15. The Rates of Chemical Reactions   Previous PageNext Page
       Formation of HBr
The rather horrendous rate law for the HBr reaction,
 

arises because the true process is a chain reaction that involves first the dissociation of Br2 molecules into atoms, then the reaction of atoms with other H2 and Br2 molecules:
 

The latter two equations constitute a chain reaction, each one yielding a molecule of HBr and producing the reactant atom for the other chain step. There also are reactions that either damp down or reverse the chain process:

 

With these reactions and a certain amount of algebra, one can arrive at the observed rate expression in a straightforward though tedious manner. Although the rate equation looks complicated, we can understand it in terms of the HBr mechanism. For example, as [HBr] increases, its presence in the denominator decreases the rate of reaction.

This happens because the chain-reversal reaction sends more HBr back to H2 molecules and Br atoms. At low HBr concentration, for which the ratio [HBr]/[Br2] is small in comparison with the rate constant k, the rate law simplifies to the 1-order expression that we saw previously:

 
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