Question

Explain why [Ti(OH2)6]3+ has a pKa of 3.9. Be sure to discuss how aquated cations can function as acids and how the charge on the cation affects acidity.

Answer 1

when water molecules interact with metal ions of high charge and small size (or having a high charge density), the pH of a solution is lowered even though there is no proton donor in the aquated metal ions. This is due to the fact that the interaction of water molecules with the metal ions weakens the O-H bond in associated water molecules which enables them to release H+ ions into the solution thereby decreasing the pH.

[M(H2O)₆ ]ⁿ⁺(aq) + H₂O(l) ⇌ H₃O⁺ (aq) + [M(H₂O)₅ (OH)]^(n-1)+(aq)

Smaller and highly charged cations such as Ti³⁺ are very good at pulling away electron density from water molecules and therefore these aquated ions are acidic in nature,

[Ti(H2O)₆ ]³⁺ (aq) + H₂O(l) ⇌ H₃O⁺ (aq) + [Ti(H₂O)₅ (OH)]²⁺ (aq) pKa = 3.9