Sciencemadness Discussion Board

Metal oxide reactivities with water

Hexavalent - 8-4-2013 at 14:02

It has intrigued me recently why calcium oxide reacts with water to form calcium hydroxide;

CaO + H2O → Ca(OH)2

whereas magnesium oxide, aluminium oxide, silicon (di)oxide, etc. do not. I have discussed this with DJF90 and the only possible explanations we have come up with are differences in crystal structures and arguments in bond enthalpies.

Can anyone verify this, or elaborate upon it? Thanks.

DraconicAcid - 8-4-2013 at 14:07

Quote: Originally posted by Hexavalent  
It has intrigued me recently why calcium oxide reacts with water to form calcium hydroxide;

CaO + H2O → Ca(OH)2

whereas magnesium oxide, aluminium oxide, silicon (di)oxide, etc. do not. I have discussed this with DJF90 and the only possible explanations we have come up with are differences in crystal structures and arguments in bond enthalpies.

Can anyone verify this, or elaborate upon it? Thanks.


Are you sure magnesium oxide doesn't do this? I think it does, it's just quite slow to do so.

Aluminum oxide and silicon dioxide have a great deal of covalent character to their bonds, so the oxides don't act like ions (which are extremely strong bases).

12AX7 - 8-4-2013 at 15:50

Al2O3 and (calcined) MgO, of course, have immense binding energies, so they're pretty much inert for the life of the universe unless changed otherwise.

Curious that CaO is comparable to MgO in binding energy (both have melting points in the... ahhh, I forget, is it 2600-2800C range?!), yet tends to be more reactive. This is probably a side-effect of its preparation (lime is typically prepared for use as a base, and therefore not calcined very hot, whereas MgO is often sintered for metallurgical purposes), and of the solubility of its hydroxide. Mg(OH)2 is hardly soluble, so a large particle of MgO will only slowly grow a thin diffusion layer of Mg(OH)2 on its surface. In contrast, sufficiently fine CaO will dissolve into solution, in part or in whole. I don't know offhand the affinities of Ca(OH)2 and Mg(OH)2 but it's likely the calcium compound is more stable (i.e., its synthesis is exothermic as well).

The same thoughts follow for most transition metal and 'ordinary metal' oxides: they tend to be poorly soluble (which ultimately is a matter of water's ability to complex and dissolve the oxides, which tend to be very stable compounds) and have high melting points.

Of course, [hydr]oxides which are renowned for their solubility are almost exclusively alkali (e.g., NaOH) and acid (e.g., NO2), which means these compounds are either ionic to begin with, or form stable ions in solution, in either case being favorably solvated by water.

Tim

Hexavalent - 9-4-2013 at 09:07

Thanks for your comments and ideas.
Quote: Originally posted by DraconicAcid  

Aluminum oxide and silicon dioxide have a great deal of covalent character to their bonds


As does phosphorus pentoxide, but that reacts violently with water.