Question

Does a precipitate form when a solution of potassium sulfide and a solution of calcium chloride are mixed together? If so, what is the net ionic reaction?

Answer 1

Yes. A precipitate of calcium hydroxide can be formed.

Explanation:

The reaction involves water. It can be considered the result of an hydrolysis of the strongly basic sulfide ion in water to hydroxide and hydrogensulfide ions, followed by a precipitation of calcium hydroxide as soon as `Ca^"2+"` and `OH^-` ionic concentrations would rise up to exceed the solubility product of `Ca(OH)_2`:
`K_"sp" = [Ca^"2+"]*[OH^-]^2 = 5.5*10^"-6"`.

`"1"`. Hydrolysis of sulfide ions in water (`"*"`):

`K_2S(aq) + H_2O(l) -> KOH(aq) + KHS(aq)`

The net reaction is a proton exchange:
`S^"2-" + H_2O -> HS^"-" + OH^-`
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`"2"`. Precipitation of calcium hydroxide:

`CaCl_2(aq) + 2KOH(aq) -> Ca(OH)_2(s) + 2KCl(aq)`

The overall reaction is:

`2K_2S(aq) + CaCl_2(aq) + H_2O(l) -> Ca(OH)_2(s) + 2KHS(aq) + 2KCl(aq)`

The net (ionic) equation is:

`2S^"2-"(aq) + Ca^"2+"(aq) + H_2O(l) -> Ca(OH)_2(s) +2HS^"-"(aq)`

`"*"` Free sulfide is not detectable in water.