Final answer:
Entropy is a measure of the energy that is not available for work due to the disorder of a system, aligning with the Second Law of Thermodynamics that indicates entropy never decreases. This inherent limitation is why no engine can be perfectly efficient, as some energy always remains unusable for doing work.
Step-by-step explanation:
Entropy is often mentioned in the context of thermodynamics as a measure of the unavailability of energy to do work. Specifically, entropy quantifies the amount of energy in a system that cannot be used to perform work due to the disorder within the system. The Second Law of Thermodynamics indicates that entropy in an isolated system can never decrease over time, meaning that not all energy can be converted into work; some energy will always be dispersed or 'lost' as heat. This is why no engine can be 100% efficient. Entropy is a fundamental concept in understanding why there is always some energy that remains unusable for doing work, regardless of the energy conversion processes applied.
The total entropy of a system can remain constant in reversible processes or increase in irreversible processes, but it never decreases. This is illustrated by phenomena such as heat transfer, which cannot spontaneously occur from a colder to a hotter body, as this would result in a decrease of entropy, violating the Second Law of Thermodynamics. Therefore, a change in entropy is associated closely with thermodynamic processes and the effectiveness of energy conversion.