Supercooled water freezes. Is this a spontaneous process?

[Dars]

Supercooled water freezes. Is this a spontaneous process??

Homework Statement

"one mole of supercooled water at -10degC freezes reversibly to ice at -10degC at constant P of 1500atm. The system is at chemical equilibrium. Assume that ΔH_fusion is independent of temperature. Also assume that the densities of ice and water are independent of temperature. State whether the following are <, > or = 0:
q, w, delta H, delta U, delta V, delta P, delta T, delta G, delta A"

I just need help understanding chemical equilibrium with respect to gibbs energy though...

Homework Equations

The Attempt at a Solution

my specific question is: what exactly do they mean when they say that "system is at equilibrium?" what does that imply? I know that Delta G refers to whether process is spontaneous or not. But is supercooled water that freezes a spontaneous process or not?

I looked up supercooling and it has to basically be disturbed to freeze (needs something to crystallize around), otherwise, if conditions don't change, it just stays the same. So does that mean that process is NOT spontaneous and therefore delta G > 0 ?

thanks for any clarification

 

[Ygggdrasil]

In this case, the water is not supercooled. Remember that the melting point of a substance is dependent on the pressure. At 1 atm pressure, water freezes at 0^oC, but at 1500 atm, water freezes at -10^oC. Since the system says that the process occurs reversibly, the ΔG for the process must be equal to zero (otherwise the process could not occur reversibly as either the forward or backward reaction would not be spontaneous).

In the case of supercooled water freezing (e.g. water at -10^oC freezing at 1 atm pressure), the process is spontaneous. Even though the system can be stably held as a liquid at this temperature, the system is not in equilibrium. Rather, we refer to a stable non-equilibrium state like this as a metastable state. The stability comes from the fact that there is not enough thermal energy available to "kick start" the reaction. Because this stability comes from the slow rate of freezing (i.e. the slow kinetics of the reaction), we call this type of stability kinetic stability.

Here's some food for thought. Graphite actually has a lower free energy than diamond at ambient temperature and pressure. So, diamonds actually represent a metastable state. Of course, the energy required to rearrange diamond into graphite is huge, so the fact that "diamonds are forever" is a good example of how great an effect kinetic stability can have.