VJC Prelim P2_Questions

 VJC 2017 9729/02/PRELIM/17 [Turn over 2 1 This question concerns some unusual nitrogen compounds. (a) Heating monomer X to 150 C produces the trimer, melamine, C3H6N6. Melamine is a cyclic planar molecule and is symmetrical. (i) Suggest a structure for melamine. [1] (ii) The average C–N bond length is 0.145 nm and the average C=N bond is 0.125 nm. Suggest why all the carbon-nitrogen bonds in melamine are intermediate in length between the given C–N and C=N values. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [1] (iii) Deduce the structure of X. Calculate the number of protons, neutrons and electrons in a monomer of X. [2] (b) Group 1 metal azides, MN 3, can be formed by passing heated dinitrogen oxide, N 2O over their corresponding amines, MNH2. (i) Explain why lattice energies of Group 1 azides become less exothermic down the group. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [1]

 VJC 2017 9729/02/PRELIM/17 [Turn over 3 (ii) Suggest why the azides become thermally more stable down the group. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [2] (c) Nitrogen has been used extensively in the research of compounds involving quadruple (bond order of 4) and quintuple (bond order of 5) bonds. These compounds typically involve transition metal atoms that are able to form bonds between themselves using their d orbitals. An example is given below: (i) Sketches of the shapes of the atomic orbitals from the d subshells are shown below, in random order. Name and label each orbital. ………………. ……………….

 VJC 2017 9729/02/PRELIM/17 [Turn over 4 ………………. ………………. ………………. [2] (ii) The d orbitals of an atom can overlap with d orbitals of the same type to form pi () and delta ( ) bonds. While a single sigma ( ) bond involves the overlap of two orbital lobes in total, and a single pi ( ) bond four lobes, a single delta ( ) bond involves the overlap of eight lobes in total. When two atoms overlap, the z-axis is used to define the internuclear axis. Suggest two different d orbitals that could be involved in delta bonds (). …………………………………………………………………………………………… [1] [Total: 10]

 VJC 2017 9729/02/PRELIM/17 [Turn over 5 2 Use of Data Booklet is relevant to this question. Aerozine 50 is a 50/50 mix of UDMH, (CH3)2N2H2 and hydrazine, N2H4. It is used as a rocket fuel, and is typically mixed with dinitrogen tetroxide, N2O4, as the oxidising agent. The equation for the reaction between UDMH and dinitrogen tetroxide under standard conditions is given as follows: (CH 3)