Crystal Formation and entropy

If a cube of ice freezes,then thaws again,where is the increase in disorder.?

The melting and freezing energy equal,just in opposite directions.

Where is the heat to re-thaw it coming from?

The same place it went when it was removed to freeze the water.

A zero net sum,not increased heat(entropy).

Heat flows from hot to cold naturally. To go the other way, one must supply additional heat to push it there. That's where the increase in disorder comes from.

Heat only flows from hot to cold.In the case of water and ice,both at zero C,there is no temperature difference.The addition of heat results in melting of some of the ice,increasing disorder,but the removal of the same amount of heat result is ice,a more orderly state.A phase change,not a temperature change.

A change in phase from liquid to solid constitutes an increase in order. The disorder needed to create the order comes from the device that supplies the "coolth" to create the solid.

Whether a reaction will take place is determined by the Gibbs Free Energy, i.e.,

ΔG = ΔH - T*ΔS,

where ΔG is Gibbs energy, ΔH is the heat absorbed by the reaction (enthalpy), T is the absolute temperature ( degrees K) and ΔS is the change in entropy.

In a nutshell, if ΔG is positive the reaction will not take place. If ΔG is negative, the reaction will take place. Heat given off or increase in entropy drive a reaction, while heat absorbed or decrease in entropy inhibits the reaction. When these forces are opposed, the stronger (ΔH or T*ΔS) will determine whether the reaction will take place or not. So, the influx of energy ΔH can drive a decrease in entropy ΔS as long as ΔG is negative. ΔH can be furnished by sunlight which evaporates a salt solution leaving the crystals or drives photosynthesis in a plant.

In practice, ΔH, T, and ΔS change as the reaction proceeds and conditions change (temperature, concentrations, etc), so the reaction proceeds while ΔG < 0.

See: https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/gibbs.php

I think the key is that entropy always increases in a closed system, i.e., neither matter nor energy can flow in or out.

Water at 0C can exist with ice at 0C.

It takes latent heat to effect a change in state from a liquid to a solid,or in the opposite direction.

It takes as much energy( 3.36 105 J Kg-1 in SI-units) to melt ice as it does to freeze it.

The latent heat is the same in opposite directions,so will entropy remain the same?

For melting ice, entropy increases, ΔS=Q/T, where Q=105 J/Kg and T=273.15 K

For freezing ice, entropy decreases the same amount

So the freeze/thaw energy is the same..entropy is the same..net zero entropy.All of the icebergs that leave the poles are absorbing energy equal to the amount they lost when frozen.This should help cushion the effects of global warming,sea level rise aside,though most of the heat will be absorbed by the oceans.

At the present,increased temperature results in more melting,more evaporation,more clouds,more rain,but not uniform in distribution.Hence we have more droughts, floods,and hurricanes and storms,(mother nature's way of moving heat).

Earth is at a triple point for water,and it constantly changes state.

When all the ice melts,we are really stewed.

So the freeze/thaw energy is the same..entropy is the same..net zero entropy.

No, in melting entropy increases, in freezing entropy decreases...

Water at 0C and ice at 0C.

Same temperature; No temperature change.One is solid,one is liquid.

The energy(heat) (latent heat) input required to change state from solid(ice) to liquid(water) is the same as the energy (output) lost to change it from liquid(water) to solid(ice).

Net entropy should be zero over the entire freeze-thaw cycle .If left in one state or the other the entropy is in one direction or the other.+ or -.

If not,why not?I correlate entropy with heat.What am I misunderstanding?

I'm only paraphrasing what others have said here.

Start with ice at zero: where does the heat come from to turn it to water?

Now where does the heat go to turn it back into ice? It can't go where it came from (PWS).

Now look at what you've got; throw away the ice which is exactly as it was, and, look again.

The source and sink of the heat is the ambient surroundings,in both directions.The point I am making is that the water goes from a disorganized state to a more organized state and the amount of heat sourced or sinked is the same.

You didn't complete the last step: ignore the ice/water and look at the starting and ending state of the whole system.

The actual starting point would be the Big Bang.

Sorry, I didn't make my self clear. Imagine a closed system and study it over a short period time, during which some ice melts and re-freezes. Now ignore the ice because it is in exactly the same state as it was in at the beginning, and, examine the state of the rest of the system at the beginning and end of the thought experiment.

OK.Start with ice,with no melt water.Add heat to melt a portion of the ice,and no more.

Now close the system from outside energy sources.

Then remove heat to freeze the water.The temperature of the water does not change,it simply changes state,the latent heat in the system is the same in either state.

Imagine a magic heat capacitor inside the container,that will absorb exactly the amount of latent heat that was put into the system,and when the last BTU is absorbed,it will discharge the exact amount of heat back into the system,then it start over.

The water and ice would be constantly alternating states.

The entropy in the system stays the same as at the start. No increase in entropy.

Of course,with no outside energy sources,we cannot observe the process,so it could be in either a water or ice state,or both, until we view it by admitting photons,which are a source of energy,which would violate the starting conditions.

Heat can only flow downhill!

How does this magic heat capacitor work unless it can somehow magically raise its temperature above "0" to melt the ice, and, below "0" to freeze the ice.

...since which event the disorder of the universe has been increasing and will, for ever more, continue to do so.

<...will entropy remain the same?...>

No, because to go from water to ice one needs a source of "coolth", which is generated by increasing the disorder of the universe elsewhere. In order to make heat go uphill, it still needs a source of additional heat to push it there.

• Law 1: one can't win.
• Law 2: one can't even draw.
• Law 3: this is the only game in town.

_Anon.