[AhmedEzatAlzawalaty]

the order of the reaction H2+I2=2HI is second order.The order of the raection H2+Br2=2HBr is 1.5
how the rate of the latter couldnt be like the rate of the former rections?

[Yggdrasil]

The two reactions probably proceed through different reaction mechanisms.

[AhmedEzatAlzawalaty]

do you know what that mechanism is?

[Yggdrasil]

Unfortunately, no.  Sorry.

[AhmedEzatAlzawalaty]

Does any one know what the mechanism is?

[ARGOS++]

Dear ahmedezat,
Mr. Bodenstein found by his experiments, that the following Formula
for the Reactiontime holds for Br₂:

     _{d[HBr]  / dt}    = [/sub]   k’  *  [H₂] * [Br₂]^½    
                               k’’ + [HBr] / [Br₂]

YES, what surprise:  [Br₂]^½  !!!

As the reaction is Light induced to build first radicals:
                     0.)   Br₂          <?>   2Br^.            (fast! )
Next Steps:     A.)  Br^.  +  H₂    ?>   HBr  + H^.     (slow!)
                     B.)   H^.  +  Br₂   ?>   HBr  + Br^.     (fast! )

That’s telling you, that reaction A.)  is relevant for the reaction speed,
and the whole "thing" is a so called "chain-reaction" ,
so in this case, (for H₂ + Cl₂ too, but NOT for H₂ + I₂!), it means,
you have to use:
   
           d[HBr] / dt    =    k’ * [H₂ ] * [Br^.]        from A.)
                                                  ________
From 0.) it follows, that:   [Br^.]  =  ? K * [Br₂]   ,  and combined:

           d[HBr] / dt    =     k’ * K^½  * [H₂] * [Br₂]^½         

Now, may be all clear?
Good Luck!
                   ARGOS++

[AhmedEzatAlzawalaty]

Thank u vey much, ARGOS++