Stability constants, Kstab
Cambridge A-Level Chemistry (9701) · Unit 28: Chemistry of transition elements · 6 flashcards
Stability constants, Kstab is topic 28.5 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 28 — Chemistry of transition elements , alongside Colour of complexes and Stereoisomerism in transition element complexes. In one line: 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.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 6 flashcards — 1 definition and 5 key concepts — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
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.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define the stability constant, Kstab, of a complex as the equilibrium constant for the formation of the complex ion in a solvent (from its constituent ions or molecules)
- write an expression for a Kstab of a complex ([H₂O] should not be included)
- use Kstab expressions to perform calculations
- describe and explain ligand exchanges in terms of Kstab values and understand that a large Kstab is due to the formation of a stable complex ion
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Stability constants, Kstab
- › Exclude the concentration of water and any solid species from the Kstab expression for complex ions.
- › Explain color as: d-orbitals split, light is absorbed for electron promotion/excitation, and the complementary color is seen.
- › Remember that electrons are removed from the 4s orbital before the 3d orbital when forming transition metal cations.
- › Check the group number to determine maximum oxidation states; for vanadium (Group 5), it is +5.
- › Define a ligand as a species that uses a lone pair of electrons to form a dative covalent bond to a metal ion.
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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Stability constants, Kstab sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Stability constants, Kstab deck
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