If #DeltaH_"fus" = 7.25# and #DeltaS_"fus" = 0.007# for calcium chloride, at what temperature does this substance freeze in a phase equilibrium?

1 Answer
Truong-Son N.
Oct 23, 2017

Since you didn't provide units for #DeltaH#, I have to assume you mean...

#DeltaH_"fus" = "7.25 kJ/mol"#
#DeltaS_"fus" = "0.007 kJ/mol"cdot"K"#

At constant temperature, we can use the ubiquitous thermodynamic relation:

#DeltaG = DeltaH - TDeltaS#

At phase equilibrium, we are at constant temperature and pressure, so #DeltaG#, being a function of temperature and pressure, is zero.

Thus:

#DeltaG_"fus" = DeltaH_"fus" - T_fDeltaS_"fus" = 0#

And so the freezing point is:

#=> bb(T_f = (DeltaH_"fus")/(DeltaS_"fus") = ?)#

You should be able to look up this value on the internet to check your work. The reference value from a quick google search is approx. #8^@ "C"# different from the result of this calculation.

At what temperature are your #DeltaH# and #DeltaS# defined? Are they for thermodynamic standard state or otherwise?

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