Chemical kinetics

 Introduction

The term chemical kinetics indicates the branch of Physical chemistry which deals about rate of reaction and mechanism following by them in each step. Chemical kinetics is focus of reaction with rate and path of it, due to which it is known as reaction kinetics. Chemical kinetics gives idea about how reaction proceeds and how many factor is related on them. It also gives diagrammatic representation with mathematical form reaction proceed with mechanism.

Peter Waage and Guldberg were the first chemist who did the concept and develop the form of chemical kinetics by expressing in terms of law of mass action, which tell us speed of chemical reaction is directly dependent to the quantity or amount of reacting component.

Van’t Hoff is famous “ Etudes de dynamic chimique” due to study of dynamics of chemical. On the basis and motivation of it, he was the first chemist who awarded first noble prize in chemistry. By following Van’t Hoff law, chemical kinetics studied in terms of rate of reaction with rate law and rate constant of a reaction. In case of zero order reaction, rate of reaction does not depend upon concentration of reactant. Similarly first order reaction, rate of reaction depends upon the concentration of only one reactant which can be shown in slow step or which is rate determining step. In case of second order reaction, rate of reaction depends upon concentration of both reactant and nucleophile or we can say base as well. In case of Pseudo order reaction, reaction depends upon only reactant but seems to be depends upon concentration of both reactant and nucleophile. So Pseudo order reaction is also known as false order reaction.

Example of first order reaction is:

$$N_2O_5\longrightarrow2NO_2 +\frac{1}{2}$$$$O_2$$

Rate of reaction

R=k$$N_2O_5$$

Example of second order reaction is :

$$CH_3COOC_2H_5+NaOH \longrightarrow CH_3COONa+C_2H_5OH$$

Example of Pseudo first order reaction is :

$$CH_3COOC_2H_5+H_2O\longrightarrow CH_3COOH+C_2H_5OH$$

Actual rate of reaction, R=k[$$CH_3COOC_2H_5$$]

Law of mass action has been fallowed in elementary reaction but combined form of rate law has been fallowed in stepwise reaction. In case of consecutive reaction, there is a rate determining step or slow step which determine the kinetics i.e, whether that reaction is first order, second order or pseudo order reaction. Also in case of consecutive reaction, steady state approximation principle simplified the rate law fallowed on that particular reaction. By applying the Arrhenious equation and support of Eyring reaction, the activation energy can be determined experimentally. The physical state of reactant, concentration of reactant, temperature in a system, presence or absence of catalyst are main factors which influence the rate or kinetics of reaction.

Gorban and Yablonsky had been told that history of kinetics can be classified into three steps. First one is Van’t Hoff wave related for general rules of chemical reactions and related kinetics to the thermodynamically. The second one is called the Semenov-Hinshelwood wave with mechanisms of reaction focus especially for chain type of reaction system. The third one is related with Aris and mathematical description related on reaction rate with their networks.

In chemical kinetics, thermodynamics can be used in terms of relation of energy between reactants and products. It is also used as to know whether reaction is exothermic or endothermic in a given system. It does not tell us how rapidly reaction form or in which path way fallow the reaction with their mechanism. But chemical kinetics is used for determination of rate with time, it means chemical kinetic observed rate of reaction with very short time. E.g. acid-base neutralization reaction, precipitation reaction etc. There are also some reactions which can take month for completion of it, e.g. rusting of iron. In some reaction, reactant changes only trace amount and in some reaction reactant consume almost. Both can be detected by using different chemical kinetics parameter.  Reaction rate can be studied theoretically as well as experimentally. Experimental fact help in calculation of chemical kinetics.

Kinetic properties of sol can be studied by applying terms as diffusion, Brownian motion and sedimentation as well. Colloidal properties can exist in ceaseless, motion and random. The kinetic activity of liquid with particle is called Brownian motion. Brownian motion is due to slow or fast collision or bombarding a particle by molecule in motion. Validity of it was given by Einstein in mathematical form.

In chemical kinetics substance which alters the rate of chemical reaction and remained on change at the inert of reaction is included which is known as catalyst. Catalyst is that substance which alters the rate of reaction but it remains unchanged at the end of chemical reaction. The reaction which is completed by the use of catalyst is known as catalysis reaction. Catalyst may be positive or negative. Positive catalyst increases the rate of reaction so it is used to complete reaction with short time. Negative catalyst decreases the rate of reaction so it is used to control the reaction in various steps. By using negative catalyst intermediate structure formed in reaction also be studied. On the stand of kinetics, reaction may be classified either homogeneous or heterogeneous. Homogeneous reaction completed within single phase and Heterogeneous reaction completed with different phases. Homogeneous reaction may be:-

• catalyzed reaction
• non-catalyzed reaction and
• chain reaction

Ionic and exclusive reaction can’t be study in terms of chemical kinetics due to very fast reaction occur. Homogeneous reaction like gaseous phase only or liquid phase only is done and homogeneous reaction in solid phase can be proceeds.

Summary

Chemical kinetics includes parts of physical chemistry with rate and mechanism of reaction in mathematical expression. Peter Waage and Gulberg is concept maker of it, which was developed by Van’t hoff and derived mathematically by Einstein. Dependent of rate of reaction on reactant show the different order of reaction. Mass action is included in stepwise reaction or chain reaction, consecutive reaction. Rate of reaction alter by the different factor which is in surrounding and included in reaction. Very slow and fast consumption of reactant can be measured by using kinetic parameters and rate law can be measured easily. Homogeneous and Heterogeneous reaction favoured by phase involve in it.

References

maron, samuel H and Carl F prutton. principle of physical chemistry. new delhi: oxford and IBH , 1972.

negi, A.S and S.C Anand. A textbook of chemistry. New Delhi: New age international, 2011.

Key words

Chemical kinetics, Pseudo order reaction, Van’t Hoff law, catalyzed  reaction, chain reaction