we usually say that a reaction is spontaneous (at fixed temp and pressure) if the standard gibbs free energy of reaction is negative. then why does a really spontaneous reaction never go to completion? why do we always end up with a mixture of reactants and products, no matter how negative the gibbs free energy change is?

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toxicsugar22 · Answer

Can you please help me

aaronq · Answer

hm you probably know that the rate of reactions (which is not dependent on thermodynamic parameters) decreases as the concentration of reactants decreases because their physical availability is reduced. extending this like of thought you can infer that there will be a point where the reactant concentration will be so low that these will never meet and react. Additionally, reactions can exist in equilibrium.

anonymous · Answer

what if it's first-order and the reactants don't have to meet? but you have a good point, the forward rate will eventually slow down until it becomes equal to the backward rate, and we settle into a (dynamic) equilibrium. 

responding to your last comment, i think reactions *always* end up in an equilibrium of reactants and products. this question is asking for the why.

aaronq · Answer

even in first order reactions the reactants have to meet, the rate of the reaction is just not dependent on them.

hm i was thinking about that when i wrote it, i'm not sure if all reactions exist in equilibrium.. statistically speaking you could say that, even in a very non-spontaneous backward process, a few molecules will react because they will have sufficient energy (gaussian distribution of energies) to convert back to the products but i don't know where the extent to say where this is valid, like when you introduce a strong acid into water, is there a full complete dissociation of all the molecules, or is there a few that remain? i imagine that there would be an equilibrium, even if the eq. constant for the backwards process is in the order of 10^-50. I could be wrong though

anonymous · Answer

well, if there isn't a large enough number of atoms, then it's very possible that the backward rate will be zero. so good point.