UK Chemistry Olympiad · 2025

Problems

Problem 1Fast-burning powders such as nitrocellulose are used in clay pigeon shooting cartridges; their burn rate is designed for lighter loads such as the 28 g clay pigeon load. Nitrocellulose is primarily composed of cellulose trinitrate, prepared by adding cellulose (empirical formula $\ce{C6H7(OH)3O2}$ ) to a mixture of concentrated nitric and sulfuric acid, giving cellulose trinitrate (empirical formula $\ce{C6H7(ONO2)3O2}$ ) and one other product. The nitronium ion $\ce{NO2+}$ , generated in the acid mixture, nitrates the alcohol groups. The nitric acid acts as a Lewis base and accepts a proton from sulfuric acid to form the intermediate $\ce{H2NO3+}$ , which then decomposes to the nitronium ion. (a) Write an equation for the reaction between cellulose and nitric acid to form cellulose trinitrate. (b) Write the equation for the formation of $\ce{H2NO3+}$ from nitric acid and sulfuric acid, and the equation for its decomposition to $\ce{NO2+}$ . (c) Complete combustion of cellulose trinitrate produces carbon dioxide, water and a gaseous element. Write the equation for the complete combustion of $\ce{C6H7N3O11}$ , and calculate the standard enthalpy of combustion using $\Delta_f H^{\circ}(\ce{CO}) = -110.5$ , $\Delta_f H^{\circ}(\ce{CO2}) = -393.5$ , $\Delta_f H^{\circ}(\ce{H2O}) = -285.8$ and $\Delta_f H^{\circ}(\text{cellulose trinitrate}) = -653.1$ kJ mol $^{-1}$ . (d) When a shot is fired there is not enough time to react with oxygen from air; instead cellulose trinitrate decomposes entirely to gaseous products (no $\ce{O2}$ formed) and the products reach temperatures over $200\ ^{\circ}\mathrm{C}$ . Write the equation for this decomposition and calculate the total volume of gaseous products from 5.00 g of cellulose trinitrate at $200\ ^{\circ}\mathrm{C}$ , in m $^3$ .Solutions: 1
Problem 2Astatine is the rarest naturally occurring element in the Earth's crust, with a total estimated mass of 30 g at any point in time; assume all astatine is the most stable naturally occurring isotope $^{219}$ At. (a) Estimate how many At atoms occur naturally on Earth at any point in time. (b) Bismuth, obtained as a by-product of lead purification, is concentrated by adding magnesium and calcium to a lead–bismuth mixture; bismuth forms alloy A containing 7.91 % Ca and 9.60 % Mg by mass, with no lead. Determine the empirical formula of alloy A. (c) Alloy A reacts with chlorine gas to form bismuth and two ionic compounds B and C in a 2:1 ratio. Write the formulae of B and C. (d) Astatine is made by bombarding $^{209}_{83}$ Bi with $\alpha$ particles; the nuclei fuse to form $^{x}_{y}$ At and two neutrons. Write the mass and atomic numbers of the astatine isotope. (e) Excess bismuth is removed with concentrated nitric acid, forming bismuth(III) nitrate, water and a colourless gas of molar mass 30.01 g mol $^{-1}$ . Write the equation. (f) BrAt is made in two steps: $\ce{Br2}$ reacts with D to give only E, and E reacts with $\ce{At2}$ to give BrAt and D; the mass spectrum of E shows two major peaks of nearly equal intensity at m/z 205.82 and 207.82. Write the formulae of D and E. (g) 3.60 g of BrAt is synthesised; the half-life of this At isotope is 433 minutes. How long (to the nearest hour) until only 3.65 mg of BrAt remains?Solutions: 1