Standard Heats and Entropies of Exchange for Alkali Metal Cations on Wyoming Bentonite1
- R. G. Gast2,
- Robert Van Bladel3 and
- K. B. Deshpande4
Standard heats of exchange, ΔH°, were determined for alkali metal cation exchange on Wyoming bentonite using a Calvet microcalorimeter. In addition, values of ΔH° for Na-Cs, Na-Rb, and Cs-Rb exchange were evaluated from selectivity measurements conducted at 25 and 50C. Standard entropies of exchange, ΔS°, were calculated by combining the standard heats with free energy values, ΔG°, estimated from equilibrium selectivity coefficient determinations using the relationship ΔG° = ΔH° − TΔS°. Values of ΔH° varied from +150 cal mole-1 for Na→Li exchange to −2,557 cal mole-1 for Na→Cs exchange. Values of ΔG° ranged from +48 to −1,081 cal mole-1 for these reactions giving values of ΔS° ranging from +0.5 to −4.9 cal mole-1 deg.-1. Both ΔH° and ΔS° were always negative for uptake of the preferred cation with the absolute values increasing with the degree of selectivity. While the entropy changes did contribute significantly to the standard free energies of exchange, they were only about 10% of the hydration entropy differences for Na-Li and Na-K exchange and 26 and 41% for Na-Rb and Na-Cs exchange, respectively.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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