Engineering Chemistry S Datta

REACTION DYNAMICS/CHEMICAL KINETICS 117
CH2OH . CH 2. CH 2. COOH
(Hydroxy butaric acid)
OCO
k 1
CH 2
k 2
CH CH
2 2
(Lactone)
+HO 2
Rate equation for the first reaction is:
dx
dt = k 1 (a – x)(b – x) – k 2 x2 .
where k 1
= rate constant for the forward reaction
k 2
= rate constant for the backward reaction
Here the reverse reaction interferes the determination of the rate constant of the main
reaction.
(c) Consecutive reaction: In many reactions, the products obtained change to a third
product. These types of reactions are known as consecutive reactions.
Examples:
k1
k2
A ⎯⎯→ B ⎯⎯→ C
(i) (CH 3
) 2
O ⎯⎯→
k 1
(ii) 2NO + H 2
⎯⎯→
k 1
CH 34
+ HCHO ⎯⎯→
k 2
N 2
+ H 2
O 2
CO + H 2
H 2
O 2
+ H 2 ⎯⎯→
k 2
2H 2
O
(iii) CH 2 . COOC2H5
CH COOC H
k
2 2 5
1
⏐ + NaOH ⎯⎯→ ⏐ + CHOH 2 5
CH . COOC H
CH COONa
2 2 5
Ethyl Succinate
2
Ethyl Sodium Succinate
CH2COOC2H5
CH COONa
k
2
2
⏐ + NaOH ⎯⎯→ ⏐ + CHOH 2 5
CH2COONa
CH2
COONa
Sodium Succinate
(iv) H 2
O 2
+ 2HI ⎯⎯→ 2H 2
O + I 2
H 2
O 2
+ I – ⎯⎯→
k 1
H 2
O + IO –
IO – + 2H + + I – ⎯⎯→
k 2
H 2
O + I 2
Of these reactions, the first proceeds very slow, which can be measured and that
determines the order of the reaction.
Period of induction
Sometimes it is seen that even though the proper conditions for the reaction are
maintained the reaction does not start. After some time the reaction starts and proceeds as
usual with normal speed. This period of static state of reaction is known as period of induction.
As for example—period of induction is noted during the reaction of H 2
and Cl 2
in the presence