Page 92 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch07 Elementary Physical Chemistry

Chemical Kinetics 77

Proposed mechanisms:

+
(1) H + HNO 2 H 2NO + rapid equil. (7.48)
2
+
(2) H 2 NO +Br → ONBr + H 2O slow (7.49)
−
2
+
(3) ONBr + C 6 H 5 NH 2 → C 6 H 5 N +H 2 O+Br − fast (7.50)
2
The rate constants for reactions (7.48)–(7.50) are respectively
(1) k 1 (forward →)and k (reverse ←),

1
(2) k 2 (forward →),
(3) k 3 (forward →)
The rate-determining step (Step 2) gives
+
r = k 2 [H 2 NO ][Br ] (7.51)
−
2
From (1) we get the equilibrium constant
+ +
K = k 1 /k =[HNO ]/{[H ][HNO 2 ]} (7.52)
2
1
which yields,
+
−
r = k 2 K[H ][H 2 NO 2][Br ] (7.53a)
or
+

−
r =(k 2k 1 /k )[H ][HNO 2][Br ] (7.53b)
1
Equating the empirical constant k with k 2k 1 /k gives the observed reaction

1
rate law.
7.14. Unimolecular Reactions
First order gas phase reactions are usually called unimolecular reactions.
But for a molecule to break up, it must acquire enough energy. How does it
do that? By colliding with another molecule. But that is a bimolecular
reaction. The overall reaction, it turns out, has both bimolecular and
unimolecular steps.

7.14.1. The Lindemann Mechanism

This was the first successful explanation of a unimolecular reaction.