∆Suniv ≥ 0 [= for equil/reversible, > for spont/irreversible]Unfortunately, abuses of this statement are many and takes but a moment's googling to find them. For many scientists, it holds a special place:
The law that entropy always increases-the second law of thermodynamics-holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations-then so much the worse for Maxwell's equations. If it is found to be contradicted by observation-well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation." – Sir Arthur Eddington And to reiterate some general statements from class, which will hopefully help you more fully grasp spontaneity and reversibility:
All spontaneous processes are irreversible.Some common misstatements about the Second Law:
For all irreversible processes, ∆Suniv > 0.
All reversible processes are at equilibrium.
For all reversible processes, ∆Suniv = 0.
All systems tend to greater disorder.Before moving onto specific applications of the Clausius relation, we paused, for bookkeeping's sake, to mention the Zeroth Law: If systems A and B are in equilibrium, and systems B and C are in equilibrium, then systems A and C are in equilibrium. Not only does this establish that a state property common to them must be equal (the temperature), it also allows for the possibility that two systems could be in equilibrium without in direct contact.
False: Not only is "disorder" a poorly constructed idea and overly dependent on human interpretation, the underlying premise is wrong.
All systems tend towards greater entropy.
False: Some systems tend towards greater entropy while others don't. The restriction rests on ∆Suniv, not on ∆S.
Lastly we began our march of calculating entropy changes for ten processes, finishing four:
02. reversible adiabatic
03. reversible isothermal
04. reversible phase transition [at const T, P]
On Friday, we will finish this list of ten, visit the revolutionary work of Boltzmann and, if time permits, elucidate the Third Law of Thermodynamics.