Systems have energy {free energy}| available to do work. Free energy is energy from order loss plus potential energy converted to kinetic energy.
purpose
Free energy can show if process is spontaneous.
heat energy
Temperature times entropy is heat energy taken from surroundings.
work
Pressure times volume is work on system.
Helmholtz free energy
For constant temperature, free energy {Helmholtz free energy} is system energy minus heat energy: E - S*T.
Gibbs free energy
For constant pressure and temperature and changed volume, free energy {Gibbs free energy} is Helmholtz free energy plus work energy: E - S*T + P*V. Gibbs free energy G is enthalpy H minus temperature T times entropy S: G = H - T*S. Gibbs free energy is net work that system can do.
Arrhenius free energy
For changed temperature, free energy {Arrhenius free energy} is net work that system can do.
chemical potential
Gibbs free energy per mole u, the chemical potential, changes with absolute temperature T and mole fraction x: u = u0 + R * T * ln(x), where R is gas constant. Gibbs free energy per mole u changes with absolute temperature T and partial pressure P: u = u0 + R * T * ln(P).
free energy change
If system is not in equilibrium, something flows from higher to lower chemical potential. Free-energy change is negative. System changes spontaneously. However, spontaneous change does not happen if no pathway exists for energy change. To minimize free energy, system can lower potential energy, by reducing pressure, or increase entropy, by increasing temperature.
Isolated systems can have no work from outside. No energy transfers in or out of closed systems. Only entropy changes affect free energy.
Isothermal systems have only work and have no entropy change, because temperature is constant.
If temperature is low, entropy is small, so reaction makes heat to lower potential energy. If temperature is high, entropy is more important, and reaction heat can be small or large. At low pressure, more gas can evolve.
free energy change: equilibrium constant
In chemical reactions, free-energy change depends on equilibrium constant. Free-energy change equals gas constant times absolute temperature times natural logarithm of equilibrium constant.
free energy change: substances
For reactants, substance chemical potential times substance moles subtracts from reactant free energy. For products, substance chemical potential times substance moles adds to product free energy. Free-energy change in systems with one substance equals chemical potential a times change in number n of moles: a * n.
Chemical-reaction product and reactant concentrations depend on free-energy changes. Free-energy change equals -R * T * ln((ap1^np1 * ap2^np2 * ... ) / (ar1^nr1 * ar2^nr2 * ... )), where R is gas constant, T is temperature, api is product chemical potential, npi is chemical-equation product number of moles, ari is reactant chemical potential, and nri is chemical-equation reactant number of moles. Chemical reactions, and all physical changes, are spontaneous if they release free energy.
reaction
To reverse reactions, second reaction, with more free energy change, must couple to reaction. Total free energy change then favors reverse reaction. Diffusion, evaporation, and solvation take energy from surroundings, or use their thermal energy, to drive other reactions.
Physical Sciences>Physics>Heat>Energy
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Date Modified: 2022.0224