Problem Set #5 –

Level 1 questions

1. The pKa for ferric ion (Fe3+) is 2.   The pKa for cupric ion (Cu2+) is 9. 

       Which is a stronger Bronsted acid?  Give and explain two reasons for the ten   
       million fold difference in their acidities, using the concepts discussed in class.

Since ferric ion has a smaller pKa and pKa = -logKa, it is the stronger Bronsted acid. The two reasons for its greater acidity are:

2.Explain the following chemical behavior observed for cobalt: Co(III) forms very strong bonds with O- and N- donor ligands (like H2O and NH3), but moderately strong bonds to P- donor ligands (like PH3) and rather weak bonds to As- donor ligands (like AsH3).

Co(3+) is a small ion with a high charge and is fairly hard. Hence it prefers hard atom donors like O- adn N- donor ligands. The 2nd and 3rd row cousins to N, P and As become progressively softer and a poorer match to Co(3+), hence bond strength trends as Co-N > Co-P > Co-As.

3. (a) The Ksp for silver tarnish, that is the black Ag2S, is 10-39, one of the smallest values indicating a VERY insoluble solid.  Why is silver sulfide so insoluble?

(b) HCl,  HBr and HI are strong acids yet HF only partially dissociates in water.  Why is HF unique among the halogen acids?


[These questions also can be easily explained using HSAB concepts]


a) The dissolution of Ag2S amounts to the reaction written as:


Ag2S  +   H2O -->   2 [Ag(H2O)x]+   +  S2-


So the “reaction” is really a competition between sulfide, S2-, and water for Ag+ ion.


Ag(+) is a nice, big, soft cation and strongly prefers to stay partnered with sulfide, not the realatively harder water oxygen donor.


b) F- and H+ are both very hard ions (base and acid respectively) and make a strong bond tha is not easily disrupted by H2O. Hence it is only slightly dissociated in water and is a weak acid.

3. The plot below shows the log of the formation constants logKf, vs. some first row metals for the formation of [M(L-L)3], from M(2+) and bidentate ligand L-L where L-L is one of the four following bidentate ligands: oxalate, glycine, ethylenediammine, aminothiolate.

Here are the questions:

(a) Write the reaction that corresponds to Kf.

M(2+) + 3 (L-L) <==> M(L-L)3

(b) Using the concepts we covered in Chapter 7, explain the trends observed in the magnitude of Kf for the four different bidentate ligands as they make complexes with the metal ions indicated in the plot.

(c) What is special about Zn(2+) that all the lines have a sharp decrease there?

Zn(II) is a d10 ion and has all d orbitals completely filled. CFSE =0 and so there is NO extra CF stability for Zn(II) complexes, as compared to Cu(II) or other M(II) complexes with the same ligands. Therefore its overall Kf values in the plot drop as compared to Cu(II).