Water absolute molar entropy

In the following, the application of this method of analysis will be illustrated for the case of Pt(l 11) in 0.1 M HCIO4 solutiom First, the evaluation of the entropies of formation of the interphase, double-layer and the interfacial water network will be described in Sections IV. 1, IV.2 and IV.3, respectively. Then, these results will be employed to evaluate the absolute molar entropy of adsorbed hydrogen and OH on Pt(lll), in Section IV.4. Finally, these results will be critically compared with those obtained with a generalized isotherm in Section IV.5. 

The second indicator of kosmotropicity is the standard molar entropy of hydration. For all ions it is highly negative the higher its absolute value, the more water is ordered upon ionic hydration, and the higher the electrolyte kosmotropicity [2,21]. 

A note on the third law of thermodynamics There is a point of lowest possible temperature, close to zero degrees kelvin, Tq 0 K, and for every solid body that forms a perfectly regular crystal the entropy at Tq equals zero. Water can crystallize so at 0 K the entropy of water is zero (or nearly so). It is perhaps a little stretch to assume that Josiah Gibbs would make a perfect crystal, even at 0 K, but we will not be too pedantic about this point right now. So what is the entropy of water at, say, T = 309.8 K It equals the difference between the entropy of water at 309.8 K and the entropy of water at 0 K, which is zero. We can calculate this or we can take the listed value it is determined for room temperature, T = 298.15 K, and is listed as the absolute entropy or standard molar entropy. For water at room temperature, S298 = 69.9 J K mol True, T = 298.2 K is not the same as T = 309.8 K, but we should not worry about this detail now the error is likely to be less than 1%. The entropy of water will change significantly between 273 and 274 K, when ice (solid) melts into... 

The standard molar entropy of hydration of an ion is A 5" = 5 - 5, , the difference between its standard molar entropy in the aqueous solution (Table 2.8) and the standard molar entropy of the isolated ion in the ideal gas phase (Table 2.3). The latter, S°, are calculated from the third law of thermodynamics and spectroscopic data without invoking any extra-thermodynamic assumptions. The former, do involve the assumptions leading to A5 (H+, aq)=-22.2 2J K" mol" for the hydrogen ion (Section 2.3.1.2). With 5°(H% g)=108.9J K" mol", the standard molar entropy of hydration of the hydrogen ion is then A,5"(H ) = -22.2 2-108.9 =-131.1 2 J-K" mor. The standard molar entropies of hydration of ions are shown in Table 4.1, derived from A 5,°° = S -S,° but also obtainable from the conventional values by use of the absolute value of the hydrogen ion. They are related to the effect that ions have on the structure of water according to various approaches. This aspect is fully dealt with in Section 5.1.1.7. 

Fig. 11. Absolute partial molar volumes, V bs> of [Ln(H20)n] is aqueous L11CI3 solutions ( ), compared with the calculated V bs values for [Ln(H20)8] and [Ln(H20)9] indicated by the upper and lower dotted curves, respectively. Interchange rate constants, P , for the substitution of S04 on [Ln(H20)J are shown as O, and water exchange rate constants, P , for [Ln(H20)8] are shown as . Activation enthalpies, A/r, entropies, AS, and volumes, AV, are shown as T, O, and , respectively.

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