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 H |