Vietnam National Chemistry Olympiad · 2025

Problems

Problem 1The gas-phase isomerisation between cis-but-2-ene and trans-but-2-ene is reversible and unimolecular, with equilibrium constant $K$ and forward/reverse rate constants $k_1$ and $k_{-1}$ : $\ce{cis-but-2-ene <=>[$ k_1 $][$ k_{-1} $] trans-but-2-ene}$ . Initially the system contains only cis-but-2-ene at concentration $C_s^0$ . (a) Write the differential rate equation for $C_s^t$ . (b) Derive the expression for the concentration ratio $C_s^t/C_s^0$ as a function of $t$ , $K$ and $k_{-1}$ . (c) When $K = 1$ , identify which of the following linear plots is valid: (H-1) $\ln(C_s^t - 0.5C_s^0)$ vs $t$ ; (H-2) $\ln(C_s^t)$ vs $t$ ; (H-3) $1/\ln(C_s^t - 0.5C_s^0)$ vs $t$ ; (H-4) $1/(C_s^t - 0.5C_s^0)$ vs $t$ . (d) At $T = 690\ \text{K}$ , $K = 1.14$ and $k_1 = 1.6 \times 10^{-6}\ \text{s}^{-1}$ . Calculate the time (in hours) required for cis-but-2-ene to convert to 30 % of its maximum equilibrium conversion. (e) Calculate the percentage of trans-but-2-ene in the mixture after 10 hours.Solutions: 1
Problem 2Cathodic protection is widely used to prevent metal corrosion by attaching a more active sacrificial metal to the structure. (a) In an experiment on protecting steel in seawater, a 25.0 g zinc block was attached to a steel apparatus. After some time, the block was reweighed at 28.0 g; assume that the only oxidation product is $\ce{Zn(OH)2}$ adhering to the block ( $M(\ce{Zn}) = 65.38$ , $M(\ce{Zn(OH)2}) = 99.40$ g mol $^{-1}$ ). Calculate the percentage of zinc that has oxidised. (b) Find the maximum service time (in hours) of this 25.0 g zinc block if the average protective current generated is 25 mA. (c) A steel ship hull with an immersed area of 1000 m $^2$ requires an average protective current density of 2.5 mA m $^{-2}$ . If zinc sacrificial anodes are used and 10 % of the zinc is lost to secondary processes, calculate the mass of zinc required per year (365 days).Solutions: 1