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 issues

1. Rate of homo- and heterogeneous reactions.

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

3. Rate constant. Its physical meaning.

4. Dependence of reaction rate on temperature. Van't Hoff's rule.

5. The concept of catalysis.

6. Reversible and irreversible chemical reactions.

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

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

Chemical kinetics studies the course of chemical processes over time.

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

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

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

Then average rate of homogeneous reaction:

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

Heterogeneous reaction - a reaction occurs at a 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 reagents—the phase interface area S.

Average rate of heterogeneous reaction

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

Instant reaction speed– change in concentration at a specific moment, i.e. in an infinitesimal period of time 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, a 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 mass action: 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] in, where k is the rate constant.

The rate increases to a greater extent with increasing concentration of the substance whose stoichiometric coefficient in the reaction equation is greater.

The rate of reaction increases with increasing temperature because the speed of the molecules increases and, therefore, the number of active collisions leading to 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 – temperature coefficient, its value is 2 ÷ 4.

The reaction rate increases with catalysis– application catalyst- a substance that accelerates a reaction but does not interact. The catalyst does not shift the chemical equilibrium, but leads to its faster achievement, equally accelerating forward and reverse reactions. The amount of catalyst is much less than the reagents. There are homogeneous catalysis (the 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 reagents 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 « сС + dD,

or for gases , Where R- partial pressure.

The equilibrium constant depends on 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. shift of balance.

Le Chatelier's rule: if an external influence is exerted on a system that is in equilibrium, then the equilibrium of the system will shift towards 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 on temperature under the influence 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 at the time of production spontaneously decomposes to form sulfur dioxide SO 2 and free sulfur S.

The speed of the entire process is determined by the speed 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 the saturated concentration of sulfur is reached (the critical mixing point), opalescence appears in the solution - a sharp increase in light scattering (the transparent solution begins to become cloudy).

Homogeneous reaction rate v=C m /Δτ, where

Δτ – reaction time from adding 1 drop of H 2 SO 4 until opalescence appears.

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

In experiment 3 we will study the effect of the catalyst - copper sulfate CuSO 4 - on the rate of reduction of iron(III) thiocyanate Fe(SCN) 3 to iron(II) thiosulfate 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 is colored. In solution it is colored blood red. The disappearance of the color of the solution indicates the end of the reaction.

We obtain iron thiocyanate immediately before the reaction experiment

In experiment 4 we will study the shift in 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 an expression for the reaction rate for the reactions:

2NO(g) + Cl 2 (g) → 2NOCl(g)

CaCO 3 (k) → CaO (k) + CO 2 (g)

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

if we reduce the volume of the reaction vessel by 5 times?

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

How does increasing pressure and temperature affect the equilibrium of reactions?

2 H 2 (g) + O 2 (g) → 2H 2 O (g), Q>0

C(k) + CO 2 (g) → 2CO(g), Q<0

4. How will a decrease in temperature affect the state of chemical equilibrium in the system (will it 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 what direction (towards the direct or reverse reaction) in the system: 4Fe(k)+3O 2 (g)→2Fe 2 O 3 (k).

Chemical kinetics is a branch of chemistry that studies the rates of chemical reactions. Chemical reactions can occur at different rates (from small fractions of a second to decades and longer time intervals). When considering the issue of reaction rates, it is necessary to distinguish between homogeneous and heterogeneous reactions. Homogeneous systems consist of one phase (for example, any gas mixture), and heterogeneous– from several phases (for example, water with ice). Phase is a part of the system separated from its other parts by an interface, during the transition through which an abrupt change in properties occurs.

Homogeneous reaction rate is the amount of a substance that reacts or is formed during a reaction per unit time in a unit volume of the system. Speed ​​of heterogeneous reaction is the amount of a substance that reacts or is formed during a reaction per unit time per unit surface of the phase (or mass, volume of the solid phase, when it is difficult to determine the size of the surface of a solid):

v homog = ; v heterog = . Those. rate of homogeneous reaction can be defined as change in the concentration of any of the substances that react or are formed during the reaction, occurring per unit of time.

Most chemical reactions are reversible, that is, they can occur in both forward and reverse directions. Let's consider a reversible reaction:

Rates of forward and reverse reactions are related to the concentrations of reagents by the following equations:

v x.r., pr =k pr [A] a ×[B] b and v x.r. arr =k arr [C] c ×[D] d

Over time, the rate of the forward reaction will decrease due to the consumption of reagents A And IN and a decrease in their concentrations. On the contrary, the rate of the reverse reaction as products accumulate WITH And D will increase. Therefore, after a certain period of time, the rates of the forward and reverse reactions will become equal to each other. A state of the system will be established in which there are no flows of matter and energy, called chemical equilibrium. All reversible processes do not proceed completely, but only to a state of equilibrium, in which, from the condition v x.r. pr = v x.r. arr. follows:

k pr /k arr =[C] c ×[D] d / [A] a ×[B] b =K

Where K- chemical equilibrium constant, which depends on the temperature and nature of the reagents, but does not depend on the concentration of the latter. This is a mathematical expression of the law of mass action, which allows one to calculate the composition of an equilibrium reaction mixture.

The most important factors influencing the reaction rate are:

1. The nature of the reacting substances;

2. Concentrations of reacting substances;

3. Temperature factor;

4. Availability of catalysts.

In some cases, the rate of heterogeneous reactions also depends on the intensity of movement of liquid or gas near the surface on which the reaction occurs.

1) The influence of the concentration of reactants. Let us present the equation of a chemical reaction in general form: aA+bB+…=, then v ch.r. =k[A] a [B] b is essentially a mathematical notation law of mass action, discovered experimentally by K. Guldberg and P. Waage in 1864-1867. According to this law, at a constant temperature v, ch.r. is proportional to the product of the concentrations of the reacting substances, and each concentration is included in the product to a degree equal to the coefficient appearing in front of the formula of a given substance in the reaction equation. The value of the reaction rate constant (k) depends on the nature of the reactants, temperature and the presence of catalysts, but does not depend on the concentration of the substances.

2) Dependence v x.r. on temperature and on the nature of the reacting substances.Activation energy E a (in kJ/mol) is the excess energy that molecules must have in order for their collision to lead to the formation of a new substance. E and different reactions are different. This factor influences the nature of the reacting substances on v ch.r. . If E a<40 кДж/моль (т.е. мала), то скорость такой реакции велика (например, ионные реакции в растворах, протекающие практически мгновенно). Если Е а >120 kJ/mol (i.e. very significant), then the rate of such a reaction is insignificant (for example, the reaction of ammonia synthesis N 2 + 3H 2 = 2NH 3 - the rate of this reaction at ordinary T due to high values ​​of E a is so small that it can be noticed leakage is almost impossible).

In 1889, the famous Swedish chemist Arrhenius derived from experimental data an equation relating the rate constant to temperature and activation energy. Later, this equation received theoretical justification. According to Arrhenius, the rate constant is exponentially dependent on temperature: k=k max ×exp(-E a /RT), where R is the universal gas constant equal to 8.31 J/mol×K; k max is a pre-exponential factor meaning the maximum possible value of the rate constant at zero activation energy or infinitely high temperature, when all collisions of reactant molecules become active. The Arrhenius equation is often used in logarithmic form: lnk=lnk max -E a /RT.

Increasing v h.r. with increasing temperature is usually characterized temperature coefficient of reaction rate– a value showing how many times the rate of the reaction under consideration increases when the temperature of the system increases by 10 degrees. The temperature coefficient (g) is different for different reactions. At ordinary temperatures, its value for most reactions lies in the range from 2 to 4 (i.e. g hr = 2-4 times).

Catalysts are substances that are not consumed in the reaction, but affect its speed. The phenomenon of changing the reaction rate under the influence of catalysts is called catalysis, and these reactions themselves are catalytic. The action of the catalyst is due to a decrease in the activation limit of chemical interaction, i.e. decrease in activation energy. Under the influence of catalysts, reactions can be accelerated millions or more times. Moreover, some reactions do not occur at all without catalysts. Catalysts are widely used in industry.

Distinguish homogeneous And heterogeneous catalysis. At homogeneous catalysis the catalyst and reagents form one phase (gas or solution), and when heterogeneous catalysis– the catalyst is in the system as an independent phase. An example of homogeneous catalysis is the decomposition of hydrogen peroxide into water and oxygen in the presence of catalysts Cr 2 O 7 2-, WO 4 2-, etc. An example of heterogeneous catalysis is the oxidation of sulfur dioxide into trioxide using the contact method for producing sulfuric acid from waste gases of metallurgical production: SO 2 +0.5O 2 +H 2 O=(kt)=H 2 SO 4.

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2CHEMICAL KINETICS AND CHEMICAL EQUILIBRIUM

2.1 KINETICS OF CHEMICAL REACTIONS

Chemical reactions occur at different rates. Some of them are completed completely in small fractions of a second (explosion), others are carried out in minutes, hours, days and long periods of time. In addition, the same reaction can proceed quickly under some conditions (for example, at elevated temperatures), and slowly under others (for example, upon cooling). Moreover, the difference in the speed of the same reaction can be very large.

When considering the issue of reaction rates, it is necessary to distinguish between homogeneous and heterogeneous reactions. Closely related to these concepts is the concept of phase.

Phase is a part of a system separated from its other parts by an interface, during the transition through which the properties change abruptly.

A homogeneous reaction occurs in the volume of the phase [example - the interaction of hydrogen and oxygen with the formation of water vapor: H 2 (g) + O 2 (g) → H 2 O(g)], and if the reaction is heterogeneous, then it occurs at the phase interface [for example, carbon combustion: C(s) + O2(g) → CO 2 (g)].

The rate of a homogeneous reaction is the amount of substance that reacts or is formed during the reaction per unit time per unit volume of the phase:

Where n- amount of substance, mol; V- phase volume, l;τ - time; WITH- concentration, mol/l.

The rate of a heterogeneous reaction is the amount of substance that reacts or is formed during the reaction per unit time per unit surface area of ​​the phase:

Where S- area of ​​the phase interface.

The most important factors influencing the rate of a homogeneous reaction are the following: the nature of the reactants, their concentrations, temperature, and the presence of catalysts.

Dependence of the reaction rate on the concentrations of the reactants. A reaction between molecules occurs when they collide. Therefore, the rate of a reaction is proportional to the number of collisions that the molecules of the reacting substances undergo. The higher the concentration of each of the starting substances, the greater the number of collisions. For example, reaction rate A + B→ Proportional to the product of concentrations A and B:

v = k · [A] · [B],

Where k- proportionality coefficient, called reaction rate constant. Meaningful value k equal to the reaction rate for the case when the concentrations of the reactants are 1 mol/l.

This ratio expresses law of mass action This law is also called the law existing wt. : At constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

Much less often, a reaction occurs as a result of the simultaneous collision of three reacting particles. For example, reaction

2A+B → A 2 B

can proceed through a triple collision:

A+ A + B→ A 2 B

Then, in accordance with the law of mass action, the concentration of each of the reacting substances is included in the expression of the reaction rate to a degree equal to the coefficient in the reaction equation:

v = k · [A] · [A] · [B] = k · [A] 2 [B]

The sum of exponents in the equation of the law of mass action is called reaction order. For example, in the latter case, the reaction is third order (second - with respect to substance A and first - with respect to substance B.

Dependence of reaction rate on temperature. If we use the results of counting the number of collisions between molecules, the number of collisions will be so large that all reactions must occur instantly. This contradiction can be explained by the fact that only molecules with some energy enter into the reaction.

The excess energy that molecules must have in order for their collision to lead to the formation of a new substance is called activation energy (see Figure 2.1).

Figure 2.1 - Energy diagram for the reaction of formation of product AB from starting substances A and B. If the collision energy of molecules A and B is greater than or equal to activation energies E a , then the energy barrier is overcome, and movement occurs along the reaction coordinate r from starting materials to product. Otherwise, an elastic collision of molecules A and B takes place. The top of the energy barrier corresponds to the transition state (activated complex), in which the AB bond is partially formed.

As temperature increases, the number of active molecules increases Temperature is a measure of the average kinetic energy of molecules, so increasing the temperature leads to an increase in the average speed of their movement.. Therefore, the rate of a chemical reaction should increase with increasing temperature. The increase in reaction rate upon heating is usually characterized as temperature coefficient of reaction rate (γ ) - a number showing how many times the rate of a given reaction increases when the temperature increases by 10 degrees. Mathematically, this dependence is expressed rule van't Hoff :

,

Where v 1 - speed at temperature t 1 ; v 2 - speed at temperature t 2. For most reactions the temperature coefficientγ lies in the range from 2 to 4.

More strictly, the dependence of the reaction rate (or rather, the rate constant) on temperature is expressed Arrhenius equation :

,

Where A - pre-exponential a multiplier that depends only on the nature of the reactants; E a - activation energy, which is the height of the energy barrier separating the starting materials and reaction products (see Figure 2.1); R R=8.3144 J/(mol. K). In approximate calculations, R = 8.31 J/(mol. K) is often taken. - universal gas constant; T T - absolute temperature (in Kelvin scale). It is related to temperature in Celsius by the equation
T = t o C + 273.15.
In approximate calculations, the relation is used
T = t o C + 273. -

Chemical kinetics and equilibrium

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

Theoretical background:

Speed ​​of chemical reaction is the amount of a substance that reacts or is formed as a result of a reaction per unit time per unit volume (for homogeneous reactions) or per unit interface surface (for heterogeneous reactions).

If over a period of time?f = f 2 f 1 the concentration of one of the substances participating in the reaction decreases by?C = C2C1, then the average rate of the chemical reaction for the specified period of time is equal to

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

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

1) the nature and concentration of the reacting substances;

2) temperature of the reaction system;

3) presence of a catalyst;

4) pressure,

5) the size of the phase interface and the mixing rate of the system (for heterogeneous reactions);

6) type of solvent.

Effect of reagent concentrations. The rate of a reaction is proportional to the number of collisions of molecules of the reacting substances. The number of collisions, in turn, is greater, the higher the concentration of each of the starting substances.

A general formulation of the effect of concentration on the rate of a chemical reaction is given by law 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 reacting substances, taken in powers of their equalizing (stoichiometric) coefficients.

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

where K is the proportionality coefficient or speed constant;

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.

The effect of temperature on the reaction rate. As the temperature increases, the frequency of collisions of reacting molecules increases, and therefore the reaction rate increases.

The quantitative effect of temperature on the rate of homogeneous reactions can be expressed by Van't Hoff's rule.

In accordance with Van't Hoff's rule, when the temperature increases (decreases) by 10 degrees, the rate of a chemical reaction increases (decreases) by 2-4 times:

where V (t 2 ) and V (t 1 ) - the rate of chemical reaction at appropriate temperatures; f(t 2 ) And f(t 1 ) - duration of the chemical reaction at appropriate temperatures; G - Van't Hoff temperature coefficient, which can take a numerical value in the range of 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 a given reaction (expressed in kJ/mol). One of the methods of activation is to increase the temperature: with increasing temperature, the number of active particles increases greatly, due to which the reaction rate sharply increases.

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

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

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

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

Chemical balance.

All chemical reactions can be divided into two groups: irreversible and reversible. Irreversible reactions proceed to completion - 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 are completely consumed. A reversible reaction can occur in both the forward and reverse directions.

Chemical equilibrium is a state of a system in which the rates of 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 concentrations of products raised to powers equal to the stoichiometric coefficients to the product of concentrations of starting substances, also raised to the corresponding powers, is equal to some constant value called the chemical equilibrium constant.

The chemical equilibrium constant depends on the nature of the reactants and on 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 a reaction system from one state to another is called a displacement (or shift) of chemical equilibrium.

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

An increase in the concentration of one of the starting reagents shifts the equilibrium to the right (the direct reaction is enhanced); An increase in the concentration of reaction products shifts the equilibrium to the left (the reverse reaction intensifies).

If a 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 increases, the equilibrium shifts towards the endothermic reaction, and when the temperature decreases, it shifts towards the exothermic reaction.

The catalyst changes the rate of both forward and reverse reactions by the same number of times. Therefore, the catalyst does not cause a shift in equilibrium, but only shortens or increases the time required to achieve equilibrium.

Experiment No. 1 Dependence of the speed of a homogeneous reaction on the concentration of the initial reagents.

b Instruments, equipment: test tubes, stopwatch, solutions of sodium thiosulfate (III), dil. sulfuric acid (1M), water.

b Methodology: This dependence can be studied using the classic example of a homogeneous reaction between sodium thiosulfate and sulfuric acid, proceeding according to the equation

Na 2 S 2 O 3 + H 2 SO 4 = Na 2 SO 4 + Sv + SO 2 ^ + H 2 O.

At first, sulfur forms a colloidal solution with water (barely perceptible turbidity). It is necessary to measure the time from the moment of draining until a barely noticeable turbidity appears with a stopwatch. Knowing the reaction time (in seconds), you can determine the relative speed of the reaction, i.e. reciprocal of time:

chemical homogeneous kinetics

For the experiment, you should prepare three dry, clean test tubes and number them. Add 4 drops of sodium thiosulfate solution and 8 drops of water to the first; in the second - 8 drops of sodium thiosulfate and 4 drops of water; in the third - 12 drops of sodium thiosulfate. Shake the test tubes.

If we conditionally designate the molar concentration of sodium thiosulfate in test tube 1 as “c”, then accordingly in test tube 2 there will be 2 s mole, in test tube 3 - 3 s mol.

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

Perform similar experiments with the second and third test tubes. Enter the experimental data in the laboratory journal in the form of a table.

b Conclusion: with increasing concentration of sodium thiosulfate, the rate of this reaction increases. The dependence graph is a straight line passing through the origin.

Experience No. 2. Study of the dependence of the rate of a homogeneous reaction on temperature.

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

b Methodology:

Prepare three clean, dry test tubes and 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 after 1...2 minutes note the temperature. Then add one drop of sulfuric acid to the test tube, simultaneously turn on the stopwatch and stop it when a weak, barely noticeable turbidity appears. Record the time in seconds from the moment the acid is added to the test tube until turbidity appears. Record the result in the table.

Then increase the temperature of the water in the glass by exactly 10 0 either by heating it on a hot plate or by mixing it 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 its contents in a glass of water until turbidity appears. If a barely noticeable cloudiness appears, turn off the stopwatch and enter the stopwatch readings into the table. Carry out a similar experiment with the third test tube. First increase 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, while turning on the stopwatch and shaking the test tube.

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

Determine the temperature coefficient of the reaction g

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

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

Experiment No. 3 The influence of the concentration of reactants on chemical equilibrium.

b Instruments and equipment: test tubes, potassium chloride (crystal), solutions of iron (III) chloride, potassium thiocyanate (saturated), distilled water, cylinder

b Methodology:

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

FeCl3+ 3 KCNS D 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. Create 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 iron (III) chloride and one drop of a saturated solution of potassium thiocyanate . Pour the resulting colored solution equally into four test tubes. Number the test tubes.

Add one drop of a saturated solution of iron (III) chloride to the first test tube. Add one drop of a saturated solution of potassium thiocyanate to the second test tube. Add crystalline potassium chloride to the third test tube and shake vigorously. The fourth test tube is for comparison.

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

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

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

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

General conclusions: during 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); 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 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 starting substances)

Chemical kinetics

Chemical equilibrium

Chemical kinetics is a branch of chemistry that studies the rate of a chemical reaction and the factors influencing 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 a reaction under given conditions, nor does it give an idea about the speed and mechanism of the process.

Studying reaction rates makes it possible to elucidate the mechanism of complex chemical transformations. This creates a perspective for controlling the chemical process and allows for mathematical modeling of processes.

Reactions may be:

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

2. heterogeneous– do not occur in the same environment (between substances in different phases); pass at the interface.

Under speed of chemical reaction understand the number of elementary reaction events occurring per unit time per unit volume (for homogeneous reactions) and per unit surface area (for heterogeneous reactions).

Since the concentration of the reactants changes during a 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 changes in the concentration of all substances included in the reaction, since the stoichiometric coefficient in the reaction equation establishes the relationship between concentrations, i.e. at the rate of ammonia accumulation is twice the rate of hydrogen consumption.

. . because cannot be negative, so they 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 the solid (heterogeneous reactions);

6. from the environment (solutions);

7. on the shape 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 chemical reaction rate constant

Physical meaning at .

If the reaction involves not 2 particles, but more, then: ~ in powers equal to the stoichiometric coefficients, i.e.: , Where

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

The number of particles participating in this reaction event determines molecularity of the reaction :

Monomolecular ()

Bimolecular ( )

Trimolecular.

There is no more than 3, because... collision of more than 3 particles at once is unlikely.

When a reaction occurs in several stages, then the overall reaction = the slowest stage (limiting stage).

The dependence of the reaction rate on temperature is determined empirically 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 rate of chemical reaction.

Not every collision of molecules is accompanied by their interaction. Most molecules bounce off like elastic balls. And only the active ones interact with each other during a collision. Active molecules have some excess energy 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 activation energy. The smaller it is, the more particles react, the greater the speed of the chemical reaction.

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

Change during reaction:

Released (exothermic)

As the temperature increases, the number of active molecules increases and therefore 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; there are very few active radical reactions ().

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

Many reactions occur through chain mechanism. The peculiarity 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 ordinary temperatures and diffuse lighting, 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 fire), the mixture explodes.

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

Naturally, it is possible for free radicals to collide with each other, which leads to chain breakage: .

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

Photochemical processes are very diverse. During photochemical action, the molecules of reacting substances, absorbing light quanta, become excited, i.e. become reactive or break down 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 of accelerating chemical reactions in chemical practice is catalysis . Catalysts– substances that change a chemical reaction due to participation in intermediate chemical interaction with the components of the reaction, but restore their chemical composition after each cycle of intermediate interaction.

The increase in catalytic reaction is associated with fewer new reaction pathways. Because in the expression for is a 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 effect of catalysts.

Reactions that proceed in only one direction and go to completion - irreversible(precipitate formation, gas evolution). They are few.

Most reactions - 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, : for an endothermic reaction it increases, for an exothermic reaction it decreases and remains constant.

The influence of various factors on the position of chemical equilibrium is determined La Chatelier's principle: if any impact is exerted on a system that is in equilibrium, then processes in the system that seek to reduce this impact are intensified.