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Characteristic Reactions of Lead Ions (Pb²⁺)

  • Page ID
    97271
    • Most common oxidation states: +2, +4
    • M.P. 328º
    • B.P. 1750º
    • Density 11.35 g/cm3
    • Characteristics: Lead is a soft metal having little tensile strength, and it is the densest of the common metals excepting gold and mercury. It has a metallic luster when freshly cut but quickly acquires a dull color when exposed to moist air.
    • Characteristic reactions of \(\ce{Pb^{2+}}\): The +2 oxidation state is the more stable state.

    Chloride Ion

    Soluble chlorides, such as hydrochloric acid, precipitate white lead chloride from \(\ce{Pb^{2+}}\) solutions, when the solutions are not too dilute:

    \[\ce{Pb^{2+}(aq) + 2Cl^{-}(aq)<=> PbCl2(s) }\]

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    Lead chloride is a slightly soluble salt, with a solubility of 10 g/L at 20º. The solubility of \(\ce{PbCl2}\) increases very rapidly as the temperature rises. At 100º it has a solubility of 33.5 g/L. However, \(\ce{PbCl2}\) precipitates very slowly, particularly when other ions that form insoluble chlorides are not present. The precipitation can be speeded up by vigorously rubbing the inside of the test tube with a stirring rod. Even then the precipitate may not form until 3 to 5 minutes after mixing the solutions. \(\ce{PbCl2}\) dissolves in excess chloride ion as a result of the formation of the tetrachloroplumbate(II) complex ion:

    \[\ce{PbCl2(s) + 2Cl^{-}(aq)<=> [PbCl4]^{2-}(aq) }\]

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    Sulfate Ion

    Soluble sulfates, including dilute sulfuric acid, precipitate white lead sulfate, which is much less soluble than lead chloride:

    \[\ce{Pb^{2+}(aq) + SO4^{2-}(aq)<=> PbSO4(s) }\]

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    \(\ce{PbSO4}\) dissolves in concentrated solutions of hydroxide or acetate ions.

    \[\ce{PbSO4(s) + 4OH^{-}(aq)<=> [Pb(OH)4]^{2-}(aq) + SO4^{2-}(aq) }\]

    \[\ce{PbSO4(s) + 2CH3CO2^{-}(aq)<=> Pb(CH3CO2)2(aq) + SO4^{2-}(aq) }\]

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    The lead acetate, though only slightly dissociated, is soluble.

    Aqueous Ammonia

    Lead(II) ion reacts with aqueous ammonia to precipitate a white basic salt, \(\ce{Pb2O(NO3)2}\), rather than the expected lead(II) hydroxide:

    \[\ce{Pb^{2+}(aq) + 2NH3(aq) + 3H2O(l) + 2NO3^{-}(aq)<=> Pb2O(NO3)2(s) + H2O(l) + 2NH4^{+}(aq) }\]

    Pb3ppt.gif

    The basic salt is insoluble in excess ammonia.

    Sodium Hydroxide

    Sodium hydroxide precipitates lead(II) hydroxide, which dissolve with excess hydroxide:

    \[\ce{Pb^{2+}(aq) + 2OH^{-}(aq)<=> Pb(OH)2(s) }\]

    Pb4appt.gif

    \[\ce{Pb(OH)2(s) + 2OH^{-}(aq)<=> [Pb(OH)4]^{2-}(aq) }\]

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