The capacity of a system to do work, or use force to move an object, is its free energy. This is often expressed as Gibbs free energy, named after the American scientist Josiah Willard Gibbs, known for his work in thermodynamics in the late 1800s. Gibbs free energy (G) is the amount of work that can be done by a system, and it is expressed as , where H is enthalpy in kilojoules (which is the total internal energy of the system plus the product of the pressure and volume), T is temperature, and S is entropy.
For any chemical reaction, the change in Gibbs free energy () can be calculated. Assuming that temperature is standard (25°C, which is 298.15 K), the change in Gibbs free energy is , with T = 298.15 K. Importantly, if is less than 0, then the reaction is spontaneous; it does not need energy input to proceed. If is greater than 0, then the reaction is nonspontaneous and requires energy input to proceed.