9701 · 28.5
Stability constants, Kstab flashcards
Revision flashcards for Cambridge 9701 Stability constants, Kstab (syllabus 28.5). Flip, recall, then mark a real past-paper question.
Card
What is the stability constant, Kstab?
Kstab is the equilibrium constant for the formation of a complex ion in a solvent from its constituent metal ion and ligands.
Card
Write the general expression for Kstab.
For the equilibrium M(aq) + nL(aq) ⇌ [MLn](aq), the stability constant is Kstab = [[MLn]] / ([M][L]ⁿ).
Card
What does a large value of Kstab indicate?
A large Kstab (e.g., > 10⁴) indicates that the complex ion is very stable and the equilibrium lies far to the right, favouring the products.
Card
How are the units of Kstab determined?
The units depend on the stoichiometry of the formation equation. They are calculated by substituting and cancelling the units of concentration (mol dm⁻³) in the Kstab expression.
Card
What is a stepwise stability constant?
It is the equilibrium constant for the addition of a single ligand to a metal ion or a complex ion that already contains ligands. For example, K₁ is for M + L ⇌ ML.
Card
What is the relationship between the overall stability constant (βn) and stepwise constants (K₁, K₂, etc.)?
The overall stability constant is the product of the individual stepwise stability constants. βn = K₁ × K₂ × K₃ × ... × Kn.
Card
How can Kstab values be used to predict the outcome of a ligand substitution reaction?
The ligand that forms the complex with the significantly larger Kstab value will displace the other ligand(s). The equilibrium favours the formation of the more stable complex.
Card
What is the chelate effect?
The observation that complexes formed by polydentate ligands (chelating agents) are significantly more stable than those formed by a comparable number of monodentate ligands. This is due to a large, favourable entropy change.
Card
Why is the Kstab for [Cu(EDTA)]²⁻ much larger than for [Cu(NH₃)₄]²⁺?
EDTA⁴⁻ is a hexadentate ligand, so its complex benefits from the chelate effect, resulting in a large positive entropy change and thus a much higher stability constant compared to the complex with the monodentate ligand NH₃.
Card
A common exam trap: What is the difference between Kstab and Kc?
Kstab is a specific type of Kc. While both are equilibrium constants, Kstab specifically refers to the equilibrium for the formation of a complex ion from its components in solution.