Helmholtz Free Energy, symbolized by \(A\), is a thermodynamic potential that quantifies the energy within a closed system that is available to perform work. It measures the maximum amount of useful work that a system can deliver to its surroundings. Understanding this energy value allows scientists to determine if a physical or chemical process is energetically favorable.
Conditions for Using Helmholtz Free Energy
Helmholtz Free Energy requires the system to operate under constant temperature and constant volume. The fixed temperature means the system is isothermal, ensuring heat exchange keeps the temperature steady.
The constant volume condition requires the system to be contained within a rigid, sealed vessel, preventing any change in size. Since the volume cannot change, the system cannot perform mechanical work by expanding against external pressure. The change in Helmholtz Free Energy thus represents the absolute maximum work that can be extracted, excluding expansion work. This makes it ideal for modeling systems like gases in sealed containers or solid-state reactions.
Breaking Down the Components
The mathematical definition of Helmholtz Free Energy links three fundamental properties using the relationship \(A = U – TS\). \(U\) represents the system’s Internal Energy, which includes the kinetic energy of molecules and the potential energy stored in chemical bonds.
The term \(TS\) is subtracted from \(U\) because it represents the energy unavailable to perform useful work. This product combines the absolute temperature \(T\) with the system’s Entropy \(S\). The \(TS\) term is often called the bound energy because it is tied up in the system’s thermal motion and disorganization.
A system with high entropy has a large portion of its total energy dispersed, leaving less available for directed tasks. Subtracting this disorganized energy (\(TS\)) isolates the remaining “free” energy (\(A\)). This free energy can theoretically be converted into mechanical or electrical work. A spontaneous process occurs when the system’s Helmholtz Free Energy decreases.
The Difference Between Helmholtz and Gibbs
While Helmholtz Free Energy (\(A\)) is suited for systems at constant volume, Gibbs Free Energy (\(G\)) is used for systems at constant pressure. This difference in the fixed constraint is the fundamental distinction. Most chemical and biological reactions occur in open environments, such as a beaker, where pressure remains constant at atmospheric pressure.
Because the system is open, its volume can expand or contract. Gibbs Free Energy accounts for this volume change by including pressure-volume or expansion work. Helmholtz Free Energy excludes this expansion work because its volume is fixed. This makes Gibbs Free Energy the most commonly used measure in chemistry, relating to spontaneity under constant-pressure conditions.
Helmholtz is appropriate for systems where volume is strictly controlled, such as solid-state materials or reactions in high-pressure bombs. Gibbs is used when the system adjusts its volume to maintain equilibrium with external pressure.
Where Helmholtz Free Energy is Applied
Helmholtz Free Energy is used in fields where sealed or rigid systems are the norm. In materials science, it models the behavior of solids and fluids, helping researchers predict the phase transitions of substances under fixed-volume conditions. It is also used to define fundamental equations of state for pure substances, such as industrial refrigerants.
Helmholtz Free Energy is applied in high-energy density scenarios, such as explosives research. Explosive reactions occur in a confined space, meaning the system volume remains fixed during the crucial period of the reaction. This potential is also used in electrochemistry to calculate the maximum electrical work output of devices like batteries or fuel cells.