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Chemical Kinetics

Hints to selected problems

(7)The reaction is 3rd

order, Rate [NO]2[O 2], When the volume is doubled the conc, becomes

half , the rate becomes , [] 3= 1/8

(11) The rate of reactionremains the same with respect to any reactant or product.( The rate of

consumption or formation can vary )

(12) The rate of disappearance of H2is thrice that of N2.

(20) The rate of reaction =

[NO2]/t= 15.210

/100 =1.3 10mol/s

(23) Rate of reaction = []/ =0.14/30 =4.67 10-3

mol/s

(25)k = 0.693/t =0.693/480 =1.44010-3

/s

(26) No. of moles of O2=36/32 = 1.125 moles/min

No. of moles of H2O = 2 1.125 = 2.250/ min

(27) For a zero order reaction , the conc. Of reactant decreases linearly with time.

(28) The rate constant is independent of conc.

(31) Rate depends on the slowest step,

(32) because rate

(35) After two half lives th of the substance reacts and th is left so t =2t

(38) Rate const of a first order reaction is time-1

(40) 87.5% change takes place in 3 half lives therefore t =15/3 = 5 min

(46) by formula

(54) Rate = k [k] 2

(56) For a zero order reaction t is proportional to initial conc. Of reactants.

(59) Rate =k [A]2[B ], = 0.22 = k [1]

20.25 , k = 0.22/0.250= 0.88

(71) Rate = k[ A ], [ A ] = 2.4010-6

/4 10-6

= 0.6

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(90) Rate = k[ A ], k = 1 10-2

/ 2 = 0.5010-2,

t = 0.693/ 0.5010-2

= 138 sec approx

(107) After 3 half lives 12.5 % is left , after 4 half lives 6.25 % is left so for 10% to be left , time

should be between 3 and 4 half lives.

(117) For an exothermic reaction , inc of temp favours backward reaction .

(119) r2=[2A]a[B/2]

b, r 1 =[A]

a[B ]

btherefore r2/ r 1 = 2

(a-b)

(120) E a does not depend on stoichiometry

(122) When conc of A is increased , there is no change in rate but when conc of B is doubled ,

the rate becomes 8 time.

(125) Rate = k [A]2

or moles dm-3

s-1

= k moles2dm

-6therefore k = moles

-1dm

3s

-1.

(130) 0.8/0.60= 1/0.75 = 1.33 it shows that ratio is same.

(134) slope of the graph is k therefore k = 2.50 10-3

s-1

.

(141) For every 10 degree rise rate gets doubled so for a rise of 70 degree rate increases by 27

=128 times approx

(144) by considering the unit of rate const

(154) k = Rate/ [A] [B] = 4 10-2

/11 moles dm-3

s-1

, 90% of reactants have been converted to

products therefore [A] and [B] = 10% = 10-1

so rate = k 10-1

10-1

= 4 10-4

(155) same as 154

(158) [A] ---------------[A/3] reacts in 1 hr so [2A/3] is left , in second hr 1/3 of [2A/3] or 2A/9

reacts

(160) theoretically the conc of reactants in a first order reaction never becomes zero.

(166) According to Arrhenius eqn k = A-Ea/RT

as T the value of-Ea/RT = 0 and

e0=1

Therefore k = A and k = 6.0 1014

s-1

(168) k =.

log

= 1.386 10

-4

(169) since the conc of X changes from 0.1 M to 0.025 M i.e. it becomes so it takes two half

lives , half life must be 40/2 = 20 min.

Rate = k [X] = 0.693[X]/ t = 0.693[.01]/ 20 = 3.465 10-4

moles min-1

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