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CBSE Class-12 Chemistry Quick Revision NotesChapter-04: Chemical
inetics
Chemical kinetics:It is the branch of chemistry that deals with
the study of reaction rates and theirmechanisms.
Rate of reaction:It is the change in concentration of reactant
(or product) in unit time.
The unit of rate of reaction is mol L ! s ! . " # $ % C #
&
Rate of disappearance of[ ]d A
Adt
=
where d'" is small change in conc. of "* and dt is small
inter+al of time
Rate of disappearance of[ ]d B
Bdt
=
,here d'$ is small change in conc. of $* and dt is small
inter+al of time
Rate of appearance of[ ]d C
C dt
+=
,here d'C is small change in conc. of C* and dt is small
inter+al of time
Rate of appearance of [ ]d D
D dt
+=
,here d'& is small change in conc. of &* and dt is small
inter+al of time
Rate[ ] [ ] [ ] [ ]d A d B d C d D
dt dt dt dt
+ += = = =
Rate law or rate e-uation:It is the e pression which relates the
rate of reaction with concentration of the reactants.The constant
of proportionality k* is known as rate constant.
"+erage rate:It is the rate of reaction measured o+er a long
time inter+al.
"+erage rate xt
=
where is / change in concentration and /t is large inter+al of
time. Instantaneous rate:
It is the rate of reaction when the a+erage rate is taken o+er a
particular moment of time.
Instantaneous ratedxdt
= .
where d is small change in conc. and dt is the smallest inter+al
of time.
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It is the e pression which relates the rate of reaction with
concentration of the reactants. Rate constant:
,hen the concentration of reactants is unity0 then the rate of
reaction is known as rateconstant. It is also called specific
reaction rate.
The constant of proportionality k* is known as rate constant.
1olecularity of a reaction:
The total number of atoms0 ions or molecules of the reactants
in+ol+ed in the reaction istermed as its molecularity. It is always
in whole number and is ne+er more than three. Itcannot be 2ero.
3rder of a reaction:The sum of the e ponents (power) of the
concentration of reactants in the rate law istermed as order of the
reaction. It can be in fraction. It can be 2ero also.
If rate law e pression for a reaction isRate 4 k '" '$ yThen its
order of reaction 4 # y
3rder cannot be determined with a gi+en balanced chemical
e-uation. It can bee perimentally determined.
Integrated rate law for 2ero order reaction:R % 5
0[ ]dx
k Rdt
=
0[ ] [ ] R R
k t
=
If we plot a graph between concentration of R +s time t0 the
graph is a straight line withslope e-ual to k and intercept is
e-ual to 'Ro .
6alf life of a reaction:The time taken for a reaction0 when half
of the starting material has reacted is calledhalf life of a
reaction.
7or 2ero order reaction0 the half life time is0
1/2
[ ]2 R
t k
=
7or first order reaction0 the half life time is
1/2
0.693t
k =
where k* is rate constantIt is independent of initial
concentration for first order reaction.
Rate law for first order reaction:R 5
0[ ]2.303 log[ ] R
k t R
=
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where k* is rate constant or specific reaction rate0 'Ro is
initial molar conc.0 'R is finalmolar conc. after time t*.
2.303log
ak
t a x=
where a* is initial conc. reacted in time t* final conc.0 after
time t* is (a 8 ). If we plot a graph between ln'R with time0 we
get a straight line whose slope 4 k and
intercept ln'Ro . To calculate rate constant for first order gas
phase reaction of the type
" (g) $(g) # C(g)
12.303 log(2 )i t
pk
t p p=
,here pi is initial pressure of "0 pt is total pressure of
gaseous mi ture containing " 0 $0
C p t 4 p " # p $ # p C 5seudo first order reaction:
The reaction which is bimolecular but order is one is called
pseudo first order reaction.This happens when one of the reactants
is in large e cess.9 ample "cidic hydrolysis of ester (ethyl
acetate).
3 2 5 2 3 2 5 H CH COOC H H O CH COOH C H OH
+
+ +
"cti+ation energy (9 a):It is e tra energy which must be
possessed by reactant molecules so that collision
between reactant molecules is effecti+e and leads to the
formation of product molecules. "rrhenius e-uation of reaction
rate:
It gi+es the relation between rate of reaction and temperature.
/ a E RT k Ae
= ln ln / ak A E RT =
log log2.303
a E k A RT
=
where k 4 rate constant0 " 4 fre-uency factor0 9a 4 energy of
acti+ation R 4 gas constant0T 4 temperature in el+in0
2 2 1
1 1 2
log2.303
a E k T T k R TT
=
5robability factor or ;teric factor
.a E
RT AB Rate PZ e
=
,here
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It is the se-uence of elementary processes leading to the
o+erall stoichiometry of achemical reaction.
"cti+ated comple :It is an unstable intermediate formed between
reacting molecules. ;ince0 it is highlyunstable and it readily
changes into product.
Rate determining step:It is the slowest step in the reaction
mechanism.
The number of collisions per second per unit +olume of the
reaction mi ture is known ascollision fre-uency (
