Engineering Chemistry S Datta

114 ENGINEERING CHEMISTRY
Pseudo Unimolecular Reactions
Let us take the reaction
C 12
H 22
O 11
+ H 2
O ⎯⎯→ C 6
H 12
O 6
+ C 6
H 12
O 6
Sucrose Glucose Fructose
If we determine the rate of the reaction, we see that the reaction is of the second order.
So, the reaction is dependent on the concentration of both the reactants. So, the rate equation
of the reaction is
– dc
= k[C
dt
12
H 22
O 11
] [H 2
O]
In solution the amount of water is excessively high compared to sugar. After complete
hydrolysis of sugar, the change of concentration of sugar is very negligible. So [H 2
O] is constant.
The rate equation comes to
– dc
= k′[C
dt
12
H 22
O 11
]
dx
or
= k′(a – x)
dt
or k′ = 1 t . ln a
a – x
So, characteristics of first order reaction is seen actually in second order reaction. Due
to presence of excess water, the incident so happens. Such solvolytic first order reactions are
called Pseudo unimolecular reactions. Other pseudo unimolecular reactions are:
(i) COS + H 2
O ⎯⎯→ CO 2
+ H 2
S
(ii) CH 3
COOCH 3
+ H 2
O ⎯⎯→ CH 3
COOH + CH 3
OH
(iii) (CH 3
CO) 2
O + 2C 2
H 5
OH (excess) ⎯⎯→ 2CH 3
COOC 2
H 5
+ H 2
O
Methods for the Determination of the Order of a Reaction
(i) Substitution methods (with the help of rate equations)
The following are the expressions of rate constants for different reactions:
First order reaction ⇒ k = 1 a
ln
t a – x
Second order reaction ⇒ k = 1 x
.
t aa ( – x)
Third order reaction ⇒ k = 1 1 1
–
2 2
2t ( a – x)
a
L
N
M
To determine the rate of a reaction a definite amount (a) of the reactant is taken and
with the passage of time ‘t’ the change in concentration (x) is determined mechanically or by
chemical analysis, i.e., (a – x) is determined. By putting the value ‘a’ and (a – x) the value of ‘k’
is determined. The order of the reaction is determined by that equation which gives the
satisfactorily constant value of ‘k’. The method is laborious and too complicated to give a
satisfactory constant value. But is still employed for simpler reactions.
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