## Pre-equilibrium approach and steady-state approach

Michael Lonsway 3O
Posts: 43
Joined: Wed Sep 21, 2016 2:57 pm

### Pre-equilibrium approach and steady-state approach

What's the difference between the pre-equilibrium approach and steady-state approach? The course reader states that they give the same result but I'm confused with which method goes to which approach. Also, how would the direct computation method relate to these two approaches?

Ranica Hortelano 2D
Posts: 9
Joined: Wed Nov 18, 2015 3:00 am

### Re: Pre-equilibrium approach and steady-state approach

Direct computation is used experimentally and uses software.
For the steady-state method you would have to assume that the concentration of the intermediate is constant (rate of change of concentration is 0) and the rate-determining step is unknown while the pre-equilibrium method assumes that the reactants and the intermediates are in equilibrium.

Marisa_Woo_2G
Posts: 24
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Pre-equilibrium approach and steady-state approach

To my understanding, the steady-state approximation is when you determine if the concentration of the intermediate product/reactant is constant in the rate limiting slow step. If the proposed mechanism is correct, the intermediate that is formed and used is in a low amount and therefore constant during the reaction. The intermediate doesn't change with time and is therefore not included in the overall rate law since the amount is constant. As there are no intermediates in the overall rate law, the proposed mechanism is likely to be correct. The intermediate is in a steady-state.

The pre-equilibrium approximation is when you determine that the reaction before the rate limiting slow step is at equilibrium. You would substitute the intermediate concentration in the rate law with the equivalent value derived from the equilibrium constant K. If the rate law then matches the overall rate law, this means that the proposed mechanism is likely to be correct. This means that the intermediate canceled out in equilibrium before the rate limiting slow step. The intermediate is gone in pre-equilibrium before the slow step that determines the rate law.

These two approaches will give the same result as both ultimately determine that the intermediate is canceled out or constant and therefore not included in the overall rate law. We are using the pre-equilibrium approximation, while the steady-state approximation is sometimes used in the textbook.

They are both a part of the approaches to identify the rate limiting step. Both examine the rate limiting slow step and compare the proposed mechanism to the experimentally observed value. This is different from the approach of direct computation. Direct computation uses a computer to make a numerical approximation of the rate constants k. The identification of the rate limiting step is one that we can use without a computer.

Michael Lonsway 3O
Posts: 43
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Pre-equilibrium approach and steady-state approach

Are there any problems in the hw (or course reader) that show the three methods being used in one problem? I haven't been able to find any so far.