Engineering Chemistry S Datta

REACTION DYNAMICS/CHEMICAL KINETICS 105
To consider the rate of a reaction, we should take into account the ‘order’ and the
‘molecularity’ of a reaction.
Order of a Reaction
The rate of reaction will definitely depend on the concentration of the reactants.
Therefore, the rate equation will involve the concentration of the reactants. In the
experimentally determined rate equation, the powers of the concentration terms are added to
get the order of a reaction. As for example, for a reaction:
aA + bB ⎯⎯→ Products
rate of the reaction (r) = k . [A] a . [B] b .
So, the order of the reaction will be = (a + b).
The experimentally determined rate equation for the decomposition of H 2
O 2
is:
r = k . [H 2
O 2
]
Here we see rate is proportional to the first power of the concentration of H 2
O 2
. So it is
a first order reaction.
Table 5.1
Reactions Experimentally determined Order
rate
(i) 2HI = H 2
+ I 2
r = k . [HI] 2 2
(ii) H 2
+ I 2
= 2HI r = k . [H 2
][I 2
] 2
(iii) CH 3
COOC 2
H 5
+ NaOH r = k[CH 3
COOC 2
H 5
] [NaOH] 2
= CH 3
COONa + C 2
H 5
OH
Highlights:
• The number of molecules of the equation for the chemical change has got no relation
with the order of a reaction.
• The order of a reaction will be determined by the power terms of the concentrations
of the rate equation.
• The order of a reaction may be zero or fraction.
Molecularity of a Reaction
The minimum number of molecules, atoms or ions taking part in a chemical reaction is
known as molecularity of the reaction. As for example
2HI = H 2
+ I 2
For this reaction, minimum two molecules of HI will be required. So, the molecularity is
‘two’. Again for the reaction:
2H 2
O 2
= 2H 2
O + O 2
But the reaction can be written as
H 2
O 2
= H 2
O + O
So, for decomposition of H 2
O 2
, one molecule is needed and hence the molecularity is one.