StatMolThermo 05.07 Enthalpy

Enthalpy (Module 5 of University of Minnesota Chemistry 4501)
Erratum: The numerators in the last 2 slides should have units of kJ, not J, and the sign of the last term on the right hand side of the last slide should be negative, not positive. These errors have been corrected in the pdf files of the slides that are provided for download, but the errors persist in the video itself.

[@radeyavasquezromero8294]

Hi Dr. Cramer. Greetings from Colombia. I've got a little question. Why do I have to provide 40,7 kJ/mol to water molecules (at 273,15 K and 1 atm) when it is already boiling? Pd. You're amazing.

[@ChemProfCramer]

Well, phase changes are funny things... You can have liquid water at 373.15 K AND gaseous water (steam) at 373.15 K, but they have substantially different enthalpies because they have such different molar volumes (a fairly small part) AND because all of the favorable hydrogen bonding interactions in the liquid were broken in becoming a vapor (the major part!) That is, you put a lot of energy into pulling the molecules apart from one another, even though you did NOT add any kinetic energy to their translational, rotational, or vibrational modes, and so the temperature stays the same over the phase change (that's a little simplified, as molecules in the liquid don't have isolated modes like molecules in the gas phase, but it's the right idea). Module 9 of this series goes into phase changes in much more detail -- you might find it interesting to sample that.

[@solsticetwo3476]

Chris Cramer Hi Dr. Cramer, possibly I'm wrong, but I had understood that during the change of phase you indeed add energy (excite) to the rotational and vibrational degrees of freedom, as also to the configurational change (potential energy); only that that energy does not contribute to the change of temperature (a measure that depends only on the translational kinetic energy).