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Standard Heats and Entropies of Exchange for Alkali Metal Cations on Wyoming Bentonite
Author(s) -
Gast R. G.,
Van Bladel Robert,
Deshpande K. B.
Publication year - 1969
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1969.03615995003300050014x
Subject(s) - chemistry , alkali metal , standard molar entropy , selectivity , standard enthalpy of formation , metal , bentonite , mole , enthalpy , analytical chemistry (journal) , ion exchange , inorganic chemistry , thermodynamics , chromatography , ion , catalysis , organic chemistry , physics , paleontology , biology
Standard heats of exchange, Δ H °, were determined for alkali metal cation exchange on Wyoming bentonite using a Calvet microcalorimeter. In addition, values of Δ H ° for Na‐Cs, Na‐Rb, and Cs‐Rb exchange were evaluated from selectivity measurements conducted at 25 and 50C. Standard entropies of exchange, Δ S °, were calculated by combining the standard heats with free energy values, Δ G °, estimated from equilibrium selectivity coefficient determinations using the relationship Δ G ° = Δ H ° − T Δ S °. Values of Δ H ° varied from +150 cal mole ‐1 for Na→Li exchange to −2,557 cal mole ‐1 for Na→Cs exchange. Values of Δ G ° ranged from +48 to −1,081 cal mole ‐1 for these reactions giving values of Δ S ° ranging from +0.5 to −4.9 cal mole ‐1 deg. ‐1 . Both Δ H ° and Δ S ° 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.