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Is there a trend in alkali metal ion hydration entropy?

There's much talk of hydration enthalpy, but I have a hard time finding information about the change in entropy.

Am I right in thinking that Li$^+$ would have a more negative $\Delta S^o_{hyd}$ than Cs$^+$ because more water ligands will be ordered around it, due to it's higher charge density?

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Yes, there is. The following is a plot of the hydration enthalpy versus row in the Periodic Table. Data was extracted from the Wikipedia, not the primary source (Ref 1).

Your thinking is right. The hydration entropy is evidently negative due to structuring of water (including loss of rotational degrees of freedom), becoming less negative for the larger ions (smallest loss of entropy for the cesium cation).

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References

  1. Burgess, John (1978). Metal Ions in Solution. Chichester: Ellis Horwood. ISBN 978-0-85312-027-8.
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