# Why does the rate equation between hydrogen peroxide and iodide ions not follow the stoichiometry of the rate-determining step?

The ionic equation of the reaction between hydrogen peroxide and iodide ions can be written as $\ce{H2O2 +2I- + 2H+ -> 2H2O +I2}$ which is the rate-determining step as well.

From what I have learnt, the order of reaction with respect to each reactant in the rate equation should be the coefficients of the respective reactants in the rate determining step.

So that makes the rate equation $= K\ce{[H2O2][I-]^2[H+]^2}$

However, from an experiment conducted, the rate equation I have obtained is rate $= k\ce{[H2O2][I-]}$ instead. Why is it so?

• It is extremely unlikely that the rate determining step involves 5 separate species... – Zhe Jun 8 '18 at 17:19

I would guess because your equation does not represent an elementary step in the reaction mechanism. That's hardly surprising, because it represents the simultaneous collision of five objects -- the H2O2 molecule and the four ions. That would be an extraordinarily rare occurrence, even in a very concentrated solution.

Your experiment is suggesting the rate-limiting elementary step involves a collision between the H2O2 molecule and the I- anion. Those will happen all the time.