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Name: Adam M.
Status: N/A
Age: 17
Location: N/A
Country: N/A
Date: 5/13/2003


Question:
Hi, Look im completing my studies in chemistry a high school level and am currently working with the reasoning behind why liquids are miscible and immiscible. I am aware and have been taught about polar and nonpolar bonds and the general consensus that like dissolve like and the whole intermolecular forces scene. However I have done some reading into thermodynamics and am extremely interested as to how the second law of thermodynamics relates to miscibility/immiscibility of liquids. Obviously a system desires a higher level of entropy and will change to that if possible but can someone please explain to me why separating two liquids therefore creating two layers (immiscible) is more orderly or of higher entropy than dispersing the two liquids molecules throughout (being miscible).


Replies:
Adam,

You said, " ... please explain to me why separating two liquids therefore creating two layers (immiscible) is more orderly or of higher entropy than dispersing the two liquids molecules throughout (being miscible)."

Your statement is not quite correct. Here's a corrected version: " ... separating two liquids, thereby creating two immiscible layers, is more orderly and of LOWER entropy than dispersing the two liquid's molecules throughout each other in a miscible system."

The separated (two layer) liquid system is more orderly because it has two discrete (separable) parts. When liquids mix, their entropy (state of disorder) increases. The separated layers represent a system of greater order and lower (not higher) entropy.

Entropy is a measure of system disorder. High entropy represents great disorder -- chaos. Low entropy represents a state of organization and great order.

Regards,
ProfHoff 661


Well, obviously you have noted that solubility and miscibility is a rather complicated phenomenon. You are correct in thinking that all spontaneous processes require an overall entropy increase. Entropy is related to the number of micro-states available to a system; when two liquids mix, the entire volume of the container is accessible to every molecule of both liquids, which corresponds to a lot more micro-states than if each liquid had its own distinct volume from which it could not escape. So why, you wonder, would two liquids ever not want to mix?

It comes down to the difference in entropy between the separate liquid phases and a combined single phase. It is actually possible for a mixed phase, containing molecules of more than one component, to be more orderly (have less entropy) than two phases, one rich in one of the phases and the other phase rich in the other component. There are two basic ways this can happen.

The first is for the molecules of one component to form highly stabilizing interactions with each other, which they cannot duplicate with the other component. Take, for example, a polar liquid. The polar molecules can align with each other so that positive charges are near negative charges, and vice versa. This constitutes increased order, and thus decreased entropy, right? Not necessarily. Bringing opposite electric charges near each other lowers the potential energy of a system. Since energy is conserved, this means that the kinetic energy of the system rises by the same amount that the potential energy goes down. Molecular kinetic energy is heat. So when the potential energy goes down, the temperature goes up, making more disorder (entropy) in the system. Which effect dominates - the increased ordering from biasing the molecular positions and orientations, or the decreased ordering from the higher temperature? It turns out that different substances can give different answers.

What does that have to do with miscibility? Well, if the polar liquid mixes with a non-polar liquid, on average any two molecules of the polar liquid become farther apart. This means that their stabilizing interactions become less, hence less potential energy decrease, thus less kinetic energy increase, thus less entropy.

The second way that mixing two liquids can result in less entropy than when they are dissociated into separate phases is for the mixing to cause increased ordering on a microscopic scale. Imagine, for instance, that when molecules of substance A are introduced into liquid B, the molecules of liquid B form an organized "shell" around the molecules of A. Although the molecules of A now can occupy many more positions than they could before they were introduced into the liquid, the molecules of the liquid now are much more constrained in how they can move about. These two effects oppose each other, and whether the entropy goes up or down depends on which effect is larger.

Surprisingly enough, water seems to behave in the second way when nonpolar substances are introduced.

If this seems confusing, it is because entropy ("disorder") can be manifested in several different ways, and the same process can increase entropy in one way while decreasing it in another.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois



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