When species combine to produce a coordination complex , the equilibrium constan

Question

When species combine to produce a coordination complex, the equilibrium constant for the reaction is called is the formation constant, Kf.

For example, the iron(II) ion, Fe2+, can combine with the cyanide ion, CN, to form the complex [Fe(CN)6]4 according to the equation

Fe2+(aq)+6CN(aq)[Fe(CN)6]4(aq) where Kf=4.21×1045.

The average human body contains 5.60 L of blood with a Fe2+ concentration of 2.90×105M . If a person ingests 10.0 mL of 18.0 mM NaCN, what percentage of iron(II) in the blood would be sequestered by the cyanide ion?

Explanation / Answer

     Fe2+(aq)            +       6CN(aq)                [Fe(CN)6]4(aq)              where Kf=4.21×1045

5.6 x 2.9x10-5x1000        10 x 18 x10-3

= 0.1624 millimoles           = 0.18 millimoles

Cyanide ion is the limiting reagent, so

assuming all the cyanide goes to complex, Kf is very large , so indicates that forward reaction dominates

Fe+2 millimolesunreacted = initial taken - used for complexation = 0.1624 - (0.18/6 ) =0.1324 millimoles

Fe2+ % complexed = (0.03/0.1624)x100 = 18.4 %

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