Background I'm taking grade 12 chemistry. During a lesson, my teacher briefly mentioned that enthalpy is equal to the internal energy of a system plus the product of the pressure and volume of that system. However, she remarked that the concept wasn't an important part of the curriculum and moved on after she wrote its mathematical definition on the board. (She's a good teacher; I don't mean to suggest that she short-changed us.) My Attempt to Answer my Question I spent several hours yesterday looking for examples that might give me an intuitive understanding of what that formula is describing: I read the Wikibooks chapter, spoke with an aerospace engineer, and watched the Khan Academy video. The Wikibooks chapter quickly employs more calculus than I know. The engineer provided some help, but noted that he couldn't distinguish change in enthalpy from heat. The Khan Academy video1 defines it as "heat transfer for chemical reactions". I suspect that transfer in heat transfer (a phrase that the video presenter used) is redundant, as she defines heat as "energy transfer due to change in temperature". If it is redundant, then the presenter defined enthalpy as 'heat for chemical reactions'. Additionally, I know that heat is not a state function and that enthalpy is a state function. As such, it seems to me that the video presenter's definition would make sense only if she used enthalpy to denote 'change in enthalpy' as I assume she did. My Question The two sources that I could understand both equated change in enthalpy with heat. However, that still leaves me with my original question: What does the formula for enthalpy (not change in enthalpy) describe? Some everyday examples of things that exhibit much, or little, enthalpy might help me understand. Thank you, 1 https://www.khanacademy.org/test-prep/mcat/biomolecules/principles-of-bioenergetics/v/enthalpy-1 (see 1:22).