Rates of reaction - kinetics (A2)
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I am doing my A2 chemistry investgation on the kinetics of bromide and bromate ions. I have found that the reaction is 1st order wrt each reactant. However, from research, I know that the reaction should be 2nd order wrt sulphuric acid. I am stuck on a suggested reaction mechanism for my reaction as they are all 1st order. If the reaction is first order wrt to each reactant, which is the rate determining step?
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Ulex replies
This is an interesting situation! You say that the reaction is first order with respect to H2SO4. The fact is, however, that sulphuric acid is not one acid, but, effectively, two. In aqueous solution it ionises completely to H+(aq) and HSO4-(aq). The latter is also an acid, but a fairly weak one. A solution which is 1 mol dm-3 H2SO4 thus has a hydrogen ion concentration of only a little over 1 mol dm-3 H+ (aq). To put it another way, the hydrogen ion concentration is virtually the same as the total sulphuric acid concentration.
A possible mechanism could thus be (omitting the (aq) because everything is in aqueous solution):
H+ + Br- -> HBr (fast)
HSO4- + BrO3- -> HBrO3 + SO42- (slow)
HBr + HBrO3 -> HBrO + HBrO2 (fast)
HBrO2 + HBr -> 2HBrO (fast)
HBrO + HBr -> H2O + Br2 (fast)
This mechanism is consistent with the first order kinetics you mention and the rate-determining step is justifiable on the grounds that it involves the collision of two negative particles, and also it involves the weak acid HSO4-.
Don’t get too excited, though – I just made it up! This is what mechanisms are all about, isn’t it? You put up a hypothesis and someone else tries to find fault with it. Over to you!
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updated: 21 February 2008
