Chemical reactions are reversible; they can proceed in both the forward and reverse directions

Question

If the ?G° for the reaction Y->X is energetically favorable, how can you explain the fact that not all of the Y molecules will be converted to X molecules?

Answer 1

Even when the forward reaction is highly favorable, it is important to keep in mind that molecules exist as part of a population, and each member of a given population has a varying level of energy per molecule. Statistically speaking, there will always be some molecules that have sufficient energy to reach the energy of activation for the back reaction X->Y, even though the proportion of molecules with this energy will be much lower than that for the forward reaction Y->X. As more and more X molecules are converted to Y molecules, eventually the Y molecules in the mixture outnumber the X molecules to such a large extent that the fluxes in the backward and forward directions become equal; it is here that the reaction reaches its equilibrium point.