CHEMISTRY COMMUNITY

What does it mean for a reaction to be non-spontaneous yet its reverse reaction to be spontaneous? Why is its reverse reaction spontaneous?

If a reaction is non-spontaneous, its deltaG is positive and therefore the formation of products is not favored. This means that it's favored for the reactants to stay reactants instead of undergoing the reaction, and if there were a great number of products under certain conditions, the reaction would proceed in the reverse to make more reactants.

In order to find the deltaG of a reverse reaction, just multiply the deltaG of the original reaction by -1, so for a non-spontaneous reaction with a positive deltaG, the reverse reaction would be a spontaneous reaction with negative deltaG.

If a reaction is spontaneous (\Delta G is negative), that means that the forward reaction is favored. Just remember that this does not mean the reaction will happen quickly, it just means that if you leave the reaction alone, it will complete by itself.
So if the forward reaction is non-spontaneous (\Delta G is positive), this means that the forward reaction is nonspontaneous. However, if you were to reverse the reaction, you would flip the sign of \Delta G (making it negative now), and this shows that now the reverse reaction is spontaneous. Again, this means that if left alone, the reaction would go to completion.

To the posts above, why does it mean that the forward reactions are favored? What does it have to do with forward vs reverse reactions?

The forward reaction is favored when there doesn't need to be an outside source, such as increasing temperature, to catalyze the reaction. If the forward reaction is favored, then the reverse reaction is not favored (and vice versa).