Protonated hydrates

The largest protonated cluster of water molecules yet definitively characterized is the discrete unit lHi306l formed serendipitously when the cage compound [(CyHin)3(NH)2Cll Cl was crystallized from a 10% aqueous hydrochloric acid solution. The structure of the cage cation is shown in Fig. 14.14 and the unit cell contains 4 [C9H,8)3(NH)2aiCUHnOfiiai- The hydrated proton features a short. symmetrical O-H-0 bond at the centre of symmetry und 4 longer unsymmetrical O-H - 0 bonds to 4... 

Actually the hydrogen ion H+ (or proton) does not exist in the free state in aqueous solution each hydrogen ion combines with one molecule of water to form the hydroxonium ion, H30+. The hydroxonium ion is a hydrated proton. The above equations are therefore more accurately written ... 

The typical strong acid of the water system is the hydrated proton H30+, and the role of the conjugate base is minor if it is a sufficiently weak base, e.g. Cl-, Br-, and C104. The conjugate bases have strengths that vary inversely as the strengths of the respective acids. It can easily be shown that the basic ionisation constant of the conjugate base KR canj is equal to Kw/KA conj, where Kw is the ionic product of water. 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

Ojamae, L., Shavitt, I., Singer, S. J., 1995, Potential Energy Surfaces and Vibrational Spectra of H502+ and Larger Hydrated Proton Complexes , Int. J. Quant. Chem. Symp., 29, 657. 

Wei, D., Salahub, D. R., 1994, Hydrated Proton Clusters and Solvent Effects on the Proton Transfer Barrier ... 

Wei, D. and D. R. Salahub. 1994. Hydrated proton clusters and solvent effects on the proton transfer barrier A density functional study. J. Chem. Phys. 101, 7633. 

Applying this to our electrochemical system could qualitatively explain some of the observed effects. Assuming that there is only a weak interaction between metal and the perchloric acid hydrate, protons being part of the clathrate structure may not be in a favorable position for a charge transfer reaction at the interface. This could result in small pre-exponential factors. The electrode potential, however, may... 

The Arrhenius theory has limitations, however. For example, H (aq), a bare proton, does not exist in water. The positive charge on a proton is attracted to the region of negative charge on the lone pair of electrons on a water molecule s oxygen atom. The combination is a hydrated proton called a hydroniiun ion, HaO+faq)-... 

The state of unit activity of hydrated proton at the standard temperature 25X and pressure 1 atm. In elecfrodiemistry of aqueous solution, the scale of chemical potential for hydrated ions takes as the reference zero the standard chemical potential of hydrated protons at unit activity in addition the standard stable state energy of element atoms is set equal to zero. 

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 ... 

Fig. 2-43. Energy balance in the reaction of normal hydrogen electrode H2(sid.p>j = hydrogen molecule in the gaseous standard state (at 1 atm) H( gro. i) = hydrated proton of unit activity = real potential of the hydrated proton of unit activity a.ajHE) = real potential of the equilibrium electron of NHE (= Fermi level cpcnhe) of NHE).

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

The energy level of hydrated proton depends on the proton concentration. For an acidic proton in Eqn. 3-32 and a basic proton in Eqn. 3-34, the proton levels Hh- are, respectively, given in Eqns. 3-37 and 3-38 ... 

It follows, then, that the proton level in pure water is located midway between the unitary level of acidic proton and the unitary level of basic proton, leading to the hydrated proton concentration at pH 7. 

The chemical thermodynamic energy scale of ions described in this section is not the same as the conventional energy scale of hydrated ions in aquatic electrochemistry (Refer to Sec. 6.4.) the conventional scale is referred to the ion level of hydrated proton of unit activity. 

As described in Sec. 2.11, the electron level in the normal hydrogen electrode (gaseous hydrogen molecules at unit fiigacity and hydrated protons at unit activity) is -4.5 eV (or - 4.44 eV in the lUPAC report [Trasatti, 1986]). We, then, obtain the equilibrium potential of the normal hydrogen electrode nhe (= in Eqn. 4-32) as shown in Eqn. 4-34 ... 

The ion that determines the p>otential of the compact layer is called the potential-determining ion. In cases in which the potentied of compact layer is determined by the dissociation reaction of adsorbed hydroxyl groups, the potential -determining ions are hydrated protons or hydroxide ions. For cadmium sulfide electrodes, the potential-determining ions are not hydrated protons but hydrated sulfide ions the iso-electric point is at the sulfide ion concentration of 4 x 10 M [Ginley-Butler, 1978]. 

It follows from Eqn. 6-22 that the standard chemical potential of hydrated ions determined from the standard equilibrium potential of the ion transfer reaction is a relative value that is to the standard chemical potential of hydrated protons at unit activity, which, by convention in aqueous electrochemistry, is assigned a value of zero on the electrodiemical scale of ion levels. 

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