Ay h2oco3 oh

Because these reactions reduce the concentrations of the original anions produced by dissolution of the salt PbS or PbC03, the position of equilibrium in the original reactions shifts to the right side, thereby dissolving more of the salt, in analogy with the process involving CaC03 that we analyzed in Chapter 13. Thus the solubilities of PbS and PbC03 in water are substantially increased by the reaction of the anion with water.

If highly acidic water comes into contact with minerals such as PbS, the "insoluble" solid dissolves to a much greater extent than in neutral waters. This occurs because the sulfide ion initially produced is subsequently converted almost entirely to bisulfide ion, HS", which in turn is converted by the acid to dissolved hydrogen sulfide gas, H2S, since both S"~ and HS act as bases in the presence of acid:

S2- + H+ ^^ HS" K = 1/JC, (HS") = 7.7 x 1012 HS * + H+ ^^ H2S K' = 1/Ka (H2S) = 1.0 X 107

When these two reactions are added to that for the dissolution of PbS into Pb2+ and S2^, the overall reaction is seen to be

PbS(s) + 2 H+ Pb2+ + H2S(aq)

Since the equilibrium constant Koveran for an overall process which is the sum of several others is the product of their equilibrium constants, in this case ^overall = KspKK' = 6.5 X 1CT8. By application of the law of mass action to this reaction, we find the expression for the equilibrium constant in terms of concentrations:

Kowrall = [Pb2+] [H2S]/[H+]2

Under conditions in which no significant amount of hydrogen sulfide gas is vaporized, but which are sufficiently acidic that almost all the sulfur exists as H2S rather than as S2~ or HS~, the stoichiometry of the reaction allows us to write that [Pb2+] = [H2S]. By substitution of this relationship into the above equation, we obtain

[Pb2+]2 - 6.5 X 1CT8 [H+]2

[Pb2+] = 2.5 X 10~4 [H+]

Thus the solubility of PbS increases linearly with the 11' concentration in acidic water. At pH = 4, the solubility of PbS and the concentration of Pb2+ ion in water is calculated to be 2.5 X 1CT8 M, whereas at pi 1 = 2, the solubility is 2.5 X 10 6 M. We conclude that dangerous concentrations of lead ion can occur in highly acidic bodies of water that are in contact with "insoluble" lead minerals.

PROBLEM 15-8

By calculations similar to those for PbS above, deduce the relationship between the solubility of mercuric sulfide, HgS (Ksp = 3.0 X 10^53), and the hydrogen ion concentration in acidic water. Is the solubility of HgS increased by exposure to acid?

Continue reading here: Ionic 4 Lead in Automobile Batteries

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