Stability constants, Kstab
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Define the stability constant, Kstab, of a complex ion.
Kstab is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions or molecules. A higher Kstab value indicates a more stable complex ion.
Write the expression for the stability constant (Kstab) of the complex ion [Cu(NH₃)₄]²⁺.
Kstab = [[Cu(NH₃)₄]²⁺] / [[Cu²⁺][NH₃]⁴]. Note that water is not included in the expression.
Describe how the magnitude of Kstab relates to the stability of a complex ion.
A large Kstab value indicates a greater tendency for the complex ion to form and remain intact in solution. This signifies a higher stability of the complex ion.
Explain ligand exchange in terms of Kstab values.
Ligand exchange occurs when a ligand with a higher affinity (forming a more stable complex with a higher Kstab) displaces a ligand with a lower affinity. The reaction favors the formation of the complex with the larger Kstab.
If [Ag(NH₃)₂]⁺ has a Kstab = 1.7 x 10⁷ and [AgCl₂]⁻ has a Kstab = 1.1 x 10⁵, which complex is more stable?
[Ag(NH₃)₂]⁺ is more stable because its Kstab value is significantly larger than that of [AgCl₂]⁻. This means Ag⁺ has a greater affinity for NH₃ than for Cl⁻.
The Kstab for the formation of [Fe(CN)₆]⁴⁻ is very high. What does this indicate about the complex ion?
A very high Kstab indicates that the [Fe(CN)₆]⁴⁻ complex is highly stable. The Fe²⁺ ion has a strong affinity for the cyanide ligands, resulting in minimal dissociation in solution.
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