Kinetic and Rate of Reaction

7/25/2019 Kinetic and Rate of Reaction

1/9

CHEMICAL KINETICS

Key Terms the students are expected to know in this chapter:

Activated complex Activation energy Average rate

Catalyst Collision frequency Complex reactions

Energy barrier Ester irst order requency factor !alf life period !ydrolysis

"nitial rate method "nstantaneous rate "nversion of cane sugar

"rreversible reaction Kinetic stability of fuels #aw of mass action

$olecularity %rder of a reaction %rientation

&hotochemical reaction &seudounimolecular 'ate constant

'ate law 'ate of reaction 'eversible reaction

(econd order (imple reactions (pecific rate constant

Temperature coefficient Thermodynamic stability Third orderThreshold energy )nits of order )nits of rate

*ero order Collision theory


2/9

CHEMICAL KINETICS

Rate

The rate of a reaction may be defined as the change in the concentration of any one of the reactants or products

per unit time+

)nit of rate $ol,#,Time

Average rate

Total change in concentration by total time taken is known as average rate+Instantaneous rate

The rate of change of concentration of any one of the reactants or products at that particular instant of time+

dxRate

dt=

Graphical method or determination o rate!

The rate can be determined by finding the slope of the tangent to the curve at the point corresponding to that

instant of time+

Examples of differential rates:

-rite differential rates for the reaction:


3/9

"ependence o rate o reaction on the concentration o the reactants#

Rate Law and Order of reaction

The dependence of the rate of a reaction on concentration is seen form the law of mass action+ "t gives a

theoretical dependence of the rate of the reaction on concentration of reactants+

.refer Chemical equilibrium for #aw of mass action

The above is the sum of law of mass action/

The rate law is determined experimentally:

"n a multi step reaction the rate depends on the slowest step that is the rate0determining step+

'ate .A/ .1/

'ate 2 k .A/ .1/

-here $is the rate constant or the specific reaction rate or the velocity constant+

K can be defined as the rate of the reaction when the concentration is unity+

%rder

"t can be defined as the sum of the exponents of the concentration terms of the rate law

%r"t may be defined as the sum of the powers to which the concentration terms are raised in a rate law expression+

This is an experimentally determined factor+

or the reaction

'ate 2 k .A/x.1/y

The order 3n4 of the reaction 2 3x4 5 3y4 where 3a4 and 3b4 may or may not be equal to 3x4 and 3y4

&hat are the dierent orders o reaction'

*ero6 irst6 (econd6 third etc+

A (ero order reactionis independent of the concentration of reactants+ The rate of the reaction does not depend

on the concentration terms or in other words an increase in the quantity of the reactants does not increase the rate

of the reaction+

&hotochemical formation of !Cl from hydrogen and chlorine is a 7ero order reaction+


4/9

)nits o *ero order reaction#

+irst order reactionthe rate of a first order reaction depends on one concentration term only+ "t is possible that

there could be more than one reactants involved in the reaction+

All nuclear fission reactions are first order decay or disintegration+!ydrolysis of esters: are first order reactions

[ ]

8 9 : 9 8 9 :

;

8 9 :

;

HCH COOC H H O CH COOH C H OH

Rate k CH COOC H

Order

+

+ +

=

=

)nit o irst order reaction

Second order reaction#

The rate of a second order reaction is dependent on two concentration terms+ This depends on the mechanism of

the reaction+ Example of a second order reaction is given below


5/9

)nit of 3k4 for second order reaction:

Molecularit,

The number of reacting species E' is one but molecularity is two is called pseudounimolecular reactions+ E+g+

!ydrolysis of Esters+

Equation to be written:

Mechanism of a reaction

A sequence of elementary reactions or steps written to account for the overall reaction is called the mechanism

of the reaction+

E-ample#

The rate0determining step is the slowest step+ The rate law is written using this step+

>istinction between rate of reaction rate constant

Rate o reaction Rate Constant

;+ 'ate if reaction is the change in

concentration of a reactant or product per

unit time+

9+ "t is a constant of proportionality in the

rate law equation and is equal to the rate of

reaction when the molar concentration of

each reactant is unity

8+ The rate of reaction at any instant of time

depends upon the molar concentration of

the reactants at that time+

?+ The rate constant is constant for a

particular reaction at a particular

temperature and does not depend upon theconcentrations of the reactants+

+ "ts unit is always $ol,#,Time @+ "ts units depends upon the order of the

reaction+2 mol;0n

litren0;

time0;

n is the orderof the reaction+

#ife Time

The time taken for the reaction to complete is know as life time


6/9


7/9

/resence o Catal,st

A catalyst is a substance that increases the speed of a reaction without itself undergoing any chemical change+

A catalyst provides an alternate path with lower energy barrier+ "t decreases the ener& of acti(ation of the

reaction+

Eect o radiation

Those reactions which take place only in the presence of electromagnetic radiation4s are known as

&hotochemical reaction4s


8/9

>ifference between order and molecularity of a reaction

%rder of a reaction $olecularity of a reaction

;+ (um of the exponents of the

concentrations in the rate law equation+

;+ "t is the number of atoms6 ions or

molecules that must collide with oneanother simultaneously so as to result

into a chemical reaction+

9+ "t can be a whole number6 7ero or afractional value

9+ "t always has to be a whole number

8+ !as to be determined experimentally 8+ "t can be calculated by adding the

molecules present in the slowest stepof a reaction

?+ "s for the overall reaction ?+ The overall molecularity of a complex

reaction has no significance6 the

slowest step is the rate determining

step

+ %rder of the reaction may or may not

be equal to the number of the reactants

as represented in the balanced equation

+ The molecularity for simple reactions

is obtained from the coefficients of a

balanced equation+


9/9

Mechanism o Reactions#

A sequence of elementary reactions or steps written to account for the overall reaction is called the

mechanism of the reaction+

E-ample#

F%95

9H F%

9 5

F%95 H F%9

(low reaction step "

ast reaction step ""9F%

95

9H 9F%

9

"n the above example the slowest step is the rate determining step+ The rate determining step involvesonly 9 reactants therefore it is a second order reaction+

'ate 2 k .F%9/.

9/ order n 2 9

Kinetic and Rate of Reaction

Documents

Transcript of Kinetic and Rate of Reaction