Ice Vs Entropy?

[Vexer]

You take energy *out* of a system and it becomes *more* "organised"?

 

Take energy out of chaotic water molecules, and they will eventually form into highly ordered

"ice" lattice.

 

Doesn't this violate a principle of Thermodynamics?

Edited June 10, 2012 by Vexer

[CraigD]

A belated welcome to hypography, Vexer! :) Thanks for all the interesting posts so far. Please feel free to start a topic in the introductions forum to tell us something about yourself.

 

You take energy *out* of a system and it becomes *more* "organised"?

As a general, empirical law of thermodynamics, yes – see below.

 

Take energy out of chaotic water molecules, and they will eventually form into highly ordered

"ice" lattice.

 

Doesn't this violate a principle of Thermodynamics?

No.

 

Your example doesn’t violate the 2nd law of thermodynamics, because the “system” that must be considered when applying it to cooling liquid water to form ice must include not only the water, but at least the surrounding medium that cooled it (typically air). It illustrates the 3rd law, as it shows that entropy of a system – in this case, considering only the water – decreases as temperature does.

 

Consider a simple thought experimental model of water being cooled to form ice:

• A big box of sub-freezing (say, 250 K) air
  
• An ice cube tray filled with room temperature (293 K) water
  

Left for a while, the water freezes into ice, reducing the entropy, or disorder, of the water in the ice cube tray. However, the temperature of the air in the box, and thus its entropy, increases. Considering the whole system – box or air and tray of water, its entropy would be constant, except that the heating of the air by the water isn’t perfectly uniform, but creates disorderly convection cells and flows. Eventually, this disorder will disappear from the air, but only through friction with the box and tray walls, etc., heating them and increasing their entropy, and so on, out into the larger universe.

 

Key to understanding the laws of thermodynamics is understanding the meaning of the term “system” in the various contexts in which it occurs in their statement.

[Vexer]

I can't say I understand. Are you saying that the energy is sucked from the larger System? And so the rule doesnt apply locally?

[CraigD]

I can't say I understand.

You’re far from the first person to find thermodynamics hard to understand, nor I the first person not to explain it very well. I’ll keep trying.

 

Are you saying that the energy is sucked from the larger System?

In my ice cube tray in a box example above, energy is actually being moved from the smaller system – the water in the icecube tray – to a larger system – the air in the box. The temperature of the water in the tray decreases, and the temperature of the air in the box increases. Temperature is the average kinetic energy of the moving parts of a system – in this case, essentially, the atoms that make up the water and air, so, since the volume of the smaller and larger system remains nearly constant (not exactly, as water expands slightly when it freezes), their decrease and increase in temperature is equivalent to their increase an decrease in energy.

 

And so the rule doesnt apply locally?

Depends on which law you’re applying.

 

The 3rd law states that, as the temperature of the system of the water in the tray decreases, its entropy/disorder decreases. So it applies to this system. The 2nd law doesn’t apply to it, because it isn’t an isolated system, but part of the larger box or air + tray of water system.

 

The 2nd law states that the entropy of the box of air + tray of water system increases. So it applies to this system. In our thought experiment, we assume this system is effectively isolated from “the larger universe”. Otherwise, it might itself be in a yet larger box of cooler air, and its temperature and entropy be decreasing.

 

One needs to be careful equating energy, heat, and entropy, because while related, they’re not the same. But for this though experiment, it’s OK not to worry too much about how these concepts differ. Eventually, though, everybody should plunge into the rabbit hole the concept of entropy leads down – it’s a truly deep, and important concept, having to do with much more than temperatures.

[modest]

I think Craig's answer is very good. Pardoning the repetition, this example approaches things from the other side of the coin as it were:

 

 

YouTube - Supersaturated Sodium Acetate

http://www.youtube.com/watch?v=0wifFbGDv4I

 

The liquid in the beaker spontaneously turns into a solid. When that happens the less organized liquid becomes a more organized solid, so—as you say—the entropy of the beaker spontaneously lowers.

 

At the same time this is an exothermic reaction. It produces a lot of heat which warms the air up around the beaker. The air becomes less organized at that higher temperature. Because the entropy of the beaker goes down and the entropy of the surrounding air goes up entropy overall stays constant or increases.

 

~modest

[Little Bang]

Curious thought, isn't entropy just a function of time? The older the Universe gets the faster entropy occurs.

[Vexer]

Thanks for your efforts.