Rate determing step and additional ?

[imapremed]

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if the rate determing step is not the first one... then all the steps before it must be in equilibrium? is that correct?

and also, a solvent stabilizes the transition state which I know… now does it also stabilize products or reactants.. I remember this coming up in my orgo class in college but im not 100% sure so I wanted to confirm – if I change a solvent I will change the thermodynamics of it in terms of changing the magnitude of delta H or delta G but I wont change the fact that it is say endothermic or exothermic… or even endergonic or exorgonic…

Is that correct? What I remember this is in terms of stabilizing say a product if it has a plus charge but not equally stabilizing a reactant since it has no plus charge and therefore off balancing the magnitude?

 

[imapremed]

bump. any ideas. anything helps at this point.

 

[MT Headed]

if the rate determing step is not the first one... then all the steps before it must be in equilibrium? is that correct?

While that might not always be true in the real world, it is an assumption we make in chemistry to make the math easier. I'm sure the MCAT would have you do the same.

and also, a solvent stabilizes the transition state which I know… now does it also stabilize products or reactants.. I remember this coming up in my orgo class in college but im not 100% sure so I wanted to confirm – if I change a solvent I will change the thermodynamics of it in terms of changing the magnitude of delta H or delta G but I wont change the fact that it is say endothermic or exothermic… or even endergonic or exorgonic…

Is that correct? What I remember this is in terms of stabilizing say a product if it has a plus charge but not equally stabilizing a reactant since it has no plus charge and therefore off balancing the magnitude?

Yeah, I'm having a hard time wrapping my head around this one. A solvent could modify the kinetics of a reaction, making it run faster or slower. More importantly it could temporarily make one side reaction occur faster than another one. But the thermodynamically favorable product should win out eventually, especially when heat is added. I have seen AAMC passages like this.

But a solvent altering G, H, and K? These are state functions. For a solvent to do that it would essentially have to participate in the reaction, and that is really stretching the definition of a solvent. It would have to show up as a reactant and/or product.

Maybe somebody else can comment on your second question.

 

[imapremed]

k cool. thanks a bunch!

anyone got ideas for second question?

 

[deleted329605]

So for the solvents I'm guessing your talking about organic chemistry and Sn1/E1 compared to Sn2/E2. So yea, when dealing with solvents, essentially if you have a species in the rate equation with a charge (like Sn2 or E2 where you have a strong base, - charge) a protic solvent (like alcohol) with slow the reaction because it will stabilize reactants, increasing energy of activation. Thus you wanna use a polar aprotic solvent in Sn2/E2 (need polar or else species won't dissolve, and they will pretty much never interact). And yea the opposite goes for species with charges in the transition state, like E1 and Sn1 where you have a carbocation. The protic solvent will help stabilize this charge, make the energy of activation lower.

In terms of the solvent changing the free energy of the reaction, I would imagine it doesn't

 

[deleted329605]

Also, in terms of the rate equation, the only rule is that you can't have a short lived intermediate in it. Thus if you slow step if #2 (most likely) for a reaction where it isn't the first, you have to substitute reactant from the first step that are equivalent to the concentration of the short lived intermediate, which is a product of those first reactants. Also, if you have both forward and reverse rates going for the first step, it starts to get really tricky, but usually they say the reverse reaction is negligible.

In terms of kinetic vs. thermodynamic, this really only comes into play when there are major steric factors or stability factors associated reactants/products. Increasing temperate will always favor thermodynamic.