Re: solubility of sulfur

From: Mohammed Farooq (farooq_w_at_hotmail.com)
Date: 09/29/04 

Date: 29 Sep 2004 06:07:14 -0700

miggy_wang@mac.com (M. Wong) wrote in message news:<bbe9f7a7.0409282248.43269ba9@posting.google.com>...

> I've been searching the web and the groups on here but I still cannot
> find an answer to my question of why S or sulfide would decompose when
> in a solvent, rather than just stay a sulfide ion.
>
> BaS
> CaS
> MgS
> AlS
> Fe(III)S
> and
> Hg(II)S04
>
> they all decompose for some reason, can anyone please explain why?

Recall the general rule that all sulfides are water insoluble except
those of alkali and alkaline earth metals. The list of compounds can
then be divided into two, the insoluble Al2S3 and FeS and soluble BaS,
CaS and MgS. Now consider a souble sulfide CaS, the hydrolysis
equilibrium can then be written as

CaS (aq) ----> Ca(2+) + S(2-)

S(2-) (aq) + 2H(+) from water <-----> H2S (aq),

thus many metal sulfides strongly smell of this gas. Note that the
value of this overall equilbrium constant of equation no. 2 is very
large, of the order of 10^20, and there is strong tendency of the gas
to form, using up H+ from water and leaving an excess of OH(-) ions.
The OH(-) then react with Ca(2+) to form a hydroxide and the gas may
escape from the solution.

With the insoluble sulfides like FeS the hydrolysis may take many
years to complete and by complex reactions may end up in Fe(OH)3 or
Fe2O3.

Mercuric sulfate hydrolyses like all other transition metal salts in
water and have acidic pH, indicating the metal ion is acting as a
Lewis acid, sulfur has no role to play here. Mercuric nitrate does
hydrolyse though no sulfur is present.