International Chemistry Olympiad · 2016

Problems

Problem 1Nitrogen trifluoride is a surprisingly stable compound that was first prepared by the melt electrolysis of a mixture of ammonium fluoride and hydrogen fluoride. (a) At which electrode does $\ce{NF3}$ form? Write the balanced half-reaction for its formation. (b) The related compounds $\ce{NH2F}$ and $\ce{NHF2}$ are very unstable and form as side products. Which of $\ce{NF3}$ , $\ce{NHF2}$ , $\ce{NH2F}$ is expected to condense at the lowest temperature? (c) The N–F bond lengths in these molecules are 136, 140 and 142 pm; assign them using a simple electrostatic (partial-charge) model. (d) When $\ce{NHF2}$ is bubbled through a solution of KF in HF, a binary nitrogen–fluorine compound is obtained as a mixture of two geometric isomers; write a balanced equation for its formation. (e) $\ce{NF4+}$ salts form from $\ce{NF3}$ and $\ce{F2}$ in the presence of a suitable reagent — propose one and write the equation. (f) $\ce{NF4+}$ hydrolyzes to $\ce{NF3}$ and $\ce{O2}$ , but less $\ce{O2}$ than expected is often obtained; write the hydrolysis equation and a possible side reaction lowering the $\ce{O2}$ : $\ce{NF3}$ ratio. (g) A tetrafluoroammonium salt (a candidate solid rocket fuel) contains 65.6% fluorine by mass, all released as $\ce{NF3}$ and $\ce{F2}$ on heating, with $n(\ce{F2}) = 2.5\,n(\ce{NF3})$ . Determine the formula of the salt.Solutions: 1
Problem 2Copper(I) oxide $\ce{Cu2O}$ is a semiconductor used in solid-state electronics and solar cells. The cubic unit cell has lattice constant $a = 427.0$ pm, with oxygen atoms (A) forming a body-centered cubic sub-lattice and copper atoms (B) occupying positions forming a face-centered cubic sub-lattice. (a) What are the coordination numbers of Cu and O? (b) Calculate the shortest $\ce{O-O}$ , $\ce{Cu-O}$ , and $\ce{Cu-Cu}$ distances. (c) Calculate the theoretical density of stoichiometric $\ce{Cu2O}$ in g cm $^{-3}$ . (d) A non-stoichiometric sample has $0.2\%$ of all Cu atoms oxidized to $\ce{Cu^2+}$ . What percentage of Cu sites are vacant, and what is $x$ in $\ce{Cu_{2-x}O}$ ? (e) Write balanced equations for the reaction of $\ce{Cu2O}$ with: (1) atmospheric $\ce{O2}$ in humid air; (2) dilute sulfuric acid; (3) hot concentrated sulfuric acid.Solutions: 1