Proton levels in aqueous solution

The formation of a hydrated proton in acidic aqueous solution from a standard gaseous proton is written as follows  

+ H sto)- H3OI, hsdration energy of acidic protons efn THjO), (3-32) 

In addition to the acidic proton level, there is the basic proton level in basic aqueous solution which is represented by the unitary vacant proton level (the 

In the same way as the acidic proton level, the unitary proton acceptor level H (OH-,A) of OH ion is equal to the unitary proton donor level cih.chjo.d) of H2O molecule  

The combination of the acidic proton hydration 3-32 and the basic proton hydration 3-34 leads to the ionic dissociation of water molecule as shown in Eqn. 3-36  

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Furthermore, since in Sec. 121 we found the value J = 0.36 electron-volt for the proton transfer (211), this gives the occupied proton level of the (HCOOII2)+ ion a position at (0.52 — 0.36) = 0.16 electron-volt above that of the (H30)+ ion in formic acid as solvent. This is shown in Fig. 65, where, for comparison, a diagram for proton levels in aqueous solution has been included, the level of the (H30)+ ion in aqueous solution being drawn opposite to the level of the same ion in formic acid solution. This choice is quite arbitrary, but was made in order to show more clearly that we may expect that one or more acids that are strong... 

Fig. 3-16. Acidic and basic proton levels in aqueous solution h (Hso /H20) = unitary energy of hydration of a standard gaseous proton to occupy the xmitary vacant acidic proton level 1h (H2cvoh-) = unitary energy of hydration of a standard gaseous proton to occupy the unitary vacant basic proton level Dh o = ionic dissociation energy of HjO.

Fig. 3-20. Comparison of the proton level diagram of aqueous solutions with the electron level diagram of semiconductors (a) proton levels in pure water, (b) electron levels in intrinsic semiconductors, (c) proton levels in week add solutions, (d) electron levels in n type semiconductors. = proton level in aqueous solutions = unitary acidic proton level of HaO /HjO = unitary basic proton level of HjO/OH" nij. , = unitary...

FIGURE 22.3 Energy levels of protons and proton vacancies in aqueous solution showing the ionic dissociation of water molecules aH+ = occupied proton level (donor), o[i = vacant proton level (acceptor), and a0 = the standard level. 

A Chart of Occupied and Vacant Proton Levels. With two exceptions, each of the values of J given in Tables 9, 10, and 11 refers to the process where a proton is raised to the vacant proton level of an HsO molecule from a lower occupied proton level of a species of molecule or molecular ion in each case the value of J gives the amount by which this initially occupied level lies below the vacant level of H20. Obviously, using these values, it is at once possible to map out a chart of the proton levels of these various particles in aqueous solution, as has been done in Fig. 36. The two exceptions in Table 9 are the values derived from the KB of glycine and alanine. In these cases, as shown in (125), a proton is transferred to a vacant level from the ordinary occupied proton level in a water molecule the value of J gives the amount by which the vacant level lies above this occupied proton level of H20. 

In Fig. 37 two areas have been shaded. The area in the upper left corner, where protons in occupied levels are unstable, we have already discussed. In the lower right-hand corner the shaded area is one where vacant proton levels cannot remain vacant to any great extent. In aqueous solution any solute particle that has a vacant proton level lower than that of the hydroxyl ion will capture a proton from the solvent molecule, since the occupied level of the latter has the same energy as the vacant level of a hydroxyl ion. Consequently any proton level that would lie in this shaded area will be vacant only on the rare occasions when the thermal agitation has raised the proton to the vacant level of a hydroxyl ion. On the other hand, there are plenty of occupied proton levels that lie below the occupied level of the H2O molecule. For example, the occupied level of the NH3 molecule in aqueous solution lies a long way below that of H20. 

There are, of course, many substances, soluble in water, whose molecules contain one or more protons, but which, like the Nll.t molecule, show no spontaneous tendency to lose a proton when hydroxyl ions are present. In each of these molecules the energy level occupied by the proton must, as in NII3, lie below the occupied level of II20. If methanol is an example of this class, the vacant proton level of the moth date ion (CH3O)- in aqueous solution must lie below the vacant level of (OH)-. 

In Fig. 38 it will be seen that for the (H2PO4)- ion there are two entries, one for its occupied proton level and one for its vacant proton level. In the aqueous solution of Nal PCh under consideration the thermal agita-... 

The Dissociation Constant of Nitric Acid. The largest value of K in Table 9 is that for the (HS04) ion. In Fig. 36 there is a gap of more than 0.2 electron-volt below the level of the (H30)1 ion. As is well known, several acids exist which in aqueous solution fall iu the intermediate region between the very weak acids and the recognized strong acids the proton levels of these acids will fall in this gap. The values of K for these acids obtained by different methods seldom show close agreement. Results obtained by various methods were compared in 1946 by Redlich,1 who discussed the difficulties encountered. 

For comparison, consider now the same ions in methanol solution. Each ionic field will contain more electrostatic energy than the corresponding ionic field in aqueous solution. Suppose that again we raise a proton from the occupied level of a (NIIi) ion to the vacant level of a (CH3COO)- ion. In this process the amount of electrostatic energy released will be greater than in water. If then the value of, / is roughly the same as before, the total amount of work required to transfer the proton will be smaller than in water. Hence, in the chart of the proton levels in methanol, we expect that the interval between these two proton levels will be narrower than in Fig. 36. 

The Sulfate Ion. In Fig. 36 we see that the vacant level of the (SO ) ion in aqueous solution lies only 0.13 electron-volt above the occupied level of HCOOH. If the interval has a comparable value when sulfate ions are present in formic acid as solvent, the thermal agitation should transfer a large number of protons from solvent HCOOH molecules to the (SO4)" ions. This was found to be the case when Na2SC>4 was dissolved in pure formic acid. Such a transfer of protons from molecules of a solvent to the anions of a salt is analogous to the hydrolysis of the salt in aqueous solution and is known as solvolysis, as mentioned in Sec. 76. In a 0.101-molal solution of Na2SC>4 in formic acid the degree of the solvolysis was found to be 35 per cent.1... 

If the occupied proton level of the CH3COOH molecule dissolved in liquid ammonia lies above the vacant level of NH3, as it does in aqueous solution, acetic acid should be a strong acid in liquid ammonia. This is found to be the case the carboxylic acids are strong acids in this solvent, the protons being transferred to NH3 to form (NH4)+. 

The indicators numbered 1 and 2 at the bottom of Table 39 both have vacant proton levels low enough for use in dilute solution the circles in Fig. 67 give the experimental results obtained in aqueous solutions of HC1. In each case the slope of the line does not differ from the theoretical slope of (218) by as much as 5 per cent. Reading off the constant vertical distance between the two curves (the length of the vertical arrow in Fig. 67), we find... 

In electrochemistry, the electron level of the normal hydrogen electrode is important, because it is used as the reference zero level of the electrode potential in aqueous solutions. The reaction of normal hydrogen electrode in the standard state (temperature 25°C, hydrogen pressure 1 atm, and unit activity of hydrated protons) is written in Eqn. 2-54 ... 

In aqueous solutions containing impurity solutes, the proton level approaches either the unitary acidic proton level > mog-) with increasing concentration... 

We consider dehydration-adsorption of hydrated protons (cathodic proton transfer) and desorption-hydration of adsorbed protons (anodic proton transfer) on the interface of semiconductor electrodes. Since these adsorption and desorption of protons are ion transfer processes across the compact layer at the interface of semiconductor electrodes, the adsorption-desorption equilibrium is expressed as a function of the potential of the compact layer in the same way as Eqns. 9-60 and 9-61. In contrast to metal electrodes where changes with the electrode potential, semiconductor electrodes in the state of band edge level pinning maintain the potential d(hi of the compact layer constant and independent of the electrode potential. The concentration of adsorbed protons, Ch , is then determined not by the electrode potential but by the concentration of h3 ated protons in aqueous solutions. 

In general, semiconductor electrodes adsorb in aqueous solutions water molecules, hydronium ions, and hydroxide ions in addition to various solute ions. As a result, the dissociation-association equilibria of the adsorbed hydronium ions and water molecules produce, in the proton dissociation-association reactions of Eqns. 9-69 and 9-70, the acidic and basic proton levels, respectively, on the electrode interface as shown in Fig. 9-21 ... 

In aquatic chemistry, the unitary proton level of the proton dissociation reaction is expressed by the logarithm of the reciprocal of the proton dissociation constant i.e. p = - log K here, a higher level of proton dissociation corresponds with a lower pK. When the pKy of the adsorbed protons is lower than the pH of the solution, the protons in the adsorbed hydronium ions desorb, leave acidic vacant proton levels in adsorbed water molecules, and form hydrated protons in the aqueous solution. Fig. 9-22 shows the occupied and vacant proton levels for the acidic and basic dissociations of adsorbed hydronium ions and of adsorbed water molecules on the interface of semiconductor electrodes. 

For metal oxide electrodes, the iso-electric point pH p is also located midway between the unitary acidic proton level and the unitary basic proton level of adsorbed water. Table 9-1 shows the iso-electric point pHi, of several metal oxides in aqueous solutions. 

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