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Solubility of Group 2 trends in a period

Hey there! So my question is to do with the solubility of the group 2 hydroxides and sulphates, how it is affected by lattice energy and hydration energy.

In the book (George Facer, pg 46 or on the pdf file I will attach, pg 17) it mentions why lattice energy is more than hydration energy for hydroxides (i.e. why does solubility increase down the group). One factor is due to the actual size of the anion (OH-) being of a similar size to the group 2 cations, thus resulting in the value of r(+) + r(-) increasing considerably (which in turn obviously affects lattice enthalpy, causing it to decrease more than hydration enthalpy and thus the high solubility down the group as it becomes more exothermic).

However, with sulphates, it is the opposite, and that the large anion results in r(+)+r(-) only changing by a small amount. Now, call me dumb but I don't actually get HOW it is possible that the sum of the ionic radius of both ions change a lot with a smaller anion than a larger one? :confused:

Any help would be much appreciated! :biggrin:

Link: http://www.philipallan.co.uk/pdfs/txtchee209.pdf (pg 17) Thank you guys so much!! Let me know if my question is unclear so I can amend it!

(edited 9 years ago)

Reply 1

theJdog

Also, I just noticed there might be a slight error on the same page of the book:

The opposite is true for the group 2 sulfates. The sulfate ion is much larger
than any of the group 2 cations. Therefore, as r(−)>> r(+), the value of
{r(+) + r(−)} changes by only a small amount. This means that the decrease in
the value of the lattice energy is more than the decrease of the hydration
enthalpy of the ions. This makes the enthalpy of solution increasingly less
exothermic as the group is descended.

That to me made no sense, because if the lattice enthalpy is decreasing MORE than hydration, which it clearly isn't according to the table data, then it would obviously be MORE exothermic wouldn't it? Or am I just totally missing the point? Thanks again!

Reply 2

theJdog

Really, no one knows this answer?! I'm a bit desperate here!!

Reply 3

Protoxylic

Relative to a small quantity say arbitrarily small number 5, if you add 5 you get 10 which is a 100 percent increase. Now say you have a number 50 and you add 5 you get 55 which is a 10 percent increase. The same increase has had different RELATIVE effects.

Reply 4

Protoxylic

Basically, because for Hydroxides, you have {r(-)+r(+)} having a greater change down the group than {r(-)+r(+)} for sulphates, the lattice enthalpy of hydroxides decreases more rapidly down the group than the sulphates. The book does have a typo as you have pointed out.