Helmholtz energy, also known as Helmholtz free energy, is a thermodynamic potential that represents the amount of work that a thermodynamic system can perform at a constant temperature and volume. It is denoted by the symbol A and is defined as the difference between the internal energy of the system and the product of its absolute temperature and entropy.
Helmholtz energy is closely related to the first law of thermodynamics, which states that energy cannot be created or destroyed, only transferred or converted. The first law of thermodynamics is represented by the equation ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. Helmholtz energy is related to this equation through the equation A = U - TS, where A is the Helmholtz energy, U is the internal energy, T is the absolute temperature, and S is the entropy of the system.
Helmholtz energy and Gibbs free energy are both thermodynamic potentials that represent the maximum amount of work that a system can perform. However, Helmholtz energy is defined at constant temperature and volume, while Gibbs free energy is defined at constant temperature and pressure. This means that Helmholtz energy is more useful for systems that undergo chemical reactions in a closed container, while Gibbs free energy is more useful for systems that undergo chemical reactions in an open container.
According to the second law of thermodynamics, for a process to occur spontaneously, the total entropy of the universe must increase. Therefore, if the Helmholtz energy of a system decreases during a chemical reaction, the entropy of the system must increase, and the reaction will occur spontaneously. This change in Helmholtz energy is represented by the equation ΔA = ΔU - TΔS, where ΔA is the change in Helmholtz energy, ΔU is the change in internal energy, T is the temperature, and ΔS is the change in entropy.
Helmholtz energy is commonly used in thermodynamics to calculate the maximum amount of work that can be obtained from a system at constant temperature and volume. It is also used in chemical thermodynamics to predict the spontaneity of chemical reactions and to calculate equilibrium conditions. In engineering, Helmholtz energy is used to optimize processes and systems for maximum efficiency.