Sarkisyan Z. M., Prokhorova L. B.. Lecture. Hemical kinetics. Rate of chemical reaction. Chemical equilibriurn

УДК 54 ББК 24.54 Х82

Х82 Lecture. Hemical kinetics. Rate of chemical reaction. Chemical equilibriurn. Manual for students. / V.V. KhorunzhQI, M.K. Davydova, Z.M. Sarkisyan, L.B. Prokhorova. - SPb.: SPbGPMU, 2018 - 28 p.

ISBN 978-5-907065-59-8

The manual is confirmed by Methodic council of SPbSPMU.

The manual includes important topics of physical chemistry. The first part of the manual gives main definitions and lows of kinetics including rate of chemical reac­ tion, kinetic equation, constant of chemical reaction, reaction mechanism, catalyst and another. The second part concerns chemical equilibrium and main lows of equi­ librium shifts.

The manual will help the students to learn general chemistry better.

Reviewers:

Chief of Department of Biological chemistry of SPbSPMU, Professor L.A. Danilova;

reader Professor of Saint-Petersburg State Institute of Technology Panina N.S.

УДК 54 ББК 24.54

Утверждено учебно-методическим советом Государственного бюджетного образовательногоучреждения высшего профессионального образования «Санкт-Петербургский государственный педиатрический медицинский университет» Министерства здравоохранения Российской Федерации

INTRODUCTION

INTRODUCTION

The end resuli of each chemical process is defined by thermodynamic laws. I losvcvcr, the thermodynamics considers only initial and final conditions of system, and it does not give the information on a rate of achievement of this final (equilib­ rium) condition and on a way of transition to it. These questions are solved by chemi­ cal kinetics chemical kinetics studies a rate and the mechanism of chemical proc­ esses, and also their dependence on various factors

Utility value of chemical kinetics is defined by that for practical use of any re­ action is necessary to be able to operate her, i.e. not only to know rate of its course in the given conditions, but also ways of change of this rate. The knowledge of the basic kinetic processes is necessary to gain a better understanding the processes that occur in nature and in particularly in human organism, maintenance of an optimum mode of carrying out of process.

RATE OF CHEMICAL REACTIONS

1. Rate of homogeneous chemical reactions

One of the basic concepts in chemical kinetics is a rate of chemical reaction. Before consideration of a question on a rate it is necessary to distinguish the reactions proceeding in homogeneous system (homogeneous), and the reactions proceeding in non-uniform system (heterogeneous).

Example of homogeneous reaction can be the reaction of formation C02 proceed­ ing in a gas mixture:

Homogeneous reactions go in all volume of system.

Let's consider concept about a rate of homogeneous chemical reaction.

If in the closed system (i.e. the structure of system varies only as a result of chemical reaction, but not due to mass exchange with environment) reaction pro­

stoichiometric coefficients of the reaction, an average rate by any substance partici­ pating in reaction, is equal:

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(3)

where Arij - the change of quantity of substance in moles. Because a rate of any chemical reaction has positive size, Дп* undertakes with a sign "+ " for products of reaction (their quantity grows) and with a sign for reagents (their quantity de­ creases); Дт - a time interval, for example, in seconds; V - system’? volume, for ex­ ample, in litres.

Thus, the quantity of the substance reacting or formed during reaction for a time unit in unit of volume refers to as rate of homogeneous chemical reaction.

If reaction occurs in system with constant volume average rate of reaction can be

Where Дс* is molar concentration changing of one of components for a time period of Дт. Rate of reaction in this case is expressed in mole-1 '' -s'1.

The equation (4) reflects most often used definition of a rate of homogeneous chemical reaction: in homogeneous system changing of concentration of any sub­ stances reacting or formed at reaction, during a time unit refers to as rate of re­ action.

The average speed of reaction is expressed by the equation

vincr

P.VAr

where Pi is stoichiometric coefficient in the equation of reaction, An/V = Cj. Average rate in this case does not depend on substance on which it is defined. As the value Дп/Р( in chemical kinetics refers to as a chemical variable, the expression (7) has the name average reaction rate by a chemical variable. Last equation is mathematical expression of other definition of speed of homogeneous chemical reaction. The number of acts of chemical transformation (expressed in mole) for a time unit in unit of volume refers to as speed of homogeneous reaction.

Example. For homogeneous reaction of oxygen with carbon(ll) oxide in gaseous phase

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Rate of CO2accumulation is equal to speed of CO consumption, but in 2 times is more than rate of СЬ consumption, because in accordance with stoichiometric coeffi­ cient 1 mole of O: and 2 moles of CO are required for 2 moles of CO2formation.

However for average rate of this reaction by a chemical variable it is necessary to measure experimentally only change of quantity for one of the listed substances only:

If concentration of substances is expressed in mole/1 expression of average rate of reaction by a chemical variable looks like:

Fig. 1. Change of a rate of chemical reaction in time

Kven under constant external conditions average rate of chemical reaction varies in time because of a consumption of reagents and accumulation of products of reac-

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lion (Fig.l). Therefore chemical kinetics besides average considers also instant rate ofreaction (vmsl). For system with constant volume

AC N 0 is the change of nitrogen concentration during the time period AT. If con­

centration of nitrogen has decreased from 0.7 up to 0.5 mole/I for 20 seconds average speed of reaction will be equal:

1.1. Dependence of Chemical Reaction Rate on the Concentration of Reacting Substances. Kinetic Equation

Necessary condition of that between particles of starting substances chemical interaction (molecules, ions, etc.) has took place, is their collision with each other. Therefore it is possible to tell, that the more collisions occurs for a time unit, the it is more reaction rate. The number of collisions, on the other side, is the more, the more the concentration is of each of starting substances.

Dependence of a rate of chemical reaction on reacting substances concentration has received the name "the kinetic equation" or "the law of acting masses" (rale low).

For reaction

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where Сл., Св, ... are concentrations of reacting substances;

x, у - exponents at concentration of reagents which refer to as partial kinetic orders of reaction on these reagents, and their sum (x + y) - the general or total kinetic or­ der of reaction.

Kinetic orders of reaction are the experimental values depending on the mecha­ nism of reaction. Orders of reaction can be any whole or fractional numbers.

Factor of proportionality (k) is a constant named as a rate constant of reaction. The rate constant of reaction (k) depends on

-nature of reacting substances,

-temperature,

-pressure,

-presence of the catalyst.

it is necessary to note, that, according to the kinetic equation, at concentration of all reacting substances (CA, Св,—) equal 1 mol/1, a rate of reaction and к are numeri­ cally equal each other, therefore a rate constant of reaction is named also specific rate of reaction.

In some cases the partial kinetic orders are equal to corresponding stoichiomet­ ric coefficients of reaction. The kinetic equation has a following appearance

v к *Сд (17)

This is right if reaction is elementary (simple) reaction (it proceeds in one stage and in one direction) or at equilibrium point.

The partial kinetic orders (x and y) are equal to stoichiometric coefficients in the equation of reaction (15) and refer to as stoichiometric orders of reaction by corre­ sponding substances.

Rate of complex (multiphase and|or reversible) reactions in some cases can be described also by the equation (17) with stoichiometric orders; such reaction is named as quasi-simple.

If the general order of reaction is equal to unit a reaction is named as reaction of

lixample. What is the general order of the reaction

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where x = 2 and у = ) and are the exponents defining the orders of reaction by NO

and СЬ accordingly.

v к • CN0 • Ct1| .

We can see that partial orders are equal to the coefficients in reaction and so this reaction may be simple or quasi-elementary (quasi-simple). The general order of re­ action (x + у = 1 + 2) is equal 3. It is reaction of the third order.

The partial orders on reactants always are equal to coefficients in chemical reac­ tion at the equilibrium conditions.

The kinetic equation gives important information for practical purposes about rate dependence on concentration of reacting substances. This is necessary for selec­ tion of optimum chemical processes conditions.

1.2. The mechanism and molecularity of chemical reactions

The equation of reaction often shows only total quantity of reagents and prod­ ucts, but actually reactions proceed through some elementary stages. These are com­ plex reactions. Elementary stages show the least number of the particles simulta­ neously participated in chemical interaction; the sequence of elementary stages describing complex reaction, refers to as the mechanism of reaction.

For example, it is established, that reaction of bromine with the hydrogen in a

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For each separate elementary stage, partial orders on reagents are equal to stoichiometric coefficients in the corresponding simple reaction (19-23). However for total reaction (18) kinetic equation received experimentally, has a following appear­ ance:

The order of reaction by Hi is equal 1, the order on Вь - 0,5, and the general or­ der of reaction has value 1,5 (1 + 0,5). Discrepancy of orders of reaction by reagents with stoichiometric coefficients speaks about complexity of reaction.

The stage of process with a minimal rate plays the greatest influence on a rate of complex reaction. This stage refers to as a limiting stage of reaction.

The concept about molecularity of reactions is connected with the mechanism of reaction. Molecularity of reactions is defined as the least number of particles (molecules, atoms, ions), taking part in an elementary stage. The number of formed particles has no importance at that.

Distinguish reactions of three types.

1. Monomolecular reactions are reactions in which only one kind of molecules (or other particles) undergoes transformation, and stoichiometric coefficient in the equation is equal to unit, for example:

The general order of such elementary reaction is equal 1 and coincides with molecularily of reactions.

2. Bimolccular reactions arc reactions in which two various kinds of molecules or two molecules of one kind undergoes transformation, for example:

The general order is equal to two.

3. Trimolecular reactions are reactions in which molecules of one or different kinds participate, for example:

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