Which statement best describes the relationship between unusable energy and entropy?

• A. Usable energy increases as entropy increases
• B. Unusable energy is entropy
• C. Entropy is unrelated to energy
• D. Unusable energy is free energy

Answer: (B)

Think about energy and disorder together. When a system converts energy, some of it can do work, but some becomes spread out as heat and random molecular motion. That spread reduces the amount of energy that’s usable to do work, which is what we describe by an increase in entropy. So as entropy grows, more energy becomes unusable. Saying unusable energy is entropy captures that relationship: the energy you can’t use for work rises with the system’s disorder. The other ideas don’t fit as well: usable energy does not increase with entropy; entropy is not unrelated to energy, and unusable energy is not the same as free energy (free energy is the portion that can do work).