Helmholtz Energy

HELMHOLTZ FREE ENERGY (F)

The Helmholtz free energy is a thermodynamic potential which measures the “useful” work obtainable from a chemical reaction in a closed thermodynamic system at a constant temperature. For such a system, the difference in the Helmholtz energy between final state and initial state is equal to the maximum amount of work extractable from this thermodynamic process, including expansion work if volume varies.

When a reacting system changes from a well-defined initial state to a well-defined final state, at constant temperature, the Helmholtz energy variation ΔF equals all the maximum work that can be exchanged by the system with its surroundings, including the work of the pressure forces. This latter is equal to the product of pressure by volume variation (P * ΔV).

The Helmholtz free energy was developed by Hermann von Helmholtz and is denoted by the letter A (from the German “Arbeit” or work), or the
letter F .

While Gibbs free energy is most commonly used as a measure of useful work obtainable from a system under constant pressure (isobaric conditions), especially in the field of chemistry, the isobaric
restriction on that quantity is sometimes inconvenient. For example, in explosives research, Helmholtz free energy is often used since
explosive reactions by their nature induce pressure changes.