hodges - 26-12-2005 at 14:30
Why is it that (for example) aluminum sulfide hydrolyses in water to form aluminum hydroxide and hydrogen sulfide, whereas calcium sulfide and sodium
sulfide do not (at least not nearly as much). OTOH, calcium carbide hydrolyzes readily in water to produce calcium hydroxide and acetylene. And none
of the carbonates hydrolyze to produce carbonic acid in a water solution.
12AX7 - 26-12-2005 at 15:27
It has to do with the alkalinity of the cation compared to the acidity of the anion conpared to the acidity of water.
H2S (hydrosulfuric acid) is a weaker acid than H2O (hydroxidic acid maybe?) so tends to be displaced. Strong bases like Na+ can keep it in solution
to some extent. Weak bases (or mild acids like Al(3+)!) won't hold it as strongly and it will tend to hydrolyze.
Any unstable carbide readily hydrolyzes because "hydrocarbidic acid" of either form (acetylidic acid, as it were, or hydromethic? acid) is a very weak
acid and attracts protons (H+) stronger than water, so as to form C(4-) + 4H+ --> CH4, or C2(2-) + 2H+ --> C2H2.
Note that methane can support four bonds, comparable to a phosphate's three or silicate's four. This would be CH4 (methane) --> CH3(-) (methyl) +
H(+) --> CH2(2-) (methylene) + 2H(+) --> CH(3-) (hydrocarbide maybe?) + 3H(+) --> C(4-) (carbide) + 4H(+). That is, if you have a strong
enough agent to ionize and remove those protons. Carbon and hydrogen really like to be together.
Note that, if precipitation can occur, it will, and you can displace even the strong nitrate with acetylide to form, for example, silver acetylide.
Stable carbides such as WC, HfC, TiC, SiC and so on I'm guessing have too high a lattice energy to be decomposed too quickly; that, and the cations
would tend to form passivating layers (WO3 except in basic conditions, HfO2, TiO2, SiO2, etc.). They are also extremely hard materials, suggesting a
strong crystal.
Tim
BromicAcid - 26-12-2005 at 18:02
In the cases of inorganic reactants it's usually a simple matter, the difficulty is determining the extent to which it occurs.
If you have a solution of sodium sulfide, it usually has a smell of hydrogen sulfide. The reaction between the water molecules and the sulfide anion
occurs in this soluion noticeably, forming the hydrogen sulfide anion and some hydrogen sulfide. However, the sodium cation is happy as is and
doesn't impact the solution. So you have the hydrolysis of sulfide which produces hydroxide and hydrogen sulfide anion. The hydroxide helps to keep
it in solution.
But in the case of aluminum sulfide, the aluminum cation forms complexes with the water, as a result aluminum salts are usually acidic in water.
Aluminum chloride dissolved in water makes an acidic soluiton, not as a result of the chloride which is for the most part neutral, sodium chloride for
example isn't appreciably acidic or basic. No, the solution becomes acidic by virture of the small, strongly positive aluminum cation. In the above
scenario if you start to acidify the soluion of sodium sulfide you get more free hydrogen sulfide, what aluminum sulfide is, is like a combo of acid
and sulfide, so when it's added to water it works toward its own decomposition.
Carbonates do indeed form carbonic acid in water when dissolved, just not much.
CO<sub>3</sub><sup>2-</sup> + H<sub>2</sub>O <---> HCO<sub>3</sub><sup>-</sup> +
OH<sup>-</sup>
Note that for the next step to occur, you have to have a solution that has increased in basicity donate another proton to the hydrogen carbonate:
HCO<sub>3</sub><sup>-</sup> + H<sub>2</sub>O <---> H<sub>2</sub>CO<sub>3</sub> +
OH<sup>-</sup>
So it's less likely to do that, although carbonic acid is a weak acid it isn't weak enough to want to go to the free acid that much. As a result it
usually hydrolyzes to the hydrogen carbonate stage. Adding acid of course forces the equilibrium to the right and makes free carbonic acid with is
unstable in solution and subsequently decomposes when present in a strong enough concentration. Other things that can force the equilibrium work too,
Vulture for example commented on putting a concentrated solution of potassium carbonate under vacuum and having CO<sub>2</sub> be evolved.
In this case the weakly soluble carbonic acid was continuilly removed from solution allowing for the equilibrium to be hard to establish.
It is said that every reaction happens to some extent, and like I said, what matters is the extent to which it happens. Just look at each reaction as
an equilibrium, and look for what factors force the equilbrium.
neutrino - 26-12-2005 at 19:20
A note I feel I should make: calcium carbide (CaC<sub>2</sub>) is not a true carbide, rather it is an acetylide. Carbide is C<sup>4
-</sup>, acetylide is ( :C#C: )<sup>2-</sup>. This seems to be the cause of some confusion.
(# = triple bond, : = lone pair)
[Edited on 27-12-2005 by neutrino]
BromicAcid - 26-12-2005 at 21:04
Indeed, the true reactive carbides that I know of are aluminum and beryllium carbide, both of which undergo hydrolysis to produce methane gas.
12AX7 - 27-12-2005 at 04:55
Ok, thanks for clearing that up 
IIRC, there's a third type of carbide, formed in rare earth compounds. I seem to recall reading about a rare earth cobalt carbide I think that had
acetylides, carbides and carbon chains (polyethylide so to speak?) in it!
Tim
hodges - 27-12-2005 at 15:35
Thanks for the replies; this makes sense.