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Posted by on Friday, November 21, 2008 at 1:23pm.

Fe+3 + SCN- ==Fe(SCN)+2
if adding NaOH will form precipitates, but what is the precipitates' formula?
And after this reaction, if we add HCl to the reaction, will the precipitates dessolve? What does the HCl affect the reaction? THX

  • science-CHEM - , Friday, November 21, 2008 at 2:06pm

    The ppt probably is Fe(OH)3 and that is soluble in HCl.

    Remember (Fe^+3)(SCN^-) = Fe(SCN)^+2
    and Fe(SCN)^+2/(Fe^+3)(SCN^-)= Kformation so a complex FeSCN^+2 forms when Fe^+3 and SCN^- are high enough. If NaOH is added, then we are raising (OH^-) from the NaOH to a high level so that Ksp for Fe(OH)3 is exceeded and a ppt of Fe(OH)3 forms.
    Remember (Fe^+3)(OH^-)^3 = Ksp.
    Probably the red colored FeSCN^+3 complex disappears, also, because the pptn of Fe(OH)3 removes so much Fe^+3.

    Adding HCl dissolves the ppt because H^+ is large (from the HCl) which reduces the OH^- (because of the
    (H^+)(OH^-) = Kw and if the OH^- is reduced enough, Ksp for Fe(OH)3 is no longer exceeded and it dissolves.

  • science-CHEM - , Friday, November 21, 2008 at 6:34pm

    Thanks a lot!!!
    I think I got it!!!

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