Question

Explain the trends in the magnitude of the enthalpy of hydration, H_hyd, values for the following ions. (Note that the negative sign indicates direction of energy transfer, i.e., exothermic.) Hints: think about Coulomb’s Law and look at your periodic table. Describe and explain each trend using complete sentences, supplemented with diagrams, and/or equations if needed.

Ion                   Enthalpy of Hydration (kJ/mol)

Mg²⁺                                          -1922

Na⁺                                             -405

Rb⁺                                             -296

Answer 1

Hydration energy (also hydration enthalpy) is the amount of energy released when one mole of ions undergo hydration which is a special case of solvation. It is a special case of dissolution energy, with the solvent being water.

The size of the hydration enthalpy is governed by the amount of attraction between the ions and the water molecules. The attractions are stronger the smaller the ion. For example, hydration enthalpies fall as you go down a group in the Periodic Table. The small lithium ion has by far the highest hydration enthalpy in Group1, and the small fluoride ion has by far the highest hydration enthalpy in Group 7. In both groups, hydration enthalpy falls as the ions get bigger. The attractions are stronger the more highly charged the ion. For example, the hydration enthalpies of Group 2 ions (like Mg2+) are much higher than those of Group 1 ions (like Na+).

The problem with Rb is that of its bigger size so lesser energy is released as much of it is used for the reaction itself.

Feel free to give a thumbs up if you like the answer. Also, I don't think there is a need to draw pictures for this, so I didn't supported my answer with pictures.