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J a m e s Arthur Campbell Harvey Mudd College Claremont, CA 9171 1
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Reversibility and Returnability Or, when can you return again?
What do chemists mean by a reversible reaction? A common, but misleading, classification of chemical reactions is divided into those which are reversihle (the ionization of acetic arid is often -.-~- - ~ cited) ~ - ~ and those which are irreversible (the combustion of methane might be typical). Then, in discussing eanilibrium. the nrincinle of micro-reversihilitv is often invdked (or idplicitly assumed) to justify and'to interpret eauilibrium states such as those in which every reaction is matched by its exact reverse: both forward and reverse reactions proceed at identically equal rates and hy thesame, but reversed, paths. Also, discussions of thermdynamic functions, and their changes, depend on the concept of reversihle processes as those.durini which the itate functions (or driving forres) never differ l)y more than an infinitesimal amount from moment to moment. Yet there are few, ifany, attemPfS to relate these uses of the word reversihle and its derivatives. Perhaps there are no links? Let us investigate. ~
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Thermodynamic Reverslbllity There can he little doubt of the usefulness and validity (at least in comnetent hands) of the conceDt of thermodvnamic reversibilit< Reversible adiabatic processes, reversible isothermal processes, reversihle isobaric processes, etc., are a t the heart of thermodynamics (even if not a t the heart of learners of thermo). After all, who can deny the second law: the change in entropy of a system equals the heat for the process (performed reversibly) divided by the Kelvin temperature, dS = dq,,/T. An important corollary is that dS 2 0 for any process in an isolated svstem. and dS > 0 for anv real change in an isolated system. (A system is iwlated if neither mass nor energy can flow hetween it and the surroundings.) Onlv at eauilibrium is dS = 0 for any known total system. ' ' ~ k r o ~is; time's arrow," and both have only increased in every known total system not a t equilibrium. All known total systems, then, increase in entropy in every observed change. ~ n the d entropy, heing a state-function, undergoes the same change whether the process iscarried out reversiblv or i r r e v e r s i h l v ~ ~ nsod we mu& conclude that "reversible3'hoesnot, and cannot, mean that isolated systems can return to earlier conditions. After all, the entropy has increased. Thermodynamic reversihility can apply to a process, but i t does not sueeest that it is nossible for the final state to revert to the original state for the system and surroundings. In fart. the usual exr)ositions of the ideaofentr0"v are larrelv hased on the impo&bility of any net change &'an isol&d svstem reversing. All changes in isolated svstems tend to daximize the entropy of the-system and preclude areturn to a state of lower entropy. Closed systems, on the other hand, can he forced, in princ i ~ l aet least. to return to anv desired earlier state bv suitable use of an external source and/or sink of energy. ( ~ s ~ s t eism closed if enernv, -- . hut not mass, can flow between i t and the surroundings.) Cyclical processes in closed systems are common, easily described, and often easily accomplished. Ice can melt to liquid, which can then be refrozen. Hydrogen and oxygen can read to give water, which can then be decomposed into hydrogen and oxygen. Many biological and other naturally orcurring cyclesare of this type. K\.ena hr(~kenegg can, hy feeding it t a chicken, he rwycled to a new egg. Of course, an additional source nnd sink of energy must he nvailahle, so the total system plus surroundings does undergo net change, including an increase in entropy.
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Micro-Reverslbillty Chemical equilibria are dynamic states for which there is overwhelming evidence that micro-reversihility is occurring. T h e random molecular motions, which have a wide distrihution of energies, lead to a tremendous variety of collisions so that all possible changes occur, and, a t equilibrium, each change in one direction is offset by an equally likely change (under the equilibrium conditions) in the reverse direction. One result of all this dynamism is no net change for the equilibrium state, AS = 0. Furthermore, all properties of the equilibrium system undergo no net change with time. Because we are anite sure that anv isolated svstem, or anv closed system at c k t a n t temperat&e, will n,me toequilihrium eventually, we can he equally sure that all chemical reartionsare reversihle in thesense that they ran and do prnreed in both directions. Furthermore, the ratio of the rate ronstants, k, for the forward and reverse reactions of any opposing pair just equals the equilibrium constant, K, for the net reaction: K., = k(reverse)/k(forward). Micro-reversibility occurs under conditions of infinitesimal changes in state functions, so meets the criterion for thermodynamic reversibility. Reversible and Irreversible Reactions We see, then, that the evidence from thermodynamics, from eauilibrium studies. and from kinetics SuDDorts without ex&&oin that every chemical reaction path >&I be traversed in both directions and that euerv chemical reaction can go backward (can reverse). The eq;ilibrinm system is the one;n which reaction in either direction is eauallv . . likelv. In other circumstances, one direction of reaction is favored and a net reaction orrlm. But, as the products increase in concentration and the reactants decrease, the forward reaction becomes lrss likely and the reverse reaction more likely. Furthermore, the reaction can he "driven"either way by use of suitable energy sources and sinks. The products can always be returned to the original reactants. Nor does it seem useful to use "irreversible') todescribe reactions which proceed "almost to completion," and "reversihle" for those with small equilibrium runstants. Why not use the simpler terms to describe extent of reaction, a n d use "reversible" in its more precise thermodynamic sense? I t is true that most chemistry courses define thermodynamic reversihility and micro-reversibility in an acceptable manner. But it is also true that many talk ahout irreversible reactions, a class that does not exist. Why not eliminate that use of "irreversible"? There are no such systems, and returning a system to a former state should not be confused with thermodynamic reversibility. Perhaps the following suggestions would he simpler for students to follow, learn, use, and adapt: (1) All chemical reactions will come to an equilibrium exhibiting micro-reversibilityif kept in a closed system at constant temperature. (2) Any claaed system can return to an earlier stste if suitable energy sources and/or sinks are available. Closed systems are returnable. (3) No isolated system can return ta an earlier state, even if the change occurred with thermdynamic reversibility. Isolated systems are not returnable. Since weappear to live in an isolated system (the univerie), "we cannot gu home again." But our world and many of the systems we use are nearly closed systems; therefnre, we can, with the use of sufficient enerm, return parts of these systems to earlier states. The price is adecrease in available energy and an increase in the entropy of the surroundings. ~~~
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Volume 57, Number 5, May 1980 / 345
