Knowra Free energy Free energy A thermodynamic potential that measures energy available to perform useful work under specified constraints. Helmholtz and Gibbs free energy apply under different controlled conditions.
Gibbs free energy : A thermodynamic potential equal to enthalpy minus temperature times entropy, useful for processes at constant temperature and pressure. Its decrease predicts spontaneous change when temperature and pressure stay constant.
Internal energy : The total microscopic kinetic and potential energy contained within a thermodynamic system. It is the energy term retained in Helmholtz free energy before entropy is accounted for.
Chemical thermodynamics : The study of energy, heat, and equilibrium in chemical systems. Reaction free-energy changes predict equilibrium composition and the maximum non-expansion work.
First law of thermodynamics : The principle that energy is conserved, changing form or moving between system and surroundings. Energy conservation alone does not tell how much energy can perform useful work.
Helmholtz free energy : A thermodynamic potential equal to internal energy minus temperature times entropy, useful at constant temperature and volume. It gives the corresponding criterion when volume, rather than pressure, is held fixed.
Entropy : A thermodynamic state function that quantifies energy dispersal and the number of microscopic arrangements compatible with a state. The entropy term subtracts energy unavailable for useful work at a given temperature.
Chemical equilibrium : A state in which forward and reverse chemical reactions occur at equal rates, leaving composition macroscopically constant. At fixed temperature and pressure, equilibrium minimizes Gibbs free energy.
Second law of thermodynamics : The principle that the total entropy of an isolated system cannot decrease in a spontaneous process. The second law constrains spontaneous change; free energy packages that constraint for fixed conditions.
Thermodynamic equilibrium : A state in which macroscopic thermodynamic properties no longer change without external influence. At fixed constraints, equilibrium occurs at a minimum of the relevant free energy.
Temperature : A thermodynamic state variable that determines the direction of heat transfer between systems in contact. Temperature weights the entropy contribution in both common free-energy potentials.
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