Why does hydration enthalpy decrease with decrease in size.
Asked by | 7th Nov, 2012, 04:57: PM
Expert Answer:
Enthalpy of hydration, Hhyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinite dilute solution in the process,
Mz+(g) + mH2O ® Mz+(aq)
where Mz+(aq) represents ions surrounded by water molecules and dispersed in the solution.
As the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration. For exmple;
Enthalpy of Hydration (Hhyd kJ/mol) of Some Typical Ions
Ion Hhyd Ion Hhyd Ion Hhyd
H+
-1130
Al3+
-4665
Fe3+
-4430
Li+
-520
Be2+
-2494
F-
-505
Na+
-406
Mg2+
-1921
Cl-
-363
Ion | Hhyd | Ion | Hhyd | Ion | Hhyd |
---|---|---|---|---|---|
H+ | -1130 | Al3+ | -4665 | Fe3+ | -4430 |
Li+ | -520 | Be2+ | -2494 | F- | -505 |
Na+ | -406 | Mg2+ | -1921 | Cl- | -363 |
Answered by | 9th Nov, 2012, 10:40: AM
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