Helmholtz Free Energy
The thermodynamic potential that determines spontaneity at constant temperature and volume.
Formula / equation
A = U − TS
Unit
kJ/mol
In depth
Helmholtz energy is the constant-volume counterpart of Gibbs energy and gives the maximum total work — expansion included — obtainable from a process. It is the natural function for bomb calorimetry and for statistical mechanics, though Gibbs energy dominates practical chemistry.
Examples in the real world
Analysing a reaction in a sealed rigid vessel, where no volume change is possible, uses ΔA rather than ΔG.