Question

When HCl is added to a solution containing PbCl2 (s) its solubility decreases, while when added to PbF2 (s) its solubility increases. Explain why?

Answer 1

PbCl2 is a sparingly soluble salt, which dissociates as PbCl2------> Pb2+ + 2 Cl-

in presence of HCl which completely dissociates in solution producing H+ and Cl- ,the concentration of Cl- in solution increases so due to common ion effect the solubility of PbCl2 decreases.

But in case of PbF2, the F- formed is a weak base that reacts with H3O+ to form HF so there is a completion of equilibrium so the solubility increases.

Answer 2

The solubility behavior of HCl when added to a solution containing PbCl2 (lead(II) chloride) and PbF2 (lead(II) fluoride) can be explained by the differences in the reactions that occur between HCl and these two lead salts.

1. PbCl2 (lead(II) chloride):

When HCl is added to a solution containing PbCl2, it reacts with PbCl2 to form a sparingly soluble lead chloride complex, which decreases the overall solubility of PbCl2. The reaction is as follows:

PbCl2 (s) + 2 HCl (aq) → PbCl2•2H2O (s)

In this reaction, HCl acts as a ligand, coordinating with the lead ion (Pb2+) to form the lead chloride complex (PbCl2•2H2O). The formed complex has limited solubility in water, leading to a decrease in the overall solubility of PbCl2.

2. PbF2 (lead(II) fluoride):

When HCl is added to a solution containing PbF2, it reacts with PbF2 to form a more soluble lead chloride salt, which increases the overall solubility of PbF2. The reaction is as follows:

PbF2 (s) + 2 HCl (aq) → PbCl2 (s) + 2 HF (aq)

In this reaction, HCl displaces the fluoride ions (F-) in PbF2, leading to the formation of PbCl2, which is more soluble in water than PbF2. As a result, the overall solubility of PbF2 increases.

In summary, when HCl is added to a solution containing PbCl2, it forms a sparingly soluble lead chloride complex, decreasing the solubility of PbCl2. On the other hand, when HCl is added to a solution containing PbF2, it forms a more soluble lead chloride salt, increasing the solubility of PbF2. The difference in solubility behavior is due to the different chemical reactions that occur between HCl and PbCl2, and HCl and PbF2, respectively.