|The Lewis inequalities for natural processes and unnatural process in the 1933 notation of English chemical thermodynamicist Edward Guggenheim. 
The notation and symbol usage in chemical thermodynamics, prior to Guggenheim's uniformity of the notation system, was chaotic and varied, to say the least, as evidenced by both Guggenheim's (1933) and Theophile de Donder's (1936) historical usage characteristic function notation tables. The central point to note is that in 1933 the symbol G was assigned to replace F in the following formula:
Rossini notation (1950) Modern notation Description δu < 0 ΔG < 0 If the value of δu is negative, useful energy is obtainable from the system and we know the change is a naturally occurring one in the direction toward the state of equilibrium. δu > 0 ΔG > 0 If the value of δu is positive, useful energy is required to be supplied to the given system to bring about the desired change and we know that the change is an unnatural one in the direction away from equilibrium.
Differential change State change Name dG < 0 ΔG < 0 Lewis inequality for a natural process (or spontaneous process), one in which the change will occur naturally or spontaneously and in which useful energy is obtainable from the system. dG > 0 ΔG > 0 Lewis inequality for an unnatural process (or spontaneous process), one in which useful energy is required to be supplied to the given system to bring about the desired change.
“The consequences of this statement for chemistry are as yet hardly realized, but with the further progress of science it is to be expected that empirical results will more and more give way to exact quantitative laws.”
“We may think of the quantity ‘– ΔF ’ as the driving force of a reaction; where, in a thermodynamic sense, a system is stable when no process can occur with a diminution in free energy.”
Newton notation (1718) Lewis notation (1923) Partington notation (1924) Modern notation (1933)
|The position of G1 is such that, in the words of Gilbert Lewis (1923), "no further process can occur with a diminution in free energy", and is thus representative of a state of maximal stability; whereas the position of G1 could decrease further in free energy, to the position of state one, and is thus not maximally stable.