How is entropy defined in thermodynamics?

Entropy is defined as a measure of disorder or randomness in a system. This concept is fundamental in thermodynamics as it reflects the degree of chaos and the number of possible microstates that a system can occupy. In general, systems tend to evolve towards states with higher entropy, indicating that spontaneous processes are associated with an increase in disorder.

For example, when ice melts into water, the structured arrangement of molecules in the solid state transitions to a more random arrangement in the liquid state, resulting in an increase in entropy. Additionally, the second law of thermodynamics states that in an isolated system, the total entropy can never decrease, reinforcing the idea that processes naturally progress towards greater disorder.

The other options do not accurately capture the concept of entropy: energy stored in a system refers to potential energy, which is not the same as entropy. Potential energy itself does not address randomness or disorder. The speed at which reactions occur pertains to reaction kinetics and not directly to the concept of entropy. Thus, the correct characterization of entropy is its relationship to disorder and randomness.