From Li to Cs, thermal stability of carbonates increases. From Li to Cs, due to larger ion size, hydration enthalpy decreases. So, solubility should decrease from Li to Cs. But, experimentally, order is reverse. I cannot wrap my head around this.

  • $\begingroup$ Order of what is reversed? $\endgroup$ – orthocresol Jan 27 '17 at 11:31
  • $\begingroup$ Found my mistake (probably) thermal stability is related to decomposition, I wanted to break lattice into gaseous ions, whose energy change is given by lattice energy. Both |hydration energy| of constituent ions and lattice energy of M2(CO3) decreases from Li to Cs as in Li>Na>K>Rb>Cs. Entropy change plays a big part too here. I meant order of solubility of M2(CO3) in water. $\endgroup$ – Mrigank Jan 27 '17 at 12:04
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    $\begingroup$ Yes. Even ignoring entropy (to a first approximation we might assume that the entropy change is constant for all the metals), solubility depends on both lattice energy and hydration enthalpy so you can't just look at one trend. While it's more favourable to solvate Li+, it's also harder to break up Li2CO3 into its constituent gaseous ions. Just so happens that here, the lattice energy term predominates. $\endgroup$ – orthocresol Jan 27 '17 at 12:08
  • $\begingroup$ A side question, as the hydrated ionic radius is highest and mobility least for Li among the above compounds, would Li2(CO3) have least entropy change of solution? $\endgroup$ – Mrigank Jan 27 '17 at 12:25
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    $\begingroup$ It's difficult to say. The entropy term also has to be split up into lattice breakage (probably roughly the same for all metals) and solvation effects. In the absence of any data my hypothesis would be that Li+, being the smallest (highest charge density), will lead to the greatest ordering of the solvent molecules around it. Consequently the entropy change would be the least positive. This effect is also reflected in the large hydrated ionic radius that you speak of. (I don't have data and I'll take your word for it.) $\endgroup$ – orthocresol Jan 27 '17 at 12:30

Lattice energy is also decreasing down the group. hence thermal stability is decreasing. But since lattice energy is decreasing squarely and hydration energy is decreasing linearly, the effect of decrease in lattice energy is higher than the effect of decrease in hydration energy. So if energy required to break lattice is very less, then the lattice of the bottom elements in alkali group will break easily compared to the lighter ones. Thus if not lattice form, then they prefer the aqueous state and hence they love to stay dissolved and not form lattice. So solubility increases.

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