[cmonsour]

Why does the thermodynamic favorability of a chemical reaction depend on the concentrations of the products and reactants? 

Here's why I ask this question.  I'm trying to understand the connection between physics and chemistry here.  It seems like for any reaction, either the forward or the reverse reaction would be energetically favorable (and the other would be disfavorable).  So why doesn't the reaction just go all the way to completion in the favorable direction?  Obviously one way to answer is to say that reaction energetics depends on concentration, so the products and reactants reach an equilibrium where the free energy change in both directions is zero.  But that just pushes the question forward, to the one I asked above: why does the energetic favorability of a reaction depend on concentration?

Thanks!

[Yggdrasil]

Short answer: entropy

The longer answer:  Consider a case A-->B where A and B have equal energy.  If your system is 100% molecule A, there is no energetic driving force to create B.  Creating more B, however, increases the entropy of the system (going from a more ordered pure substance from a less ordered mixture).  In this case, you would expect the system to try to maximize its entropy and create a 50-50 mix of A and B at equilibrium.

In the case of a real chemical reaction where A and B do not have the same energy, there is a competition between entropy and enthalpy.  Entropy would like to maintain a 50-50 mix of A and B* whereas the energetics of the reaction would like to drive the reaction toward the side with lower energy.  Thus, the system settles at a point where the entropic and enthalpic forces reach a balance.


*(the real situation is actually more complicated because A and B can have differing entropies as well, but the basic concept is correct)