calcium hydroxide is more soluble in water than magnesium hydroxide under standard conditions,even though the hydration enthalpy of cations decreases down group 2?
Although the hydration enthalpy becomes less exothermic as you go down the group, the lattice dissociation enthalpy becomes less endothermic down the group as well.
The changes in the enthalpy of solution are dependent on the relative rates of fall of the two quantities.
Down the group the decrease in lattice dissociation enthalpy is greater than the decrease in hydration enthalpy.
Hydroxides dissolve more easily down the group - the enthalpy change in solution becomes more exothermic.
With larger negative ions in the salt (like sulphates), the change in ionic radius of the cation becomes less significant because the inter-ionic distances are so large anyway.
The real factor in determining whether a substance dissolves is the change in entropy during the process:the greater the increase in entropy, the more favourable the dissolution process. The enthalpy change of solution may be endothermic, and a substance may still dissolve.
It happens to work out that group 2 hydroxides with more exothermic values of enthalpy changes of solution also have greater increases in entropy when they dissolve.
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