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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: AhmedEzatAlzawalaty on March 19, 2007, 02:53:02 PM

Title: H2+Br2
Post by: AhmedEzatAlzawalaty on March 19, 2007, 02:53:02 PM
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?
Title: Re: H2+Br2
Post by: Yggdrasil on March 19, 2007, 03:25:25 PM
The two reactions probably proceed through different reaction mechanisms.
Title: Re: H2+Br2
Post by: AhmedEzatAlzawalaty on March 19, 2007, 03:59:24 PM
do you know what that mechanism is?
Title: Re: H2+Br2
Post by: Yggdrasil on March 19, 2007, 06:34:32 PM
Unfortunately, no.  Sorry.
Title: Re: H2+Br2
Post by: AhmedEzatAlzawalaty on March 20, 2007, 08:11:17 AM
Does any one know what the mechanism is?
Title: Re: H2+Br2
Post by: ARGOS++ on March 20, 2007, 08:00:31 PM
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’ * K½  * [H2] * [Br2]½         

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


Title: Re: H2+Br2
Post by: AhmedEzatAlzawalaty on March 21, 2007, 08:37:09 AM
Thank u vey much, ARGOS++