Ionic compounds electrolytic behavior

TABLE 4.3 Summary of the Electrolytic Behavior of Common Soluble Ionic and Molecular Compounds... 

According to this equation the reversible potential of the calomel electrode depends only on the activity of the chloride ion in the electrolyte. An electrode whose reversible potential depends uniquely on the activity of anions present in the electrolyte is called electrode of the second kind, or electrode reversible to anions. This behavior implies the presence, at the electrode surface, of a poorly soluble ionic compound. In the case of the calomel electrode, the species in question is the salt Hg2Cl2 ("calomel"). 

The solution behavior of the [Ln(BuL)8](N03)3 complexes, however, is not in agreement with what one would expect for an ionic compound. In nitromethane the complexes are nonconducting, indicating covalent, rather than ionic, nitrate groups, while in DMF the compounds are 1 2 electrolytes. This indicates that the nitrato groups are bonded to the lanthanide ion in the same manner as in the [Ln(BuL)3(N03)3] series. Further evidence for the covalency of the nitrate ion in solution is obtained from the solution spectra of these complexes represented in Fig. 3. 

Curve 1 in Figure 7.14 is the type of behavior characteristic of most un-ionized organic compounds. Curve 2 is typical of inorganic electrolytes and highly hydrated organic compounds. The type of behavior indicated by curve 3 is shown by soluble amphipathic species, especially ionic ones. The break in curve 3 is typical of these compounds however, this degree of sharpness is observed only for highly purified compounds. If impurities are present, the... 

The halogen hydracids are all weaker than perchloric acid. Actually they are not ideal strong electrolytes, although they a pproach this behavior when water is used as the solvent. Certainly, these compounds differ distinctly from typical strong electrolytes such as potassium chloride and other neutral salts. The difference probably originates in the structure of the solid form. Neutral salts in the solid crystalline state possess a coordination lattice. Simple molecules do not exist in this type of lattice since the constituents of the salt are present. solely in the ionic form. Each ion is surrounded in a uniform manner by a definite number of other ions of opposite charge. Indeed it is no longer correct to speak of undissociated molecules in the solid state. 

The term ionic liquid is commonly used for the molten salts whose melting point is below 100°C (48). In particular, the salts that are liquid at room temperature are called room temperature ionic liquids. The earliest known ionic liquid (published in 1914 49, 50) was ethyl ammonium nitrate EtNHfNO, which has a melting point of 12°C. It was these initially developed ionic liquids (molten salts) that were used as electrolytes to study the electrochemical behavior of other compounds. Recently, ionic liquids with interesting properties have been synthesized and used as solvents, and studied in different areas of chemistry (26-44). 

It is well known that graphite is unstable in some aprotic electrolytes. For instance, when propylene carbonate (PC) is used as a solvent, the cointercalation of solvent molecules and the Li ions will lead to the exfoliation of graphite layers Only in some selected electrolyte systems such as LiPF in EC/DEC (EC for ethylene carbonate and DEC for diethyl carbonate), can graphite show better cycling behavior. Solvent decomposition on the surface of conductive carbon or lithium electrodes will lead to the formation of a passivating layer. Peled named this layer as solid electrolyte interphase (SEI). ° It is an ionic conductor but electron insulator, mainly composed of LijCOj and various lithium alkylcarbonates (ROCO Li) as well as small amounts of LiE, LijO, and nonconductive polymers. These compounds have been detected on carbon and Li electrodes in various electrolyte systems. Therefore, it would be an interesting question whether semiconductive nano-SnO anode is also sensitive to electrolyte and electrolyte decomposition takes place on it. This section will characterize the structures and compositions of the... 

An important point to note in solid state electrochemistiy is that electrolyte and electrode behavior may coincide in compounds showing both ionic and electronic conduction, the so-called mixed ionic-electronic conductors (often abbreviated to mixed conductors). 

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