International Chemistry Olympiad · 2005

Problems

Problem 4Gold Capital of Asia. Chiufen, northeast Taiwan, was one of the largest gold mines in Asia. PART A. KCN extracts gold from ore: $\ce{4Au(s) + 8CN-(aq) + O2(g) + 2H2O(l) -> 4[Au(CN)2]-(aq) + 4OH-(aq)}$ . (4.A-1) State the geometry of $\ce{[Au(CN)2]-}$ . (4.A-2) How many grams of KCN are needed to extract 20 g of gold? ( $M(\ce{Au}) = 197$ , $M(\ce{KCN}) = 65.12$ g mol $^{-1}$ .) Aqua regia (3:1 conc. HCl: $\ce{HNO3}$ ) dissolves gold by redox: $\ce{Au(s) + 3NO3-(aq) + 6H+(aq) + 4Cl-(aq) -> [AuCl4]-(aq) + 3NO2(g) + 3H2O(l)}$ . (4.A-3) Write the half-reactions and the balanced equation. (4.A-4) Identify the oxidizing and reducing agents. (4.A-5) Given $\ce{Au^{3+} + 3e- -> Au(s)}$ ( $E^\circ = +1.50$ V) and $\ce{[AuCl4]- + 3e- -> Au(s) + 4Cl-}$ ( $E^\circ = +1.00$ V), calculate the formation constant $K_f = \frac{[\ce{[AuCl4]-}]}{[\ce{Au^{3+}}][\ce{Cl-}]^4}$ at $25\ ^\circ\mathrm{C}$ ( $RT\ln10/F = 0.059$ V). PART B. Gold nanoparticles (AuNPs). In the Brust-Schiffrin synthesis, aqueous $\ce{HAuCl4}$ is phase-transferred to toluene by tetraoctylammonium bromide, dodecanethiol is added, and excess $\ce{NaBH4}$ reduces Au(III) to AuNPs of controlled diameter. (4.B-1) Is this top-down or bottom-up? (4.B-3) What is the function of $\ce{NaBH4}$ ? (4.B-4) If the average diameter of an AuNP is 3.0 nm and the atomic radius of Au is 0.144 nm, estimate the number of Au atoms per nanoparticle. (4.B-5) Estimate the percentage of Au atoms located at the nanoparticle surface.Solutions: 1