Expert: Mark Janus - 9/3/2009

Question
Dear M Janus:

I have checked about a dozen sources and have not grasped the physical interpretation of enthalpy. Why this term is so important? Is it only limited to a mathematical relationship, i.e. H = U + PV? Or, does it possesses any QUALITATIVE MEANINGS? Please do tell me the "meanings" and essence of enthalpy. I have been told that some people grasp it so well that they only use this property to describe a FLUID!!! Kindly tell me what it means.

Answer
Beyond its mathematical statement, enthalpy gives an alternate view of the energy of a system..  When viewed in terms of change, it represents the change in the energy of a system held at a constant pressure...  whereas internal energy (or a change in it) is measured with the system held to a constant volume...  scientists dealing with chemical reactions find great utility in this fluid property...

Hope this helps,
mj