Enthalpy refers to energy and entropy refers to chaos.
2. Different physical meanings:
The physical meaning of enthalpy is the thermodynamic energy in the system plus PV. H=U+pV. The physical meaning of entropy is the degree of chaos of the system, which is defined as DS = DQ/T.
3. Different in nature:
Enthalpy and entropy are essentially state functions, but entropy can be zero (when the number of microscopic states is 1) and cannot be negative (obviously, the number of combinations cannot be less than 1).
State function
Enthalpy is a state function of the system, which has nothing to do with the change mode. Enthalpy has a unique value as long as the state of the system is fixed. Enthalpy is introduced by isobaric change without volume work. But it doesn't mean that enthalpy only exists in the isobaric process without volume work, and it doesn't exist in other cases.
The change of enthalpy is very important for chemical reactions, because the absorbed (or released) heat is equal to the change of internal energy (δ U) plus the external work done during the change (δ PV). In many biological systems, no work is done, so the change of enthalpy is equal to the change of internal energy.
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