Standard enthalpy of hydration

Estimate the standard enthalpy of hydration of the cyanide ion. given that the lattice enthalpy of potassium cyanide is —692 kJ mol-1 and its standard enthalpy of formation is —113 kJ mol- . 

Inclusion of the absolute value of the standard enthalpy of hydration of the proton, Ahyd// (H +, g) = — 1110 kJ mol 1 (derived in Chapter 2), gives the absolute values for the enthalpies of hydration of the transition metal ions. The estimated values are given in Table 7.5. 

A A ihermochemieal cycle that may be used to give the estimate of the absolute standard enthalpy of hydration is shown in Figure 7.1. 

Q Calculate a value lor the absolute standard enthalpy of hydration of the I a ion using the data from I able 8.3. 

Although for solutions at infinite dilution the absolute enthalpy of hydration of a single ion is clearly defined, there is no purely thermodynamic way to separate the sum of the hydration energies of a cation and an anion into constituent parts. Hence it has been common practice to use conventional standard enthalpies of hydration related in the case of a cation Mz+ and an anion Xz to the absolute enthalpies of hydration by... 

The hydration enthalpies were calculated from the conventional standard enthalpies of hydration using AHh, (H ) =-1103.3 kJ mol... 

Table 7.5 Estimated values of the standard enthalpies of hydration of some + 2 and + 3 transition metal ions and that forGa ...

In order to split the experimentally available Ahydr fE° values dealing with entire electrolytes into the ionic contribution a value must be estimated for just one ion. Conventional values are obtained on setting Ahydr f° (H+) " =0 at all temperatures. The absolute value Ahydrf / (H+, aq)=-1103 7 kJ mor at 298.15 K results (Marcus 1987) according to the TPTB assumption, equating the standard enthalpies of hydration of the tetraphenyphosphonium and tetraphenylborate ions ... 

Standard enthalpy of hydration of M " ", AiiydT7 (298K)/kJmoP Standard entropy of hydration ofM " ", AhydS°(298K)/JK moP -2500 -1931 -1586 -1456 -1316 -... 

Perovskites such as BaZrOs and SrCeOs, doped as above, are able to absorb a quantity of water vapor, while others such as CaTiOs and SrTiOs, are less able to do so. The difference in the ease of hydration has been linked to the basicity of the elements constituting the oxide [1]. Alternatively, the electronegativity has been implicated. More specifically, a correlation has been found between the standard enthalpy of hydration and the difference in electronegativity between the A-site and B-site elements in the perovskite (Fig. 3) [8]. The open points are from reference [8] the solid points are for BaCeo.9 xZr ,Yo.i03 5 (BCZY) [9]. 

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