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what happens to the free energy change when water freezes to ice
- anam (age 17)
Since the water freezes, I assume the temperature is below 0°C. The free energy goes down as the water freezes. Here are the important terms:
The energy of the interactions between the water molecules goes down as they line up in the stable ice positions. This lost energy flows out into the environment as heat, increasing the entropy of the environment.
The entropy of the water molecules goes down because their positions and orientations are constrained by the crystalline structure.
So long as T<0°C, the entropy gain of the environment beats the entropy loss of the freezing water, so the water freezes.
Equivalently, the free energy U-TS (where U is the water energy, S is the water entropy, and T is the absolute environment temperature) goes down on freezing so long as T <273 K (the same as 0°C).
(published on 05/18/10)
Follow-Up #1: enthalpy and freezing
will enthalpy rise when temperature goes below 0deg C
- v.hariharan (age 70)
As the temperature gradually goes below 0°C, the enthalpy (H=U+pV) of the super-cooled water drops. That's pretty simple- its energy U is going down and its volume V is hardly changing. (p is pressure.)
At some point, however, that supercooled water will start to freeze. If the process is quick, so that there's little heat flow to the environment, there's little change in H. The only way that energy then leaves is via work (p times volume change, pdV) done on the environment. But that energy loss just cancels the pdV change in H, so there's no net enthalpy change.
O course, the freezing does release latent heat. The lowered potential energy of the contacts between the molecules in the ice leaves energy to warm up the ice-liquid slush. That can lead to gradual heat flow out to the slightly colder environment, so the slush will lose some enthalpy.
(published on 11/11/12)
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