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### Which equilibrium equation to use

Posted: Thu Mar 15, 2018 11:48 am
In the example Lavelle did in class, he used the equilibrium equation for the step 1 (K1) and subbed this in for the intermediate in the second step. How do you know to use the first equilibrium concentration for the first step? Is it because this is the one with reactants from the problem? Is this always a rule?

### Re: Which equilibrium equation to use

Posted: Thu Mar 15, 2018 1:26 pm
If a fast step is before a slow step, we can assume the fast step is in equilibrium. I believe Lavelle called it pseudo equilibrium.
We substitute equilibrium constant K to get rid of any intermediates because intermediates cannot be in the rate law.

### Re: Which equilibrium equation to use

Posted: Thu Mar 15, 2018 1:36 pm
I just wanted to elaborate on Cristina's answer. The concept behind pseudo equilibrium is that having the fast step before the slow step results in a build up of products for the fast step. As a result, the reaction begins to reverse and create reactants, creating a fake equilibrium. As a result, we can use K (equilibrium constant) for the fast step (Step 1) because technically the reaction is in equilibrium.