THE JOIJRNAL OF
PHYSICAL CHEMISTRY (Regiatered in U. 8. Patent Ofice)
VOLUME58
(Copyright, 1954. by the American Chemical Society)
JULY 31, 1954
NUMBER 7
SYMPOSIUM ON HYDRATION OF AQUEOUS IONS. INTRODUCTORY REMARKS BY WENDELLM. LATIMER Department of Chemistry, University of California, Berkeley, Calij. Received April 1.8. 1964
It is important to take an inventory, from time t o time, of our information in a given field and today we are making such an inventory of our information on the nature of the hydration of aqueous ions. The Born cycle has enabled us to calculate the sum of the total hydration energies of pairs of positive and negative gaseous ions and also the differences in the energies of hydration of ions of like charge; but we do not know how t o break down these sums and differences into absolute values. At present we can only give approximate answers to this problem . It would be nice if we could make a theoretical calculation of the energy of interaction between an ion and the solvent water, but we do not know the dielectric constant of the water in the neighborhood of the ion and the calculations are uncertain whether we attempt to use the Born expression for the field energy of a charge in a continuous dielectric medium or attempt to calculate the energy of interaction of the charge with fixed dipoles. I n most cases we do not know how many water molecules are in the first hydration layer and also, how much additional water is held in outer layers. Thus, for example, we have only approximate values for the moles of water which move with an ion in electrical conductance or in diffusion. I n recent years we have obtained considerable information on the entropy changes involved in hydration. Since these entropy terms arise largely
from the loss of entropy of the bound water, the values have given us some insight into the nature of the hydration process. We think we know the absolute partial molal entropies of the ions with considerable certainty. Hence in the case of the total entropy of hydration of a pair of positive and negative ions, we do know how the total entropy should be divided between the two ions. From the nature of the hydration process, this suggests that the energies of hydration should be divided in the same ratio as the entropies. At present this must be considered only a tentative theory. There is one more problem which should be mentioned. When the charge on a n ion is suddenly changed, as for example, the ejection of an electron from an iodide by the absorption of a photon, the resulting atom is left in an excited state because the bound water molecules do not have time to reorient themselves. What is the energy of this reorientation or relaxation? In the case of iodide the estimates of this quantity range from 4 to 70 kcal. This relaxation energy appears to be related to the activation energies in exchange reactions involving an electron transfer, as between Fe++ and Fe+++. Today we do not have this answer. In our symposium we are not going to get the answers to all of these problems. However, we are going to have a lot of evidence presented and I hope our discussion will stimulate work which will lead to eventual solutions of many of these problems.
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