What is the correct expression for the rate law of a first order reaction?

[atavistic]

OK if we are given the following first order rxn:

$$2N_2O_5 - > 4NO_2 + O_2$$

The rate constant is $$k$$. Now how would we express the rate law for the decomposition of $$N_2O_5$$.

$$- d[N_2O_5]/dt = k[N_2O_5]$$

OR

$$- d[N_2O_5]/dt = 2k[N_2O_5]$$

I think it is the latter but I see all books use the former.zzz

Its urgent.

 

[Ygggdrasil]

Well, the rate is equal to -(1/2) d[N₂O₅]/dt. You cannot, however, say a priori that the reaction will be first order with respect to the concentration of N₂O₅ without knowing the reaction mechanism.