Engineering Chemistry S Datta

REACTION DYNAMICS/CHEMICAL KINETICS 111
(iii) Dimerisation (gas phase):
CH2 = CH⎯ CH = CH2
⎯⎯→ CH2
⎯ CH = CH⎯CH
butadiene
⏐
⏐
CH ⎯ CH = CH⎯CH
2
dimer
(iv) Addition (gas phase):
H 2
+ I 2
⎯⎯→ 2HI
(v) Substitution reaction (Solution phase):
(a) I – + CH 3
Br ⎯⎯→ CH 3
I + Br –
(b) OH – + CH 3
I ⎯⎯→ CH 3
OH + I –
(vi) Hydrolysis of an ester:
CH 3
COOC 2
H 5
+ OH – ⎯⎯→ CH 3
COO – + C 2
H 5
OH
(vii) Esterification:
CH 3
COOH + C 2
H 5
OH ⎯⎯→ CH 3
COOC 2
H 5
+ H 2
O
Comparison
Points First order reaction Second order reaction
2
2
Rate equation
k = 1 a
ln
t a – x
k = 1 t
x
aa ( – x)
Linearity
log c decreases linearly with time
Unit of k sec –1 l mol –1 . sec –1
1
c
increases linearly with time.
k Independent of concentration. Dependent on concentration.
t 1/2
Independent of initial concentration. Inversely proportional to initial
Third Order Reaction
The rate equation for a third order reaction is
where,
A + B + C ⎯⎯→ Products
dx
dt
Initial concentration of A = a
Initial concentration of B = b
Initial concentration of C = c
= k(a – x)(b – x)(c – x)
no. of moles of each reactant decomposed at time t = x.
When,
a = b = c
the rate equation becomes,
dx
dt
= k(a – x)3
concentration.