Electron Transfer Reactions · Paper 2 — Free Chemistry HL Practice Questions

Vanadium is a transition metal used in high-strength steel alloys. A student investigates the stepwise reduction of ammonium vanadate(V), $\text{NH}_4\text{VO}_3$, using zinc metal in acidic aqueous solution at $298\,\text{K}$. Standard electrode potentials (acidic solution, $298\,\text{K}$): $$\text{VO}_2^+(aq) + 2\text{H}^+(aq) + e^- \rightarrow \text{VO}^{2+}(aq) + \text{H}_2\text{O}(l) \quad E^\circ = +1.00\,\text{V}$$ $$\text{VO}^{2+}(aq) + 2\text{H}^+(aq) + e^- \rightarrow \text{V}^{3+}(aq) + \text{H}_2\text{O}(l) \quad E^\circ = +0.34\,\text{V}$$ $$\text{V}^{3+}(aq) + e^- \rightarrow \text{V}^{2+}(aq) \quad E^\circ = -0.26\,\text{V}$$ $$\text{Zn}^{2+}(aq) + 2e^- \rightarrow \text{Zn}(s) \quad E^\circ = -0.76\,\text{V}$$ Colours of vanadium species in aqueous solution: $\text{VO}_2^+$ (yellow), $\text{VO}^{2+}$ (blue), $\text{V}^{3+}$ (green), $\text{V}^{2+}$ (violet).