2018 H2 Chem P1 Q19 — Transition elements Solution

The answer is C.
Ligand exchange only means the ligands around the metal ion change but the oxidation state of the metal does not.
In C, chromium stays +3 throughout; a sulfate ion acts as a monodentate ligand and replaces one water ligand, which changes the colour from purple to green:
\([\text{Cr}(\text{H}_2\text{O})_6]^{3+} + \text{SO}_4^{2-} \rightleftharpoons [\text{Cr}(\text{H}_2\text{O})_5\text{SO}_4]^+ + \text{H}_2\text{O}\)
• A is wrong: Cu(II) and Cu(0) react to give Cu(I) (comproportionation), \(\text{Cu}^{2+} + \text{Cu} + 4\text{Cl}^- \rightarrow 2\text{CuCl}_2^-\), so it is a redox reaction, not ligand exchange only.
• B is wrong: zinc displaces copper, \(\text{Cu}^{2+}(\text{aq}) + \text{Zn}(\text{s}) \rightarrow \text{Cu}(\text{s}) + \text{Zn}^{2+}(\text{aq})\). Cu(II) is reduced to Cu and Zn is oxidised, so it is a redox reaction.
• D is wrong: \(5\text{Fe}^{2+} + \text{MnO}_4^- + 8\text{H}^+ \rightarrow 5\text{Fe}^{3+} + \text{Mn}^{2+} + 4\text{H}_2\text{O}\). Fe(II) is oxidised to Fe(III) and Mn(VII) is reduced to Mn(II), so it is a redox reaction.