ConceptConceptDocsDocuments

CAIE A-Level Chemistry 28.5.4 Ligand exchanges

Practise predicting ligand exchange products and observations by comparing Kstab values for competing complex ions.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • predict the highest concentration complex when ligands compete for the same metal ion
  • use Kstab order to rank complex ions from least to most stable
  • link colour changes or no visible change to formation of the more stable complex

28.5.4—Ligand exchanges question 1

[Maximum number: 1]

Copper is a transition element and has atomic number 29.

Copper also forms the complex ions [Cu(NH3)2(H2O)4]2+\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+} and [Cu(en)(H2O)4]2+\left[\mathrm{Cu}(e n)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+} where en is the bidentate ligand ethane-1,2-diamine, H2NCH2CH2NH2\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}.

[Cu(H2O)6]2++2NH3⇌[Cu(NH3)2(H2O)4]2++2H2O[Cu(H2O)6]2++en⇌[Cu(en)(H2O)4]2++2H2O\begin{gathered} {\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+2 \mathrm{NH}_{3} \rightleftharpoons\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+2 \mathrm{H}_{2} \mathrm{O}} \\ {\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+e n \rightleftharpoons\left[\mathrm{Cu}(e n)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+2 \mathrm{H}_{2} \mathrm{O}} \end{gathered}

equilibrium 1
equilibrium 2

The table lists the values of stability constants for these two complexes.

Table for Question 28.5.4—Ligand exchanges question 1 — CAIE A-Level Chemistry A2

What do these Kstab K_{\text {stab }} values tell us about the relative positions of equilibria 1 and 2?

All question bank results loaded