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By the end of these notes, you should be able to:
Before we dive in, let's quickly recap what a complex ion is, because you need to understand this to make sense of K_stab.
A complex ion is a central metal ion (usually a transition metal, like Cu²⁺, Co²⁺, or Fe³⁺) surrounded by molecules or negative ions called ligands. A ligand is any molecule or ion that donates a pair of electrons to the central metal ion, forming a dative covalent bond (a bond where both electrons come from one atom — in this case, the ligand).
For example, when you dissolve copper(II) sulfate in water, the Cu²⁺ ion does not float around alone. Six water molecules immediately surround it, each donating a lone pair to the copper ion, forming the complex ion [Cu(H₂O)₆]²⁺.
When a metal ion is in solution, it forms a complex with the surrounding molecules or ions. This process is reversible — the complex can break apart and re-form. Because it is reversible, we can write an equilibrium for it.
For example, the formation of the hexaaquacopper(II) ion from its parts can be written as an equilibrium:
Cu²⁺(aq) + 6H₂O(l) ⇌ [Cu(H₂O)₆]²⁺(aq)
Just like any other equilibrium, we can write an equilibrium constant for this reaction. This special equilibrium constant has a specific name — the stability constant, K_stab.
Definition: The stability constant, K_stab, is the equilibrium constant for the formation of a complex ion in a solvent, from its constituent (individual) ions or molecules.
In plain English: K_stab tells you how easily and how completely a complex ion forms in solution. The bigger K_stab is, the more the equilibrium lies to the right — meaning the complex ion forms very readily and is very stable.
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