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It is said that 2nd law predicts direction of spontaneous process. It does so by saying that direction of spontaneous process is that in which entropy of the isolated system increases or stays the same, it can't decrease. What about non - spontaneous processes? They are externally driven by energy input. For them to happen entropy of the isolated system must increase or stay the same since no process can happen which would decrease entropy of the isolated system, this would violate 2nd law.  Thus 2nd law should predict the direction of non - spontaneous process as well as far as I understand. If so why is 2nd law almost always stated as to predict the direction of spontaneous procces? I think it is better to say that 2nd law predicts what processes are possible in nature regardless of their spontaenity. What are your thoughts? Is my reasoning correct and in harmony with 2nd law? 

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  • $\begingroup$ I'm not sure what you are getting at, but a reversible process is non spontaneous and the total entropy change (system + surroundings) is zero, per the second law. A spontaneous process is irreversible to the total entropy change is greater than zero, again per second law. $\endgroup$ – Bob D Apr 10 at 14:06
  • $\begingroup$ Who says it only applies to spontaneous processes? $\endgroup$ – Chet Miller Apr 10 at 14:13
  • $\begingroup$ @Chet, I think it shouldn't, but it is mostly stated that way. $\endgroup$ – Dario Mirić Apr 10 at 14:45
  • $\begingroup$ What I am saying is that for every process that happens regardless of its spontaenity entropy of the isolated system (universe is an isolated system) must increase. This is not true only for spontaneous process, but for non - spontaneous as well. There can be no process which would decrease entropy of isolated system. So, why is 2nd law usually formulated that it predicts direction of spontaneous process when it can also predict the direction of non - spontaneous process? $\endgroup$ – Dario Mirić Apr 10 at 15:25
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I think you are mixing up spontaneous process vs unspontaneous process with isolated system vs non-isolated system.

  • If you state the 2nd law of thermodynamics for isolated systems then you are right in stating that the entropy will not decrease for any processes, spontaneous or not.
  • If you state the law for non-isolated systems then you can only state that entropy will not decrease for spontaneous processes.

Because, as you rightly say, an unspontaneous process such as my action of willfully ordering a system will decrease it's entropy. But that is only because we then aren't including the body and the processes that produce the forces that let me create this local entropy decrease. If you included that - the whole picture, the isolated system - the 2nd law of thermodynamics does state that the entropy won't as a whole decrease.

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  • $\begingroup$ Yes, I agree. Whether or not process is spontaneous, entropy change of the isolated system (universe) must increase. So, there is the same condition for both types of processes. Why is than 2nd law most often stated that it predicts the direction of only spontaneous processes when it can also predict the direction of non - spontaneous processes? 2nd law predicts what processes are actually possible in nature regardless of their spontaenity if we look at the isolated system (universe) as a whole (system of interest and surrondings). $\endgroup$ – Dario Mirić Apr 10 at 15:32
  • $\begingroup$ I actually don't understand why does entropy of closed system need to increase for spontaneous process? Aren't free energies predictors of what processes are possible for closed systems (Helmholtz and Gibbs) depending on how process are carried? Gibbs energy of closed syetem should decrease for any process in nature carried at constant pressure. This is equivalent with that entropy of both system and surrondings should always increase. $\endgroup$ – Dario Mirić Apr 10 at 15:58
  • $\begingroup$ @DarioMirić "Why is than 2nd law most often stated that it predicts the direction of only spontaneous processes" Because that is the most generalised version of the law. As I explained, this is how you must state the law if you are talking generally about all possible systems. It cannot predict the direction of non-spontaneous processes unless you also know that it is an isolated system. $\endgroup$ – Steeven Apr 10 at 17:09
  • $\begingroup$ @DarioMirić "2nd law predicts what processes are actually possible in nature regardless of their spontaenity i" Hmm, I would argue that processes in nature in general are spontaneous. I'd presume that your definition of "spontaneous" comes with human interaction, with conscious choices. Otherwise we might not be talking about the same thing. $\endgroup$ – Steeven Apr 10 at 17:14
  • $\begingroup$ @DarioMirić "I actually don't understand why does entropy of closed system need to increase for spontaneous process?" Actually, this law is not entirely certain, to be honest. It is a statistically based law. It is actually not impossible to obserbe entropy reductions even for isolated systems, if the systems is simple. But with more complexity, the tendency towards entropy increase for any process happening grows rapidly... $\endgroup$ – Steeven Apr 10 at 17:17

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