to an active state is the activation energy. The smaller it is, the more particles react, the greater the rate of the chemical reaction.

The value depends on the nature of the reactants. It is less than dissociation - the least strong bond in the reagents.

Change in the course of the reaction:

Released (exothermic)

As the temperature increases, the number of active molecules increases, so it increases.

The chemical reaction constant is related to

where is the pre-exponential factor (related to the probability and number of collisions).

Depending on the nature of the reacting substances and the conditions of their interaction, atoms, molecules, radicals or ions can take part in the elementary acts of reactions.

Free radicals are extremely reactive, active radical reactions are very small ().

The formation of free radicals can occur in the process of decomposition of substances at temperature, lighting, under the influence of nuclear radiation, during electric discharge, and strong mechanical influences.

Many reactions proceed chain mechanism. A feature of chain reactions is that one primary act of activation leads to the transformation of a huge number of molecules of the starting substances.

For example: .

At normal temperature and diffused light, the reaction proceeds extremely slowly. When a mixture of gases is heated or exposed to light rich in UV rays (direct sunlight, light from a burning one), the mixture explodes.

This reaction proceeds through separate elementary processes. First of all, due to the absorption of a quantum of energy of UV rays (or temperature), the molecule dissociates into free radicals - atoms: , then , then , etc.

Naturally, free radicals can also collide with each other, which leads to chain termination: .

In addition to temperature, the reactivity of substances is significantly affected by light. The effect of light (visible, UV) on reactions is studied by the branch of chemistry - photochemistry.

Photochemical processes are very diverse. During the photochemical action, the molecules of the reacting substances, by absorbing light quanta, are excited, i.e. become reactive or decompose into ions and free radicals. Photography is based on photochemical processes - the effect of light on photosensitive materials (photosynthesis).

One of the most common methods in chemical practice for accelerating chemical reactions is catalysis . Catalysts- substances that change a chemical reaction by participating in an intermediate chemical interaction with the reaction components, but restoring their chemical composition after each cycle of the intermediate interaction.

The increase in the catalytic reaction is associated with a smaller new reaction path. Because in the expression for is included in the negative exponent, then even a small decrease causes a very large increase in the chemical reaction.

Exist 2 types of catalysts :

homocatalysts;

heterocatalysts.

Biological catalysts - enzymes .

Inhibitors- Substances that slow down chemical reactions.

promoters- substances that enhance the action of catalysts.

Reactions that proceed in only one direction and go to the end - irreversible(precipitation, gas evolution). They are few.

Most reactions are reversible : .

According to the law of mass action: – chemical equilibrium .

The state of a system in which forward reaction = reverse reaction is called chemical equilibrium .

With increasing temperature, : increases for an endothermic reaction, decreases for an exothermic reaction, and remains constant.

The influence of various factors on the position of chemical equilibrium is determined principle of La Chatelier: if a system in equilibrium is affected in some way, then processes in the system are intensified, seeking to reduce this impact.

Chemical kinetics

Chemical equilibrium

Chemical kinetics is a branch of chemistry that studies the rate of a chemical reaction and the factors that affect it.

Reactions can be:

1. homogeneous– flow in one medium (in the gas phase); pass in its entirety;

2. heterogeneous- do not occur in the same medium (between substances in different phases); pass through the interface.

Under chemical reaction rate understand the number of elementary reactions taking place per unit time per unit volume (for homogeneous reactions) and per unit surface (for heterogeneous reactions).

Since the reaction changes the concentration of the reactants, the rate is usually defined as the change in the concentration of the reactants per unit time and is expressed in terms of

. In this case, there is no need to monitor the change in the concentration of all substances involved in the reaction, since the stoichiometric coefficient in the reaction equation establishes the ratio between the concentrations, i.e. when the rate of accumulation of ammonia is twice the rate of consumption of hydrogen.
, , because cannot be negative, so put "-".

Speed in time interval

– true instantaneous speed– 1st derivative of concentration with respect to time.

The rate of chemical reactions depends:

1. from the nature of the reacting substances;

2. on the concentration of reagents;

3. from the catalyst;

4. on temperature;

5. on the degree of grinding of a solid substance (heterogeneous reactions);

6. from the environment (solutions);

7. from the form of the reactor (chain reactions);

8. from lighting (photochemical reactions).

The basic law of chemical kinetics is law of mass action: the rate of a chemical reaction is proportional to the product of the concentrations of the reactants in the reaction

: , is the rate constant of a chemical reaction

physical meaning

at .

If not 2 particles participate in the reaction, but more

, then: ~ in powers equal to stoichiometric coefficients, i.e.: , where is the index of the order of the reaction as a whole (reactions of the first, second, third ... orders).

The number of particles involved in this reaction act determines reaction molecularity:

monomolecular ( ) bimolecular ( ) trimolecular.

More than 3 does not happen, because collision of more than 3 particles at once is unlikely.

When the reaction proceeds in several stages, then the total

reactions = slowest stage (limiting stage).

The dependence of the reaction rate on temperature is determined by the empirical van't Hoff's rule: when the temperature increases by

, the rate of a chemical reaction increases by 2–4 times: . ,
is the temperature coefficient of the chemical reaction rate.

but in comparison with inactive molecules, therefore, in active molecules, the bonds between them are weakened.

The energy to transfer the molecule to the active state is the activation energy

. The smaller it is, the more particles react, the greater the rate of the chemical reaction.

Value

depends on the nature of the reactants. It is less than dissociation - the least strong bond in the reagents.

Change

during the reaction: released (exothermic)

As the temperature increases, the number of active molecules increases, so

Chemical kinetics

Chemical equilibrium

Chemical kinetics is a branch of chemistry that studies the rate of a chemical reaction and the factors that affect it.

Reactions can be:

1. homogeneous– flow in one medium (in the gas phase); pass in its entirety;

2. heterogeneous- do not occur in the same medium (between substances in different phases); pass through the interface.

Under chemical reaction rate understand the number of elementary reactions taking place per unit time per unit volume (for homogeneous reactions) and per unit surface (for heterogeneous reactions).

, , because cannot be negative, so put "-".

Speed in time interval – true instantaneous speed– 1st derivative of concentration with respect to time.

The rate of chemical reactions depends:

1. from the nature of the reacting substances;

2. on the concentration of reagents;

3. from the catalyst;

4. on temperature;

5. on the degree of grinding of a solid substance (heterogeneous reactions);

6. from the environment (solutions);

7. from the form of the reactor (chain reactions);

8. from lighting (photochemical reactions).

The basic law of chemical kinetics is law of mass action: the rate of a chemical reaction is proportional to the product of the concentrations of the reactants in the reaction

where is the rate constant of the chemical reaction

Physical meaning at .

If more than 2 particles participate in the reaction, then: ~ in powers equal to stoichiometric coefficients, i.e.: , Where

- an indicator of the order of the reaction as a whole (reactions of the first, second, third ... orders).

The number of particles involved in this reaction act determines reaction molecularity:

Monomolecular ()

Bimolecular ( )

Trimolecular.

More than 3 does not happen, because collision of more than 3 particles at once is unlikely.

When the reaction goes in several stages, then the total reaction = the slowest stage (limiting stage).

The dependence of the reaction rate on temperature is determined by the empirical van't Hoff's rule: with an increase in temperature by , the rate of a chemical reaction increases by 2–4 times: .

where is the temperature coefficient of the chemical reaction rate .

The energy to transfer a molecule to an active state is the activation energy. The smaller it is, the more particles react, the greater the rate of the chemical reaction.

The value depends on the nature of the reactants. It is less than dissociation - the least strong bond in the reagents.

Change in the course of the reaction:

Released (exothermic)

As the temperature increases, the number of active molecules increases, so it increases.

The chemical reaction constant is related to

where is the pre-exponential factor (related to the probability and number of collisions).

Depending on the nature of the reacting substances and the conditions of their interaction, atoms, molecules, radicals or ions can take part in the elementary acts of reactions.

Free radicals are extremely reactive, active radical reactions are very small ().

Many reactions proceed chain mechanism. A feature of chain reactions is that one primary act of activation leads to the transformation of a huge number of molecules of the starting substances.

For example: .

Naturally, free radicals can also collide with each other, which leads to chain termination: .

In addition to temperature, the reactivity of substances is significantly affected by light. The effect of light (visible, UV) on reactions is studied by the branch of chemistry - photochemistry.

One of the most common methods in chemical practice for accelerating chemical reactions is catalysis. Catalysts- substances that change a chemical reaction by participating in an intermediate chemical interaction with the reaction components, but restoring their chemical composition after each cycle of the intermediate interaction.

Target:

solving experimental problems related to determining the dependence of the rate of a chemical reaction on the concentration of reactants, temperature, the presence of a catalyst and calculating the conditions of chemical equilibrium in systems with reversible chemical reactions.

Theoretical questions

1. Rate of homo- and heterogeneous reaction.

2. Law of mass action for velocity in a homogeneous system.

3. Rate constant. Its physical meaning.

4. Temperature dependence of the reaction rate. Van't Hoff's rule.

5. The concept of catalysis.

6. Reversible and irreversible chemical reactions.

7. Chemical balance. Equilibrium constant. Its physical meaning.

8. Shift in chemical equilibrium. Le Chatelier's principle.

Chemical kinetics studies the course of chemical processes in time.

Chemical reaction rate n – this is the amount of substance Dn that reacts or is formed in the reaction per unit time Dt per unit volume of the reaction space n

Homogeneous reaction - proceeds in the entire volume, the reactants and reaction products are in the same phase.

Amount of substance per unit volume Dn/V – is the molar concentration of C.

Then average rate of a homogeneous reaction:

The unit for measuring the rate of a homogeneous reaction is mol l -1 s -1.

Heterogeneous reaction - the reaction proceeds at the phase boundary, the reactants and (or) reaction products are in different phases.

For a heterogeneous reaction, the rate depends on the contact surface area of the reactants - the phase separation area S.

Average rate of a heterogeneous reaction

The unit of measurement for the rate of a heterogeneous reaction is mol m -2 s -1.

Instant reaction rate- change in concentration at a particular moment, i.e. for an infinitesimal time interval dt

The rate of a chemical reaction is always positive. The plus sign "+" or "-" indicates whether the change in the amount of substance Δn is positive or negative, that is, the substance is formed or consumed during the reaction.

The reaction rate depends on the nature of the reactants, their concentration, temperature, and the presence of a catalyst.

Law of acting masses: The rate of a homogeneous reaction is proportional to the product of the molar concentrations of the reactants, taken in powers equal to the stoichiometric coefficients.

aA + bB → cc + dD v= k[A] a [B] c, where k is the rate constant.

The rate increases to a greater extent with an increase in the concentration of one of the substances whose stoichiometric coefficient in the reaction equation is greater.

The reaction rate increases with increasing temperature, because the speed of the molecules and, consequently, the number of active collisions leading to the interaction increases. The dependence of the reaction rate on temperature is expressed by the van't Hoff rule: v 2 = v 1 ∙γ (t 2 - t 1)/10 , where

v 1 – reaction rate at initial temperature t 1 ;

v 2 - reaction rate at temperature t 2

γ is the temperature coefficient, its value is 2 ÷ 4.

The reaction rate increases with catalysis– application catalyst- a substance that accelerates the reaction, but does not interact. The catalyst does not shift the chemical equilibrium, but leads to its faster achievement, equally accelerating the forward and reverse reactions. The amount of catalyst is much less than the amount of reactants. There are homogeneous catalysis (catalyst substances are in the same phase) and heterogeneous (in different phases).

Reversible reactions- chemical reactions occurring simultaneously in the forward (®) and reverse () directions.

Chemical equilibrium- the state of the system in which the rates of forward and reverse reactions are equal, the concentrations of reactants and reaction products are constant.

Equilibrium constant- is equal to the ratio of the product of the equilibrium concentrations of the reaction products to the product of the equilibrium concentrations of the reactants to the power of the stoichiometric coefficients in the equation and shows how many times the rate of the forward reaction is greater than the rate of the reverse reaction.

aA + bB « sС + dD,

or for gases , Where R- partial pressure.

The equilibrium constant depends on the temperature, the nature of the reactants, and does not depend on their concentration. At K c >>1, the reaction gives a high yield of reaction products, at K c<<1 выход продуктов мал, преобладают исходные реагенты.

A change in at least one of the parameters of the system leads to an imbalance, a change in concentrations and the establishment of a new equilibrium with other equilibrium values, i.e. balance shift.

Le Chatelier's rule: if an external influence is exerted on a system in equilibrium, then the equilibrium of the system will shift in the direction of the reaction that weakens this influence.

In experiments 1 and 2 we will study the dependence of the rate of decomposition of sodium thiosulfate of different concentrations and temperature under the action of acid H 2 SO 4 in the homogeneous stage of the reaction

Na 2 S 2 O 3 + H 2 SO 4 → Na 2 SO 4 + S + H 2 O + SO 2.

When Na 2 S 2 O 3 and H 2 SO 4 interact, unstable thiosulfuric acid H 2 S 2 O 3 is instantly formed, which spontaneously decomposes at the time of production with the formation of sulfur dioxide SO 2 and free sulfur S.

The rate of the whole process is determined by the rate of this slowest stage: H 2 S 2 O 3 → H 2 SO 3 + S

The resulting sulfur is poorly soluble in water, so the process can be divided into two stages:

homogeneous - sulfur is in solution, the sulfur concentration is less than saturated and

heterogeneous - sulfur precipitates, the saturated concentration is exceeded.

At the moment when the saturated concentration of sulfur (the critical mixing point) is reached, opalescence appears in the solution - a sharp increase in light scattering (the transparent solution begins to become cloudy).

The rate of the homogeneous reaction step v=C m /Δτ, where

Δτ is the reaction time from adding 1 drop of H 2 SO 4 to the appearance of opalescence.

C m - molar concentration of Na 2 S 2 O 3.

In experience 3 we will study the effect of a catalyst, copper sulfate CuSO 4 , on the rate of reduction of iron(III) thiocyanate Fe(SCN) 3 to iron(II) thiocyanate Fe(SCN) 2 under the action of sodium thiosulfate Na 2 S 2 O 3 .

2Fe(SCN) 3 + 2Na 2 S 2 O 3 → Na 2 S 4 O 6 + 2Fe(SCN) 2 + 2NaSCN

Of all the substances taking part in this reaction, only Fe (SCN) 3 has a color. In solution, it is colored blood red. The disappearance of the color of the solution indicates the end of the reaction.

Iron thiocyanate will be obtained immediately before the experiment by the reaction

In experience 4 we will study the shift of chemical equilibrium with a change in concentration using the example of a reversible reaction:

FeCl 3 + 3KSCN → Fe(SCN) 3 + 3KCl

A change in the concentration of iron(III) thiocyanate Fe(SCN) 3 , which has a red color, leads to a change in the color intensity of the reaction mass and makes it possible to judge in which direction the equilibrium is shifting.

Practical task:

1. Write a reaction rate expression for reactions:

2NO(g) + Cl 2 (g) → 2NOCl(g)

CaCO 3 (c) → CaO (c) + CO 2 (g)

2. How will the reaction rate 2NO (g) + O 2 (g) → 2NO 2 (g) change,

if the volume of the reaction vessel is reduced by 5 times?

3. Determine the initial concentrations of chlorine and hydrogen, if the equilibrium in the system H 2 (g) + Cl 2 (g) → 2HCl (g) is established at = 0.025 mol / l, = 0.09 mol / l.

How does an increase in pressure and temperature affect the equilibrium of reactions?

2 H 2 (g) + O 2 (g) → 2H 2 O (g) , Q>0

C(c) + CO 2 (g) → 2CO(g), Q<0

4. How will the decrease in temperature affect the state of chemical equilibrium in the system (will not be disturbed; will it shift to the left or to the right)?: 2NO + O 2 → 2NO 2, ∆H<0.

5. Will the equilibrium shift with increasing pressure and in which direction (toward a direct or reverse reaction) in the system: 4Fe (c) + 3O 2 (g) → 2Fe 2 O 3 (c).

Tasks for the section chemical kinetics and equilibrium of a chemical reaction. Chemical kinetics and equilibrium Chemical kinetics. Chemical equilibrium

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