In heat exchange, heat lost by object equals heat gained by other object {conservation of heat energy} {heat energy conservation}| {law of heat exchange} {heat exchange law}.
Energy exchange can change potential energy, translational kinetic energy, and heat energy and change pressure and volume {enthalpy}|. Enthalpy equals total system energy E plus product of pressure P times volume V: H = E + P*V. Pressure times volume is work. Under constant pressure or volume, enthalpy is heat that system makes. For solids or liquids, enthalpy equals energy, because volume does not change.
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.
Heat can exert force in direction and so do work {work, heat}|. Possible work energy is difference in heat energy between hotter region and colder region, which is available heat energy. Machines have ratio {efficiency, work} between work actually done and heat available or input work. Efficiency is high temperature Th minus low temperature Tc divided by high temperature: (Th - Tc) / Th. Engines have efficiency of 30%.
Ideal engines have four stages {Carnot cycle}|: isothermal heat gain, adiabatic gas expansion, isothermal heat loss, and adiabatic gas contraction.
Temperature increase causes material to increase random translational kinetic energy and so absorb heat {heat capacity}|. Material can absorb heat and gain random translational kinetic energy, so temperature rises. Heat capacity is heat needed to raise one gram of material one degree Celsius. Heat H equals mass m times heat capacity c times temperature change T: H = m * c * T.
factors
Heat capacity depends on material type. Chemicals can hold more or less heat depending on possible electric dipole states. Metal atoms have no vibrations and rotations. Metals have low heat capacity, because all heat goes into random translational motion, rather than into vibrations or rotations. Diatomic molecules are linear molecules. Diatomic molecules have medium heat capacity, because they have few vibrations and rotations. Water is triatomic, is asymmetric, and has hydrogen bonds between molecules. Water has high heat capacity. Large complex molecules in gasoline, clays, and ceramics have high heat capacity. Crystal structure can have chemical bonds, hydrogen bonds, van der Waals forces, or ionic bonds, allowing many vibration modes and high heat capacity.
material heat capacity divided by water heat capacity {specific heat}|.
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Date Modified: 2022.0225