CBSE Class 12 Chemistry 2015 Solved Paper

Question : 24

Total: 26

(i) Show that it follows pseudo first order reaction, as the concentration of water remains constant.
(ii) Calculate the average rate of reaction between the time interval 30 to 60 seconds.
(Given log‌2=0.3010,log‌4=0.6021 )
OR
(a) For a reaction A+B→P, the rate is given by Rate =k[ A][B]‌2
(i) How is the rate of reaction affected if the concentration of B is doubled ?
(ii) What is the overall order of reaction if A is present in large excess?
(b) A first order reaction takes 30 minutes for 50% completion. Calculate the time required for 90% completion of this reaction.
(log‌2=0.3010)

Solution:

‌k=‌

2.303

‌log‌

[A0]

[A]

‌k=‌

2.303

‌log‌

0.60

0.30

‌k=‌

2.303

×0.301=0.023 s−1
‌k=‌

2.303

‌log‌

0.60

0.15

‌k=‌

2.303

×0.6021=0.023 s−1
(ii) As k is constant in both the readings, hence it is a pseudo-first order reaction.
‌ Rate ‌‌=−‌

∆[R]

∆t

‌=−‌

[0.15−0.30]

60−30

‌=0.005‌mol‌L−1‌ s−1
OR
(a) (i) Rate will increase 4 times of the actual rate of reaction.
(ii) Second order reaction
(b) t1∕2‌=‌

0.693

30‌min‌=‌

0.693

k‌=0.0231min−1
t‌=‌

2.303

‌log‌

[A0]

[A]

t‌=‌

2.303

0.0231

‌min
t‌=99.7‌min

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