Helmholtz free energy (Arbeitfunktion)

(A) A thermodynamic property that can be used to predict whether a process will occur spontaneously at constant volume and temperature. Helmholtz free energy A is defined as A = U - TS where U, T and S are the internal energy , temperature , and entropy . Changes in A correspond to changes in free energy for processes occuring at constant temperature and volume. The sign of Delta A is negative for spontaneous processes and zero for processes at equilibrium .