The values D E and D H are very nearly equivalent in biological reactions Stated above was the fact that state functions, like D H and D E, do not depend on the path taken during a reaction. These functions pertain only to the differences between the initial and final states of a reaction. This demonstrates that some of the heat of the surroundings has to be absorbed by the system in order to perform the work of changing the system volume. Reversible reactions differ from irreversible in that the former always proceeds infinitely slowly through a series of intermediate steps in which the system is always in the equilibrium state. Whereas, in the irreversible reaction no equilibrium states are encountered.
Irreversible reactions are also spontaneous or favorable processes.
Thermodynamic calculations do not give information as to the rates of reaction only whether they are favorable or not. This concept is defined by the term entropy, S . Whereas, enthalpy is a term whose value is largely dependent upon electronic internal energies, entropy values are dependent upon translational, vibrational and rotational internal energies. Entropy also differs from enthalpy in that the values of enthalpy that indicate favored reactions are negative and the values of entropy are positive. Together the terms enthalpy and entropy demonstrate that a system tends toward the highest entropy and the lowest enthalpy.