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The rate law isn't determined by the overall equation. Certain reactions might proceed in multiple steps. In this case, the rate is determined by the slowest step of the reaction, not the overall equation. HI is second order because the rate determining step is bimolecular with respect to HI (two HI molecules are involved in the step). N2O5 is first order because the rate determining step is first order with respect to N2O5 (only one molecule is involved in the rate determining step).
To second off that, you can never determine a rate law like you do equilibrium constants or Q, as there are usually a bunch of intermediate steps in reactions that you have to take into account. Therefore, unless you know the whole reaction mechanism, rate laws can only really be determined experimentally.
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