Question

Answer the following questions based on HSAB theory. If using Fajans Rules, give the rule number and explain why it applies.

1. Explain why aluminum perchlorate is less soluble than aluminum cyanide. This is unusual since perchorates tend to be very soluble.

2. Explain why the Ksp for FeBr2 is 4.1 x 10–4 and the Ksp for FeF2 is 3.2 x 19–7.

3. Explain why the Ksp for Fe(OH)3 is 1.2 x 10–36 and the Ksp for Fe(OH)2 is 7.8 x 10–16.

4. Explain why VS is more soluble than NiS.

Answer 1

According to Hard-Soft acid base (HSAB) theory hard acids form more stable compounds with hard bases and soft acids form more stable compounds with soft bases.

Hard acids: These are having less size and more charge. Hard acids also having less polarizability.

   Eg. H+, Li+ , Na+, K+, Mg2+, Al3+, Fe3+ etc

Soft acids: These are having large size and less charge. Hard acids also having more polarizability.

   Eg. Ag+, Au+ , Cu+, Pt2+, Pd2+ etc

Hard bases: These are the anions or ligands which are having oxygen, nitrogen and F as bonding atoms. etc

                OH-, OH2, C2O42-, ClO4- , F-, eyc

Soft bases: These are having carbon, sulfur, phosphorus as bonding atoms.

              Eg. CN-, CO, S2-, PPh3, etc

a) Al3+ is hard acid so can form more strong bond with hard base perchlorate. Cyanide is soft base as it has carbon as bonding atom so it can very weak bond with hard acid Al3+, so aluminum cyanide is less stable than aluminum perchlorate and it is more soluble.

Hence aluminum perchlorate is less soluble than aluminum cyanide.

b) Br- is soft base so it can form weak bond with Fe2+. F- can form more strong bond with F2+ than Br-. FeF2 is more stable and less soluble, so it has less solubility product than FeBr2.

c) Fe3+ in Fe(OH)3 is hard acid than Fe2+ in Fe(OH)2. OH- is hard base so Fe3+ can form more srong bond with OH- and Fe(OH)3 is more stable and it has less solubility product.

d) There is steady decrease in size of ions or atoms from left to write in a period. So V2+ has larger size than Ni2+. Hence Ni2+ can form more stable bond with S2- and it is less soluble.