Which principle of thermodynamics deals with the direction of spontaneous processes?

The second law of thermodynamics addresses the direction of spontaneous processes by establishing that these processes tend to increase the overall entropy of a system. Entropy is a measure of disorder or randomness, and the second law posits that natural processes will evolve toward a state of greater disorder or higher entropy.

This principle has significant implications in various natural phenomena, such as heat transfer and the efficiency of engines. For example, in a closed system, heat will always flow from warmer areas to cooler ones without external work being performed, reflecting the second law's insight into the nature of energy transfer.

By contrast, the first law of thermodynamics focuses on the conservation of energy, indicating that energy cannot be created or destroyed, only transformed. The third law concerns the behavior of systems as temperature approaches absolute zero and establishes a reference point for measuring entropy. The Zeroth law lays the groundwork for temperature measurement but does not pertain to spontaneous processes. Thus, the second law of thermodynamics is the one that directly addresses how spontaneous processes occur and the direction they take.