Engineering Chemistry S Datta

106 ENGINEERING CHEMISTRY
Highlights:
• For the reaction
2N 2
O 5
= 4NO 2
+ O 2
,
The observed rate = k . [N 2
O 5
]
So, the order of the reaction is one and the molecularity is two.
• 2NO + O 2
= 2NO 2
Rate = k . [NO] 2 [O 2
]
So, both the order and the molecularity of the reaction are three.
• For the reaction
BrO 3
–
+ 5Br – + 6H + = 3Br 2
+ 3H 2
O
The observed rate = k . [BrO 3 – ] [Br – ] [H + ] 2
So, the order of the reaction is four but the molecularity is twelve.
• For the reaction
CO + Cl 2
= COCl 2
Rate = k . [CO] [Cl 2
] 1.5
So, the order of the reaction is 2.5 and the molecularity is 2.
Mathematical Formulation of First Order Reaction
The reactions, where the experimentally determined rate of the reaction is dependent
on the first power of the concentration of the reactant, are called first order reactions.
If c be the concentration, then the rate of the reaction = – dc
dt = kc.
where
k = rate constant.
or
Let the reaction be
A ⎯⎯→ Products.
(a – x) x
At time ‘t’, a = initial concentration of A.
x = concentration of the product at time t.
Therefore, the rate equation will be
Integrating we have,
dx
dt
= k . (a – x)
dx
= k . dt.
a−
x
– ln (a – x) = kt + z
where z = integration constant
when t = 0, x = 0
Then,
– ln a = z
So, – ln (a – x) = kt – ln a
or ln
a
= kt
a – x