Question about Molar Enthalpy

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Nathan Sanchez 1F
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Joined: Sat Jul 09, 2016 3:00 am

Question about Molar Enthalpy

Postby Nathan Sanchez 1F » Thu Jan 12, 2017 9:43 pm

I understand that the enthalpy of vaporization is the difference in molar enthalpy between the vapor and the liquid states, but how does one calculate the molar enthalpies of those states? Are they usually given? Is there an equation to determine the molar enthalpies? Thank you.

Jonathan Sarquiz 3F
Posts: 32
Joined: Fri Jul 15, 2016 3:00 am

Re: Question about Molar Enthalpy

Postby Jonathan Sarquiz 3F » Thu Jan 12, 2017 10:54 pm

Both the enthalpy of fusion (solid to liquid) and enthalpy of vaporization (liquid to gas) are constants. They are experimentally determined, they are not calculated by an equation. For water, the enthalpy of fusion is 6.01 kJ/mol while the enthalpy of vaporization is 40.7 kJ/mol. However, you should realize that if the question asks how much heat is needed to vaporize 2.0 mol of liquid water at 100 degrees to steam at 100 degrees, you use those values in the calculation: (2.0 mol)(40.7 kJ/mol) = 81.4 kJ.

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