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Standard XII

Chemistry

Electrolytes

A recent inve...

Question

A recent investigation of the complexation of ${S C N}^{-}$ with ${F e}^{3 +}$ represented by constant $K_{1}, K_{2}$ and $K_{3}$ as $130, 16$ and $1.0$ respectively. What is the overall formation constant of $F e {(S C N)}_{3}$ from its component ions and what is the dissociation constant of $F e {(S C N)}_{3}$ into its simplest ions on the basis of these data?

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Solution

$F e^{+ 3} + S C N^{-} \to [F e (S C N)]^{2 +} k_{1}$
$[F e (S C N)]^{2 +} + S C N^{-} \to [F e (S C N)_{2}]^{- 1} k_{2}$
$[F e (S C N)_{2}]^{-} + S C N^{-} \to [F e (S C N)_{3}] k_{3}$
formation constant $= k_{1} k_{2} k_{3} = 130 \times 16 \times 10$
$= 20800$
dissociation constant $= \frac{1}{20800}$
$= 4.81 \times 10^{- 5}$

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Fe+3+SCN−→[Fe(SCN)]2+k1[Fe(SCN)]2++SCN−→[Fe(SCN)2]−1k2[Fe(SCN)2]−+SCN−→[Fe(SCN)3]k3formation constant =k1k2k3=130×16×10 =20800dissociation constant =120800 =4.81×10−5