Copper compounds, free energy

Free energy values for various copper compounds have recently been critically reviewed and collected by Randall, Nielsen, and West.88 The proper values from this compilation have been used in calculating the equilibrium conditions for the reactions ... 

Disproportionated systems have been discussed in Section 10.7 above. Several elements have many accessible oxidation states, as well as missing oxidation states. Equation 17.2 simply expresses that a disproportionation reaction has the free energy AG. In the CuO case, however, there is no disproportionation (AG > 0). In Figure 17.5, we show the same Marcus parabolas as in Figure 10.18, but in this case AG > 0. The oxidation states Cu+ and Cu " " are well known. Cu + is known from NaCu02 and a few other similar compounds, and as impurities, as mentioned above. In most other cases, copper has the oxidation state -1-2 (or -i-l or 0). Judging from spectral data, the Cu " " disproportionation reaction may be written... 

The conditional stability constants (log K<.) obtained for copper with humic compounds extracted from soils and natural waters are invariably greater than those for other transition metals (see Table IV). This is expected from the enhanced levels of crystal field stabilisation energy which result fi-om the splitting of the 3d electronic orbitals on Cu by an octahedral field (Mackay and Mackay, 1969). The divei ence in the values of log Kc shown in Table IV, may, in part, have arisen from intrinsic variations in the copper-binding properties of the various humic samples. However, these deviations may also be explained in terms of the different experimental conditions employed (pH, ionic strength, temperature, for example) and the assumptions made in the calculations. For example, an increase in the pH will enhance the availability of dissociated binding sites (see Section 6) which are then free to participate in further complexation of copper and... 

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