Why is the phase change of water freezing favourable under 0 degrees Celsius?

Question

When water freezes, the phase change that occurs is exothermic (DH = -6.02 kJ). Based on the change in enthalpy, you would expect that water would always freeze. Use the concepts of entropy and free energy to explain why this phase change is favourable only below 0 degrees C.

Answer 1

Yikes, everyone is doing DG = DH - T DS.  

So, you have an exothermic process (-DH) with a decrease in entropy (-DS).  Long story short, processes like these are only spontaneous (that is, DG negative) at low temperatures.  See my explanation of the math behind DG  = DH - T DS at the link below for more details.

http://answers.yahoo.com/question/index;...