The chelate effect can be partially explained in terms of the entropy change inv

Question

The chelate effect can be partially explained in terms of the entropy change involved in chelation reactions. The reaction forming CoEDTA? from the hexaaqua complex of Co3+,

Co(H2O)63+(aq) + EDTA4-(aq) ? CoEDTA-(aq) + 6H2O(l)

leads to a net increase in the number of particles in the solution.

Part A: In which of the following reactions will the entropy change, ?S, be a positive value?

The porphine molecule(Figure 2) is a tetradentate ligand and is the basis for many porphyrins found in biological systems. Porphyrins are complexes with metal ions and have different substituents attached to the carbons of the porphine ligand. Heme (which contains Fe2+) and chlorophyll (which contains Mg2+) are two common examples.

Part B: Consider the reaction of a generic metal ion (Mn+) with a generic porphine molecule (por):

M(H2O)mn+(aq)+por(aq)?M(por)n+(aq)+mH2O(l)

Based on the values of standard enthalpy, ?H?, and entropy, ?S?, shown here,

what is the value of the formation constant, Kf , for the reaction at 25 ?C?

CaEDTA?(aq)+3en(aq)?Ca(en)33(aq)+EDTA4?(aq) Cu(H2O)62+(aq)+3NH3(aq)?Cu(H2O)3(NH3)32+(aq)+3H2O(l) Co(NH3)4(acac)3+(aq)+2acac(aq)?Co(acac)33+(aq)+3NH3(aq) ZnCl2(en)(aq)+4OH?(aq)?Zn(OH)2?4(aq)+2Cl?(aq)+en(aq) FeCl64?(aq)+EDTA4?(aq)?FeEDTA2?(aq)+6Cl?(aq)

Explanation / Answer

Co(NH3)4(acac)3+(aq)+2acac(aq)?Co(acac)33+(aq)+3NH3(aq)

FeCl64?(aq)+EDTA4?(aq)?FeEDTA2?(aq)+6Cl?(aq)

In the above two reactions entropy change is positive, due to increase in no.of particles which leads to increase in randomness.

deltaG0 = deltaH0 - T deltaS0 = -20.5 x 10^3 - 298x6.32 = -22383.36 J

deltaG0 = -RTlnKf

-22383.36 = - 8.314 x 298 x lnKf

Kf = 8.3 x 10^3

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