I read the answers to What does enthalpy mean? and both answerers mentioned that it's the energy required to "make room for the system". What does this mean? How do we provide that energy and how does it succeed in "making room"? Does this also involve the building up of matter as the gas didn't even exist at absolute zero?

  • $\begingroup$ Did you actually read the answers? This is explained fairly non-technically in that link. Which part of those answers is not clear? $\endgroup$
    – Steeven
    Sep 28, 2017 at 5:36
  • $\begingroup$ @Steeven I didn't understand the "gas at absolute zero" part. $\endgroup$
    – Archer
    Sep 28, 2017 at 6:00
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    $\begingroup$ I have some advice for you. I strongly recommend not trying to ascribe any physical significance to enthalpy. It is just a convenient function to work with which comes into play in many kinds of thermodynamics problems. If you just treat it as that, you will eliminate much frustration in your thermodynamics studies and applications. $\endgroup$ Sep 28, 2017 at 21:55

2 Answers 2


Think of the "the energy needed to make room" as the energy needed to place a given system in a specific condition. For example, when you boil water, the water vapor that is produced needs to push away the air molecules to make room for itself. The energy we associated with this process is accounted for in the enthalpy. Another example would be when you move an object from place A to place B. Removing it from place A causes the air molecules to rush into its place fill it in (work done by environment on the system) and by placing it at B you need to push away air molecules to make room for the object.

  • $\begingroup$ what about the internal energy in the water vapour conversion process? $\endgroup$
    – Archer
    Sep 28, 2017 at 6:04
  • $\begingroup$ Enthalpy=(energy needed to make room)+(energy needed to create the system, aka its internal energy) $\endgroup$
    – Ptheguy
    Sep 28, 2017 at 14:17

You provide the energy in any way that is appropriate to the system. It's a general statement, to account for the full energy required to establish a system, Displacing fluids by using energy to pump away liquids or gases are only examples of the " book keeping". you need to do to keep track of the processes in establishing the system.

Try to think generally, more than just the examples given, so in space there may be electric or magnetic fields opposing the establishment of a system, tthat need to be accounted for in your energy calculations.

Another way of looking at it is the original viewpoint, as in why it's called free energy, as the Gibbs and HelmHoltz free energies are also called. They are the energies you get back if the system disappears and the fluid, or the original conditions before the system was created, returned.


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