Engineering Chemistry S Datta

116 ENGINEERING CHEMISTRY
Disturbing Factors in the Determination of an Order
The reaction which takes a single path has no interfering influence in the main reaction.
But many reactions are complicated, so the interpretation of the order of a reaction from their
velocity data becomes difficult. The rate constant values are not obtained constants for a number
of reactions. There are certain disturbing factors which influence the reaction and constant
values for k are not obtained. Some of these factors are given below:
Simultaneous Reactions
In a simple reaction, the reactants react to give products. But many of the reactions are
not so simple. The other reactions occur simultaneously. These reactions are called
simultaneous reactions. These reactions interfere to give a constant value of k. There are
three types of simultaneous reactions.
Simultaneous reactions
Side reactions
Opposing reactions
Consecutive
reactions
(a) Side reaction: Sometimes the reactants undergo two or more independent reactions
simultaneously. Each reaction gives a set of products. The reaction which gives the major
product is called main reaction and the other is called side reaction.
B + C; yield 90% (main reaction)
Example:
CH .CH .Cl
3 2
A
P + Q; yield 10% (side reaction)
CH = CH 2 2
CH .CH OH
3 2
CHOH 2 5
dx
C 2H4 + H2O
= k 1 c
dt
CH CHO + H
3 2
dy
dt
= k 2 c
[ k and k are not same]
1 2
(b) Opposing reaction: The reversible reactions also interfere to give the constant value of ‘k’.
A + B C + D
So, the experimentally determined rate for this reaction is a net result for the two opposing
reactions.
Example:
CH 3
COOH + C 2
H 5
OH
k 1
k 2
CH 3
COOC 2
H 5
+ H 2
O