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NCERT Class XII Chemistry
Chapter - Chemical Kinetics
Questions with Solutions

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Question : 20
Total: 39
For the decomposition of azoisopropane to hexane and nitrogen at 543 K, the following data are obtained.
 t (sec)  P(mm of Hg)
 0  35.0
 360  54.0
 720  63.0
Calculate the rate constant.
Solution:  
‌‌‌‌‌‌‌‌‌‌‌‌‌
(CH3)2CHN=NCH(CH3 )2( g)
Azoisopropane
––––––▶N2( g)+
C6H14(g)
Hexane
Initial‌pressure‌‌‌‌P0‌‌‌‌‌‌‌‌‌0‌‌‌‌‌0
Pressure‌‌‌‌P0−P‌‌‌‌‌‌‌‌‌p‌‌‌‌p
after‌time‌t
Total pressure after time t (Pt )= (P0−p )+p+p
=P0+p
or p=Pt−P0
[R]0∝P0 and [R]∝P0−p
On substituting the value of p,[R]∝P0−(Pt−P0 ),
i.e.[R]∝2P0−Pt
As decomposition of azoisopropane is a first order reaction
∴‌‌k=
2.303
t
‌log‌ ‌
[R]0
[R]

=
2.303
t
‌log‌ ‌
P0
2P0−Pt

When t = 360 sec,
k=
2.303
360
‌log‌ ‌
35.0
2×35.0−54.0

=
2.303
360
‌log‌ ‌
35.0
16

=2.175×10−3 s−1
When t=720 sec,
k=
2.303
720
‌log‌35.0‌2×35.0−63
=
2.303
720
‌log‌5
=2.235×10−3s−1
∴ Average value of k=
2.175+2.235
2
×10−3 s−1
=2.20×10−3 s−1
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