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CAIE A-Level Chemistry A2 28.5.4 Ligand Exchanges Questions

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

CAIE A-Level Chemistry A2 28.5.4 Ligand Exchanges Questions 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 CAIE A-Level Chemistry A2 28.5.4 Ligand Exchanges Questions 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?

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