Helmholtz free energy function The thermodynamic function Helmholtz free energy, F is defined as,

F = U - TS

where U is the internal energy, S the entropy and T the thermodynamic temperature. From the above eq.,

dF = dU - SdT - TdS

But dU = dQ - pdV

and for reversible change,

dQ = TdS

\ dU = TdS - pdV

Þ dF = -pdV - SdT

If dV = 0 and dT = 0

dF = 0

that is, for reversible change taking place at constant volume and temperature the Helmholtz free energy is constant.