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A non-spontaneous change occurs when an external effort is being done to it. Since the external effort is also a natural source , does this mean that there is no truly any non-spontaneous process in nature (universe) ?

Example to illustrate my point :

An electrochemical process (like in a galvanic cell) can be reversed by providing an external voltage in opposite direction whose magnitude is higher than the cell potential. This external voltage is given by humans. Humans are basically converting some form of energy like mechanical energy of rivers in a dam into electrical energy. Also, humans are doing so because they eat food and they get their energy to do through spontaneous metabolic processes in their body. Also, the conversion of energy in above process is spontaneous because humans are making use of a spontaneous process ( a river flowing down from a height) .

So, is there a truly non-spontaneous process ?

An_Elephant
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    You are making pedantic technical points. Do not do that. Technically all processes are natural (i.e. in accordance with natural laws of physics) and thermodynamic. But it is useful to indicate to people that, say, a distribution is not of the usual thermal distribution, even though something else inside it will be thermodynamic. – naturallyInconsistent May 04 '23 at 06:45
  • @naturallyInconsistent Please expand it into an answer. Thanks ! – An_Elephant May 04 '23 at 09:08
  • I think that you miss-use the term non-spontaneous ,this link makes it very clear applied to reactions https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/20%3A_Entropy_and_Free_Energy/20.03%3A_Spontaneous_and_Nonspontaneous_Reactions – anna v May 04 '23 at 09:16
  • In a non-spontaneous process, the system is only allowed to deviate slightly from thermodynamics equilibrium. If this slight deviation is removed, the system will not proceed. If the initial deviation is increased so that it is finite, the system will change spontaneously. – Chet Miller May 04 '23 at 12:05
  • A human being is not a thermodynamic process. Thermodynamics is really thermostatics. It can't say much, and very little useful stuff, about systems that are not homogeneous and not near equilibrium. You are simply using the wrong theory. – FlatterMann May 04 '23 at 16:06
  • @FlatterMann Thermodynamics is not limited to reversible processes. For example, both first and second laws are valid for all processes regardless of the fact that they are reversible or not. – An_Elephant May 04 '23 at 16:52
  • I can give you a trivial system that thermodynamics can't predict: the dynamics of a boiling liquid. No, forget about the boiling... one can't even to the most trivial fluid dynamics with a TD approach. A theory that can't even deal with such simple systems can not be expected to deal with more complex ones. It's really that simple. TD is a very limited theory. That's it's strength. It is only so general because it is highly specialized. – FlatterMann May 04 '23 at 18:19
  • @FlatterMann It depends on what one considers as trivial. – An_Elephant May 05 '23 at 05:58
  • TD makes the (trivial) assumption that all molecules of the same kind in a system (like a gas or mix of gases) behave interchangeably. That assumption is simply not valid for boiling water (some molecules are moving up, other are moving down in a convection cell) and it definitely is not true for a living organism where all the different molecules have very specific functions. – FlatterMann May 05 '23 at 15:51
  • @FlatterMann First law says ∆U = Q + W which is true irrespective on the identity of system , let it be an ideal gas or otherwise. Similarly , second law talks about perpetual motion. These are very powerful but not sufficient for everything. The choice of triviality or power of a model is arbitrary and personal. – An_Elephant May 05 '23 at 16:23
  • Heat is not even a well defined quantity, except in a sufficiently large system in equilibrium. A biological system in equilibrium is dead. The second law simply defines temperature. A biological system without temperature differences is... dead. – FlatterMann May 05 '23 at 16:30

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You define the difference between spontaneous and nonspontaneous processes when you define a system: spontaneous processes are the ones that happen without adding or subtracting anything from the system.

Definitions are arbitrary and can't be true or false.

...Although definitions can be useless if they define something that doesn't mean anything ("let $x$ represent the wavelength of the angular momentum of pi five minutes before the beginning of time"), or define a symbol to mean something completely different from what everybody else means by the symbol ("let $=$ represent velocity").

g s
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The question is not as simplistic as the down voting suggests. It alludes to the problem of how life, and in particular intelligent life that is capable of creating new things that did not exist before, should be viewed in the context of the second law. As asked, however, it is about the definition of spontaneity, which is a much easier question to answer:

A spontaneous process is one which, once initiated moves on its own without further assistance from the outside. The classical example is a box divided into two parts, each with its own pressure, temperature and composition. If we puncture the wall that separates the two parts, thus initiating the process, the rest happens entirely on its own.

Of course, humans made the box, filled it with gases and punctured the wall. But what happens after that, the equilibration of temperature, pressure and chemical potentials, requires no humans.

The waterfall (minus the hydroelectric plant) is an example of a spontaneous process that involves no humans whatsoever: water falls down the hill, not up, and in the process it converts kinetic energy into internal energy while also increasing the entropy of the universe.

Two examples of a non spontaneous process:

  • Separating the gases of the first example into the pure components

  • Pumping the water from the bottom of the waterfall back to the top

Themis
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  • If I get it right, difference between spontaneity and non-spontaneity is not anything absolute but it's defined according to whether there is or not any human intervention in that process , right ? Thanks ! – An_Elephant May 04 '23 at 15:09
  • The second law of thermodynamics does not apply to life, which is in an extreme state of disequilibrium. You are simply using the wrong theory. – FlatterMann May 04 '23 at 16:04
  • @An_Elephant Yes. Remember, thermodynamics started as the study of engines: we feed it heat, it gives us work. The "surroundings" is always us. – Themis May 04 '23 at 16:09
  • @FlatterMann You are making a blanket comment that we cannot take it too seriously. A lot (in fact all) engineering processes operate away from equilibrium, yet we routinely and successfully apply thermodynamics. Under assumptions and approximations, to be sure, but we don't say "your reactor is irreversible, you can't apply thermodynamics". – Themis May 04 '23 at 16:19
  • Biology is nowhere near thermodynamic equilibrium and biological systems are not even close to homogeneous. The same molecule in one part of your body has one function and a totally different one in another. They are in different chemical environments and subject to different regulatory loops that are extremely sensitive. Once these loops break down death follows within minutes. If you love thermodynamics, please let it live within its range of application. Biology is simply not the same as a three component chemical system in a pressure vessel. – FlatterMann May 04 '23 at 16:26
  • @FlatterMann Biological Thermodynamics – Themis May 04 '23 at 16:49
  • You don't even have to go to biology to reach the limits of TD: it can't even model fluid dynamics, a boiling liquid or a laser. The most trivial non-homogeneous systems are out of the reach of the theory. – FlatterMann May 04 '23 at 18:21
  • @FlatterMann your general statement "The most trivial non-homogeneous systems are out of the reach of the theory" is clearly false for there are many non-homogeneous systems that can be very well modeled by thermodynamic theory. For a beautiful example see the Belousov-Zhabotinsky oscillation or the Rayleigh–Bénard convection both being explained quite well by the "Belgian school". Another, by now classic example, is Bridgman's way of explaining steady-state thermoelectricity using material characteristic entropy production or using the same by Eckart for fluid dynamics. – hyportnex May 04 '23 at 20:49
  • @An_Elephant there's no requirement that the intervention be human, just whether something interacts with the system under consideration that was not itself part of the system under consideration. – g s May 04 '23 at 21:19
  • @hyportnex I just gave a number of trivial systems that are out of the reach of the theory. The assumption that living organisms can be described with thermodynamics when it can't even handle a boiling pot of water is a bit of a stretch, IMHO. If by "Belgian School" you mean the competing group to the German "Synergetics" community, then I am not sure what you mean by "thermodynamics". Either group does not use thermodynamics techniques alone to describe complex dynamic phenomena. They are simply not sufficient. – FlatterMann May 05 '23 at 01:49
  • @FlatterMann The Belgian School of Thermodynamics: de Donder, Prigogine, Glansdorff, Nicolis, etc., – hyportnex May 05 '23 at 02:15
  • @hyportnex Yes, Prigogine is the competitor of Haken. He got the Nobel and Haken was said to have been very "unhappy" about having been left out, even though his theoretical descriptions of e.g. laser thresholds seem to have been more accurate. What they are using is a variation of perturbation theory, though, not thermodynamics. It basically boils down to time-scale separation and the search for order-parameters. Time dependence is certainly not part of classical TD. – FlatterMann May 05 '23 at 02:28
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I completely agree with @Themis and I do not understand the downvotes either. The problem of what constitutes spontaneous or non-spontaneous process is not a trivial one because the adjective describes human thinking and is not easy to define it in mathematical language. The issue is similar to the unease most of us must have felt having, the first time, learned of Lord Kelvin's formulation of the 2nd law:

It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects

The same question could be asked how to put the "some other change" of Clausius into, say, a differential equation form:

Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.

I have never understood what is being an "inanimate material agency" or "some other change" has to do with entropy, isothermal heating/cooling, or a differential inequality?

There has always been a stressful and strained relationship, pun intended, between real irreversible processes and classical thermo-statics whose concepts are based on reversible, i.e., non-real processes, and I believe your question is one manifestation of that. A possible way, though not explicitly emphasized by the author, is Pippard's "hole-in-the-wall" argument that formulates the 2nd law as "It is not possible to vary the constraints of an isolated system in such a way as to decrease the entropy".

If you restrict the 2nd law this way then the question what spontaneity is becomes easier to answer: spontaneous is what happens when you remove a constraint in an isolated system but do nothing else from the outside. Entropy is well-defined before and after the removal of the constraint because the isolated system starts and ends in equilibrium, thermo-statics to start and thermo-dynamics in between to proceed, and if anything happens in between that will increase the entropy. The system being isolated may include in and with all its complexity reservoirs, work bodies, fields, etc.

hyportnex
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  • Thanks for your answer. Unfortunately it's too high level for me to understand as I have only learnt introductory thermodynamics. – An_Elephant May 04 '23 at 16:03