| Physical Chemistry V & P Chapters
Purely from view point chemical kinetics is as important as the IInd Law of thermodynamics Neither humans nor any other organism could exist in air if activation energy (Eact) was not an energetic barrier to second Law because all organic compounds are thermodynamically unstable in oxygen.
Action of IInd law. Without chemical kinetics there would be no life of universe Air makes life impossible unless the barrier of Eact is present.
Fortunately force "The IInd law of thermodynamic is time's arrow but chemical kinetics is time's variable clock."
Study of speeds of the rates of chemical reaction of factors affecting the rates of reactions.
Dumb question:- Why ionic reaction are instantaneous?
Ans. Because in ionic reaction they do not involve any breaking of bonds as ions are already present in aq. Solution.
Rate of reaction :- it is change in conc. Of anyone of reactants products per unit time.
Rate of reaction =
Dumb question:- what is significance of -ve & + ve sign ?
Ans:-ve sign indicates that conc. Of reactant is decreasing & sign indicates that conc. Of products is increasing.
Units:-Unit of rates of rxn. Is mol/L/sec.
In case of gaseone rxn, pressure or used in place of moles conc.
So, rate of rxn. are atm min- 1
Avg. rate of rxn:- Avg. rate is defined for a particle time interval.
Avg. rate rxn =
of time which is change in conc. of any of reactants at that particular instant of time.
Instantaneous rate =
Expressing rates of reaction in terms of different reactants & products :-
In genral for rxn.
aA + bB xX + yY - a, b, x, y stoichiometre coefficiet.
eg. 2N2O5 4NO2 + O2
Illustration:- Following rxn mass studied in closed vessel
2N2O5(I) 4NO2 + O2
It was found conc. of NO2 increases by 2 X 10- 2 mol- 1
(i) rate of rxn. (ii) rate of change of conc. of M2Os
Ans :- (1) Rate of rxn =
Rate of rxn =
Factors affecting reaction rate:-
(1) Nature of reactants:-
(2) Conc. Of reactants:- Greater are conc. of reaction faster is reaction.
(3) Temperature:- Rate of reaction increases with increases of temperature.
(4) Presence of catalyst :- Catalyst speed of reaction without being consumed.
(5) Surface area of reactants :- Smaller particle size, more surface area, faster is reaction.
Law of mass action :- Rate at which substance reacts is directly proportional to its active mass & rate at which reaction proceeds is proportional to product of active masses of reacting substances.
Rate Law According to this rate of reaction is directly propotional to conc. termes raised to, some exponent which is determined exprimentally.
aA + bB Products
According to RAte law
Rate = K
According to law of mass action
Rate = K [A]a[B]b
Rate = K
[A] = 1 & [B] = 1 Rate = K
Characteristics :- (1) Greater is value of rate constant test is reaction.
(2) value of rate constant for same rxn. change with time.
Order of reaction :- It is some of exponents (pomers) to which conc. terms inrate law eq. Are raised.
Rate = = K BY Rate law
oredr of rxn =
(1) Reaction of zero order :-
H2(I) + Cl2(I) 2HCl
Rate = K[H2]0[Cl]0 = K
(2) Reactions of Ist ordwer :- NH4 NO2 N2 + 2H2O
Rate = K[NH4 NO2
(3) reaction of IInd order :-
2HI H2 + I2
Rate = K[HI]2
Illustration :- Rate of gaseones rxn. Is halved when vol. Of vessel is doubled. What is order of rxn.
Ans:- Suppose rxn. Is
A Products & order of rxn is n.
Rate = K[A]n or r = Kan ........................................... (1)
when volume of vessel is doubled, molecules conc. Is helved. As rate be comes half. So,
2 = 2n n = 0
Units of Rate Constant :-
(1) For zero order:-
(2) For Ist order:-
(3) For nth order :-