2NH3->N2+3H2. Rate=k[NH3]/[H2], what is the mechanism?

[Akersar]

Homework Statement

Ammonia decomposes to form nitrogen gas and hydrogen gas. The rate law for the reaction is Rate=k[NH3]/

.
Devise a plausible mechanism for this reaction, taking into account both the balanced equation and the rate law.

Homework Equations

Rate = k[NH3]/

2NH3->N2 + 3H2

The Attempt at a Solution

I've tried the following mechanisms:
1. NH3 -> NH2 + H
NH2 -> N + H2
N + H + NH3 -> N2 + 2H2

2. NH3 -> N + H2 + H
NH3 + N -> N2 + H2 + H
H + H -> H2

3. NH3 -> 3H + N
NH3 + H -> N + 2H2
H + H -> H2
N +N -> N2

 

[epenguin]

You say you've tried. You don't say what was the result. Do any of those mechanisms fit the kinetics? What other assumptions do you need to make? (About rates).

As well as fitting the kinetics, it would be nice for it to be chemically and mechanistically plausible. Mechanistically plausible means a molecule not doing too many things all at once. I don't like the last line of your first try. You are requiring a trimolecular collision - take it this is so improbable it never happens - and then a complicated rearrangement. In your second try you ask a molecule split into three all at once. Try doing anything by easier stages even if there are more of them.

Play around a bit more, but show in all cases how (if) you get the stated kinetics.

I thought of a mechanism that does the kinetics and relates to some known chemistry but I am not overconfident about it so just try to get something as reasonable as you can. I am assuming from phrasing of the question that this is a pure gas phase reaction without catalyst or free radical initiator. You will not easily find info about this online. This is a very well known and important reaction - recognise it? - but we are always told it is done with high a catalyst and at high temperature.