5.2.1/5.2.2/5.2.3 Enthalpy and entropy, redox and electrode potentials GapFill

The energy released during the formation of one mole of an ionic solid from its constituent ions in the gas phase is called the   crystalmatrixlatticeconstitutional enthalpy. This can be difficult to measure experimentally, so it is often calculated with a   Michaelis−MentenBrønsted−LowryCurie−WeissBorn−Haber cycle, which contains enthalpy values that are easier to study. These include the energy released when an electron is added to an atom in the gas phase, called the electron   activityaffinityassociationaffiliation, and the energy required to separate a compound in its standard state into its constituent atoms in the gas phase, called the enthalpy of  sublimationatomisationfissionvaporisation.

As well as enthalpy change, the feasibility of a reaction also depends on the entropy change; entropy is a measure of the dispersal of energy in a system, represented by the symbol   SPHE. Together, the enthalpy and entropy changes, along with temperature, determine the   GibbsGaussGalvaniGould free energy change, ΔG, of a process. Only processes for which ΔG  ≥<=≠ 0 are feasible.

Oxidation and reduction reaction processes are often written in the form of  half-part-sub-hemi-equations. These equations, and the corresponding electrodes, can be assigned an E^⦵ value, which is known as the standard electrode potential. It is measured by connecting the electrode to a standard hydrogen electrode (SHE), which is always assigned a value of 0 V, to form an electrochemical cell. The two beakers are joined by  a saltan osmotica galvanican electrolytic bridge, and the instrument that gives the reading is called   a voltmeteran ohmmetera wattmeteran ammeter. All solutions must have a concentration of 1 mol dm⁻³ (except solutions of two ions of the same element, where the concentrations need only be the same). The temperature must be  373273398298 K and any gases must be at  100 kPa100 Pa1 kPa1 Pa. In a standard hydrogen electrode, or any other electrode where none of the reactants or products are solids, the redox reaction takes place on a  silvergoldplatinumtitanium surface. The difference between the E^⦵ values of two electrodes is called the standard cell potential, or simply E^⦵_cell. If E^⦵_cell is positive for a pair of electrodes, the redox reaction involving them is feasible. 

However, even if ΔG or E^⦵_cell suggests that a reaction is feasible, it may not take place under standard conditions because of a high  activationinitiationstimulationcommencement energy.