Question

To a solution containing a precipitate of PbS, an excess of H2S was added. What is the ion present in the primary adsorption layer?

a. hydrogen ion (+1)

b. sulfide ion (-2)

c. lead ion (+2)

d. hydroxide ion (-1)

Answer 1

Option:      b. sulfide ion (-2)

H₂S + PbS --.> PbS (precipitate) due to common ion effect.

S^2- ion is in excess and forms the primary adsorption layer on the surface of the PbS precipitate. The particles have a negative surface charge and repel each other; the colloid is stabilized by this. Beyond the equivalence point, Pb²⁺ is in excess and the surface of the precipitate becomes positively charged, with the primary layer being Pb²⁺. The sulfide ions are only weakly adsorbed to the precipitate after the equivalence point is reached.

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