How can you tell if a reaction is spontaneous?
Not necessarily. The entropy of the system can in fact decrease as long...a negative gibbs free energy corresponds to an increase in the entropy of the system
"Shooting down" your post was not at all my intention. After all, you dida negative gibbs free energy change corresponds to an increase in the entropy of the system
Is a correct statement... Q.
This is notYou must ask one simple
question: Does the entropy of the system increase?
? You could say that an increase in the entropy contributesA negative gibbs free
energy change corresponds to an increase in the entropy of the
Whatbecause the enthalpy releases energy to
the surroundings that then become "The system" in order to compensate for
the fact that the "universe" is a closed system in itself.
. Since when is the enthalpy a gauge of the overallThis is the energy released
to/absorbed by the reaction and is a gauge of the overall entropy change of
. What precisely is "overall energy change"? And why doThe gibbs
free energy, this is a measure of the overall energy change for the
reaction (and therefore entropy), a negative gibbs free energy corresponds
to an increase in the entropy of the system and therefore a spontaneous
That's correct. We are too much used to constant T and P processes and we can easily forget the others.[...]
You failed to mention, however, that the particular criterion for
spontaneous change, a decrease in the Gibbs free energy change, only
applies to reactions which occur under the restraint of constant
temperature and pressure. You are in excellent company, for Andy_Resnick and Lightarrow also leave out this important qualifier.
for reactions restrained to occur at constant temperature and
volume, for example, it is the Helmholz free energy that one must use.
Certainly, but you know that spontaneity in chemical physics is a different concept from kinetics.But for a reaction to be spontaneous in human terms we need something more than just universe entropy increasing or Gibbs free energy (for constant T and P) decreasing. There ar e a number of termodynamically spontaneous reactions that will not happen un human time scales. As an example, you won't see a diamond burning at 20º even when this reaction should be spontaneous.
This is exactly what I was trying to point out.spontaneity in chemical physics is a different concept from kinetics