Introduction to entropy: Difference between revisions

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{{Thermodynamics|cTopic=[[List of thermodynamic properties|System properties]]}}

{{Introductory article|Entropy}}

In [[thermodynamics]], '''entropy''' is a numerical quantity that shows that many physical processes can go in only one direction in time. For example, you can pour cream into coffee and mix it, but you cannot "unmix" it; you can burn a piece of wood, but you can't "unburn" it. The word 'entropy' has entered popular usage to refer a lack of order or predictability, or of a gradual decline into disorder.<ref name="lexico">{{cite web |title=Definition of entropy in English |url=https://www.lexico.com/en/definition/entropy |website=Lexico Powered By Oxford |accessdateaccess-date=18 November 2020}}</ref> A more physical interpretation of thermodynamic entropy refers to spread of energy or matter, or to extent and diversity of microscopic motion.

If you reversed a movie of coffee being mixed or wood being burned, you would see things that are impossible in the real world. Another way of saying that those reverse processes are impossible is to say that mixing coffee and burning wood are "irreversible". Irreversibility is described by an important law of nature known as the [[second law of thermodynamics]], which says that in an isolated system (a system not connected to any other system) which is undergoing change, entropy increases over time.<ref>Theoretically, coffee can be "unmixed" and wood can be "unburned", but for this you would need a "machine" that would generate more entropy than was lost in the original process. This is why the second law only holds for isolated system which means they cannot be connected to some external "machine".</ref>

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===Microscopic explanation and interpretation===

'''Microstate dispersal''': [[Edward A. Guggenheim]] proposed an ordinary language interpretation of entropy that may be rendered as 'dispersal of modes of microscopic motion throughout their accessible range'.<ref name="Dugdale 101">Dugdale, J.S. (1996). ''Entropy and its Physical Meaning'', Taylor & Francis, London, {{ISBN|0748405682}}, Dugdale cites only Guggenheim, on page 101.</ref><ref name="Guggenheim1949">Guggenheim, E.A. (1949), Statistical basis of thermodynamics, ''Research: A Journal of Science and its Applications'', '''2''', Butterworths, London, pp. 450–454; p. 453, "If instead of entropy one reads number of accessible states, or spread, the physical significance becomes clear."</ref> Later, along with a criticism of the idea of entropy as 'disorder', the dispersal interpretation was advocated by [[Frank L. Lambert]],<ref name=Lambert/><ref name="Lambert2005">{{cite journal |last1=Kozliak |first1=Evguenii I. ||last2=Lambert |first2=Frank L.|date=2005 |title=“Order-to-Disorder” for Entropy Change? Consider the Numbers!|journal=Chem. Educator |volume=10 |pages= 24-25|access-date=December 13, 2020}}</ref> and is used in some student textbooks.<ref>For example: Atkins, P. W., de Paula J. Atkins' Physical Chemistry, 2006, W.H. Freeman and Company, 8th edition, {{ISBN|9780716787594}}. Brown, T. L., H. E. LeMay, B. E. Bursten, C.J. Murphy, P. Woodward, M.E. Stoltzfus 2017. Chemistry: The Central Science, 10th ed. Prentice Hall, 1248pp, {{ISBN|9780134414232}}. Ebbing, D.D., and S. D. Gammon, 2017. General Chemistry, 11th ed. Centage Learning 1190pp, {{ISBN|9781305580343}}. Petrucci, Herring, Madura, Bissonnette 2011 General Chemistry: Principles and Modern Applications, 10th edition, 1426 pages, Pearson Canada {{ISBN|9780132064521}}.</ref>

The interpretation properly refers to dispersal in abstract microstate spaces, but it may be loosely visualised in some simple examples of spatial spread of matter or energy. If a partition is removed from between two different gases, the molecules of each gas spontaneously disperse as widely as possible into their respectively newly accessible volumes; this may be thought of as mixing. If a partition, that blocks heat transfer between two bodies of different temperatures, is removed so that heat can pass between the bodies, then energy spontaneously disperses or spreads as heat from the hotter to the colder.