Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: LHM on January 15, 2011, 05:48:18 PM

Title: Rate law and mechanism
Post by: LHM on January 15, 2011, 05:48:18 PM
For the reaction:
2 H2(g) + 2 NO(g) :rarrow: N2(g) + 2 H2O(g)
This mechanism is proposed:
step 1: H2 + NO :rarrow: H2O + N
step 2: N + NO :rarrow: N2 + O
step 3: O + H2 :rarrow: H2O

If we're given that the 2nd step is the rate law, then how come rate=k[H2][NO]2, instead of rate=k[H2][NO]2/[H2O]? Like why can just disregard the [H2O], since it's a gas in the overall equation?
Title: Re: Rate law and mechanism
Post by: samiam on January 15, 2011, 05:59:10 PM
what?
You are saying step 2 is the rate law? so the rate determining step i.e. the elementary step that limits the overall rate of reactions.

than the rate law would be r=k[N][NO] it does not depend on the products.
Title: Re: Rate law and mechanism
Post by: LHM on January 15, 2011, 06:03:29 PM
Quote from: samiam on January 15, 2011, 05:59:10 PM
what?
You are saying step 2 is the rate law? so the rate determining step i.e. the elementary step that limits the overall rate of reactions.

than the rate law would be r=k[N][NO] it does not depend on the products.


Oh oops, I'm sorry! I meant that step 2 is the rate determining step.
Title: Re: Rate law and mechanism
Post by: LHM on January 17, 2011, 12:22:07 PM
But it's highly unlikely that the answer key's wrong, which means that the rate law is rate=k[H2][NO]2, so do you or anyone else know why it's this?