Transition metals tend to be good electron acceptors (Lewis acids). In solution,

Question

Transition metals tend to be good electron acceptors (Lewis acids). In solution, anions can act as electron donors (Lewis bases) and coordinate to the transition metal. The Keq's for these types of reactions are called formation constants (Kf). For example, consider the following balanced chemical reaction showing how the cyanide anion coordinates to Fe2+:

This reaction is what makes cyanide so toxic to human beings. The cyanide ion is a stronger Lewis base than oxygen and binds to the iron that red blood cells use to carry oxygen around the body (displacing the oxygen), thus interfering with the blood's ability to deliver oxygen to the tissues.

Part A

The average human body contains 5.10 L of blood with a Fe2+ concentration of 2.10 x 10-5 M. If a person ingests 8.00 mL of 10.0 mM NaCN, what percentage of iron (II) in the blood would be sequestered by the cyanide ion?

Express the percentage numerically ( %Fe2+)

Explanation / Answer

Moles of NaCN = Molarity x Volume(in L) = 10.0 x 10^-3 M x 0.00800 L = 8 x 10^-5 mol

Moles of Fe^2+ = Molarity x Volume(in L) = 2.10 x 10^-5 x 5.10 = 1.1 x 10^-4 mol

Fe^2+ + 6CN- <-------------> [Fe(CN)6]^4-

As the moles of cN- are less than moles of Fe^2+, NaCN is the limiting reactant.

From the given reaction, it is clear that 6 moles of CN- require 1 mol of Fe^2+.

Thus, 8 x 10^-5 mol of CN- require (8 x 10^-5 mol) / 6 = 1.33 x 10^-5 mol of Fe^2+.

Percentage sequestered = [(Fe^2+ used up)/(Initial Fe^2+)] x 100

= [(1.33 x 10^-5 mol) / (1.1 x 10^-4 mol)] x 100

= 12.09 %

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