Which term refers to energy that is available to do work?

• A. Kinetic energy
• B. Free energy
• C. Potential energy
• D. Thermal energy

Answer: (B)

Energy that can do work is called free energy. It represents the portion of a system’s energy that can be transformed into work under the conditions of interest (often at constant temperature and pressure in biology).

Kinetic energy is energy of motion, and potential energy is stored energy due to position; both are forms of energy, but they don’t inherently specify how much can be used to perform work in a given environment. Thermal energy is the total microscopic energy related to temperature, which isn’t all available to do work because some energy is tied up in disorder.

Free energy accounts for that distinction by subtracting the energy unavailable due to entropy. The Gibbs free energy change, ΔG = ΔH − TΔS, tells us how much energy can actually be used to do work: negative ΔG means the process can release usable energy to drive work. In biological systems, this concept explains why reactions that release free energy can power cellular processes and why ATP hydrolysis is a common energy source.