Basic thermodynamic functions and parameters

In turn, in its differential form the change in enthalpy is defined as  

under the normal environmental conditions encountered by biopoly-mer solutions and food colloids, i.e., constancy of pressure, the change in enthalpy is defined as  

Let us denote the equilibrium state of the system existing before the occurrence of the interactions as A, and the equilibrium state reached as 

In order to take into account the spontaneity and irreversibility of real processes (heat always goes from a hot substance to a cold one, but not the reverse), thermodynamics invokes the notion of the entropy S. In statistical terms entropy is defined as the probability of accessible states for each molecule in the system  

In equation (5.5), k is the Boltzmann constant (1.38066 x 10 JK ), which is related to the (thermal) energy of one molecule, and V is the number of all accessible states of the molecule. Generally, in physical chemistry, we consider not a single molecule (or particle), but rather one mole, i.e., 6 x 10 molecules (or particles). Thus equation (5.5) becomes 

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