In thermodynamics, the Helmholtz free energy is a thermodynamic potential that measures the “useful” work obtainable from a closed thermodynamic system at a constant temperature and volume. The negative of the difference in the Helmholtz energy is equal to the maximum amount of work that the system can perform in a thermodynamic process in which volume is held constant. If the volume is not held constant, part of this work will be performed as boundary work. The Helmholtz energy is commonly used for systems held at constant volume. Since in this case no work is performed on the environment, the drop in the Helmholtz energy is equal to the maximum amount of useful work that can be extracted from the system. For a system at constant temperature and volume, the Helmholtz energy is minimized at equilibrium.
- Mathematical development
- Minimum free energy and maximum work principles
- Relation to the canonical partition function
- Relating Free Energy to other Variables
- Bogoliubov inequality
- Generalized Helmholtz energy
- Application to fundamental equations of state
The Helmholtz free energy was developed by Hermann von Helmholtz, a German physician and physicist, and is usually denoted by the letter A (from the German “Arbeit” or work), or the letter F . The IUPAC recommends the letter A as well as the use of name Helmholtz energy. In physics, the letter F can also be used to denote the Helmholtz energy, as Helmholtz energy is sometimes referred to as the Helmholtz function, Helmholtz free energy, or simply free energy (not to be confused with Gibbs free energy or free enthalpy).
While Gibbs free energy is most commonly used as a measure of thermodynamic potential, especially in the field of chemistry, it is inconvenient for some applications that do not occur at constant pressure. For example, in explosives research, Helmholtz free energy is often used since explosive reactions by their nature induce pressure changes. It is also frequently used to define fundamental equations of state of pure substances.
The Helmholtz energy is defined as:
The Helmholtz energy is the Legendre transform of the internal energy, U, in which temperature replaces entropy as the independent variable.
From the first law of thermodynamics in a closed system we have
Applying the product rule for differentiation to d(TS) = TdS + SdT, we have:
The definition of A = U - TS enables to rewrite this as
Because A is a thermodynamic function of state, this relation is also valid for a process (without electrical work or composition change) that is not reversible, as long as the system pressure and temperature are uniform.
Minimum free energy and maximum work principles
The laws of thermodynamics are most easily applicable to systems undergoing reversible processes or processes that begin and end in thermal equilibrium, although irreversible quasistatic processes or spontaneous processes in systems with uniform temperature and pressure (uPT processes) can also be analyzed based on the fundamental thermodynamic relation as shown further below. First, if we wish to describe phenomena like chemical reactions, it may be convenient to consider suitably chosen initial and final states in which the system is in (metastable) thermal equilibrium. If the system is kept at fixed volume and is in contact with a heat bath at some constant temperature, then we can reason as follows.
Since the thermodynamical variables of the system are well defined in the initial state and the final state, the internal energy increase,
The volume of the system is kept constant. This means that the volume of the heat bath does not change either and we can conclude that the heat bath does not perform any work. This implies that the amount of heat that flows into the heat bath is given by:
The heat bath remains in thermal equilibrium at temperature T no matter what the system does. Therefore, the entropy change of the heat bath is:
The total entropy change is thus given by:
Since the system is in thermal equilibrium with the heat bath in the initial and the final states, T is also the temperature of the system in these states. The fact that the system's temperature does not change allows us to express the numerator as the free energy change of the system:
Since the total change in entropy must always be larger or equal to zero, we obtain the inequality:
We see that the total amount of work that can be extracted in an isothermal process is limited by the free energy decrease, and that increasing the free energy in a reversible process requires work to be done on the system. If no work is extracted from the system then
and thus for a system kept at constant temperature and volume and not capable of performing electrical or other non-PV work, the total free energy during a spontaneous change can only decrease.
This result seems to contradict the equation dA = -S dT - P dV, as keeping T and V constant seems to imply dA = 0 and hence A = constant. In reality there is no contradiction: In a simple one-component system, to which the validity of the equation dA = -S dT - P dV is restricted, no process can occur at constant T and V since there is a unique P(T,V) relation, and thus T, V, and P are all fixed. To allow for spontaneous processes at constant T and V, one needs to enlarge the thermodynamical state space of the system. In case of a chemical reaction, one must allow for changes in the numbers Nj of particles of each type j. The differential of the free energy then generalizes to:
In case there are other external parameters the above relation further generalizes to:
Relation to the canonical partition function
A system kept at constant volume, temperature, and particle number is described by the canonical ensemble. The probability to find the system in some energy eigenstate r is given by:
Z is called the partition function of the system. The fact that the system does not have a unique energy means that the various thermodynamical quantities must be defined as expectation values. In the thermodynamical limit of infinite system size, the relative fluctuations in these averages will go to zero.
The average internal energy of the system is the expectation value of the energy and can be expressed in terms of Z as follows:
If the system is in state r, then the generalized force corresponding to an external variable x is given by
The thermal average of this can be written as:
Suppose the system has one external variable
If we write
This means that the change in the internal energy is given by:
In the thermodynamic limit, the fundamental thermodynamic relation should hold:
This then implies that the entropy of the system is given by:
where c is some constant. The value of c can be determined by considering the limit T → 0. In this limit the entropy becomes
Relating Free Energy to other Variables
Combining the definition of Helmholtz free energy,
along with the fundamental thermodynamic relation,
one can find expressions for entropy, pressure and chemical potential:
These three equations, along with the free energy in terms of the partition function,
allow for an efficient way of calculating thermodynamic variables of interest given the partition function and are often used in density of state calculations. One can also do Legendre Transforms for different systems. For example, for a system with a magnetic field or potential, it is true that
Computing the free energy is an intractable problem for all but the simplest models in statistical physics. A powerful approximation method is mean field theory, which is a variational method based on the Bogoliubov inequality. This inequality can be formulated as follows:
Suppose we replace the real Hamiltonian
where both averages are taken with respect to the canonical distribution defined by the trial Hamiltonian
The Bogoliubov inequality is often formulated in a slightly different but equivalent way. If we write the Hamiltonian as:
Here we have defined
And thus the inequality
holds. The free energy
For a classical model we can prove the Bogoliubov inequality as follows. We denote the canonical probability distributions for the Hamiltonian and the trial Hamiltonian by
then holds. To see this, consider the difference between the left hand side and the right hand side. We can write this as:
it follows that:
where in the last step we have used that both probability distributions are normalized to 1.
We can write the inequality as:
where the averages are taken with respect to
Since the averages of
Here we have used that the partition functions are constants with respect to taking averages and that the free energy is proportional to minus the logarithm of the partition function.
We can easily generalize this proof to the case of quantum mechanical models. We denote the eigenstates of
We assume again that the averages of H and
still holds as both the
On the right-hand side we can use the inequality
where we have introduced the notation
for the expectation value of the operator Y in the state r. See here for a proof. Taking the logarithm of this inequality gives:
This allows us to write:
The fact that the averages of H and
Generalized Helmholtz energy
In the more general case, the mechanical term (
where we are now using Einstein notation for the tensors, in which repeated indices in a product are summed. We may integrate the expression for
Application to fundamental equations of state
The Helmholtz free energy function for a pure substance (together with its partial derivatives) can be used to determine all other thermodynamic properties for the substance. See, for example, the equations of state for water, as given by the IAPWS in their IAPWS-95 release.