## Rate Law

$aR \to bP, Rate = -\frac{1}{a} \frac{d[R]}{dt} = \frac{1}{b}\frac{d[P]}{dt}$

Jina Kwon
Posts: 100
Joined: Fri Aug 09, 2019 12:15 am

### Rate Law

for 2 N2O5 ---> 4 NO2 + O2
why is the rate law: kr[N2O5]

for 2 HI ---> H2 + I2
why is the rate law: kr[HI]2

how come for the reaction, the reactant is not squared?

Shail Avasthi 2C
Posts: 101
Joined: Fri Aug 30, 2019 12:17 am

### Re: Rate Law

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).

Jessica Li 4F
Posts: 115
Joined: Fri Aug 09, 2019 12:16 am

### Re: Rate Law

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.