Gibbs free energy is defined as

• A. The energy of a system available to do work at a constant temperature and pressure
• B. The energy of a system available to do work at changing temperature and pressure
• C. The energy stored in chemical bonds only
• D. The heat content of the system

Answer: (A)

Gibbs free energy tells us how much of a system’s energy can be turned into useful work when temperature and pressure are kept constant. It’s defined by G = H − TS, which combines enthalpy (overall heat content) and the entropy term scaled by temperature. Under constant T and P, a process that lowers G can proceed spontaneously, because there’s available free energy being released to do work; if ΔG < 0 it’s favorable, and if ΔG = 0 the system is at equilibrium.

This idea matches the option describing energy that can be used to do work at constant temperature and pressure. The other statements aren’t correct: Gibbs free energy is defined with fixed T and P, not changing conditions; it isn’t just the energy stored in chemical bonds (that’s a narrower view), and it isn’t simply the heat content (that relates to enthalpy, not free energy).