Undissociated molecules

Dimerization involves addition of the cyanamide anion to the nitnle group of an undissociated molecule to give the anion of cyanoguanidine, or dicyandiamide. This reaction takes place most readily at pH 8—10 where the reactants are present in favorable proportion. The product is a weaker acid than cyanamide and is protonated at once with generation of a new cyanamide anion. 

The equilibrium constant at room temperature corresponds to pKi, = 4.74 and implies that a 1 molar aqueous solution of NH3 contains only 4.25 mmol 1 of NH4+ (or OH ). Such solutions do not contain the undissociated molecule NH4OH, though weakly bonded hydrates have been isolated at low temperature ... 

In connection with the adsorption of organic molecules at the surface of an electrode it is possible to distinguish two types (a) adsorption of undissociated molecules and (b) adsorption of intermediates formed by dissociation of the original molecule. The variation of coverage of the surface of a... 

When an ionic solution contains neutral molecules, their presence may be inferred from the osmotic and thermodynamic properties of the solution. In addition there are two important effects that disclose the presence of neutral molecules (1) in many cases the absorption spectrum for visible or ultraviolet light is different for a neutral molecule in solution and for the ions into which it dissociates (2) historically, it has been mainly the electrical conductivity of solutions that has been studied to elucidate the relation between weak and strong electrolytes. For each ionic solution the conductivity problem may be stated as follows in this solution is it true that at any moment every ion responds to the applied field as a free ion, or must we say that a certain fraction of the solute fails to respond to the field as free ions, either because it consists of neutral undissociated molecules, or for some other reason ... 

Because of dissociation and the resulting increase in the total number of particles in solution, the parameters of the colUgative properties assume higher values. These values are proportional to the total concentration, c, of particles (ions and undissociated molecules) in the solution, which for a binary electrolyte is given by [1 + a(X(, - l)]q. The isotonic coefficient i is the ratio of and the concentration c, that would be observed in the absence of dissociation ... 

The third point of Arrhenius s theory was amplihed in 1888 by Wilhelm Ostwald. He introduced the idea of an equilibrium between the ions and the undissociated molecules ... 

Using the expressions for the concentrations of ions and undissociated molecules, we can write this equation as well in the form... 

At the beginning of the twentieth century the idea was put forward that in solutions of strong electrolytes the degree of dissociation is not simply high but dissociation of the solute is complete (i.e., equilibrium between ions and undissociated molecules does not exist). This point is particularly evident for ionophors, which in the solid state do not possess individual molecules and for which it is unlikely that undissociated molecules should appear in a solution. 

Ion-pair formation lowers the concentrations of free ions in solution, and hence the conductivity of the solution. It must be pointed out that ion-pair formation is not equivalent to the formation of undissociated molecules or complexes from the ions. In contrast to such species, ions in an ion pair are linked only by electrostatic and not by chemical forces. During ion-pair formation a common solvation sheath is set up, but between the ions thin solvation interlayers are preserved. The ion pair will break up during strong collisions with other particles (i.e., not in all collisions). Therefore, ion pairs have a finite lifetime, which is longer than the mean time between individual collisions. 

Hence the pH does not change to an appreciable extent. When an alkali (even a strong alkali such as NaOH) is added, the OH ions would combine with the H+ ions of CH3COOH, dissociating more and more of the undissociated molecules ofCH3COOH, and H20 would be formed ... 

S Riegelman, WJ Crowell. The kinetics of rectal absorption. III. The absorption of undissociated molecules. J Am Pharm Assoc Sci Ed 47 127-133, 1958. 

Undissociated molecules penetrate skin better than do ions. 

Osmotic pressure is a colligative property and is dependent on the number of particles of solute(s) in a solution. The total number of particles of a solute in a solution is the sum of the undissociated molecules and the number of ions into which the molecule dissociates. The number of ions, in turn, depends on the degree of ionization. Thus, a chemical that is highly ionized contributes a greater number of particles to the solution than the same amount of a poorly ionized chemical. When a chemical is a nonelectrolyte such as sucrose or urea, the concentration of the solution depends only on the number of molecules present. The values of the osmotic pressure and other colligative properties are approximately the same for equal concentrations of different nonelectrolyte solutions. 

Since osmotic pressure depends upon the number of particles of solute(s) in solution, the osmotic pressure of an electrolyte is directly proportional to the degree (or extent) of dissociation. The dissociation factor, symbolized by the letter i, can be calculated by dividing the total number of particles (which include undissociated molecules and ions) in a solution by the number of particles before dissociation, i.e.,... 

The ion-ion electrostatic interaction contribution is kept as proposed by PITZER. BEUTIER estimates the ion - undissociated molecules interactions from BORN - DEBYE - MAC. AULAY electric work contribution, he correlates 8 and 8 parameters in PITZER S treatment with ionic standard entropies following BROMLEY S (9) approach and finally he fits a very limited (one or two) number of ternary parameters on ternary vapor-liquid equilibrium data. 

Case 3.2 When the interfacially reactive species are the undissociated molecules of the extractant adsorbed at the interface [i.e., the first rate-determining step of the two-step mechanism is the reaction between the metal ion and HB(ad)], the following equations will hold ... 

The nature of the titanium-containing active site has been investigated with different techniques, including theoretical calculations. The formation of a hydroperoxidic species or of a bidentate side-on titanium peroxo structure was suggested by many authors . Alternatively, some DFT calculations indicated an undissociated molecule of H2O2 weakly interacting with Ti centers or an active Ti-O-O-Si peroxo moiety as a reactive site . Recently, Lin and Frei reported the first direct detection, obtained using in situ FT-infrared spectroscopy, of a Ti-OOH moiety as active species in the oxidation of small olefins like ethylene or propylene . 

Acids react with HgO to produce corresponding Hg(II) compounds. Two classes of Hg(II) compounds maybe defined covalent and ionic. The covalent compounds HgCl2, HgBr2, HgD, and Hg(CN)2 go into HOH solution chiefly as undissociated molecules, which undergo little hydrolysis. The ionic compounds which include HgF2, Hg(N03)2, HgS04, and Hg(C104)2 go into... 

Electrolytes contain ions in more or less solvated (hydrated) forms and solvent molecules however, undissociated molecules or ion associations, and so on may also be present. The composition of... 

The first step always occurs since, attractive forces between the undissociated molecule and the surface usually exist. This step may involve adsorption into a so-called precursor state where the molecule is mobile and diffuses across the... 

If a compound, on fusion, forms a conducting liquid phase, the process is not one of simple dissociation because the ions are not completely free, but are either in contact with ions of opposite charge or combined with undissociated molecules. It is to be expected, however, that the number of free ions formed at a given temperature will be greater, the smaller the energy required for the dissociation process, and therefore that fused compounds will be better conductors, the smaller the charge and the greater the radius of the ions. 

A further complicating effect in the problem of solubility is that not all compounds decompose into ions on solution. A typical example of such a compound is HgCl2, and obviously the energy is lower in a solution containing undissociated molecules. The energy change in the solution of a compound to undissociated molecules is... 

When we look at the series NaCl -> AgCl HgCl2 -> CC14, then we see that by increasing polarization or transition to a more covalent type of bond, the solubility is first decreased, followed by the formation of solutions containing undissociated molecules, and finally the compound becomes completely insoluble. 

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