[FouRRaW]

Question is...

The balanced equation for the decomposition of dinitrogen pentoxide is

       2 N₂O₅ ------> 4NO₂  + O₂

The experimentally determined rate law is Rate - k [ N₂O₅ ]
(i.e. The reaction is first order in N₂O₅ )

attempt:  the answer is this i'm just kind of confused...

The reaction is not elementary because the rate law shows it is first order, and yet, the balanced equation has a coefficent of "2" in front of the N₂O₅  .  If elementary, the rate law should be second order in N₂O₅ .

So could anyone make more sense of this for me??  And so does this mean that if elementary does the coefficient of the reactant always have to equal the exponent of the rate law in order to be elementary?

Thanks

[Albert]

The experimentally determined rate law is Rate - k [ N₂O₅ ]

To quote my physical chemistry professor: 'Rate laws have nothing to do with the stoichiometry of the reaction, since they are experimentally determined.'
Put in other words, there are many occasions where stoichiometry is misleading.

For what concerns your question about elementary reactions, check this out: http://www.science.uwaterloo.ca/~cchieh/cact/c123/elmntary.html

[Donaldson Tan]

The actual reaction mechanism consist of my steps.

When these steps are combined, you get the overall material balance equation, which is the one which you had depicted above.

However, the rate equation only depicts the stoichiometry of the slowest step.

This means the elementary equation related to the rate law is not necessary the overall material balance equation.