ACS Physical Chemistry: Thermochemistry Practice Test

In thermodynamics, spontaneity refers to the natural direction of a process without the need for external intervention. This concept is often evaluated through changes in free energies and other thermodynamic potentials.

The correct criteria for spontaneity involve the change in the Helmholtz free energy (A) and Gibbs free energy (G). The Helmholtz free energy change at constant temperature and volume must be less than or equal to zero for a process to be spontaneous. A negative change in A indicates that the system can do work on the surroundings, reinforcing the notion of spontaneous processes.

Moreover, for processes occurring at constant temperature and pressure, the Gibbs free energy change must also be less than or equal to zero. This criterion is essential as it encapsulates both the enthalpic (heat exchange) and entropic (disorder) contributions to spontaneity.

In summary, both conditions outlined in the correct choice reflect the fundamental principles of the second law of thermodynamics, which states that for a process to be spontaneous, available energies such as A and G must decrease or remain constant. This underpins the thermodynamic framework for predicting the direction of chemical reactions and physical processes.