Enthalpy is a thermodynamic property of a thermodynamic
system.
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there are three systems :
1 open system
2 close system
3 isolated system
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You can search the relevant meaning of the availability
condition for thermodynamic system in the wikipedia since there's
so much to learn and laws of thermodynamic equilibrium that
explains what and those condition works.
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please read the 3 kinds of thermodynamic system maybe it can
help you to recognize and you can identify which one can be
described.