Spontaneous electrolytic dissociation

One of the difficulties in understanding the true mechanism of the process of electrolysis at the time that Faraday enunciated his laws of electrolysis was the absence of the idea of spontaneous electrolytic dissociation, postulated much later by Clausius and by Arrhenius. In fact, Faraday believed that the electric force at (between) electrodes split up molecules in the electrolyte, giving rise to conductivity. This idea was connected with Freiherr von Grotthus s theory of a series of dissociations and recombinations of charged species in the conductance of aqueous solutions. 

The theory of electrolytic dissociation was not immediately recognized universally, despite the fact that it could qualitatively and quantitatively explain certain fundamental properties of electrolyte solutions. For many scientists the reasons for spontaneous dissociation of stable compounds were obscure. Thus, an energy of about 770kJ/mol is required to break up the bonds in the lattice of NaCl, and about 430kJ/mol is required to split H l bonds during the formation of hydrochloric acid solution. Yet the energy of thermal motions in these compounds is not above lOkJ/mol. It was the weak point of Arrhenius s theory that this mismatch could not be explained. 

The Electrolytic Dissociation Theory. —From his studies of the conductances of aqueous solutions of acids and their chemical activity, Arrhenius (1883) concluded that an electrolytic solution contained two kinds of solute molecules these were supposed to be active molecules, responsible for electrical conduction and chemical action, and inactive molecules, respectively. It was believed that when an acid, base or salt was dissolved in water a considerable portion, consisting of the so-called active molecules, was spontaneously split up, or dissociated, into positive and negative ions it was suggested that these ions are free to move independently and are directed towards the appropriate electrodes under the influence of an electric field. The proportion of active, or dissociated, molecules to the total number of molecules, later called the degree of dissociation, was considered to vary with the concentration of the electrolyte, and to be equal to unity in dilute solutions. 

Horiuti J, Polanyi M (1935) The basis of a theory of proton transfer. Electrolytic dissociation prototropy spontaneous ionization electrolytic evolution of hydrogen hydrogen-i[Pg.423]

In electrolyte solutions the molecules dissociate into ions spontaneously, so that the solution becomes conductive. Different electrolytes exhibit different degrees of dissociation, a, which will influence the actual values of molar conductivity A the two parameters are interrelated as... 

The concept that charged particles are responsible for the transport of electric charges through electrolytic solutions was accepted early in Ihe hisioiy of electrochemistry. The existence of ions was first postulated by Michael Faraday in 1834 be called negative ions anions and positive ones cations." In 1853. Hiltorf showed that ions move with different velocities and exist as separate entities and not momentarily as believed by Faraday. In 1887. Svante Arrhenius postulated that solute molecules dissociated spontaneously into five ions having no influence on each other. However, it is known that ions are subject to coulombic forces, and only at infinite dilution do ions behave ideally, i.e.. independently of other ions... 

The accumulated evidence for the electrolytic nature of dilute metal solutions is overwhelming—metal atoms introduced into a variety of nonaqueous solvents spontaneously dissociate into localized excess electrons and positive ions (37, 39, 164). 

At the pristine water-water vapour Interface spontaneous polarization of the water molecules takes place, leading to the fpotentIal. Prlstlnlty implies that there are no other ions or dissolved molecules apart from minute amounts of H and OH Ions, created by spontaneous dissociation of water molecules and which may give rise to a weak superimposed Ionic double layer. There Is no operational procedure to establish this f-potentlal but present-day consensus has It that the alr-slde Is negative, see sec. 3.9. At Issue Is now the formation of ionic double layers in addition to this when the solution contains simple electrolytes. The more dramatic changes caused by adsorbed or spread surfactants will not be addressed here. 

In a DMFC, the oxidation of methanol occurs in an acidic medium to avoid the carbonation of the electrolyte. Unfortunately, under such conditions, only platinum is stable and is able to dissociate the methanol molecule at low potentials. However, this dissociation leads spontaneously to the formation of an adsorbed layer of poisoning species (identified as adsorbed carbon monoxide). This poison can be removed from the electrode surface only at higher potentials when it is oxidized into carbon dioxide. 

In addition to chemical cogeneration studies, where the anodic and cathodic reactions are driven spontaneously by the cell generated voltage, several other electrocatalytic reactions have been investigated in solid electrolyte cells via external voltage application. These include the dissociation of H2O and NO, the hydrogeneration of CO, and the partial oxidation of CH4 as reviewed elsewhere. ... 

S. Arrhenius developed, in his doctoral thesis published in 1887, a theory pretty much similar to the modem concepts of 1883. He was the first to point out that eonductivity and a departure of coUigative properties from normal/expected values always occur togedier, concluding, this way, from his observations, that the two effects should have the same origin. Sueh observations could be summarized in three main points 1) in electrolyte solutions the molecules dissociate into ions spontaneously, turning the solution conductive different electrolytes exhibit different degrees of dissociation, called a, which will influence actual... 

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