Why is a high pH needed to precipitate the group 3 cations as sulfide solids and

Question

Why is a high pH needed to precipitate the group 3 cations as sulfide solids and a low pH needed to precipitate the Group 2 cations as sulfide solids? Be specific in your explanation and include the chemical equilibrium reaction that is shifted right or left based on pH Why is a high pH needed to precipitate the group 3 cations as sulfide solids and a low pH needed to precipitate the Group 2 cations as sulfide solids? Be specific in your explanation and include the chemical equilibrium reaction that is shifted right or left based on pH

Explanation / Answer

In general,

group II metals will preciptiate easily with H2S presence; this means that actually, when we add H2S

H2S <--< H++ HS-

HS- <--> S-2 + H+

So there is plenty of S-2 ions, therefore saturations

Then, group III metals require high pH so there is much more S-2 presence (ammonium sulfide) and the H+ media must be low to aviod metallic aquoeus solution

Therefore

previous metals (groupII) would have precipitates with OH- ions is not S-2

Mn(OH)2, Fe(OH)3 and so on...

Clearly, this is shifted ni the equilbirium, if H+ is high in concentraiton ( that is, low pH) then, this will not precipitate as hydroxides, rather as Sulfides

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