The mechanism for the reaction between Hydrogen and Bromine is given here which can take place both under thermal as well as photochemical conditions.

1. Mechanism of the reaction
between Hydrogen and Bromine

2. • Mechanism is comparatively simple.
• The results could be fitted into the following expression for the
rate of consumption of H2 and Br2
• m and k are constants; m value is about 10 and is independent
of temperature .
  
 
 
 1
1
2
2
1
22



Brm
HBr
BrHk
vt

3. The generally accepted mechanism is as follows:
   
 
 
 
 
   nterminatio21
2
3
2
21
2
2
2
2
2
BrBr
BrHHBrH
npropagatiochain
BrHBrBrH
HHBrHBr
InitiationBrBr










4. Applying the steady state hypothesis for H is :
For Br:
When you add these equations:
            2022322   HBrHkBrHkHBrk
dt
Hd
                30
2
12232221   BrkHBrHkBrHkBrHkBrk
dt
Brd
    0
2
121   BrkBrk
     42
1
2
2
1
1
1








Br
k
k
Br
                30
2
12232221   BrkHBrHkBrHkBrHkBrk
dt
Brd

5. (4) Gives the equilibrium concentration of Br atoms.
An expression for [H] can be obtained by inserting this
expression for [Br] into (2) or(3). Inserting into (2) becomes,
So that
           502232
2
1
2
2
1
1
1
2








HBrHkBrHkHBr
k
k
k
 
  
   
 6
223
2
1
22
2
1
1
1
2











HBrkBrk
BrH
k
kk
H

6. The rate of reaction is the rate of consumption of H2, which is ,
Subtraction of eqn. (2) gives,
Insertion of the expression for [H] gives,
       7222   HBrHkHBrkvt
    823  BrHkvt
  
   
 9
223
2
3
22
2
1
1
1
32











HBrkBrk
BrH
k
k
kk
vt

7. Also given by,
This is of the same form as the empirical equation (1) where k is
equal to k2(k1/k-1)½ and m = k3/k-2
  
   
 10
/1 2
3
2
5.0
22
2
1
1
1
2
















BrHBr
k
k
BrH
k
kk
vt

8.  All the constants k1,k2,k3,k-2 and k-1 have been evaluated.
 k which is experimental , together with k1/k-1 leads to k2.
 From the temperature of k2 dependence E2 can be obtained
 Reaction -2 is the reverse of reaction 2, so that from the
equilibrium constant k2/k-2 and from thermochemistry for the
reaction, k-2 and E-2 can be obtained.
 The values of k3 and E3 can be found from the value of m: since
this is temperature independent, E3 is equal to E-2.
 The values of k1 and k-1 were obtained by comparing the thermal
and photochemical reactions of H2-Br2

9.  It was found that the photochemical reaction proceeds
according to the equation:
 K’, m’ are constants and I is the intensity of light absorbed.
 Similarity with (1) shows that mechanisms are similar.
 Difference being that initiation reaction in the photochemical
reaction is absorption of a photon by a bromine molecule.
 
 
 
 11
'
1
'
2
2
1
2



Brm
HBr
IHk
vp
BrhBr 22  

10. This process is followed by the same four reactions as in the
thermal reaction. The steady state equation for bromine atoms
is now,
Eqn (2) is still the steady state equation for H, and addition of eqns
2 and 12 gives,
Or,
              1202
2
122322   BrkHBrHkBrHkBrHkI
dt
Brd
  )13(02
2
1   BrkI
   14
2 2
1
1







k
I
Br

11. The rate equation is obtained from (10) by replacing k1[Br2] with 2I
Which is in agreement with eqn. 11.
 
   
 15
/1
2
2
3
2
2
1
2
2
1
1
2
















BrHBr
k
k
IH
k
k
vp

12. SOME FEATURES ABOUT THE REACTION:
 Br. atom concentration in the thermal
reaction is the same with or without the
presence of hydrogen. (why ?)
Change in nature of surface or its area
cannot affect the rate of reaction.(why ?)
Presence of third bodies has no effect.
   
   nterminatio21
21
2
2
MBrMBr
InitiationMBrMBr



13. It follows from 10 and 15 that the ratio of the photochemical and
thermal rates is
At the same concentrations of H2 and Br2. Therefore if vp/vt is
measured at known values of I and [Br2] , the rate k1 can be
obtained.
If the experiments are repeated at different temperatures, the
parameters A1 and E1 are obtained.
 
 16
2
2
2
12
1
1







Br
I
kv
v
t
p

14. # Using the activation energies estimate the overall activation
energy of thermal hydrogen-bromine reaction under the
following conditions
a. At the beginning of the reaction
b. In the presence of large excess of HBr.
Also estimate the overall activation energy for the photochemical
reaction under the same conditions.
   
 
 
 
 
    0
5
5
72
192
nterminatio21
2
3
2
21
2
2
2
2
2
BrBr
BrHHBrH
npropagatiochain
BrHBrBrH
HHBrHBr
InitiationBrBr








