Hydrogen, electrode standard entropy

Fig.1 Calculated free energy diagram for hydrogen evolution at a potential U = 0 V relative to the standard hydrogen electrode at pH = 0. The free energy of H+ + e is by definition the same as that of j - i at standard conditions. The free energy of H atoms bound to different catalysts is then found by calculating the free energy with respect to molecular hydrogen including zero-point energies and entropy terms (reprinted from Ref 83 with permission).

Table 3.1.2 Values of the standard enthalpies and entropies of formation for the products and reactants of the chemical reaction proceeding in the electrochemical cell of Figure 3.1.5, see Equation (8). Note that the listed values for H+soin are zero because by convention the hydrogen electrode under standard condition is the reference system.

Since it has been postulated that the standard potential of the hydrogen electrode is zero at all temperatures, it follows that AS for the reaction JHa(gf) H -f must also be zero. An alternative convention would therefore be to take the standard entropy of the hydrogen ion as equal to that of iH2(flr) this is equivalent to p>ostulating zero entropy for the el trons. It is immaterial, however, which convention is employed, provided it is adbmd to ctly. Since the convention that S for the hydrogen ion is zero is widely used in the literature, it will be adopted here. 

Another one is that the thermodynamic functions of the standard hydrogen electrode (SHE) are taken as the reference of AG and so in this definition. This reference is called the "conventional scale" where all thermodynamic functions of SHE reaction including its free energy change, enthalpy change and entropy change as well as its electrode potential are designated to zero at any temperature. 

Thermo- electrochemistry and the entropy change of the standard hydrogen electrode reaction. Acta Metall. Sinica. Vol. 9,189-192. 

Since AG° = —F , this corresponds to a standard electrode potential of —2..11 V. The hydrated electron is thus an extremely potent reducing agent, much more powerful than the hydrogen atom, for which is —2.10 V. The free energy and heat of hydration are —37.4 and —38.1 Real mole", respectively. The entropy of hydration is —1.90 cal mole" °K" [31]. 

Thermodynamic cycles involving standard electrode potentials obtained by cyclic voltammetry have also been used to provide thermochemical information on organometallic compounds. This so-called electrochemical method leads to Gibbs energies of reaction in solution, from which bond dissociation enthalpies may be derived using a number of auxiliary data that are often estimated. For example, the derivation of a metal-hydrogen bond dissociation enthalpy in an L MH species requires (i) an estimate of the reduction potential of in the same solvent where the experiments were carried out (ii) an estimate of the solvation entropies of L MH, L M, and H and (iii) the knowledge of the pK of... 

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