Why does CdS dissolve in HCL, when CuS and Bi2S3 will not?

Who says Bi2S3 and CuS will not dissolve? In most circumstances they will

Sulfur combines with H+ => H2S(g) and shifts the solubility equilibrium toward more ionization and increased solubility.

The formation of H2S essentially is removing S-2 from the solution forcing the metal-sulfide to decompose in order to replace the sulfur combined with the added H+.

CdS, CuS, and Bi2S3 are all sulfide compounds that have different solubilities in different solvents. In the case of HCl (hydrochloric acid), CdS dissolves while CuS and Bi2S3 do not due to the differences in their solubility product constants and acid-base properties.

1. CdS (Cadmium Sulfide)
CdS is more soluble in HCl due to the formation of a soluble complex with the chloride ion (Cl-). When CdS is added to HCl, the following reaction occurs:
CdS (s) + 2HCl (aq) → CdCl2 (aq) + H2S (g)
The formation of CdCl2 (cadmium chloride) and H2S (hydrogen sulfide) gas allows the dissolution of CdS in HCl.

2. CuS (Copper Sulfide)
CuS is not soluble in HCl because copper does not form a soluble complex with chloride ions. Thus, when CuS is added to HCl, it does not react and remains insoluble:
CuS (s) + 2HCl (aq) → No reaction

3. Bi2S3 (Bismuth Sulfide)
Similar to CuS, Bi2S3 does not dissolve in HCl. Bismuth is less likely to form soluble compounds compared to cadmium. Therefore, the reaction between Bi2S3 and HCl does not occur:
Bi2S3 (s) + 6HCl (aq) → No reaction

In summary, CdS dissolves in HCl due to the formation of a soluble complex, while CuS and Bi2S3 do not dissolve in HCl because they do not form soluble complexes with chloride ions. These differences in solubility are determined by the solubility product constants and acid-base properties of the compounds involved.

The solubility of different compounds in a particular solvent depends on their chemical properties and the nature of the interaction between the solute particles and the solvent molecules. In the case of CdS, CuS, and Bi2S3, their solubility in HCl can be explained by considering the acid-base reactions occurring between the sulfide compounds and the hydrochloric acid (HCl).

CdS dissolves in HCl due to the formation of a complex ion. When CdS is added to HCl, the acid behaves as a proton donor (H+) and can react with the S2- ions in CdS, which are basic in nature. The reaction can be represented as follows:

CdS(s) + 2HCl(aq) → CdCl2(aq) + H2S(g)

In this reaction, the S2- ions from CdS react with H+ ions from HCl to form H2S gas (hydrogen sulfide). The Cd2+ ions combine with chloride ions (Cl-) from HCl to form a soluble compound, CdCl2. The formation of the H2S gas and the soluble CdCl2 compound allows CdS to dissolve in HCl.

On the other hand, CuS and Bi2S3 do not dissolve in HCl. This is because the reaction between sulfide compounds and HCl does not occur to the same extent as with CdS. CuS and Bi2S3 are less soluble in HCl due to the differences in the reactivity of the metal ions and the acid. The acid-base reactions are either much slower or do not happen at all, resulting in limited or no dissolution of CuS and Bi2S3 in HCl.

It's important to note that the solubility of compounds can also be influenced by other factors such as temperature, concentration, and presence of complexing agents. In some cases, additional chemical treatment or a different solvent may be required to dissolve certain compounds that are insoluble in a specific solvent.