Editing Irreversible process (section)

{{Short description|Process that cannot be undone}}
{{For multi|the concept in evolutionary theory|Dollo's law of irreversibility|other uses|reversibility}}
{{Thermodynamics|cTopic=[[Thermodynamic system|Systems]]}}

In [[thermodynamics]], an '''irreversible process''' is a [[thermodynamic processes|process]] that cannot be undone. All complex natural processes are irreversible,<ref>{{cite journal|last1=Lucia|first1=U|year=1995|title=Mathematical consequences and Gyarmati's principle in Rational Thermodynamics|journal=Il Nuovo Cimento|volume=B110|issue=10|pages=1227–1235|bibcode=1995NCimB.110.1227L|doi=10.1007/bf02724612|s2cid=119568672}}</ref><ref>{{cite journal|last1=Grazzini|last2=Lucia|first2=U.|year=1997|title=Global analysis of dissipations due to irreversibility|journal=Revue Gènèrale de Thermique|volume=36|issue=8|pages=605–609|doi=10.1016/s0035-3159(97)89987-4}}</ref><ref>
{{Cite journal|last1=Lucia|first1=U.|year=2008|title=Probability, ergodicity, irreversibility and dynamical systems|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=464|issue=2093|pages=1089–1104|bibcode=2008RSPSA.464.1089L|doi=10.1098/rspa.2007.0304|s2cid=34898343}}</ref><ref>
Grazzini G. e Lucia U., 2008 Evolution rate of thermodynamic systems, 1st International Workshop "Shape and Thermodynamics" – Florence 25 and 26 September 2008, pp. 1-7
</ref> although a [[phase transition]] at the coexistence temperature (e.g. melting of ice cubes in water) is well approximated as reversible.

A change in the [[thermodynamic state]] of a system and all of its surroundings cannot be precisely restored to its initial state by [[infinitesimal]] changes in some property of the system without expenditure of [[energy]]. A system that undergoes an irreversible process may still be capable of returning to its initial state. Because entropy is a [[state function]], the change in entropy of the system is the same whether the process is reversible or irreversible. However, the impossibility occurs in restoring the [[Environment (systems)|environment]] to its own initial conditions. An irreversible process increases the total [[entropy]] of the system and its surroundings. The [[second law of thermodynamics]] can be used to determine whether a hypothetical process is reversible or not.

Intuitively, a process is reversible if there is no [[dissipation]]. For example, [[Joule expansion]] is irreversible because initially the system is not uniform. Initially, there is part of the system with gas in it, and part of the system with no gas. For dissipation to occur, there needs to be such a non uniformity. This is just the same as if in a system one section of the gas was hot, and the other cold. Then dissipation would occur; the temperature distribution would become uniform with no work being done, and this would be irreversible because you couldn't add or remove heat or change the volume to return the system to its initial state. Thus, if the system is always uniform, then the process is reversible, meaning that you can return the system to its original state by either adding or removing heat, doing work on the system, or letting the system do work. As another example, to approximate the expansion in an internal combustion engine as reversible, we would be assuming that the temperature and pressure uniformly change throughout the volume after the spark. Obviously, this is not true and there is a [[flame front]] and sometimes even [[engine knocking]]. One of the reasons that Diesel engines are able to attain higher efficiency is that the combustion is much more uniform, so less energy is lost to dissipation and the process is closer to reversible.{{Citation needed|date=October 2018}}

The phenomenon of irreversibility results from the fact that if a [[thermodynamic system]], which is any system of sufficient complexity, of interacting molecules is brought from one thermodynamic state to another, the configuration or arrangement of the atoms and molecules in the system will change in a way that is not easily predictable.<ref name=Lucia2009>{{cite journal |last1=Lucia |first1=Umberto |title=Irreversibility, entropy and incomplete information |journal=Physica A: Statistical Mechanics and Its Applications |date=October 2009 |volume=388 |issue=19 |pages=4025–4033 |doi=10.1016/j.physa.2009.06.027 |bibcode=2009PhyA..388.4025L }}</ref><ref>{{cite journal | last1 = Lucia | first1 = U | year = 2008 | title = Statistical approach of the irreversible entropy variation | journal = Physica A: Statistical Mechanics and Its Applications | volume = 387 | issue = 14| pages = 3454–3460 | doi = 10.1016/j.physa.2008.02.002 |bibcode = 2008PhyA..387.3454L }}</ref> Some "transformation energy" will be used as the molecules of the "working body" do work on each other when they change from one state to another. During this transformation, there will be some heat energy loss or [[dissipation]] due to intermolecular friction and collisions. This energy will not be recoverable if the process is reversed.

Many [[Biology|biological]] processes that were once thought to be reversible have been found to actually be a pairing of two irreversible processes.  Whereas a single enzyme was once believed to catalyze both the forward and reverse chemical changes, research has found that two separate enzymes of similar structure are typically needed to perform what results in a pair of [[Thermodynamics|thermodynamically]] irreversible processes.<ref>Lucia U., "Irreversible Entropy in Biological Systems", EPISTEME<br /> {{cite journal | last1 = Lucia | first1 = U. | last2 = Maino | first2 = G. | year = 2003 | title = Thermodynamical analysis of the dynamics of tumor interaction with the host immune system | journal = Physica A: Statistical Mechanics and Its Applications | volume = 313 | issue = 3–4| pages = 569–577 | doi=10.1016/S0378-4371(02)00980-9|bibcode = 2002PhyA..313..569L }}</ref>