My teacher told us that only the ""Change in Enthalpy "" is definable. Whereever I looked for it, I got only an "equivalent " type definition like: It's a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume. But change in enthalpy also holds the same kind of definition. Thanks

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    $\begingroup$ There is a reason why only change in enthalpy can be measured (experimentally). See, you can conduct a chemical experiment and measure the heat produced / lost in the reaction. Then you do some algebraic manipulation and calculate the change of enthalpy. But see, for a constant change in enthalpy c, final state and initial state enthalpy can be anything as long as their difference is c. So, you can't really know absolute enthalpy but the chane of it. $\endgroup$ Commented Sep 23, 2017 at 7:01
  • $\begingroup$ Actually, enthalpy is definable. What your teacher really meant was that you can't know absolute enthalpy. $\endgroup$ Commented Sep 23, 2017 at 7:03
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    $\begingroup$ Can energy be defined? How about change in energy? ;) $\endgroup$ Commented Sep 23, 2017 at 7:04
  • $\begingroup$ @Paracetamol Energy is defined. Actually, every concept in science is defined whether you can measure it or not. $\endgroup$ Commented Sep 23, 2017 at 7:11
  • $\begingroup$ @Mockingbird That comment was supposed to be "food for thought" <---- Intended for OP to ponder over it :) $\endgroup$ Commented Sep 23, 2017 at 7:19


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