Question

Why does CdS dissolve in HCl, when CuS and Bi2S3 will not? please explain!!

Why does CdS dissolve in HCl, when CuS and Bi2S3 will not? please explain!!

Homework Answers

Answer #1

The solubility can be explained in terms of the solubility product constant, Ksp.

1) CdS + 2 HCl ------> CdCl2 + H2S

CdCl2 (aq) -----> Cd2+ (aq) + 2 Cl- ; Ksp = [Cd2+][Cl-]2

2) CuS + 2 HCl -----> CuCl2 + H2S

CuCl2 (aq) -----> Cu2+ (aq) + 2 Cl-; Ksp = [Cu2+][Cl-]2

3) 2 Bi2S3 + 6 HCl -----> 2 BiCl3 + 3 H2S

BiCl3 (aq) -----> Bi3+ (aq) + 3 Cl- (aq); Ksp = [Bi3+][Cl-]3

A compound is soluble when the product on the ionic concentrations is less than the solubility product of the compound. CdS is soluble because the ionic product Q = [Cu2+][Cl-] is less than the solubility product and hence CdS is soluble in HCl. Moreover, CdCl2 dissolves in excess HCl forming a complex [CdCl4]2- anion.

The solubility product values for CuCl2 and BiCl3 are much easily reached in HCl and hence the solids precipitates out.

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