What characterizes a spontaneous process in thermodynamics?

A spontaneous process in thermodynamics is characterized by its ability to occur naturally without the need for external intervention or energy input. This means that the process can proceed on its own due to the inherent properties of the system and its surroundings, driven by changes in enthalpy, entropy, and temperature.

The correct characterization highlights that spontaneous processes happen without external work being applied, meaning they are driven by internal forces rather than requiring energy input from an external source. For example, the melting of ice at room temperature is spontaneous because it proceeds without additional energy input; the system moves towards a state of increased entropy as the structured solid (ice) transitions to a more disordered state (liquid water).

The other options do not accurately capture the nature of spontaneous processes. For instance, a process that occurs with no net change in the system contradicts the very definition of spontaneity, which involves a transition from one state to another. Similarly, a process that requires energy input cannot be spontaneous, as spontaneity implies that the process can occur "spontaneously" without such input. Lastly, a process that reverses spontaneously does not directly define spontaneous processes since not all spontaneous processes are easily reversible. Rather, spontaneous processes are defined by their tendency to move toward equilibrium