[Violet89]

Consider the reaction, Cl2 + H2S => 2 HCl + S, which is found to be first order in Cl2. Which step of the proposed mechanism must be slow in order to agree with this rate law?

Cl2 => Cl+ + Cl-

Cl- + H2S => HCl + HS-

Cl+ + HS- => HCl + S
     
1. only 3       
2.only 2       
3. Either 2 or 3       
4. 1 - answer


How do you solve for this?

Do I start off by setting up: Cl2 + Cl- + H2S + Cl+ + HS- ---> Cl- + HCl + HS- + HCl + S ?

Even after reading my book, I still don't understand... I would appreciate any help.

[Corribus]

You need an overall expression for d[Cl₂]/dt in terms of all your rate constants and just your reactants (which lucky for you is only Cl₂).  Then you can start to think about limits.

So why don't you start with that and see where that gets you.

[Corribus]

Violet I have to apologize.  When I read your question earlier I did so very fast in the middle of the meeting.  I just read it again and I see I gave you some pretty poor help.  Also I didn't realize this was in the undergraduate general chemistry forum.

I see now that the series of equations you wrote are going to be pretty difficult to solve the way I suggested.  A better way is probably to just consider the options given as answered and see if any of them give you a first order expression with respect to [Cl2].

Problem is, I did this myself and I think the question is a little vague.  It's unclear whether they mean first order with respect to Cl2 ONLY or first order with respect to Cl2, but possibly dependent on the concentration of another reactant as well.  Also, the answer choices you've given are a bit unclear.  Especially, I don't know what option 4 ("1 - answer") is supposed to mean.  Finally, is this the way the equations are really written, or are some of them supposed to be equilibria?

Anyway, that probably still isn't that helpful but maybe a little more helpful than my initial hasty response.