If a reaction is spontaneous in the forward direction is it always non spontaneous in the reverse direction?

Answer 1

I can't give you an example but I would be suspicious of the word never because with reactions with low activation energiers and small differentials between the stability of the starting and ending compounds then the equilibrium between the relative stabilities of the start and end compounds could eb altered by the different conditions such as pressure and temperature etc.... so I would not go for never.

The sort of example that might exist is where a salt is produced by a reaction and that salt is insoluble at low temperature.Thus there will be a spontaneous reaction in one direction and low temperate as the low solubility will cause it to simply precipitate out continuously but at higher temperatures with greater solubility dynamic equilibrium may be reached and with the presence of more more of one reagent drive it back the otehr way.

just a thought

Answer 2

Yes - a spontaneous reaction is one for which the change in Gibbs Free Energy (delta G) is negative. By definition, the reverse process would have delta G positive, and hence would be non-spontaneous. If both the forward and reverse reactions were spontaneous, then going from A-->B would release energy and going from B--> A would also release energy. Congratulations, you have created a perpetual motion machine! Ooops, but you've just broken the Law of Conservation of Energy. Oh dear, either Physics is wrong or both reactions cannot be spontaneous - I think I'll place my bets on the side of Physics, Gibbs, Helmholtz, Boltzmann, Einstein, Rutherford........

Answer 3

Yes. The sign of delta G would be negative for the forward rxn and positive for the reverse.

Answer 4

Yes - just think of a ball bearing on a slope ...