VJC Prelim P2_Answers

 VJC 2017 9729/02/PRELIM/17 [Turn over 1 Victoria Junior College 2017 H2 Chemistry Prelim Exam 9729/2 Suggested Answers 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. C N C N C N NH2 NH2 H2N C N C N C N NH2 NH2 H2N N N N NH2 NH2 H2N OR OR [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. The structure of melamine is resonance–stabilised due to delocalisation of p orbital electrons between carbon and nitrogen atoms. Hence all carbon-nitrogen double bonds are partial double bonds. [1] (iii) Deduce the structure of X. Calculate the number of protons, neutrons and electrons in a monomer of X. 22p, 20n, 22e– [2] (b) Group 1 metal azides, MN3, 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. Lattice energy  |q+.q– / (r+ + r–)|. Down Group 1, cationic radius increases, thus lattice energy becomes less exothermic. [1] (ii) Suggest why the azides become thermally more stable down the group. Down Group 1, cationic radius increases while charge remains the same. The charge density and polarising po wer of cations decrease. Extent of polarisation of anionic charge cloud N 3– is less. More energy is required to decompose and break the N-N bonds within N 3– down Group 1. Hence azides become more stable to heat. [2]

 VJC 2017 9729/02/PRELIM/17 [Turn over 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. dz2 dx2 – y2 dyz dxy

 VJC 2017 9729/02/PRELIM/17 [Turn over 3 dxz [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 (). d x2 – y2 and dxy [1] [Total: 10]

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