[blokeman]

Hi,

I'm actually a linguist, not a chemist, and I'm having to classify my data (verb phrases) for whether they denote reversible or irreversible processes. The two problematic processes are highlighted in their original contexts below:

1. The Electrolyte Genome's first major scientific finding -- that magnesium electrolytes are very prone to forming ion pairs, which impacts several crucial aspects such as conductivity, charge transfer and stability of the electrolyte -- was published in February in the Journal of the American Chemical Society. Source.

2. In particular, the pharmaceutical industry would gain huge benefit from being able to reliably predict crystal structure because pharmaceutical molecules are prone to crystallise in more than one crystal structure (or polymorph), depending on the conditions under which the molecule is crystallised. Source.

So the question is, is the forming of ion pairs reversible, such that a pair (?) of electrolytes that have formed an ion pair may break up and become single electrolytes again, or is it irreversible like the scrambling of an egg? Likewise, can a crystallized pharmaceutical molecule uncrystallize and assume its original form again?

All informed input is warmly welcomed.

[Borek]

Dilution kills ion pairs, dissolution of a crystal produces back its solution.

[blokeman]

Borek: In other words, both processes are comparable to the freezing and melting of water/ice, i.e. they can go back and forth a virtually unlimited amount of times with no permanent damage/alteration to the original, um...particles?

[Borek]

Yes.

[Corribus]

Borek: In other words, both processes are comparable to the freezing and melting of water/ice, i.e. they can go back and forth a virtually unlimited amount of times with no permanent damage/alteration to the original, um...particles?

Do we want to be pedantic here? Reversibility is a statistical concept. An irreversible process is one in which one state (call it B) is statistically favorable over another state (call it A) such that the transformation of A to B is spontaneous, and the transformation from B to A is not. A fundamental principle of chemistry is that all microscopic processes are reversible. Correspondingly, ANY macroscopic process is reversible (practically speaking) provided you are willing and able to supply enough energy to change the system/environment so as to make the reverse process spontaneous. But if the environment doesn't change, then spontaneous changes/processes are not reversible because the environment favors one state over another.

Your ice/water example is a good one. Melting is an irreversible process because if you take an ice cube and lay it on a table in the sun on a hot day, it will melt. But it will not spontaneously reverse and reform an ice cube under those same conditions. The only way to reverse the process is to change the system's conditions - i.e., gather that water and put it in an ice cube tray and stick it in the freezer. So in colloquial language we may consider melting to be reversible, in the sense that we can easily undo it, but this is not the thermodynamic definition of reversible. (You will find much confusion on this point by searching the web.)

A more rigorous thermodynamic way you may look at irreversibility is that in complex transformations, the efficiency is never 100%. You always get some energy loss, so there is no way to go back to the original state without supplying more energy into the system. In the case of ice/water it is easy in practice to reverse the process by simply sticking the melted water back in the freezer. Other cases (e.g., unscrambling an egg) would be virtually impossible. Theoretically, if you were able to microscopically untangle every protein molecule and allow it to refold in its native state, maybe you could reconstitute the albumin to be "unscrambled" - in fact, if your egg white were composed of a single protein molecule (how many proteins does it take before it's considered an egg white?), the process would be quite reversible potentially. But the statistical concept of irreversibility kicks in with macroscopic egg whites, and short of a massive amount of time and energy, this process could never be reversed.

The essential criterion for an irreversible change is whether the entropy of the universe increases as a result of the change. The interpretation of this is that if the entropy of the universe increases, energy is lost from the system to the environment (universe), so it is not possible to restore the system AND the environment to the exact same state it was in before the process started. For example, if the sun melts an ice cube, the sun loses some energy. Refreezing the ice cube doesn't restore that energy to the sun. So although it looks the process is reversed by refreezing the ice cube, the environment has still changed. Therefore the process isn't truly reversible.

So, to sum, any complex, real (macroscopic) process is fundamentally irreversible, because no process is 100% efficient. Truly reversible processes are purely theoretical (isentropic adiabatic process), because it would require that the entropy gain by the universe is zero, or that the system is perfectly insulated from the surroundings.

EDIT: Relating to the original question, whether or not to classify those as reversible or irreversible depends I guess on whether you want them classified according to the strict thermodynamic definition or something more akin to the linguistic meaning of "reversible", i.e., easy to undo with very little effort. That's subjective, of course. As you say, an ice cube melting is reversible if you have a freezer nearby. Much harder if you're in the desert under the blazing afternoon sun .

[blokeman]

Corribus: Thanks, that was very enlightening. My "definition of irreversible" here is how the original authors of the quoted sentences presumably conceptualize the two processes from the perspective of their goals. I'm not sure whether the forming of ion pairs and crystallization are micro- or macroscopic phenomena (the latter sounds like it might be detectable by the naked eye), but judging from what Borek says above, it sounds like they are both "reversible at a minor energy loss" by scientists who have the right tools, as is probably the case here. I.e. it is not the same degree of irreversible as death, or even a scrambled egg.

[Corribus]

Microscopic in this context doesn't necessarily refer to spatial size, but rather the magnitude of the change for the process.The idea is that if you perturb a system in equilibrium by an infinitesimally small amount, the system responds by moving back toward equilibrium in the opposite direction. In fact, we can define chemical equilibrium as a state in which imposing an infinitesimally small change on the system results in an infinitesimally small change in the opposite direction. The law of microscopic reversibility is a central idea in chemistry because it offers a definition of equilibrium. Importantly, infinitesimally small changes to a system in equilibrium are fully reversible. If you were to perform a long series of microscopic, reversible changes in sequence to effect a macroscopic change, this, too, would be a reversible change, because at no time does the system leave equilibrium, and you could always go the other direction by applying a sequence of microscopic changes in the opposite direction. Of course, we don't actually change thermodynamic systems by infinitely small incremental changes. Real, macroscopic changes result in wasted work, which makes those changes irreversible for the reasons I previously explained. Another way to view it is that large changes bring a system instantly far from the equilibrium point, where additional small perturbations do not result in an opposite response by the system. Energy becomes wasted.

Anyway, I guess the point is that from a thermodynamic view, both processes are irreversible in the sense that in the real world, they cannot be 100% efficient. Both processes result in wasted work (by the system on the environment, or by the environment on the system), energy loss to the greater environment, or entropy gained by the universe. On the other hand, they could both be viewed as reversible in that it is possible to cycle between the respective loosely-defined* states with only a reasonably small amount of energy or effort expended (based on whatever you determine as being reasonable). I think most chemists in the academic literature implicitly understand that "reversibility" has a strict thermodynamic definition (even if they don't exactly remembered it ) and rather use the term in the looser sense because they're simply trying to communicate how chemical systems tend to behave in practice.

Nevertheless, it doesn't hurt to remind people that some terms used frequently in science have multiple operational definitions, some more rigorous than others, and if the author uses a term one way and the reader understands it differently... then miscommunication results.   

*By loosely-defined, I mean that we tend to define transformations as binary, such as "crystalized/non-crystalized", but a more granular view brings this simplification into question. In a binary view, we could think of "crystal A --> dissolved --> crystal B" as being a reversible process. But crystal A could, strictly speaking, have a different composition than crystal B. Crystal A could have a different microstructure than crystal B. Different grain boundaries. Different crystallite sizes or orientations. Etc. In other words, can you dissolve a crystal and get back exactly the same crystal, with the exact same atoms in exactly the same position? Is this therefore a truly reversible process? Since you're a linguist, you can probably appreciate how fun this can get if we really start to parse the language .

[blokeman]

My takeaway is that the two processes are irreversible in the sense that both are brought about by creating the conditions in which they occur spontaneously and en masse, rather than in a controllable, particle-by-particle fashion.

On the other hand, they are reversible in the same sense as melting and re-creating ice cubes with a freezer and ice-cube tray is -- though the re-created ice cubes will not be identical to the previous ones at the microscopic level, they are identical for the intents and purposes of the practitioner and (most importantly) for the intents and purposes of anyone reporting the experiment to a lay audience.

This seems now resolved. Many thanks for all input!