CHEMICAL KINETICS
CHEMICAL KINETICS
Chemical kinetics deals with
study of rate of reactions, factors
affecting rate of reaction and
mechanism of reaction.
Unit of rate of
reaction
(Mol) /L / S
or
atm / s if time is in second
1.
1
2.
3.
4.
Characteristics of rate
constants:
K rate
rate.
K T
“K” is independent of
initial concentration of
reactants.
Unit of “K” varies with
order of reaction.
1. Order is experimentally
determined quantity.
2. Order cannot be written from
the balanced chemical equation.
3. Order may be zero, whole
number, fraction or negative.
4. Rate Constant of reaction is
independent of concentration of
reactant.
5. Half life period of zero order
reaction is directly proportional
t the
to
th initial
i iti l concentration
t ti off
reactant.
6. A catalyst does not influence in any way
the
ee
enthalpy
a py o
of the
e reaction.
eac o The
ee
energy
e gy
states of product or reactants remain
unchanged
7. Increase in reaction rate with rise
temperature is not due to increase in the
total numbers of collisions but due to the
increase in total number of effective
collisions
t1/2
Relation between half life
1 and order of reaction:
an-1
where t1/2 - half life
a - Initial concentration of reactant
n - order of reaction
Amount of reactant left after
“n” half lives
a
2n
Temperature coefficient of
reaction:
Temp coefficient = K(T+10) = 2 to 3
Temp. coefficient =
= 2 to 3
KT
That is for every rise 100c
temperature rate of reaction
increase by two to three times.
If the temperature coefficient is
“2” than increased rate of reaction = 2n
I
Important
t t relations:
l ti
t75%
t87.5%
87 5%
t93.75%
t99.9%
t87.5%
2 t1/2
= 3 t1/2
= 4 t1/2
= 10t1/2
= 3/2 t75%
=
t93.75% = 2 t75%
t64% = 2 t40%
t99% = 2 t90%
t99.9%
99 9% = 3 t90%
t96% = 2 t80%
t99.9% = 1.5
1 5 t99%
Q.I In the reaction 2N
Q
2 2O5Æ4NO
4 O2+O
O2
the rate of formation of NO2 is
0.0072 mol L-1 sec-1 .what is the
rate of change of [ N2O5] at the
same time?
1)o.oo18
3) 0.0036
2) 0.0144
4) 0.0072
=> rate = -½ d[N2O5]= ¼ d[NO2]
dt
dt
d[N2O5]=-2/4x0.0072
dt
=-0.0036mol L-1 Sec-1
Ans : 3) 0.0036 mol L-1 Sec-1
Q.2) A substance “A” decomposes in
g the first order
solution following
Kinetics ,Flask I contains 1 liter of 1M
Solution of “A” and flask II contains
100ml of 0.6M solution of “A” after 8
hours the concentration of “A” in flask
I becomes 0.25M what will be the time
for concentration of “A” in flask II to
become 0.3M?
1)) 0.4 hours h
2)) 2.4 hours h
3) 4 hours 4) 0.24 hours
=>In flask I conc.of “A” changes from
1M to 0.25M
i.e.; t75% =8 hours t1/2 =4 hours
i flask
in
fl k II conc.off “A” changes
h
ffrom
0.6M to 0.3M
i.e.; half life =4 hours
Ans: “3”
3 4hours
Q.3)
Q
3) If the initial concentration is
reduced to ¼th in a zero order
reaction
ti
th
the ti
time ttaken
k
ffor h
half
lf
the reaction to complete
1) Remains same 2) Becomes 4 times
3) Becomes ¼
4) Doubles
=> As t1/2 ∝ [Ro]
If Ro becomes [¼ Ro] then t1/2 also
¼th
Ans:
(3) becomes ¼
th
Q.4)Half
Q
4)H lf lif
life off a fi
firstt order
d reaction
ti
is 3 hours, how many grams of
substance
b t
will
ill remain
i after
ft 18 h
hours
from 300 gram of a substance?
1) 4.6 gram
3) 9.2
9 2 gram
2) 5.6 gram
4) 6.4
6 4 gram
=> No of half lives =18/3=6=n
Ro=300gram
Amount of
substance=Ro=300=4.6gram
=300=4 6gram
2n 26
Ans:
1) 4.6 gram
Q.5)In
Q
5)In the reaction 2N2O5Æ4NO2+O2
the rate is expressed as
1) –d[N
d[N2O5]=K1[N2O5]
dt
2) d[NO2]=K2[N2O5]
dt
3) d[O2]=K3[N2O5]
dt
Relation between K1, K2 & K3 is
1) 2K1=4K2=K3
2) 2K1=K2=4K3
3) 2K1=K2=2K3
4) K1=4K2=2K3
=> Rate = -½
½ d[N
[ 2O5] = ¼ d[NO
[
[ 2]
2] = d[O
dt
dt
dt
= -½ K1[[N2O5] =¼ K2[[N2O5] = K3 [[N2O5]
Dividing by [N2O5] & multiply by 4
2K1=K2=4K3
Ans: (2) 2K1=K2=4K3
Q.6) t1/4 can be taken as the
ti
time
taken
t k
for
f th
the concentration
t ti
of a reaction to drop to ¾ of it’s
initial value .If the rate constant
for a first order reaction is K,,
the t1/4 can be written as.
1) 0.10/K
2) 0.29/K
3) 0.69/K
4) 0.75/K
=>25% of reaction is completed then
time taken is t1/4
At 25% completion of reaction
ti
time
xK
K=0.29
0 29
time = 0.29
K
Ans: (2) 0.29/K
0 29/K
Q.7)
Q
7) Consider the reaction
2A+B--------> Products. When
concentration of “B”
B alone was
doubled the half life did not change
,when
when the concentration of “A”
A alone
was doubled the rate increased by two
times , the unit of rate constant for the
reaction is
1) L mol-1 S-1
2) S-1
3) no unit
4) Mol. L-1. S-1
=>t1/2 is independent of conc. Of “B”
order w.r.t “B” is 1
rate ∝ [A]
order w.r.t “A”
A is 1
over all order of reaction is “2”
Unit of rate constant for second order is
L mol-1.S-1
Ans: (1) L mol-1 S-1
Q.8)For the reaction system
2NO +O2Æ 2NO2 volume is suddenly
reduced to half of its value by
increasing the pressure on it .If
If the
reaction is first order with respect to
O2 and second order w
w.r.t
r t NO
NO, the
rate of reaction will be.
1) Increase by four times of it’s
initial value
2) Diminish to one fourth of it’s
initial value
3) Diminish to one eight of it’s initial
value
4) Increased to eight times of it’s
initial value
=> rate
t = K [NO]2 [O2]
Decrease of volume increases the
pressure. Increased
I
d rate=2
t 22x2
2 by
b
times
r=8
8 .Increases
I
b
by 8 ti
times
Ans: (4) Increased to eight times of
it’s
it
s initial value
9 A ffollowing
9.
ll i mechanism
h i
has
h
been proposed for a reaction.
2A+B
A+B
A+C
C+D
C+D (Slow)
C (fast)
rate equation for the reaction is
1)
2)
3)
4)
r = k [A]2 [B]
r = K [A]2
r=k [A] [C]
r = k [A] [B]
Slow step is a rate determining
step r=k [A] [B]
Ans: 4
r = k [A] [B]
10. Unit of rate of reaction and
10
unit of rate constant are same
th the
than
th order
d off reaction
ti is
i
1) 2
2) 0
3) 1
4) fraction
unit
it off rate
t off reaction
ti and
d unit
it
of rate constant are same only
y
when order of reaction is zero
Ans: 2) 0
11. The rate constant of a reaction
11
is 3.25 x 10-3 mol-2 (dm3)2 s-1.
Th order
The
d off the
th reaction
ti is
i
1) Zero
2) 1
3) 2
4) 3
Unit of rate constant is mol-22 (dm3)2 ss 1.
comparing with general unit of rate
constant mol1-n(dm3)n-1 ss 1. 1-n=-2
1 n= 2 n=3
Ans: 4 ) 3
12 A first order reaction 87
12.
87.5%
5%
completed in 300 min. What
is it’s half life period.
1) 150 min
min. 2) 100 min
min.
3) 200 min. 4) 250 min.
t87.5% = 3t1/2
t1/2 = t87.5%
87 5% = 300 = 100
3
3
Ans: 2
13 75% of a first order reaction was
13.
completed in 32 minutes. what is
the time required to complete
87.5% of the reaction?
1) 32 min.
i
3)) 48 min.
2)
4))
16 min.
i
64 min.
t87.5% = 3t75%
2
= 3 x 32 = 48 min
2
A
Ans:
3
14. A first order reaction 93
14
93.75%
75%
completed in 100 min. Calculate the time
required for 75% completion of the
reaction. 1)200
2) 300
3) 250
4) 50
t93.75% = 2t75%
t75% = t93.75%
2
= 100 = 50 min.
2
Ans: 4
15. 40% off a fi
15
firstt order
d reaction
ti
is completed in 75 min. At what time
64% of the reaction complete?
2)) 128 min.
1)) 100 min.
3) 150 min.
4) 138 min.
t64% = 2t40%
= 2x75 = 150
Ans: 3
16 96% of first order reaction
16.
was completed in 300 min.
when was it completed 80%
1)) 150 min. 2)) 600 min.
3) 100 min. 4) can’t be predicted
t96% = 2t80%
t80% = t96% = 300 = 150
2
2
Ans: 1
17 The first order reaction has a
17.
specific rate 10-3/s. How much
ti
time
will
ill it take
t k for
f 10g
10 off
reactant to reduce 5g?
1) 360 s.
3) 693 s.
2)
4)
180 s.
270 s.
t1/2 = 0.693 = 0.693 = 693
K
10-3
Ans: 3
18 Th
18.
The minimum
i i
amountt off energy
possessed by the reacting molecules
att th
the time
ti
off collision
lli i in
i order
d to
t
produce effective collision is called.
1)
2)
3)
4)
Threshold energy
Activation energy
Internal energy
P t ti l energy
Potential
Ans: 1
19. The rate of reaction increases
by
y the increase of temperature
p
because,
1) collision frequency is increased
2) energy of product decreases
3) fraction
f ti off molecules
l
l possessing
i
energy > ET increases.
4) mechanism of a reaction is Changed.
Ans: 3
20 T
20.
Temperature
t
coefficient
ffi i t off
reaction is 2, how many times
the rate increases when
p
is raised from
temperature
300K to 350K.
1) 27 ti
times
2) 16 ti
times
3) 8 times
4) 32 times
(Temperature coefficent)n = no. of
times rate increase where
n = diff.
diff off temperature
t
t
= 50 = 5
10
10
2n = 25 = 32
Ans: 4
21 Data for a hypothetical
21.
reaction A Æ product is
Expt
1
2
3
4
[A]
[B]
0.012
0
012
0 035
0.035
0.024
0.070
0.024
0.035
0 012
0.012
0 070
0.070
What is order of the reaction?
initial rate
0.1
0
1
0.8
0.1
08
0.8
1) 1
2) 2
3) 3
4) 0
order w.r.t “A”
increased rate = (increased conc)n
01 = 2n
n=0
order w.r.t “B”
increased rate = ((increased conc))n
8 = 2n
n=3
overall order of reaction is 03
Ans: 3
22 For a reaction A+B Æ product,
22.
product
by doubling the concentration
off reactant
t t “A” rate
t iincreases
by four times, by tripling the
concentration of “B” rate
increases by nine times.
What is order of reaction?
1) 2
2) 4
3) 3
4) 1
order w.r.t “A”:
increased rate = ((increased conc.))n
04 = 2n n = 2
order w.r.t “B” :
increased rate = (increased conc.)n
9 = 3n
n=2
overall order of reaction is 04
Ans: 2
23. First order reaction has half life
period 25 min. 10g of reactant is
used for decomposition. how many
grams of reactant left after 75
minutes ?
1) 2.5g
3) 1.25g
2) 5g
4) 10g
number of half lives = 3
amount off
substance
substa
ce left
et=
a = 10 10 1.25g
= =
n
3
2 2
8
Ans: 3
24. Rate of gaseous reaction is
given by the expression K [A] [B].
if the volume of the reaction vessel
is reduced to one fourth its original
volume, the rate will be
1) 1 times of original rate
10
2) 1 times of original rate
8
3) 16 times of original rate
4) 6 times of original rate
decreasing volume by four times
concentration
t ti off A & B iincrease
by four times therefore overall
order of reaction increases by
4x4 times = 16 times
Ans: 3
25. 8 gram of radioactive substance
undergoes disintegration 0.5grm
0 5grm of
substance left after 20 days. what is
half life period?
1) 7.5 days
3) 60 days
2) 15 days
4) 30 days
amount of substance left = a
2n
n=4
t1/2 =
ti
time___
= 120 = 30
no. of half lives 4
Ans: 4
26.According to Arrhenius equation rate
constant k is equal to Ae-Ea/RT. Which of the
following options represents the graph of ln
k vs 1/T ?
Ans: (1)
27.Consider the reaction A
B. The
conc. Of both reactants and products
p
y with time. Which of
varies exponentially
the following figures correctly describes
the change in conc
conc. Of reactants and
products with time ?
Ans: (2)
28. A catalyst
28
t l t llowers th
the activation
ti ti
energy of the forward reaction by
10 kj mol-1.What effect does it have on
gy of the backward
the activation energy
reaction?
1) Increase by the 10kj mol-1
2)) Decrease by
y 10 kjj mol-1
3) Remain unaffected
4) Can not be predicted
Catalyst brings the equilibrium faster by
decreasing the activation energy for both
forward and backward reaction, hence
activation energy decreases by 10kj mol-11
Ans: 2) Decrease by 10 kj mol-1
29.The rate constant is given by the
equation
equat
o k=p.ze
p e-E/RT which
c factor
acto
should register a decrease for the
reaction to proceed more rapidly
rapidly.?
?
1) T
3) E
Ans: 3) E
2)
4)
Z
P
30 Consider the endothermic reaction
30.
X
Y with activation energy Eb and
Ef for the backward and forward
reactions respectively in general
1) Eb<Ef
2) Eb>Ef
3) Eb=Ef
4) No definite relation between Eb & Ef
A
Ans:
1) Eb<E
Ef
31. The
31
Th d
decomposition
iti
off a
compound is a first order reaction.
Three fourth of the compound
taken had decomposed at the end
of two hours. The quantity left at
the end of next three hours will be
about.
1)1%
3) 3%
2) 2%
4) 4%
t3/4 = t75% = 2 hr
t1/2 = 1 hour
total time = 5 hours, 5 half
lives = t97%
amount left is 3%
Ans: 3) 3%
32 A chemical reaction carried
32.
out at 300K and 280K. The
a rate constant were found
p
y
to be K1 and K2 respectively
then
1) K2 = 2 K1
2) K2 = 4 K1
3) K2 = 0.5K1
4) K2 = 0.25K1
K 1 = 22 K 2
K1 = 4K2 or K2=0.5
0.5 K1
Ans: 4) K2 = 0.25K1
33 In which of the following case
33.
temperature has not effect on
th rate
the
t constant?
t t?
1)) Ea > 0
2) Ea < 0
3) Ea
E =0
4)) Ea can be p
positive or negative
g
Ans: 3) Ea = 0
34. The value of activation energy
can be found from one of the
following plots it is
1) ln K vs T
3) K vs T
Ans: 2)
2) ln K vs 1
T
4) 1 vs T
K
ln K vs 1
T
35. Which of following expressions is
used to describe the instantaneous
rate of the reaction 2A+3B
C + 2D?
1) - 1 d[A]
2 dt
2) + 1 d[A]
2 dt
3)) - 3 d[B]
4)) + 3 d[B]
dt
dt
Ans: 1
36 For a reaction 2X+Y Z
36.
Z, the
rate of appearance of “Z” is
0.05 mol/dm3/min. The rate of
pp
of “X” will be
disappearance
1) 0.05 mol/dm3/min
2) 0.25
0 25 mol/dm
l/d 3/min
/ i
3)) 0.1 mol/dm3/min
4) 0.05 mol/dm3/hour.
rate = + d[Z] = - 1 d[X]
dt
2 dt
rate of disappearance of X
d[X]= 2 d[Z]
dt
dt
= 2 x 0.05 = 0.1 mol/dm3/min
Ans: 3
37.Which
37
Whi h off the
th following
f ll i does
d
nott
affect the rate of reaction.
1) Amount of the reactants taken
away from
f
reaction
ti mixture.
i t
2)) Physical
y
state of the reactants.
3) Size of the vessel
4) Enthalpy
E th l change
h
off reaction.
ti
Ans: 4
38. If th
38
the reaction
ti rate
t att given
i
temperature
p
becomes slower
1) free energy of activation is higher.
2) free energy of activation is lower.
3) entropy changes.
changes
4) initial concentration of reactants
remain constant.
Ans: 1
39 Data for a hypothetical reaction
39.
A product is
Ti
Time
((s)) R
Rate
t off reaction
ti
(mol/dm3/s)
0
10
20
30
1.60
1.60
1.60
1.59
x
x
x
x
10-2
10-2
10-2
10-2
What is order of reaction?
1) 1
2) 0
3) 2
4) Unpredictable
Rate of reaction is independent of
time i.e.
i e independent of
concentration of reactant.
Ans: 2
40 If “a” is the initial concentration
40.
of reactant the half life period of
a nth order
d reaction
ti is
i proportional
ti
l to.
t
t1/2
t1/2
1) an
3) a n+1
1
an-1
a1-n
Ans: 4
2) ann-11
4) a 1-n
41. A zero order reaction is one:
1 C
1.
Conc. Of one off the
th reactant
t t is
i taken
t k large
l
excess.
2. Rate is directly
yp
proportional
p
to conc of
reactant .
3) Whose rate does not change with time
4) None of the above
Ans: 3) Whose rate does not
change
g with time.
42 Activation energy of reaction is :
42.
1.The energy released during the reaction
2.The energy evolved when the activated
complex is form
3.Minimum amount of energy needed to
overcome the
th potential
t ti l b
barrier
i off reaction
ti
4.Energy needed for 1 mole of the product
Ans: 3)
43. Number of molecules of the reactants
taking part in a single step of the reaction tells
about:
1.Molecularity of the reaction
2. Mechanism of the reaction
3. Order of the reaction
4 All the above
4.
Ans: 1)
44. Number of mole of substance present in 1
liter volume is known as
1.Activity
2. molar Concentration
3. Active mass
4. None of these
Ans: 2)
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